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EFFECT OF THE INVASION OF
‘DIPSACUS SYLVESTRIS 0N PLANT
COMMUNITIES IN EARLY
OLD'FIELD SUCCESSION

Thesis for the Degree of Ph. D.
MICHIGAN STATE UNIVERSITY
PATRICIA WERNER
1 9 7 2

II IIIIZI

31

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221

This is to certify that the
thesis entitled

Effect of the Invasion of Dipsacus givestris

on Plant Communities in Early Old-field

Suc c e 3 Si on
presented by

Patricia We rne r

has been accepted towards fulfillment
of the requirements for

PhD degree in Botany

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ABSTRACT

EFFECT OF THE INVASION OF DIPSACUS SYLVESTRIS ON PLANT
COMMUNITIES IN EARLY OLD-FIELD SUCCESSION

BY

Patricia Werner

The response of early old-field plant communities to
colonization by an experimentally introduced biennial
species, Dipsacus sylvestris Huds. (teasel) is studied over
a three-year period (1969 to 1971) in eight fields in
Kalamazoo County, Michigan. The dynamics of community
change in natural and teasel-treated areas were measured in
terms of changes in species composition, community diversity,
net primary productivity of species, various reproductive
strategies, and over—all community physical structure.

The study was designed to explore the response of a
plant community and the changes in the partitioning of the
site’s resources when a new plant species successfully in-
vades. The empirical evidence provided by this study should
contribute to testing theoretical models of species coloni-
zation and species co-existence.

Results showed that teasel communities had significantly
higher diversities (using the Shannon-Weaver function, H')

and greater "evenness" values (J') than check communities

Patricia Werner

from one to three years after teasel introduction. An
over—all increase in number of species other than teasel
was found in teasel communities.

Annual net primary productivity of the two communi—
ties was not significantly different when teasel was in
its rosette form. When teasel produced flowering stalks,
annual net primary productivity of the teasel community was
significantly greater than in the check communities. The
observed increase is attributed to teasel itself since the
productivity of individual indigenous species was the same
in both communities.

Qualities inherent in "biennialness" and in "tall
diffuse" morphology are discussed in relation to the effects
seen in this study. A conceptual model of terrestrial
secondary succession in plant communities based on these

data and current literature is proposed.

EFFECT OF THE INVASION OF DIPSACUS SYLVESTRIS ON PLANT

COMMUNITIES IN EARLY OLD-FIELD SUCCESSION

BY

Patricia Werner

A THESIS

submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Botany and Plant Pathology

1972

ACKNOWLEDGEMENTS

I gratefully acknowledge the patient guidance, time
and support of Dr. Stephen N. Stephenson, my major professor.
The enthusiasm and challenges of Dr. William E. Cooper
greatly stimulated my pursuit of this work. Dr. Peter G.
Murphy and Dr. Melinda Denton provided valuable suggestions
on the research and manuscript. During the initial stages
of this study, Dr. John E. Cantlon and Dr. John H. Beaman
were of much assistance.

Other persons who contributed their time and effort in
various ways include Bodil Burke, Christopher Wolf, Frank
Reed, Carol Heppe, and Darlene Valasek. Particular thanks
go to Earl Werner who often helped me sort out my thoughts
through lively discussion and buoyed my spirits with "good
times." Finally, the constant encouragement and three years
of assistance cheerfully given by Catherine Caswell have
made me infinitely grateful to her.

The research presented in this paper was supported by

National Science Foundation grants No. GB-6941X and 61-20.

 

ii

TABLE OF CONTENTS

INTRODUCTION . . . . . . . . . .
MATERIALS AND STUDY SITE . . . .
METHODS. . . . . . . . . . . . .
Treatment . . . . . . . . .
Field Data Collection . . .
Additional Determinations .

ANALYSIS . . . . . . . . . . . .

Statistics. . . . . . . . .
Diversity . . . . . . . . .

The Evenness Component of Diversity
The Variety Component of Diversity.

Productivity. . . . . . . .
Functional Groups . . . . .
Physical Structure . .
Biological Structure .

RESULTS. . . . . . . . . . . . .

Teasel Introduction . . . .
Teasel Effect on Diversity.
Number of Species. . .
Information Measure of
The Evenness Component
Productivity. . . . . . . .
Physical Structure . .
Biological Structure .

DISCUSSION . . . . . . . . . . .

Diversity

of Diversity.

I. The Effect of the Invasion of

Plant Communities. . .
Summary of Results . .
Diversity. . . . . .

Number of Species .

Evenness. . . . .'
Productivity and Niches.

iii

Page

12

12
13
15

20

20
20
25
26
26
29
32
37

4O

4O
41
41
50
61
64
74
79

89

89
89
9O
9O
92
93

TABLE OF CONTENTS-—Continued

II.

III.

Changes in Niche Size During Invasion. . .
Case I. . . . . . . . . . . . . . . .
Case II . . . . . . . . . . . . . . .
Case III. . . . . . . . . . . . . . .
Explanation: Life Forms . . . . . . . . .
Relationship to Nutrients . . . . . .
Relationship to Light . . . . . . . .
General Considerations of Life Forms.
Other Considerations of Results. . . . . .
Secondary Terrestrial Succession of Plant
Communities in Temperate Forest Areas. . .
Current Knowledge. . . . . . . . . . . . .
Primary Productivity and Succession .
Diversity and Succession. . . . . . .
Changes in This Study. . . . . . . . . . .
General Model. . . . . . . . . . . . . . .
The Relationship Between Productivity and
Diversity. . . . . . . . . . . . . . . . .
The Negative Relationship. . . . . . . . .
Lack of Relationship . . . . . . . . . . .
The Positive Relationship. . . . . . . . .
General Model. . . . . . . . . . . . . . .

LITERATURE CITED 0 O O O O O O O O O O O O O O O O 0

iv

Page

94
94
94
97
98
103
104
106
107

108
109
109
110
111
112

116
116
119
120
122

128

10.

11.

12.

13.

14.

LIST OF TABLES

Page
Mean Above-Ground Dry Weight of Teasel Rosettes
in Nine Diameter Classes. . . . . . . . . . . . l7
Above-Ground Teasel Biomass and Estimates of
Below—Ground Biomass (i'gms'm‘z). . . . . . . . 30
Corrected Values for Teasel Annual Net Primary
Productivity (i’gm3°m-2). . . . . . . . . . . . 31
The Physical Structure Classification . . . . . 35
Biological Structure. . . . . . . . . . . . . . 38
Percent Cover of Dipsacus . . . . . . . . . . . 42
Number of Plant Species in Teasel and Check
Communities . . . . . . . . . . . . . . . . . . 46
The Mean Difference, Over All Fields, Between
the Number of Species in Teasel Communities and
Check Communities . . . . . . . . . . . . . . . 49
Slopes of the Motomura Regressions as Measures
of the Variety Component of Diversity . . . . . 49
Differences in Diversity (H') in Teasel and
Check Communities in Eight Fields from 1968 to
1971. O O O O O O O O O O I O 0 O O O O O O 0 O 59
Differences in Evenness (J') in Teasel and
Check Communities in Eight Fields from 1968 to
1971. O O O O O O O O O O O O O O O O O O O I O 62
Above-Ground Productivity: Teasel and Check
Communities (fi'gms-m‘z) . . . . . . . . . . . . 65
Primary Productivity of Species in Field B in
1970 and 1971 O O O O O O O O O O O O O I O O O 72
Primary Productivity of Species in Field M in
1970 and 1971 . . . . . . . . . . . . . . . . . 73

FIGURE

1.

2.

10.

11.

LIST OF FIGURES

Distribution of Blocks and Teasel Introduction
Fields. 0 O O O O O O O O O O O O O O O O O O O

The Relationship Between Teasel Rosette Weight
and Teasel Rosette Diameter . . . . . . . . . .

The Technique Used to Estimate H' om with Corre—
sponding Variance Term. . . . . . . . . . . . .

Diagramatic Representation of Categories in
the Physical Structure Classification . . . . .

Percent Cover Values of Teasel in Each of Eight
Fields from August I968 to August 1971. . . . .

Graphical Representation of the Difference Be-
tween the Number of Species in Teasel Communi-
ties and the Number of Species in Check Commun-
ities (Nt — Nc) for Each of Eight Fields from

August 1968 to August 1971. . . . . . . . . . .

Diversity Measures for Teasel Communities and
Check Communities in Fields A and B from 1968
to 1971 . . . . . . . . . . . . . . . . . . . .

Diversity Measures for Teasel Communities and
Check Communities in Fields D and C from 1968
to 1971 . . . . . . . . . . . . . . . . . . . .

Diversity Measures for Teasel Communities and
Check Communities in Fields J and K from 1968
to 1971 O O O O O O O O I O O O O O O O O O O 0

Diversity Measures for Teasel Communities and
Check Communities in Fields L and M from 1968
to 1971 O O O O O O O O O O O O O O O O O O O 0

Net Primary Productivity in Teasel Communities

and Check Communities in Field B in 1970 and
1971. . . . . . . . . . . . . . . . . . . . . .

vi

Page

11

19

24

34

45

48

52

54

56

58

69

LIST OF FIGURES——Continued

FIGURE

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

Net Primary Productivity in Teasel Communities
and Check Communities in Field M in 1970 and
1971. O O O O O O O O O O O O O O O I I O O I O

The Physical Structure of Field B Expressed as
Percent Portions of Each Category for the Check
Community and for the Teasel Community from
1968 to 1971. . . . . . . . . . . . . . . . . .

The Physical Structure of Field M Expressed as
Percent Portions of Each Category for the Check
Community and for the Teasel Community from
1968 to 1971. . . . . . . . . . . . . . . . . .

The Physical Structure of Fields B and M Ex-
pressed as Productivity Values in Each Category
for the Check Communities and for the Teasel
Communities in 1971 . . . . . . . . . . . . . .

The Biological Structure of Field B Expressed
as Percent Portions of Each Category for the
Check Community and for the Teasel Community
from 1968 to 1971 . . . . . . . . . . . . . . .

The Biological Structure of Field M Expressed
as Percent Portions of Each Category for the
Check Community and for the Teasel Community
from 1968 to 1971 . . . . . . . . . . . . . . .

The Biological Structure of Fields B and M
Expressed as Productivity Values in Each Cate-
gory for the Check Communities and for the
Teasel Communities in 1971. . . . . . . . . . .

Diagramatic Representation of Three Cases Possi—
ble When a New Species Successfully Invades a
comun ity O O O O O O O O O O O O O O O O O O O

Diagramatic Representation of Relative Produc-
tivities of Plant Species in Field B, Teasel
and Check Communities, 1970 and 1971. . . . . .

Diagramatic Representation of Relative Produc~

tivities of Plant Species in Field M, Teasel
and Check Communities, 1970 and 1971. . . . . .

vii

Page

71

76

78

81

83

85

88

96

100

102

LIST OF FIGURES--Continued

FIGURE

22.

23.

Annual Primary Production During Secondary
Terrestrial Succession of a Plant Community . .
Relationships in a Plant Community Among
Resources, Plants, Consumers, and Decomposers,
Relative to the Potential Productivity,
Realized Productivity, and Indices of Diversity

viii

Page

114

124

INTRODUCTION

An understanding of the processes underlying com-
munity organization is central to the science of ecology.
In order to study such processes, one must be able to
detect and examine changes occurring in communities.
Accordingly then, succession, the developmental phase
of a sere, is one of the most fruitful areas of study for
ecologists interested in community dynamics.

