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This is to certify that the

thesis entitled

Seasonal Conductivity Patterns in the Roots and Stems
of the Two Clonal Ring—Porous Trees,
Sassafras albidum and Rhus typhina

presented by

Laura L. Jaquish

has been accepted towards fulfillment
of the requirements for

Masters—_ degree in JeiencL

y

Major professor

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1M W14

 

SEASONAL CONDUCTIVITY PATTERNS IN THE ROOTS AND STEMS OF THE
TWO CLONAL RING-POROUS TREES, SASSAFRAS ALBIDUMAND RHUS
TYPHINA.

By

Laura L. Jaquish

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Botany and Plant Pathology

1998

ABSTRACT

SEASONAL CONDUCTIVITY PATTERNS IN THE ROOTS AND STEMS OF THE TWO
CLONAL RING-POROUS TREES, SASSAFRAS ALBIDUM AND RH US TYPHINA.

By
Laura L. Jaquish

Seasonal xylem (wood) conductivity and embolism (air blockage) patterns were
monitored in roots versus stems of two clonal tree species, Sassafi'as albidum and Rhus
typhina, throughout 1996 and 1997. Stems of both species were embolized in the early
spring following thaw, while the roots were already conductive. Stems became conductive
by late June following leaf expansion and maturation of the new earlywood vessels.
Stems reached their maximum hydraulic conductivity rates by mid-July and August, and
then became totally embolized by early November, following leaf drop. The dye patterns
in xylem growth rings indicated Rhus stems were only conductive in the currents year’s
growth ring. Sassafi'as stems were more variable in the number of conductive rings, but
most of the conductivity was in the outer growth ring. In contrast, the roots of both
species had many growth rings of conductive xylem throughout the monitoring period.
No positive root pressure was detected in either species. The stems of these ring-porous
species appeared to fit the expected pattern of high vulnerability to fieezing-induced
embolism. The maximum frost depth of 204mm :1: 11 for 1996 and 149mm :I: 10 for 1997
was much deeper than the sampled root depth of Rhus (71mm 2t 6; 57mm i 3) and but not
deeper than many sampled roots of Sassafi'as (225mm :i: 17; 299 :I: 35). Despite
occasional winter freezing of the soil water, the roots either avoided xylem embolism
altogether or they were able to reverse embolism by a mechanism other than positive root

pressures.

ACKNOWLEDGEMENTS

I would like to thank Dr. Frank Ewers for serving as my advisor and mentor. I
would especially like to thank him for all the invaluable advice and encouragement.
Thanks to my committee members, Dr. Stephen Stephenson and Dr. Frank Telewski, for
advice and editorial assistance. Special thanks to Michele Pruyn and Dr. Telewski for
help with NIH Image and the image analysis equipment. I would also like to thank David
Dominic with the DNR for permitting me to use state game area land throughout Mid-
Michigan.

Thanks to Tim Tibbets for showing me the ropes and teaching me how to operate
the ganglion of tubes and clamps also known as the Sperry Apparatus. And also for
instructing me in the creation of and implementation of bubble manometers. Many
thanks to the two undergraduates, Elizabeth Webster and Amy VanSluyters, who assisted
me in the field and in the lab. Amy VanSluyters accompanied me on excursions to
collect samples for hydraulic conductivity measurement and long hours of wrestling with
the Sperry Apparatus. Elizabeth Webster assisted in slide making and image analysis.

I would especially like to thank Cal Steinorth who never complained once about
accompanying me on pre-dawn bubble manometer checks in the dark with an extra dose
of mosquitoes. And for help scouting out Sassafi'as clone locations along miles of