Successional changes are directional (therefore
predictable), self-regulating, and culminate in a stab-
ilized community in which "maximum biomass and symbiotic
functions between organisms are maintained per unit of
available energy flow” (Odum, 1969). Once a steady—state
is reached, further change presumably occurs through the
longer—term process of evolution. \

Most studies of succession have been descriptions of
communities (or parts of whole communities, as plants,
insects, phytoplankton, etc.) in various stages of develop-
ment (Drew, 1942; Oosting, 1942; Keever, 1950; Bard, 1952:
Quarterman, 1957; Olson, 1958; Odum, 1960; Golley, 1965;
Margalef, 1965, 1967; Golley and Gentry, 1966; Witkamp,

Frank, and Shoopman, 1966; Monk and McGinnis, 1966; Cooke,

1967; Bazzaz, 1968). This approach yields information

about the structure of communities of various ages and
often allows one to make inferences about possible
mechanisms that account for the directional changes.

The time involved in community development general—
ly necessitates that comparisons be made across space
(sites) as well as across time (stages of development);
hence it is difficult to separate effects due to the
many variables on different sites and those effects due
to time (development). The ideal approach is one where
time is one variable and any others are quantitatively
and qualitatively identified. In this way one is more
confident in identifying processes responsible for given
changes in community structure. The experimental ap-
proach can often be useful in this respect. Recent in-
vestigators have explicitly called for experimentation
to help explain many-species interactions (Milthorpe,
1961; Pianka, 1966a; Miller, 1967, 1969; Cavers and
Harper, 1967; Whittington and O'Brien, 1968; Harper,
1969; Price, 1971).

Processes thought to be operating in the community
may also be isolated and experimentally tested in the
laboratory. Indeed, laboratory experimentation is often
necessary to understand phenomena observed in the field.
However, the inferences from such experiments are often
limited when one applies them to interpretations of the
complexities found in the natural system (Harper, 1964;

McIntosh, 1970).

Of course, the problems of obtaining experimental
data on the community level are great. Obstacles in—
clude the difficulty in replication of experimental
units, the length of time often required for changes to
occur, the uncontrollability of many variables, and the
still uncertainty as to which parameters are important
which results in the current time-consuming practice of
measuring "everything."

Experiments which have been performed on whole com—
munities have contributed insight into processes that
organize community structure (Likens, g§_§l,, 1967;
Simberloff and Wilson, 1969; Hall, Cooper, and Werner,
1970; Hurd gt 91., 1971; Stephenson, 1972). Additional
experimentation manipulating the biotic component against
a natural physico—chemical background would be in order.

Since every species found in a community was at one
time a successful colonizer on the site, the following
questions appear crucial: what allows the species to
become established at one time and not another? When a
new species successfully invades the community, what is
the response of the individual indigenous species? What
changes in the partitioning of the site's resources are
observed, if any?

Theoretical models of species colonization and
species co—existence have been developed that may predict

answers to these questions (Margalef, 1957, 1963;

MacArthur and Levins, 1964, 1967; Schoener, 1965;
MacArthur and Wilson, 1967; MacArthur, 1967, 1969, 1970;
Pielou and Pielou, 1967; Levins, 1968; Simberloff, 1969;
McNaughton and Wolf, 1970; Horn, 1971; Price, 1971).
Empirical evidence to test the theoretical models,
however, has mainly been limited to studies of pest out—
breaks, epidemic diseases, and post—disturbance changes
in natural populations of plants and animals such as
those discussed by Elton (1958). Studies of experimental
additions of a species to a natural community have been
concerned mainly with the population dynamics of the new
species rather than with community response (Sager and
Harper, 1960, 1961; Cavers and Harper, 1967; Putwain

and Harper, 1970). "It would be . . . convincing to be
able to show with appropriate controls, that the experi-
mental addition . . . of a species affects the realized
niche distribution of another. This has seldom been
attempted, in spite of the potential value of such experi-
ments" (Miller, 1967).

The objective of my study is to gain experimental
evidence on the response of early old-field plant communi-
ties to colonization by an introduced plant species,
Dipgacus sylve§§£i§_fluds. The dynamics of community change
over three years time in natural and treated areas were
measured in terms of changes in species composition,

community diversity, net primary productivity of species,

various reproductive strategies, and over-all community
physical structure. Interpretation of the results pro-
vides insight into some of the processes that may be
Operating in the develoPment of a plant community.

A generalized conceptual model of terrestrial succession
of primary producers based on these data and current

knowledge is proposed.

MATERIALS AND STUDY SITE

Two factors which are important in a study of the
response of a plant community to an introduced alien
species are (l) the selection of a plant species that
is easy to census, and (2) the presence of study sites
where the species is absent, even though it would not be
unusual to find it growing there. The latter require-
ment allows the experimenter to control the level of
input into the community and to compare treatment quadrats
with natural community quadrats.

In this study, Dipsacus sylvesgris Huds.,1 commonly
called teasel, was chosen to be introduced into early
old-field communities in Southern Michigan. Dipsacus is
usually found in Openings undergoing later stages of
succession, in meadows, and in ruderal communities where
turnover periods are longer than one year (Ehrendorfer,
1965). In Michigan, as in Southern Ontario (Cavers 22.21;:
unpubl.), Dipsacus seeds are normally dispersed in the
autumn within a few meters of the parent plant (Tallon,

unpubl.) and typically germinate the following spring,

 

1Or Dipsacus fullonum L. See Ferguson and Brizicky,
1965, for a discussion of the taxonomic dispute on the
binomial.

 

although a few seeds germinate throughout the summer
months. No cold treatment is required for germination
but perhaps an after—ripening period is necessary. Some
delay in germination to the second or third spring after
dispersal has been observed. Seedlings form rosettes
which, as their horizontally oriented leaves enlarge,
become physically oppressive to adjacent vegetation.

The rosettes overwinter and those surviving may produce

a flowering stalk 0.5 to 2.5 meters high in a subsequent
summer and die after seeds are formed. Only occasionally
will a rosette bolt and form a flowering stalk in the
first growingseason. As is common with most "biennials,"
the duration of the rosette phase is variable (Harper and
Ogden, 1970).

A census of the species is relatively simple since
individual plants are easily recognized in all stages of
the life cycle and no vegetative reproduction occurs.

The study area was a 100 x 100 meter portion of a
former corn field located on Midhigan State University
W. K. Kellogg Biological Station property at the inter—
section of Gull Lake Drive and B Avenue, Ross Township
(T. l S, R. 9 W.), Kalamazoo County, Michigan. The soil
is well-drained Fox Sandy Loam (Typic Hapludalf) on flat
to gently rolling glacial drift of Cary Age. The site
had been farmed since about 1850. Its more recent history

includes a hybrid walnut tree crop planted in 1938 (Holt,

1969, USDA photo BDB—3-50). General farming was employed
between 1950 and 1955 (USDA photos BDB-lG-95 and BDW—lP-47).
Between 1960 and 1964 the site was planted with wheat.
alfalfa, and corn. Fertilizer (250 pounds/acre 6—24-24
and 100 pounds actual nitrogen) was last applied in 1964
on a corn crOp. Various herbicides were last applied in
November 1962 and May 1963, for a demonstration of quack-
grass (Agropyron repens (L.) Beauv.) control.

Studies of old-field succession were initiated on the

site in the autumn of 1964 (Cantlon gt a1., unpubl.).

 

The area was divided into twenty-five 16 x 16 meter plots
separated by four-meter buffer strips, and grouped into
five blocks. Blocks III, IV, and V were established with
respect to topography; Blocks I and II, both on level
ground, were partitioned to minimize effects due to two

surviving black walnut (Juglans nigra L.) trees left in

 

the field. Each year from 1964 to 1968 one plot from each
block was selected at random, ploughed, and left fallow.
In 1970, the former 1965 plots were again ploughed and
left fallow. Davis (1968) and Cantlon gt al. (unpublished
data) have documented the plant community composition each
year from 1964 to 1971 in the 100 x 100 meter study area.
During this time Dipsacus was completely absent from the
naturally-occurring plant communities.

Sites for the present study were available only on

the east and west sides of each block in the large study

area. These strips of ground were approximately 2.5 x
13 meters in size, and were parallel to the plow furrows.
Eight of these sites were chosen in 1969, four in blocks
left fallow since 1967 and four in blocks left fallow
since 1968. Hence, the fields would be entering their
third and second growing seasons after abandonment,
respectively. For convenience, strips within the third
year (1967) blocks are designated Fields A, B, C, and D;
those within the second year (1968) blocks are designated
Fields J, K, L, and M. (Figure 1)

Vascular plant nomenclature follows Gleason and
Cronquist (1963). Voucher specimens, collected in c00pera-
tion with Darlene Valasek, have been deposited in the Beal—

Darlington Herbarium, Michigan State University.

10

F IGURE 1

Distribution of Blocks and Teasel Introduction Fields.
The numbered squares represent the portions of the study
area under study by Cantlon §E_al. The Roman numerals
indicate the block number, the arabic numerals indicate
the year of fallowing, and the letters indicate fields
used in the teasel introduction study.

11

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ic— H?
i -65 3'67 W16 v - E -
1‘70 _ - E.
.1 n
_ E-os -
“66 '-"°" 117-70 1' 2
IT
E-as _
1-68 _ m-67 TV— 7
fl-70 - -
J
(T
1‘64 11°64 '67 E-u
.J
FA— Fil "M1
71-65
“7'68 3-66 __
'— .m-7o
LI L. L

 

 

[I I Area of Dipsqcus and check quadrats

0 12119 Inn1-_nj 915:

FIGURE 1

 

METHODS

Treatment

Dipsacus seeds were collected October 15, 1968, from
a naturally-occurring pOpulation in Lenawee County
(Hudson Twp., Sec. 19), Michigan, and stored in ventilated
glass containers in the dark at room temperature (230—28o
C).

Within each study field 52 randomly selected one-half
by one-half meter quadrats1 were measured out and perman-
ently marked with wooden stakes. Half of these quadrats
were randomly selected to receive teasel seeds, the other

half were designated as "chec quadrats.
Seeds were sown at the rate of 150 seeds per treat—
ment quadrat during March 15-17, 1969. The seeds had
previously been divided into lots of 150 seeds in the
laboratory, put into sealed envelopes, and then opened in

the field and broadcast by hand to simulate the pattern

of natural dispersal.

 

1In Fields K and L, adjustments in number of quadrats
had to be made for two black walnut trees. Forty-eight
and eighty quadrats were selected, respectively.

12

13

Estimates of potential first-year field germination
were assessed from laboratory germination tests. In
February 1969, twelve lots of 50 seeds each were placed
on moist blotters in petri dishes, then three replicates
were left at room temperature (24°C) and the remainder
were put in a 4°C cold room. The seeds left at room
temperature showed 100%,germination after 8 days. The
cold treatment was terminated after the eighth day since
it was no longer necessary to determine the length of
time for any possible obligate cold period. Cold treat—
ment was not applied to seeds used in the field.

Estimates of potential second—year field germination
were assessed in similar laboratory tests conducted at
room temperature. Results showed germinability had dropped
to 63.3 i 12.0%m A tetrazolium test showed the ungermi-

nated seeds were dead.

Field Data Collection

In a subsample of 24 quadrats, teasel seed germié
nation and seedling survival were assessed from April 1,
1969, until June 1, 1969, the end of the initial germi-
nation pulse, each week marking newly-germinated teasel
seedlings with different-colored plastic toothpicks.
Second-year seedling germination counts were conducted in
the same way, only at 2-week intervals. Percent cover

readings and census of teasel plants by seedling, rosette

14

size, and flowering plant size were taken in each of the
study's 220 treatment quadrats within three days before
or after the following dates: September 1, 1969, June 1,
1970, September 1, 1970, June 1, 1971, and September 1,
1971.

Floristic composition was assessed in each treatment
and check quadrat by visual estimation of the percent
cover of each species as well as the recording of the life
stage of each species (seedling, rosette, flowering) in
August 1969 and August 1970. Floristic composition values
for 1968 were obtained from my analysis of unpublished
data collected by Cantlon _e_t._al., which was in the form of
estimates of percent cover for the species.

Above-ground standing crop of individual species and
their life stage was determined in August 1970 and August
1971. In each field a subsample of nine of the treatment
quadrats and nine of the check quadrats were randomly
selected for sampling. Vegetation within the vertical
boundaries of each selected quadrat was clipped at ground
level, placed in a plastic bag for transport to the labora-
tory, cooled to 4°C, separated by species and life stage,
then dried for 24 hours at 100°C, and weighed. The litter
(dead, horizontal plant material at ground level) was
similarly removed from each sample quadrat, transported to
the laboratory, separated into monocotyledonous or dicoty—

ledonous litter, dried, and weighed.

15

Flowering heads of teasel plants were removed prior
to seed dispersal in August 1970 with the exception of
one quadrat in each of Fields L and M. Establishment
success (ecesis) of the teasel pOpulation was determined
in June 1971, by counting new germinated seedlings in
natural vegetation near these untouched quadrats.

A three—year study of the population dynamics of Dipsacus
will appear at a later date.

Any quadrat that was clipped for sampling or had
flowering heads removed in 1970 was not chosen for clip-
ping in 1971.

The term "teasel community" as used in this paper
refers to the plant community within the boundaries of
quadrats sown with teasel seed (teasel quadrats). The
terms "indigenous community" or ”natural community" refer
to the plant community outside teasel quadrats. and usually

within marked check quadrats not treated with teasel seeds.

Additional Determinations

 

In August 1970, a separate study was made to determine
the relationship between the above-ground biomass and the
diameter of a teasel rosette in order to be able to esti-
mate dry weight without sacrificing the plant, i.e., by
measuring its diameter. One hundred and thirty-two
rosettes of various sizes were measured for diameter in the

field and then removed to the laboratory, dried at 100°C

16

for 24 hours, and weighed. The following weight—diameter
relationship was determined by regression analysis using
the method of least squares: y = 0.0466 + 0.0011x2

(r = 0.8561, n = 132), where x is the teasel rosette di—
ameter (median in each of nine classes) in centimeters and
y is the above-ground weight in grams (Table 1; Figure 2).
This mathematical relationship is used to estimate above—
ground dry weight of teasel rosettes for selected fields
in 1969.

Estimates of teasel below-ground biomass were obtained
by shoot/root ratio techniques (Bray, 1963; Monk, 1966a).
Whole rosette plants from field collections in June and
August, 1970, and from greenhouse plantings in March and
April, 1970, were dried at 100°C for 24 hours. divided into
shoot and root portions and weighed separately. A shoot/
root ratio of 5.66 t 0.92 (n = 96) was calculated for
teasel rosettes. Whole flowering plants were collected
in August 1970, and prepared similarly. Results yielded a
shoot/root ratio of 9.17 t 0.87 (n = 7) for the flowering

plants.

17

TABLE 1

MEAN ABOVE-GROUND DRY WEIGHT OF TEASEL ROSETTES
IN NINE DIAMETER CLASSES

 

 

Above—Ground Weight (gm§)_

 

Diameter class (cm) Mean Standard Error
<:2.5 0.0028 0.00004
2.5 5.0 0.0249 0.0052
5.1 12.6 0.1857 0.0195
12.7 17.7 0.3435 0.0375
17.8 27.9 0.8135 0.1741
28.0 35.5 0.8600 0.0748
35.6 50.7 2.1800 0.2458
50.8 60.9 3.4567 1.2952
61.0 72.0 4.7333 2.3447

 

18

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ANALYSIS

Statistics

Tests of significance are based on standard procedures
(t-test, Wilcoxon rank-sum), given by Steele and Torrie
(1960) and Sokal and Rohlf (1969). Tests of significance
between regression lines follow procedures presented by
Ostle (1963). Means cited in the text and in tables are
accompanied by their standard errors. Points on graphs

representing means are shown with 95%.confidence limits.

Diversity

Plant community diversity within each field was com-
puted using the Shannon-Weaver (1963) formulation
3
H 2-4? 911092 Pi
where s is the number of species in proportions p1, p2,
....p8. Diversity is equated with the amount of uncer-
tainty that exists regarding the species of an individual
selected at random from a pOpulation. Ecologists are

making increasing use of information content as a measure

of diversity. (MacArthur, 1955, 1964; Margalef, 1957, 1958a;

20

21

Hairston, 1959; MacArthur and MacArthur, 1961; Crowell,
1961; Patten, 1962; Paine, 1963, 1966; Lloyd, 1964;
Lloyd and Ghelardi, 1964; Pianka, 1966a; Pielou, 1966a,
b, c).

The Shannon formulation assumes random selection
and independent observations of units. Because of the
patchiness of vegetation, that is, the tendency for
species to occur in large single clumps, and the usual
necessity of measuring plants by weight or percent cover
rather than by discrete enumeration, it is impossible to
obtain a random sample of independent observations of
the species in a field. One quadrat will contain only a
small portion of the vegetation pattern and only part of
the species in the plant community. Therefore, any H'
(the amount of uncertainty per individual unit) calculated
on the species content within one quadrat will be smaller
than the H' calculated on the entire community and will
not be representative of the vegetation in the whole field
(McIntosh, 1962, 1967; Lloyd and Ghelardi, 1964; Pielou,
1966a, b, d; Margalef, 1967; Hurlbert, 1971). Special care
must be taken to ensure an accurate estimate of H'community
(H'Com) whenever one is considering communities of plant
species.

A good estimator of H' with corresponding variance

com
term was calculated for each field and treatment by the

method that follows (Good, 1953; Pielou, 1966a, b).

22

A total of 2 number of quadrats in each field were
examined and chosen in random order for the mathematical
operations. Hi is the calculated diversity of the first
quadrat. Data from the second quadrat are added to
those of the first and diversity is recalculated to obtain
(the diversity of the pooled data). Continuing, a
sequence of values H', H2, H5,...Hi,...Hé is obtained
which are the diversities per individual unit of the pooled
contents of the first k quadrats. A graph of the curve of
Hi against k shows Hi increasing with sample area, then
leveling off (Figure 3). A subjective decision is made
as to where Hi levels off; this k is labeled t. It is
correct to assume that t or more random quadrats provide
an adequate representation of the community.

The sequence {Hi} for k ;>'t are dependent estimates
of H'com and hence do not directly allow for a determina-
tion of standard error. However, a standard error can be
estimated as follows:

For each k ;> t, calculate the increment in diversity

per individual unit (hk) that results from adding the kth

quadrat to the first (k—l) combined quadrats:

”‘ka " Mitt-lHk-l
Mk " Mk-l

 

k _

where Mk 2 total units of all species in k-combined quadrats.

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24

m mgOHm

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1N.—

okzus 1:

 

25

For all k >-t, a sequence h h ... of independent

t+1' t+2'

random variables is obtained such that E(h )‘t’H'

t+r t+r

(where r = z—t).
Since when k.;7t, no change in diversity is expected

as sample size is increased, it follows that E(hk) = h»-
fi' d 1 v i? «7’ H' (B 1953 P' 1
com an a so ar ( ) = ar ( com) aer, , 1e ou,
1966a, b). All estimates of Hcom from 1969 to 1971 in
this paper were derived by the method above, and will be
designated H'

com°

The Evenness Component of_piver§ity

As a measure of evenness with which the total plant
biomass is divided among species, it is common to calcu-
late a ratio of the observed diversity to the maximum

possible for the same number of species (Pielou, 1966a, b):

H n H I
com com

 

com = H'

max log 3

where s = number of species. This same value is sometimes
calculated from the Shannon—Weaver equation directly, using
log to the base 5:

S

Jcom = _ E Pi logs pi

The evenness measure of diversity allows simple comparison
among fields and treatments since the maximum value of

Jcom is always 1. Again, valid comparisons of species

26

evenness are possible for collections of equal size or

if a variance is calculated (Hurlbert, 1971).

The Variety Component of Diversity

The measure of diversity that is very sensitive to
the variety (number of species) component of diversity
is the slope of the line resulting from the regression of
individual species biomass (loglo) determinations against
their respective ranks (Motomura, 1932). SlOpe values
are always negative, ranging from 0 to — 00, or from
maximum to minimum diversity; that is, as the slope ap—
proaches 0 diversity approaches maximum.

Slopes of the Motomura (1932) regression line, used
as a measure of the variety component of diversity, indi-
cated that teasel communities increased the number of

species in a field or had no significant effect.

Productivity

The above ground standing crop in a field of herbaceous
vegetation where all above ground parts die each winter is
a reflection of the annual net primary productivity of the
site (Wiegert and Evans, 1964; Golley, 1965). For any one
growing season, the following should be considered.
Annual plants and rosettes of biennials are produced during

a given growing season. Net primary productivity

27

contributed by biennials flowering during the same grow-
ing season can be compensated for by estimating and
subtracting the previous year's stored underground
reserves. Herbaceous perennials produce their above-
ground portions during the current season from either
stored or newly-made materials; if herbaceous perennials
have maintained or increased above-ground biomass over
the previous year, the increased yield most probably re-
flects net primary productivity of the current year.

Thus, in fields containing annuals, biennials, and
herbaceous Perennials (increasing or steady-state pOpula-
tions), it is valid to use standing crop biomass (dry weight
yield) as an estimate of annual net primary productivity.
This technique is especially useful in comparisons among
various treatments within the same field where the stand—
ing crop is expected to be the same throughout.

Of course, a measure of the above-ground standing
crop for a woody perennial does not give much information
about the net primary productivity of that particular grow-
ing season, so other estimates must be employed (Ovington,
1957; Whittaker, 1961).

The fields chosen for teasel introduction are composed
mainly of herbaceous plants as described earlier. A woody

perennial, Rhus typhina (staghorn sumac), is gradually

 

increasing in the larger 100 x 100 study area forming a

shrub canopy over the older fields. In the fields used in

28

this study Bh2§ is a recent invader and is patchy in
distribution, usually being recorded as a zero in any
sample quadrat. However, because of its relatively
greater biomass, Rhus represents 75% of the standing
crop in a few quadrats. Most of this weight is dense
stem tissue produced in a previous year and never
occupies more than two percent of the ground surface area
of the quadrat. In quadrats where §hg§_is recorded, the
plant composition and total biomass of the remaining
vegetation remain statistically unchanged from quadrats
lacking Rhg§_(Tab1e 12). Since this was true, I chose
(1) to eliminate the problem associated with the inclusion
of Rhg§_by subtracting woody perennial values to obtain
corrected figures of total biomass, and (2) to make con-
clusions only about the annual net primary productivity of
the herbaceous vegetation.

Herbaceous vegetation values were corrected further,
where necessary, for "biennialness." This was found to
be a minor correction in check communities since biennials
made up less than five percent of the total biomass.
However, in teasel communities, this became quite important.
The correction methods applied to estimates of plant
productivity for flowering teasels were designed so that,
in any one season, the maximum possible biomass (both above
and below ground weights) formed in rosettes the previous

year was subtracted from the biomass measurement for the

29

flowering plants formed in the current year. This tech-
nique assumes that all of the biomass in the previous
year's rosette was stored in underground parts during

the winter, and then emerged with the above ground parts
the next spring into the new flowering stalk. Even though
the estimate of annual net productivity is conservative,
it will add to the validity of later conclusions.

(Tables 2 and 3)

The study assumes equal within sample turnover,
export, and herbivory in the two communities (teasel and
check) in any one field. All standing crop (biomass) and
annual primary productivity values are given on the basis

of grams per square meter.

Functional Groups

Some ecologists have described vegetation on the basis
of plant life forms (Raunkiaer, 1934; Dansereau, 1951) or
horizontal layers (MacArthur and MacArthur, 1961; Golley,
1965). No causal factors were claimed in choice of cate-
gories, although in some studies these have become predic-
tive tools. For example, MacArthur and MacArthur (1961)
found that the number of bird species breeding in a small
uniform area could be predicted in terms of the layers of
vegetation and seemed independent of the number of plant
species (MacArthur, 1967). Other investigators have

recognized the possibility of taxocenoses (Margalef, 1967;

30

TABLE 2

ABOVE-GROUND TEASEL BIOMASS AND ESTIMATES OF
BELOW-GROUND BIOMASS

 

(f'gms~m‘2)

 

i Rosettes

 

fi'Flowering Plants

 

 

Year, Above Below Above Below
Field Ground Ground Total Ground Ground Total
1969
A 13.582 2.400 15.982 0
B 5.772 1.020 6.792 0
C 1.093 0.194 1.292 0
D 2.292 0.405 2.694 0
J 3.816 0.674 4.490 0
K 0.349 0.062 0.410 0
L 2.736 0.483 3.219 0
M 10.177 1.798 11.975 0
1970
A 25.715 5.123 30.838 40.000 4.360 44.360
B 14.467 2.233 16.700 0
C 1.257 0.251 1.508 0
D 2.743 0.549 3.292 0
J 2.350 0.470 2.820 3.900 0.425 4.325
K 1.600 0.320 1.920 0
L 17.880 3.576 21.456 2.933 0.320 3.253
M 10.200 2.040 12.240 8.600 0.937 9.537
1971
A 6.114 1.080 7.194 119.829 13.062 132.891
B 28.300 5.000 33.330 171.600 18.706 190.306
C 11.371 2.009 13.380 20.171 2.199 22.370
D 25.143 4.442 29.585 0
J 1.920 0.339 2.259 0
K 1.867 0.330 2.197 0
L 10.200 1.802 12.002 105.000 11.446 116.446
M 18.465 3.262 21.72? 204.600 22.303 226.903

 

Rosette S/R = 5.66
Flowering Plant S/R = 9.17 i 0.87

i 0.92

31

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32

Hutchinson, 1967), or assemblages of species populations
that are "likely to be of about the same size, to have
similar life histories, and compete over both evolution-
ary and ecological time" (Deevey, 1969).

In an attempt to look at the plant community in some
way other than as a collection of interacting taxonomic
species, I constructed two other sets of classifications
which might have biological significance, one based on
plant life forms and another based on reproductive

strategies.

Physical Structure

The delineation of categories in this classification
scheme was made prior to the collection of data in 1969.
Plants were recorded by physical form throughout the
study, in addition to species designations. The categories
include (1) forms with long, linear, mainly vertical
leaves, as grasses; (2) seedlings of herbaceous plants,
usually less than 5 centimeters in height; (3) rosettes,
usually over 5 centimeters in height and diameters greater
than height measurements; (4) tree seedlings; (5) "diffuse"
forms, 5 to 100 centimeters in height; (6) vines;
(7) "diffuse" forms, greater than 100 centimeters in height;
(8) shrub canopy; (9) appressed to the ground, living;
(10) on the ground, dead. Any single species does not
necessarily remain in the same category for its entire life

span (Figure 4; Table 4).

33

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35

TABLE 4

THE PHYSICAL STRUCTURE CLASSIFICATION

 

 

1.

Grass-like Forms

Agropyron repens
Agrostis stolonifera
Bromus inermis

Carex spp.

Dactylis glomerata
Digitaria sanguinalis
Juncus spp.
Muhlenbergia frondosa
Panicum capillare
Panicum spp.

Phleum pretense

Poa spp.

Setaria glauca
Setaria viridus

Seedlings of Herbaceous
Plants

Ambrosia spp. seedling
Asclepias syriaca sdlg.
Aster pilosus sdlg.

Aster sagittifolius sdlg.
Erigeron annuus sdlg.
Erigeron canadensis sdlg.
Erigeron strigosus sdlg.
Lactuca spp. sdlg.
Melilotus spp. sdlg.
Potentilla norvegica sdlg.
Potentilla recta sdlg.
Rumex acetosella sdlg.
Seedlings, unknown
Solidago spp. sdlg.

 

Rosettes

Achillea millefolium sdlg.
Barbarea vulgaris sdlg.
Circium spp. rosette

Daucus carota sdlg.
Dipsacus sylvestris rosette
Oenothera biennis rosette
Rumex crispus sdlg.
Taraxacum officinale
Verbascum thapsus rosette

4. Tree Seedlings

Acer rubrum sdlgs.

Acer saccharum sdlgs.
Cornus racemosa sdlgs.
Prunus virginiana sdlgs.
Rhus typhina sdlgs.

Diffuse Forms, 5-100

cm. height

 

Acalypha virginica

Adhilles millefolium
adult

Ambrosia spp. adult

Arabis spp.

Barbarea vulgaris adult

Berteroa incana

Capsella bursa-pastoris

Cerastium vulgatum

Chenopodium album

Euphorbia spp.

Galium spp.

Geranium spp.

Hieracium spp.

Hypericum perforatum

Lepidium spp.

Lotus corniculata

Lychnis alba

Malva neglecta

Medicago lupulina

Nepeta cataria

Oxalis stricta

Plantago spp.

Polygonum aviculare

Polygonum pensylvanicum

Polygonum persicaria

Potentilla argentea

Potentilla norvegica
mature

Potentilla recta mature

Rumex acetosella

Salvia spp.

Sonchus oleraceus

Stellaria media

continued

36

TABLE 4—-Continued

 

Thlaspi arvense
Tra90pogon pratensis
Trifolium pratense
Trifolium repens
Veronica arvense
Veronica peregrina

Vines

Lonicera spp.
Parthenocissus quinquefolia
Polygonum convolvulus

Ribes spp.

Rubus spp.

Vicia villosa

Vitis spp.

Diffuse Forms, > 100 cm.
height

Asclepias syrica mature
Aster pilosus mature

Aster sagittifolius mature
Aster hybrid mature
Circium spp. mature

Daucus carota mature
Dipsacus sylvestris mature
Epilobium angustifolium
Erigeron annuus mature
Erigeron canadensis mature
Lactuca biennis mature

10.

Lactuca canadenSis mature

Melilotus spp. mature

Rumex crispus mature

Solidago canadensis
mature

Solidago graminifolia
mature

Verbascum blattaria
mature

Verbascum thapsus mature

Shrub (canopy)
Rhus typhina

Appregged to Ground,
Living

Mosses

Tree trunk

On Ground, Dead

Bare ground, Rocks

Corn litter

Dicot litter
(excludes wood)

Monocot litter
(excludes corn)

Wood litter

Species names with author citation
may be found in Table 5.

 

37

Biological Structure

In this classification, categories include (1) annual
grasses; (2) perennial grasses; (3) perennial monocots,
exclusive of grasses; (4) summer annual dicots; (5) winter
and spring annual dicots; (6) biennials; (7) woody peren-
nials; (8) herbaceous perennial dicots; (9) mosses;

(10) miscellaneous (Table 5).

38

TABLE 5

BIOLOGICAL STRUCTURE

 

 

1.

Annual Grasses

Digitaria sanguinalis (L.)
Sc0p.

Panicum capillare L.

Panicum spp.

Setaria glauca (L.) Beauv.

Setaria viridus (L.)
Beauv.

Perennial Grasses

Agropyron repens (L.)
Beauv.

Agrostis stolonifera L.

Bromus inermis Leyss.

Dactylis glomerata L.

Muhlenbergia frondosa
(Poir.) Fern.

Phleum pratense L.

Poa spp.

Other Perennial Monocots

Carex spp.
Juncus spp.

Summer Annuals (Dicots)

Acalypha virginica L.
Ambrosia spp.
Cerastium vulgatum L.
Chenopodium album L.
Galium spp.
Lychnis alba Mill.
Malva neglecta Wallr.
Oxalis stricta L.
Stellaria media (L.)
Cyrill.

5. Winter, Spring Annuals

(Dicots)

Arabis spp.

Barbarea vulgaris R.Br.
Berteroa incana (L.) DC.
Capsella bursa-pastoris L.
Erigeron annuus (L.) Pers.

Erigeron canadensis L.
Erigeron strigosus Muhl.
Euphorbia spp.

Geranium spp.

Lepidium spp.

Medicago lupulina L.
Polygonum aviculare L.
Polygonum convolvulus L.
Polygonum pensylvanicum L.
Polygonum persicaria L.
Thlaspi arvense L.
Veronica arvense L.
Veronica peregrina L.

Biennial§,(Dicot§L

Daucus carota L.
Dipsacus sylvestris Huds.
Lactuca biennis (Moench.)
Fern.
Lactuca canadensis L.
Melilotus spp.
Oenothera biennis L.
Sonchus oleraceus L.
Tragopogon pratensis L.
Verbascum blattaria L.
Verbascum thapsus L.

Woody Perennials

Acer rubrum L.

Acer saccharum Marsh.

Cornus racemosa Lam.

Lonicera spp.

Parthenocissus quinquefolia
(L.) Planch

Prunus virginiana L.

Rhus typhina

Ribes spp.

Rubus spp.

Vitis spp.

continued

39

TABLE 5-—Continued

 

 

8. Herbaceous Perennials (Dicots)

10.

Achillea millefolium L.
Asclepias syriaca L.

Aster pilosus Willd.

Aster sagittifolius Willd.
Aster hybrid

Cirsium spp.

Epilobium angustifolium L.
Hieracium spp.

Hypericum perforatum L.
Lotus corniculata L.
Nepeta cataria L.
Potentilla argentea L.
Potentilla norvegica L.
Potentilla recta L.

Rumex acetosella L.

Rumex crispus L.

Salvia spp.

Solidago canadensis L.
Solidago graminifolia (L.) Salisb.
Taraxacum officinale Weber.
Trifolium pratense L.
Trifolium repens L.

Vicia villosa Roth.

Mosses
Miscellaneous

Bare ground, Rocks

Corn litter

Dicot litter (excludes wood)
Monocot litter (excludes corn)
Seedlings, unknown

Tree trunk (ground level only)
Wood litter

 

RESULTS

Teasel Introduction

Although teasel seeds were introduced at the same
rate and time in the eight fields, the success of teasel
germination and growth varied among fields due to the
interaction between the introduced teasel plants and the
natural vegetation. Success of teasel introduction was
examined in the light of the various ages of fields,
previous herbicide treatments, litter cover, amount of
bare ground, initial amounts of Agropyron repens, Eggs
typhina, and biennials, and the dominance and diversity
of the natural plant communities. A detailed accounting
and systems analysis of these and other factors as varia-
bles affecting teasel pOpulation dynamics in old fields
is in preparation (Werner and Caswell, unpubl.). Data
from a separate two-year field study on the effects of
litter on teasel invasion are also undergoing analysis
(Werner, unpubl.).

Since the current analysis deals with the effects of
teasel on the community enumeration data is not reported
here, but rather, measurements of teasel that relate it
to the other plant species, i.e., percent cover, standing

crop biomass, etc. To serve as background information,

40

41

percent cover values of teasel in each field from August
1969 to August 1971 are presented in Table 6 and Figure 5.
The total percent cover of teasel was estimated inde—
pendently from separate readings for rosettes and flowering

plants.

Teasel Effect on Diversity

Number of Species

A simple comparison of the number of plant species
found in teasel communities vs. check communities was made
in each field and at each sample time (August 1969, 1970.
and 1971). Results of a Wilcoxon rank—sum test showed that
over all fields and times the number of species in teasel
communities significantly exceeded that in check communi-
ties (P <:0.005, T=40, N224). This held true even when a
correction was made excluding Dipsacus in the species count
(Table 7; Figure 6).

Also, the difference between the number of species in
teasel communities and check communities, averaged over all
fields, increased each year after treatment (Table 8).

Later in this paper, Fields B and M are singled out
for further analyses; comparisons of slopes as a measure of
diversity are presented in Table 9 for these two fields.
For any one year and field, the t-value tests the hypothesis
that the slope values for the teasel and check communities

are the same. Results show that slopes of regression lines

PERCENT COVER OF DIPSACUS

42

TABLE 6

 

 

 

Rosettes Flowering
Alone Plants Alone Total Cover
Field Date x': s.e. i i s.e. i’: s.e.

A 8/69 10.06 2.46 0 10.06 2.46
5/70 17.06 4.29 3.46 3.09 20.52 5.09

8/70 19.84 4.83 1.65 0.96 21.50 5.28

5/71 19.22 5.87 2.78 2.26 22.83 6.39

8/71 16.64 4.40 14.72 5.56 29.52 7.36

B 8/69 7.01 1.42 0 7.01 1.42
5/70 22.13 3.34 0.87 0.87 23.00 3.86

8/70 24.54 3.73 1.15 1.15 25.69 4.02

5/71 28.73 3.83 10.48 4.02 41.29 7.74

8/71 40.10 4.77 11.14 4.16 53.50 7.86

C 8/69 1.16 0.35 0 1.16 0.35
5/70 5.39 1.13 0 5.39 1.13

8/70 6.87 1.63 0.69 0.69 7.56 1.97

5/71 15.32 2.97 2.63 2.63 16.37 3.37

8/71 19.21 3.76 3.00 3.00 22.21 4.09

D 8/69 1.40 0.35 0 1.40 0.35
5/70 5.07 0.91 0 5.07 0.91

8/70 6.65 1.27 0 6.65 1.27

5/71 20.42 3.61 0 20.42 3.61

8/71 28.37 4.91 0 28.37 4.91

J 8/69 4.80 1.37 0 4.80 1.37
5/70 6.58 1.99 0 6.58 1.99

8/70 5.49 1.74 0 5.49 1.74

5/71 4.31 1.93 0 4.31 1.93

8/71 5.06 2.36 0 5.06 2.36

K 8/69 0.59 0.18 0 0.59 0.18
5/70 2.09 1.09 0 2.09 1.09

8/70 2.55 1.20 0 2.55 1.20

5/71 3.00 1.38 0 3.00 1.38

8/71 2.25 1.43 0 2.25 1.43

continued

TABLE 6--Continued

43

 

 

 

Rosettes Flowering
Alone Plants Alone Total Cover
Field Date E i s.e. i’i s.e. i': s.e.
L 8/69 3.86 1.48 o 3.86 1.48
5/70 8.04 2.68 0.50 0.35 7.99 2.95
8/70 9.57 3.56 0.55 0.39 10.12 3.84
5/71 8.74 3.62 4.19 2.88 12.94 6.37
8/71 6.07 2.54 5.00 3.42 11.06 5.83
M 8/69 13.00 2.80 0 13.00 2.80
5/70 15.75 3.19 1.92 1.15 18.16 4.25
8/70 24.46 4.62 1.35 0.76 25.80 5.13
5/71 23.83 5.79 8.44 2.78 32.28 7.51
8/71 21.28 4.92 17.78 6.37 39.05 8.55

 

44

FIGURE 5

Percent Cover Values of Teasel in Each
of Eight Fields from August 1968 to
August 1971.

% Cover Ilium:

45

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

45‘ ~
30‘ A /o . J
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O /'
45‘ 5
301 D “ M /./
15‘ ’/////, ‘ o
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r . I ; ===g==
68 69 70 71 68 69 7O 71
Years

FIGURE 5

46

TABLE 7

NUMBER OF PLANT SPECIES IN TEASEL AND
CHECK COMMUNITIES

 

 

 

Teasel Comm. Check Comm. Difference
Field Date No. species No. species (Nt -
A 8/68 34
8/69 17 12 5
8/70 21 18 3
8/71 17 11 6
B 8/68 36
8/69 13 14 —1
8/70 2 25 0
8/71 18 14 4
C 8/68 34
8/69 9 9 0
8/70 19 17 2
8/71 17 17 0
D 8/68 43
8/69 15 17 -2
8/70 37 31 6
8/71 2 16 8
J 8/68 41
8/69 24 19 5
8/70 26 21 5
8/71 16 11 5
K 8/68 32
8/69 4 3 1
8/70 9 11 —2
8/71 5 3 2
L 8/68 41
8/69 23 21 2
8/70 32 31 1
8/71 14 10 4
M 8/68 38
8/69 13 14 -1
8/70 26 18 8
8/71 14 15 -1

 

47

FIGURE 6

Graphical Representation of the Difference Between
the Number of Species in Teasel Communities and

the Number of Species in Check Communities (N - N )
for Each of Eight Fields from August 1968 to AugusE
1971. The solid line indicates a base line where
there is no difference in number of species.

48

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FIGURE 6

49

TABLE 8

THE MEAN DIFFERENCE, OVER ALL FIELDS, BETWEEN THE
NUMBER OF SPECIES IN TEASEL COMMUNITIES

AND CHECK COMMUNITIES

 

 

 

 

Year d (Nt - NC) s.e.

1969 1.1250 0.9531

1970 2.8750 1.1716

1971 3.5000 1.0690
TABLE 9

SLOPES OF THE MOTOMURA REGRESSIONS AS MEASURES
OF THE VARIETY COMPONENT OF DIVERSITY

 

 

Teasel community

Check community t

 

Field Year Slope s.e. Slope s.e. value df P

B 1970 -0.3469 0.0543 —0.6690 0.1334 2.4461 14 *
1971 -0.2369 0.0200 -0.3074 0.0374 1.7625 28 n.s.

M 1970 -0.2316 0.0210 -0.5280 0.0744 4.3144 20 **
1971 -0.2922 0.0205 -0.2458 0.0226 -1.4900 25 n.s.

 

n.s. P> 0.05,

3..
ll

**

P<< 0.01

not significantly
0.01<P<0.05

different

50

in teasel communities are significantly greater than those
for check communities in both fields in 1970; hence, there

were a greater number of species.

Information Measure of Diversity

Estimates of community diversity with accompanying
standard errors were calculated in each community at each
sample point in time (Figures 7, 8, 9, and 10).

Differences in diversities (Hcom values) between teasel
communities and check communities in each of the eight
fields and for each time, were used in a Wilcoxon rank-sum
test to determine, on an overall basis, the effect of teasel
treatment on the indigenous plant communities. Results show
that teasel communities had a significantly higher Hcom
(P-< 0.005, T=29, N=24).

Within each field, a non—pooled t-test was used to
compare the two communities at each time (Table 10). In
three of the eight fields (A, B, and M) teasel communities
had significantly higher diversities than their correspond-
ing check communities for three summers (1969, 1970, 1971)
after teasel was introduced. In three fields (C, J, and K)
the diversity of the teasel community was significantly
higher than the check communities for two years after teasel
introduction. In one field (L) diversity was higher in the
teasel community for the first year only; in Field D, no

significant difference in diversity is demonstrated for any

year.

51

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61

It appears that the introduction of teasel tends
to increase diversity in almost all fields (87.5%) during
the first year after treatment, even when the percent
cover of teasel rosettes is as low as L% (as in Fields C
and K). This influence of teasel introduction on diversity
may continue for the following two growing seasons, either
leveling off, or continuing to increase. There was no
correlation between percent cover of teasel and the change

in diversity (3' ) (r = 0.5212, n = 24).
com

The Evenness Component of Diversity

In an attempt to determine if the higher diversity
in teasel communities was due mainly to a difference in
"evenness", H000 and standard error values were converted
to an H' and s.e° in base 3 (s = number of species),
designated Jcom’ The results of a Wilcoxon rank—sum test,
using values over all fields and points in time, show that
teasel communities have a significantly higher Jcom than
check communities (P<:0.005, T=53, N=24); that is, the
plant species are more evenly distributed in relative
amounts within the teasel communities than within the
check communities.

Examination of Jcom values in individual fields
(Table 11) show that in five of the eight fields, teasel

communities had a more even distribution of species than

did the check communities. Three of these (A, B, and M)

62

 

 

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64

show significant differences for three summers (1969, 1970,
1971) after teasel-introduction and two (K and L) show sig—

nificant differences for the first two years.

Productivity

Mean values of above—ground standing crop (grams
dry weight per square meter) and accompanying standard
errors, corrected for woody perennials and biennials, are
given in Table 12 for 1970 and 1971. These values esti—
mate the herbaceous plant above-ground annual net produc-
tion (henceforth called "productivity").

Differences in productivity between teasel and check
communities in each of the eight fields and for the two
sampling times were used in a Wilcoxon rank-sum test to
determine, on an over-all basis, the effect of teasel
introduction on the productivity of the indigenous plant
communities. Results failed to show any over—all effect
(P>0.05, T=44, N016).

When the fields with flowering plants of teasel
(A, 1970, 1971; B, 1971: C, l97l; J, 1970; L, 1970, 1971;
M, 1970, 1971) are considered separately from those con-
taining only teasel rosettes (B, 1970; C, 1970: D, 1970,
1971; J, 1971; K, 1970, 1971) and Wilcoxon rank—sum tests
are applied to each of the groups, the results are dif-
ferent. In fields where some teasel plants have reached

flowering stage, productivity is significantly greater in

65

 

 

 

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teasel communities than in check communities (P=0.027,
T=6, n=9). In fields where all teasel plants are in the
rosette stage, no significant differences in productivity
occur (PI>0.05, T=9, n27). That is, in fields where the
introduced teasel has developed to the point of producing
flowering stalks, there is a significant increase in
community primary productivity over that of the indigenous
non—teasel plant community.

A more detailed look at differences between teasel
community productivity and check community productivity by
individual field and date (Table 12) show significantly
higher productivities in teasel communities in Fields B
(1971) and M (1970 and 1971). These two fields promised
to be the most interesting to analyze further.

Graphic representation of 1970 and 1971 productivity
in Fields B and M are found in Figures 11 and I2. Here
it is more readily evident that (1) total community pro—
ductivity increased from 1970 to 1971, and that (2) teasel
communities had a greater productivity than check communi-
ties, the differences being accounted for by productivity
of the flowering plants of teasel.

Field B and Field M productivity totals are broken
down into species values in Tables 13 and 14. Where com-
parison of means and standard errors are possible, there is
no significant difference in the productivity of any
species (other than Dipsacus) between the teasel community

and check community for either 1970 and 1971.

68

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74

Thus, it would seem that not only does the total
community productivity increase when teasel is success—
fully introduced, but that the indigenous plant species
are relatively unaffected in their respective annual

accumulations of dry weight biomass.

Physical Structure

In each field, the teasel and check communities were
analyzed on the basis of the ten categories in the
Physical Structure classification (Figure 4 and Table 4),
and the results expressed as percent portions of the total
plant community for each year from I968-l971. Fields B
and M are used as examples. (Figures 13 and 14; categories
less than one percent are excluded.)” In the check communi—
ties of both fields, the general tendency is for the grass-
like forms to increase in relative amounts (from 72 and
71% to 100 and 96%, respectively), with decreasing values
for rosettes, 5-100 cm. diffuse forms, and >100 cm. dif-
fuse forms.

However, in the teasel communities of both fields,
the grass-like forms do not achieve such relative dominance.
Instead, the rosette forms increase greatly the first year
after teasel introduction (1969) (from 2 and 2% to 21 and
20%, respectively), then level off to between 3 to 7% for
the next two years. The 5-100 cm. diffuse forms achieve
the same relative percentages as those in the check communi-

ties, and are not found after 1969; the >100 cm. diffuse

75

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79

forms also mimic those in the check communities until the
third year after teasel introduction, when relative
amounts go from near 0 and 2% in 1970 to 28 and 3I%, in
Fields B and M, respectively.

Absolute values of above-ground standing crop ex-
pressed as mean grams dry weight in Fields B and M in 1971
are presented in Figure 15. These data support the idea
that (a) new physical form(s) (rosettes and >100 diffuse
forms) had been added to the indigenous plant community,
without decreasing the dominant grass-like forms in net

productivity.

Biological Structure

 

In each field, the teasel and check communities were
analyzed on the basis of the ten categories in the Biological
Structure classification (Table 5), and the results expressed
as percent portions of the total plant community for each
year from 1968 to 1971. Fields B and M serve as examples
(Figures 16 and 17; categories less than one percent ex-
cluded). In the check communities of both fields (in 1968,
respectively entering the second and first growing season
after abandonment), the general tendency is one found in
much of the literature on early succession (Costing, 1942;
Odum, l960f Bazzaz, 1968). There was an increase in rela-
tive amount of perennial grasses (from 71 and 67% to 99
and 96%, respectively) from 1968 to 1971, and a decrease in

annual grasses and annual dicotyledonous plants (from a

80

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Diffuse >100 cm. height

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FIGURE 15

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86

total of 26 and 29% to a total o'f‘O and 1%, respectively).

In these fields, neither biennials nor herbaceous

dicotyledonous perennials achieved more than a 7% portion

of the total check communities for the years 1968-1971.

However, in the teasel communities, a different pat—

tern emerged. Biennial reproductive forms expanded their

relative portions from 8 and 14% in 1969 to 32 and 29% in

1971, in Fields B and M respectively. All other reproduc—

forms except perennial grasses remained at the same
The peren-

titres
relative percentages as in check communities.
nial grasses became relatively less important, moving from
about 70% in 1968, up to 92%, then back to about 70% in
197 l - when the biennials greatly increased.

Absolute values of standing crop in mean grams dry
wei-Saint in Fields B and M in 1971 are presented in Figure
18 ° These data support the idea that a new reproductive
fc>3=Irt (biennials) had been added to the indigenous plant

Corrlqunity without decreasing the dominant perennial grasses

11" net productivity .

87

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88

 

 

 

 

 

 

 

 

 

 

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FIELD M

FIGURE 18

DISCUSSION

I° The Effect of the Invasion of Teasel
on Plant Communities

Summary of Results

 

Teasel communities had significantly higher diversi-
ties (H') and greater "evenness" values (J') than check
communities from one to three years after teasel introduc-
tion. An over-all increase in number of species other
than teasel was found in teasel communities. Annual net
primary productivity of the two communities was not sig-
nificantly different when teasel was in rosette form.

When teasel produced flowering stalks, annual net primary
productivity of the teasel community was significantly
higher than in the check communities. The observed increase
is attributed to teasel itself since the productivity of
individual indigenous species was the same in both communi—
ties.

When one looks at the fields as collections of certain
physical forms of plants, the increased productivity in the
teasel communities may be attributed to an increase in dif-
fuse forms over 100 centimeters in height. If the fields
are analyzed on the basis of differences in reproductive

strategies of plants, the increased productivity may be

89

t_)

0—4-
a

(D

y.

COM

, J i
:1

9O

attributed to an increase in biennials in the teasel com-

munities.

Diversity

The diversity index, H', is a function of both the
number of species and the "evenness" with which these
species are represented in relative numbers or biomass
within the community. Increases in species other than
teasel in the teasel communities had only a small effect on
increasing H' because the individual biomass values were
low. The successful addition of teasel itself was the main
contribution to a higher H' in the teasel communities.

Number of Species: An examination of "extra" species

 

in teasel communities shows that they were mainly dicotyle—
donous annual species, usually good colonizers (ggngg

Baker, 1965) taking advantage of any openings in vegetation.
Such species are usually found in fields of an earlier
successional status than the fields used in this study.

When a young teasel rosette died, an opening was left
in the vegetation which was quickly colonized by the "extra"
species. A second- or third-year rosette that is greater
than 20 centimeters in diameter may form a flowering stalk;
when this happened in the study fields the leaves of the
old rosette died back, thus forming litter and subsequent
openings in the ground layer vegetation. Such openings are
not found in natural vegetation where a perennial grass

(e.g. Agropyron) predominates. In such cases, there are

91

few rosettes (Figures 13 and 14). In effect, the teasel
rosettes acted as a perturbation at the ground level and
opened up space in which seedlings became established.
Where before the plant community structure was one of a
relatively homogeneous cover of perennial grass, it became
more heterogeneous, interrupted by patches of rosettes,
dead rosette leaves, and ultimately exogenous annual
species.

These rosette openings help explain the frequent
presence of the annual species found in the teasel communi-
ties, even when teasel rosettes occupied 1% cover in a
field. Each added species was represented mainly by
seedlings and had a mean biomass measurement of 0.1 to 1.0
grams/m2; in contrast, the very infrequent species found
in both teasel communities and check communities had mean
biomass measurements of less than 0:1 grams/m2. The added
seedlings were not observed to mature or to make up more
than I% of the biomass of the teasel community. They were
always dependent on the rosettes for their presence, and
did not reestablish themselves through reproduction on the
site. Evidence of a more slowly-growing perennial species
being established in these openings has not been detected
to date.

It has been pointed out that the openings in vegeta-
tion around a dying teasel rosette or flowering stalk might

be considered "islands“ to be colonized. However, patches

92

of open habitat are different from the oceanic islands
described by MacArthur and Wilson (1967) in that the space
surrounding the former is full of the colonizers, not
barren as an ocean. A constant overflow from adjacent
competing vegetation might not allow colonization by
species that immigrated some greater distance. Thus, some
species might find it harder to colonize a habitat island
than a true island.

Present competition theory cannot directly handle
second-order interactions such as occur when added species
enter a community with teasel. It is known that certain
species can provide spatial structure for the community
and may create another level of diversity which then results
in a potential increase in number of species (Margalef,
1958b; Whittaker, 1969). A someWhat analogous situation to
this teasel introduction study was found in field experi-
ments by Harper (1960) where the presence of wheat increased
the frequency of microsites suitable for poppy (Papaver sp.)
establishment.

Evenness; Evenness, measured by J', increased when
one species (teasel) showed an increase in net production
relatively greater than other species. In the few cases
where check communities were more "even" than the teasel
community (Fields C, 1971; D, 1970; J, 1970, 1971), the
total productivity of the teasel community had dropped from

the previous year due to a decrease in teasel, while the

93

total productivity of the check community increased as it

did in all fields.

Productivity and Niches

An expression of community evenness (especially when
measured in terms of productivity) is an expression of
total niche differentiation condensed into one term, J'.
Possible insight into the relationships of various indi—
vidual parts may be lost in the condensed term.

By niche differentiation I mean the manner in which
the site's resources are partitiohed among the biota,
creating certain sized realized niches for each population.
It is assumed here that a species population fills a reali-
zed niche, smaller than its potential or absolute niche,
and regulated mainly by competitive interaction with others
on the same trophic level which partially overlap, i.e.,
require parts of the same niche or common resource pool.

Numbers of and sizes of niches are hard to measure
since the investigator observes only realized niches, the
result of competition. As Connell and Orias (1964) have
pointed out, it is also impossible to define a priori how
many potential niches there are in an area since the
"number of niches is partially a function of the number and
type of species present." An estimate of relative niche
size of species within a community may be obtained from
production measurements if the realized productivity of a

species is assumed to have some correspondence to the

94

amount of resources it utilizes (Whittaker, 1969).

Changes in realized niche sizes is most easily observed
experimentally (Connell, 1961) or when a species invades

an area (Price, 1971). Data from well-designed and well—
executed experiments will be useful in testing theoretical
models of species packing, i.e., the number of species that
can coexist in an area (MacArthur and Levins, 1964, 1967;

Schoener, 1965; Levins, 1968; MacArthur, 1969, 1970).

Changes in Niche Size During Invasion

What happens to the realized niche sizes of indigenous
species when a new one on the same trophic level success-
fully invades? Conceptually, we may envision three cate—
gories of possible outcomes (Figure I9).

Case I: Total productivity of the site is increased
by some amount. The added amount may be equal to the pro-
ductivity of the new species, in which case a new niche
effectively has been added. Or, the added productivity may
be more than that of the new species and some one or more
indigenous species enlarges its previous realized niche
(positive feedback). In MacArthur's (1970) model of species
packing, a new species can enter the system where resource
utilization is not at its maximum if the addition of one
reproductive unit will produce a total utilization even
closer to the potential production of the community.

Case II: Total productivity is not changed. The new

species must appropriate resources (niche space) from one

95

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OH mmDOHm

 

97

or more of the indigenous species. That is, the new
species must help complete the total community utilization
of the site's resources (MacArthur, 1969, 1970). In this
case, the invading species must have competitive superior-
ity to survive (Price, 1971). A problem of interpretation
of results arises from the fact that it is impossible to
distinguish between smaller niches and increased niche
overlap (Pianka, 1966b).

Case III: To maintain a consistent argument, the case

 

is included where total productivity is decreased as the
new species is added. Such a situation might occur if
there were a large negative feedback to the other species,
such as might result from invasion by a fast—growing,
shading liana. It is thought that Case III is unlikely to
occur in the early stages of natural successional communi-
ties since the group of species that have been evolutionarily
selected to take part in the development of a plant commun-
ity probably do so with an ever greater utilization of site
resources (Harper, 1967a).

In all three cases above, second—order interactions
may occur among the indigenous species. That is, even though
as few as one indigenous species is directly in competition
with the new species, any changes in that one interacting
species will result in changes in other indigenous species.
Thus, internal community adjustments in individual species

productivity may occur secondarily to the direct effect of

98

the teasel introduction. (One example is the added species
in teasel communities discussed earlier.)

In the present study, teasel communities containing
only teasel rosettes did not significantly increase or
decrease total productivity over check communities (Case II).
A slight, but not statistically significant, decrease in

Agropyron was noted. Teasel communities containing teasel

 

flowering stalks increased the total productivity in an
amount equal to the productivity of the flowering stalk
(Case I), thus, in effect, exploiting new resources or
occupying a previously unexploited niche.

Figures 20 and 21 show the relative productivity of
various species in Fields B and M for 1970 and 1971. Here,
each enclosed area represents a species; the size of an
area represents the amount of production of that particular
species relative to the others.

Since teasel communities with only rosettes fit Case II
and teasel communities with flowering plants fit Case 1,,
it can be concluded that the rosettes compete for resources
but flowering stalks have effectively escaped competition

with the indigenous vegetation.

Explanation: Life Forms

How can these results be explained? Answers to this
question might be found in data showing differences in
reproductive and morphological groups between teasel com-

munities and check communities. Recall that the increased

99

 

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103

production of teasel communities could be accounted for
by an addition of biennials or by an addition of diffuse
forms over 100 centimeters in height. Teasel made up the
majority of the community biomass found in each of these
two categories. Qualities inherent in "biennialness" and
in "tall diffuse" morphology are most likely related to
the effects seen in this study.

Relationship to Nutrients: Many biennials, teasel
included, produce a long thick tap root which is thought
to serve as a storage organ during non-reproductive years.
In contrast, annuals generally have shallow diffuse or
relatively shallow tap roots and perennial grasses usually
have diffuse or rhizomatous underground parts. The effects
of plants with long tap roots "upon grasses is usually not
marked except where they occur in unusually dense stands
. . . (they) may have resulted from long adjustment to
competition with the roots of grasses" (Weaver, 1958). Any
competition that does occur between roots begins long before
shoots are sufficiently developed to cause serious mutual
shading (Donald, 1958, 1961; Aspinall, 1960; Milthorpe,
1961).

Results of the present study indicate that some compe-
tition occurs between teasel rosettes and indigenous
species since total productivity in the teasel community
does not increase above that of the check community and

resources are divided among the new species (teasel) and

c7:

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104

the indigenous species (Case II). More time than this
study allowed would be necessary to determine the final
outcome of the competition between populations.

The changing pattern of nutrient supply with time to
various plant species is unknown (Milthorpe, 1961).
However, it is speculated in this study that at some point
in development the teasel tap root reaches soil not yet
reached by the indigenous plants and is able to exploit
a new resource or a supply of common resources effectively
unavailable to the other vegetation at that time. Thus,
by the time the rosette produces a flowering stalk, the
plant has been released from much of its interspecific root
competition. It becomes greatly productive, effectively
not infringing on soil resources of other species (Case I).

Relationship to Light: The teasel flowering stalk, a
diffuse form greater than 100 centimeters in height, is
taller than the indigenous vegetation. This large new
physical form may have a mean percent cover value of up to
54% in a field and shades the indigenous species somewhat;
yet the productivities of the various indigenous species
are not significantly decreased. Apparently light is not
limiting in the community at that point in time.

The actual area of leaves per unit area of ground
(called the Leaf Area Index or LAI) is a relatively good
indicator of primary production (Whittaker, 1963, Harper,

1967b). In a community dominated by one species, the LAI

105

is not high enough to take maximum advantage of incoming
light, due to a self-shading effect. Golley (1965) used
figures of broomsedge production to estimate a reduction

of 70% in community production due to leaf attitude and
litter shading. An increase in the number of strata of
photosynthetic tissue could compensate for this loss by
more efficient use of the light (Odum, 1960). In fact,

the development of vegetational strata in communities by
the addition of species of varying heights and growth forms
is observed in plant communities. Horn (1971) has produced
a theoretical model of the development of plant communities
on the basis of light interception and selective advantages
of different morphologies at various points in time. Leaf
Area Indices of 5.0 (Brougham, 1958), 2.0 to 3.0 (Blackman
and Black, 1959), and 3.0 to 5.4 (Donald, 1963) have been
calculated as being optimum to trap 95% of the sunlight

in various plant mixtures. Apparently if there was any
increase in LAI in teasel communities over check communities,
it was not great enough to cause a reduction in effective
sunlight utilization by the indigenous species.

In summary, annual net primary productivity of the com-
munity was not influenced by teasel rosettes but was sig—
nificantly increased by teasel flowering stalks. The repro-
ductive strategy and physical form of teasel appear to

explain these results.

106

General Considerations of Life Forms: If teasel had
not been a biennial or a tall diffuse form, different
results might have been expected, depending on how much
teasel differed from indigenous forms. Also, if one of
these biological or physical forms had been in the natural
community as a dominant, teasel might not have been able to
compete with it successfully. The biology of each of the
organisms is important to the outcome, including combinations
of reproductive strategies, growth forms, physiological
requirements, etc. These qualities cut across taxonomic
lines that are themselves important in reproduction and
natural selection.

Functional groups of organisms may be the ecological
units of communities, not taxonomic species. Recognition of
a taxonomic species implies a recognition that there once
was some isolation that allowed divergence of characters,
usually floral, but this does not necessarily recognize
similarities in major functions of one species relative to
another such as productivity, mineral cycling, shading, etc.
Looking at each taxonomic species for answers to some types
of questions on the community level may be confusing to the
picture of the whole. In this study, reports of changes in
diversity and productivity of plant groups, based on repro-
ductive strategies and physiognomy, and ignoring individual
species, yielded information on possible mechanisms

responsible for observed differences; at the same time this

107

approach can save a vast amount of time in sorting field

samples.

Other Considerations of Results

Some consideration is given here to a discussion of
ground-level space in the communities. In the teasel
communities, rosettes of teasel often covered up to 50 to

80% of the ground. Agropyron percent cover readings were

 

80 to 90% in check communities and only 30 to 50% in

teasel communities. Even so, there was no significant
difference in production of Agropyron in teasel and check
communities. This lack of difference may have been due to
some decrease in competition in Agropyron in teasel communi-
ties which allowed increased densities in isolated micro—
sites; or the effect may have been due to an artifact of
the technique of reading percent cover estimates where
density is hard to account for. I tend to support the latter
explanation as the former one does not help account for the
increase in productivity of Agropyron in both communities
between 1970 and 1971.

Within both teasel and check communities, it can be
said that resources were not fully utilized in 1970 because
total production increased in 1971. Perhaps the reason that
the 1970 total community primary production is lower than
in 1971 is due in part to the allelopahthic qualities of
decomposing plant tissue of Agropyron (Welbank, 1960, 1963;

Grfimmer, 1961; Winter, 1961; Ohman and Kommendahl, 1964;

 

108

Carley and Watson, 1968). These parts might have inhibited
germination and growth of potential competitors while the

more slowly-growing Agropyron gradually increased in the

 

communities. The question remains in successional studies
as to the extent that specific toxic substances may be
responsible for the composition of plant communities
(Bonner, 1950; Rice, Penfound, and Rohrbaugh, 1960; Rice,
1964; Muller, 1966; Tukey, 1970).

Herbivory is also important in determining plant com-
munity structure (Odum, Connell, and Davenport, 1962,
Harper, 1969); however, few community studies have shown
the proportion of annual net primary production that is
eaten by herbivores. It is estimated from studies (Golley,
1960; Odum, §E_§l., 1962; Teal, 1962; Bray, 1964; Wiegert
and Evans, 1964) that 88 to 99% of the annual net primary
production is uneaten and subsequently enters the litter—
soil component (Weigert, Coleman, and Odum, 1969). In this
study, the amount of predation on the natural vegetation is
not known but is assumed to be equal in both teasel and
check communities. No evidence of mammal herbivory on
teasel plants was found, though results of minor insect

herbivory were occasionally observed.

II. Secondary Terrestrial Succession of Plant
Communities in Temperate Forest Areas

From the results of this study and current literature,

I will construct a generalized conceptual model of

 

109

secondary terrestrial succession of plant communities in
terms of primary productivity, diversity, and the series

of vegetational dominant forms.

Current Knowledge

Primarnyroductivity and Succession: By itself, a

 

measurement of net primary productivity is important in a
study of community eCOIOgy because it is a measure of
energy fixed by plants that potentially supports all life
in the community (Woodwell and Whittaker, 1968). Produc-
tivity, amounts of standing crop biomass, and diversity of
communities are thought to be related in some way to the
stability of communities (see Brookhaven Symposium, 1969).
It is assumed that both gross and net primary produc-
tivity on a particular site increase and level off at
climax (Whittaker, 1953, 1963, 1966; Monsi and Oshima,
1955; Olson, 1963: Takeda, 1961; Odum, 1969), perhaps de—
clining slowly after a maximum is reached (Loucks, 1970).
Some studies on the early stages of succession in labora-
tory aquatic microcosms (Beyers, 1962; Cooke, 1967; Margalef,
1968) and fields (Odum, 1960) show relatively higher primary
productivity initially, then a lowering to some relatively
steady-state level. The microcosms in the laboratory are
closed systems; in these systems more complex life forms
do not migrate in and become established as does happen in
later stages of natural terrestrial succession. It may be

that open systems do not always experience an initial

110

decline in net or gross productivity as do the closed
(Odum, 1969), but in general, the pattern remains the same.
The high levels of productivity at initiation of the
secondary successional sere can be attributed to "loose"
nutrients (Odum, 1960; Ovington, Heitkamp, and Lawrence,
1963; Golley, 1965; Cooke, 1967). The decline comes as
the available nutrient supply is depleted. A plateau
(Ryther, gt a1., 1958, McAllister, §t_§l., 1961) occurs
at some level determined by the decomposition rate of the

dead organisms, i.e., the rate of supply of "new,' avail-
able nutrients. It has been demonstrated that phytoplankton
productivity largely depends on nutrient availability
(regeneration), not standing crop of nutrients (Ketchum,
1961; Pomeroy, 1960, 1970). Laboratory microcosms main-
tain productivity at this first plateau; field terrestrial
productivity moves upward in a series of discontinuous

steps (Margalef, 1968) or relays (Dansereau, 1951) or
periods of adjustment toward specific levels (Odum, 1960;

Olson, 1963) for the particular community.

Diversity_and Succession. Diversity (number of species)

 

on any trophic level climbs steadily through the seral
stages (Whittaker, 1953, 1963, 1966; Connell and Orias,
1964; Odum, 1969; Wilson, 1969), depending on increased
number of niches as a result of increased biomass and
stratification (Odum, 1969). Auclair and Goff (1971) have

postulated that this is true for the more xeric or lowland

111

areas in the western great lakes region, but that develop—
ing communities in mesic areas experience a slowly declin-
ing number of species after an early peak.

Quality of species change more rapidly than do the
totals of gross or net primary production; often whole
species arrays change without affecting total productivity

(Odum, 1960; Golley, 1965).

Changes in This Study

In the current study, net primary productivity of the
community increased with the addition of teasel flowering
stalks. I would expect this new level of site productiv-
ity to drop and level off in time as the new pool of under-
ground nutrients is reduced to a steady state level and at
some value relative to the decomposition rate of the new
teasel litter. The new value will probably be higher than
that of the check community because there will probably be
more nutrients cycling in the teasel community; that is,
the new nutrients tapped by the teasel flowering stalks
will be potentially available to all of the plant species
on the site as the nutrients are released from the decompos-
ing litter.

At the end of the first three years of this study,
teasel and Agropyron were co—dominants. Shifts in species
dominance may occur as the teasel population exhausts its
exclusive source of nutrients. This latter factor is some-

what related to a similar situation that occurs when

 

112

prairie soil is broken and planted in alfalfa. The alfalfa

grows taproots up to 30 feet long and gives high yields for
3 to 4 years; however, subsequent yields are much lower
because of depletion of water and nutrients at those depths

(Kiesselbach, Russel, and Anderson, 1929).

General Model

Assuming diversity (number of species) to be increasing
in a relatively constant manner and primary productivity
to be increasing in steps"and plateaus during terrestrial
s uccession, I propose that each step in the productivity
c urve shows a peak and subsequent decline before leveling
o ff on some new higher plateau (Figure 22). The peaks in
F igure 22 could represent the invasion by perennial grasses,
1Z—Tl'len shrubs, then trees. Any new life form that is able
to survive and also tap some new resource will cause a
sudden increase in total productivity; this will peak and
then level off as part of the biomass of the new invader
eI'mters the decomposer pool.

The time period for each plateau and the distance
between plateaus is more predictable in earlier stages,
1:‘lmen progressively less so since many variables determine

the survival of the increasing number of species. Some

of these variables include the availability of propagules
(Bazzaz, 1968), which may be related to size of the area
(Golley, 1965; Davis, 1968), allelochemic effects of inter-

mediate successional species (Rice, 9; _a_l_., 1960), timing

 

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115

of life cycles (Keever, 1950), and different growth rates
(Bard, 1952) .

As each new life form invades, the fate of the forms
originally present is not known. The teasel introduction
3 tudy showed they are unaffected, at least initially.
Ewentually the earlier forms probably do decrease in pro-

dmctivity. This would help to explain the measurements in
c Jimax forest communities where the tree canopy productiv-
i ty is greater than that of the shrub layer which in turn
is greater than that of the herbaceous layer (Whittaker,
1 966) .

Again, the life form of the plant seems to be impor-

tant in describing one level of ccmmunity development.

Within each growth form, species composition may change

more than once. An example of species change within a
growth form may be found in the data of Cantlon gt; pi.

( unpubl.) where Poa sp. (bluegrass) replaces Agropyron

‘1: epens (quackgrass) after 3133; typhina (staghorn sumac)
enters the plant community.

Natural selection may be said to be operating on the
Species level and on a higher level, between whole groups
Of populations, selecting for various strategies which
allow more efficient environmental exploitation which then
results in increased total productivity on the site. This

strengthens the concept of a community as an integrated

whole and not merely an assemblage of individuals or even

taxonomic species.

116

III. The Relationship Between Productivity
and Diversity
A commonly-held notion is that productivity and
cizirversity are negatively related in communities. This
.r1<::tion has gained some support from information on yield-
diversity relationships in agricultural crops, from nutrient
enrichment studies, and from a misunderstanding of Margalef's
1Jl=ESe of the term productivity. On the other hand, Whittaker
( 21.966, 1969) finds no correlation between net or gross
];>J:rimary productivity and diversity of communities. Further,
<:=<3mparisons of climax communities on a worldewide basis,
‘tzilje results of this teasel introduction study, and Patten's
‘(IJL962) phytoplankton community show a positive correlation
between net primary productivity (biomass accumulation) and
CEl.‘:i_versity (number of species).
I submit that general statements about the productivity—
(Elaijersity relationship (henceforth called the P-D relation?
astit‘nip) can be made only within defined limits and that there

jL-ss only an indirect relationship between the two in any

<==Eise.

3E1t3e Negative Relationship

Some support for the notion of a negative relationship
between productivity and diversity has been gained from the
‘Iast amounts of information on yield-diversity relationships

in agricultural crops or weeds (Harper, 1967b). (See Reviews

in deWitt, 1960; Donald, 1963; Whittington and O'Brien,

117

1968; Loomis, Williams, and Hall, 1971.) Harper (1967b)
describes diallel analysis where pairs of species are
gnzrown together and in mixed stands for analysis of produc-
1:ji.vity. He states that a rigid demonstration that "a mix-
tztslre of plant species outyields pure stands seems not to
IIEEELVG been made." However, in those few studies where there
:i_=Es an increase in yield in mixtures, various explanations
linealve been given: the species were not synchronous in
‘Ejncrowth, reducing interference (Harper, 1967b), the species
‘nrweare of different growth habit (Baeumer and deWitt, 1968;
‘Nijbzittington and O'Brien, 1968), or the experiment was con-
(Elsuzcted for more than one growing season (Harper, 1961).
The answer to the contradiction lies in the degree to which
1:23k1e forms have been mutually selected, that is, their
" eacological combining ability" (Harper, 1964). The tech-
n ique used in paired species studies are not likely to
EEB<=>lve problems of the relationship between productivity and
(El-ijersity in natural communities (Harper, 1964; MacIntosh,
JL5370; Scarisbrick and Ivins, 1970).

When the changes in productivity and diversity are
‘Exatamined in nutrient enrichment experiments on communities,
jLtlitial results show impoverished fauna and flora (Patrick,

1949; Williams, 1964; Hall, Cooper, and Werner, 1970;
Eitephenson, 1972). Productivity increases and diversity
<iecreases; the result is a "bloom“ and the effects are

attributed to a release from competition with the fastest

118

growing populations taking advantage of the new nutrient

source. If one examines any available data from a later

date in similar studies, he often finds that the produc-

t. :‘Lvity has peaked and leveled off and diversity climbs

again. Thus, if one looks within one site or pond, the

cil‘nanges in productivity and diversity are inversely

r elated during the recovery period after the experimental
p erturbat ion .

Margalef (1969) hypothesizes a negative correlation

between productivity and diversity. It should be made

clear that Margalef almost always uses the term

" productivity" to mean the productivity to biomass ratio

( P/B) and states so in the beginning of most of his papers

( e.g., "Primary productivity per unit biomass will be

named here productivity, gross or net, " 1967, p. 260). He

also has said that productivity, meaning net primary pro-

ductivity per _sg, increases during succession, but that

the ratio (P/B) of primary productivity to total biomass

drops (Margalef, 1965, 1968). Indeed, his experimental

anuatic microcosms in the laboratory do show a negative
czorrelation between the P/B ratio and diversity (number of
species) .

Margalef rarely discusses net or gross primary pro-
ductivity pg; §§_and is more often concerned with the P/B
ratio which he relates to community stability and efficient

use of resources. Perhaps his system has a more or less

119

constant rate of net or gross productivity since his
1.aboratory aquatic systems are closed to the addition of
c>rganisms and/or nutrients from the outside. Frank (1968)
ss.tates that P/B goes down if one assumed constant produc-
't:.ivity throughout succession. However interesting, the
nur'alidity of the diversity-stability hypothesis is not en-
1:; irely evident from studies of plant communities (Loomis
_Sgggt al,, 1971).
Some investigators have attempted to support Margalef's
S tatements that productivity (meaning P/B) and diversity
are negatively related by correlating diversity with primary
‘];xroductivity only (McNaughton, 1968; Hurd 23 31., 1971).
Such a misapplication of Margalef's statements concerning
itihe P/B and diversity relationship hinders the accuracy of
jignterpretations of the investigators' results since they

«azure not referring to the same "productivity."

Lack of Relationship

 

Whittaker (1966, 1969) states that he finds no rela-
1tLionship between gross or net primary productivity and
diversity. Perhaps his results can be explained by con-
ssaidering scale; he is mainly comparing communities from

ssite to site within one geographical, climatic zone, as op-
Iposed to successional or nutrient-augmentation studies on
<one site, or a world-wide comparison of biomes. Most

likely the difference in productivity he observes is related

to fertility of the various sites as well as the stage in

120

succession. In pioneer stages of upland temperate forests,
ciiversity has been shown to be higher in mesic (fertile)
ssites and lower at both ends of the "fertility" scale
(TWhittaker, 1960, 1965, 1966; Monk, 1966b, 1967; Auclair
ea:.nd Goff, 1971). Then as the various seres move toward
<:=rlimax communities, diversity peaks and drops on the mesic
:55 ites, but slowly rises to some leveling-off point in both
‘t:;he less fertile (xeric and lowland) sites (Auclair and
(Cszoff, 1971). An understanding of this reveals that diversity
does not correlate with productivity in measurements across
iaLll these communities within one geographical region.
Ovington gt 31. (1963) and Monk and McGinnis (1966)
lhiave examined productivity and diversity, respectively, in
<::1imax and successional communities and found no time-
Jc-elated general pattern;‘ Ovington (1964), in a comparison
<:>f’net annual primary productivity in three ecosystems,
:EEound no difference between a maize field and oak woodland;
'1:>oth produced less plant material than a nearby savanna
«secosystem. Again, within one geographical region, compari-
sons among sites of various ages and management do not

?§?ield good correlations of diversity to productivity.

Ehe Positive Relationship

Comparisons of climax communities made on a more world-
xnide scale tend to show a general increase in annual net or
gross primary production toward the tropics (Ogawa, Yoda,

and Kira, 1961; Bray and Gorham, 1964; Whittaker, 1966).

121

This is perhaps partially dependent on soil fertility
(here, in terms of a faster turnover of nutrients due to
higher temperatures over the entire year), though the
productivity level varies greatly from region to region
within a climatic area. The greater diversity (numbers of
species) in the tropics is perhaps due to many factors,
only one of which may be higher primary productivity (more
resources to partition, MacArthur, 1969b), the others being
lack of thermal seasonality (MacArthur, 1969b), faster turn-
over rates (Olson, 1963; Margalef, 1968), longer evolution-
ary time (Wilson, 1969), and longer food webs (Hutchinson,
1959). (See Odum, Cantlon, and Korniker, 1960 and Pianka,
1966b.)

The results of the teasel introduction study show both
productivity and diversity increasing when teasel reaches
the flowering stage. Diversity was increased by both
teasel and "extra" annuals that invaded with teasel and
annual production increased as a new source of nutrients
enlarged the site's total potential for primary production.
Though productivity and diversity measurements may be
mathematically correlated positively, I am not prepared to
state that productivity and diversity are positively related
generally. Indeed, there is probably no direct relationship,
only an indirect one such that (if we insist on correlating
diversity and productivity) yields a positive correlation in

some situations and a negative correlation in others.

122

In studies over time on one site the positive or negative
correlations may merely reflect relative rates of change in

diversity and productivity.

General Model

Many of the processes in a terrestrial community that
determine productivity and diversity, and any subsequent
relationship between the two, depend upon (1) the amount of
available nutrients in the system (due to natural conditions
in terms of amount of water, soil pH and composition, etc.,
as well as initially "loose" nutrients present because of
artificial additions, fertilizer residues, initiation of
succession, or a new source), (2) the turnover rate of the
nutrients by decomposers, which in turn is regulated by
moisture content and temperature conditions, and (3) the
biology (physiology, life form, competitive abilities, etc.)
of the available organisms, the outcome of whose interactions
we record as diversity.

Figure 23 shows diagramatically the relationships
among these important factors. Any change in the amount of
any compartment (primary producers, consumers, decomposers,
nutrient pool) or in flow rates, whether naturally or
experimentally induced, will cause changes in the whole sys—
tem; also, considerable time, on the order of years, is
required for readjustment.

An experimental enrichment of a community directly

manipulates the resources by artificially increasing the

123

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125

amount of "loose" nutrients. If certain organisms are
present that can take advantage of the increased nutrient
pool by rapid rates of growth, they do so and increase
greatly relative to those with slower growth rates: the
result is lower diversity of the enriched community. The
same sort of situation holds true in aquatic laboratory
microcosms or terrestrial cropland after abandonment when
the initial amount of available nutrients is quite high.
In all the above cases, the initial increased amounts of
biota move eventually into the decomposer compartment and
the amount of nutrients in the system becomes dependent on
the rate of release from the decomposers.

The role played by detritus in nutrient regeneration
becomes more and more important through seral stages
(Margalef, 1968; Odum, 1969). In a study of revegetation
of ground by kudzu, Witkamp §t_§l, (1966) found an increase
in microbial activity over time up to "a fixed rate of break-
down for a given substrate . . . regardless of composition
or density of the microflora." Olson (1963) estimates a
matter of centuries for the decomposition rate in forests
to reach 95% of its steady-state level; thus these communi-
ties continue to show an increase in primary productivity
for that time. In the kudzu succession studies, large
portions of the cycling minerals (84% nitrogen, 79% phos-
phorus) were locked up in litter and soil dead organic

matter by the ninth year. Witkamp g§_§1, (1966) attributed

126

the leveling off of kudzu growth to stagnation of mineral
cycles. Thus, the potential productivity of communities
become dependent on the turnover of material within the
system more than the standing crop of nutrients (Ketcham,
1961; Pomeroy, 1960, 1970; Olson, 1963; Westlake, 1963).

When teasel was added to the plant community a new
source of nutrients was reached. Earlier I proposed a
general terrestrial succession.model which showed new life
forms (shrubs, trees, etc.) tapping new pools of nutrients
with increases in total amount in the living system. It
is evident that some organisms might increase the resource
(potential productivity) compartment.

A measure of diversity in the plant community reflects
the result of the competition among organisms. Again, the
plant biomass will eventually move into the decomposer
compartment, often yia_the consumers. The consumers may
also influence diversity in the plant community by differ—
ential feeding or by increasing competition (Odum §t_§l,,
1962; Harper, 1969).

Statements have been made that increased productivity
is generated by increased dominance (McNaughton, 1968),
and, alternately, that "species diversity increases produc-
tivity efficiency of the soosystem while dominance makes
the system stable, though less efficient for production"
(Singh and Misra, 1968). Golley (1965) relates productivity

and diversity directly with a "system of regulation of the

127

production process through the diversity of the vegetation."
Such statements of a direct causal relationship between
productivity and diversity bypass either the very important
decomposer role or fail to consider the "biology filter,"
and should be reconsidered.

Perhaps explanations of the productivity—diversity
relationship take on a hierarchial framework. One level of
potential primary production is set by the amount of light
exposure and the temperature and moisture regimes (thus,
by climate within a geographical area); another, lower,
level is set by the amount of available nutrients in the
system (fertility). A still lower level of realized pro-
duction in each locality is determined by the biology of
the organisms living there. A measure of diversity in the
plant community then reflects the outcome of the competi-

tion among these organisms.

LITERATURE CITED

L I TERATURE C ITED

Aspinall, D. 1960. An analysis of competition between
barley and white persicaria. II. Factors determin—
ing the course of competition. Ann. Appl. Biol.
48:637—654.

Auclair, A. N. and F. G. Goff. 1971. Diversity rela—
tions of upland forests in the western Great Lakes
area. Amer. Natur. 105:499-528.

Baer, R. M. 1953. Information theory in biology, p.
21-24. In. H. Quanstler led.) Information theory
in biology. Univ. Illinois Press.

Baeumer, K. and C. L. de Witt. 1968. Competitive inter-
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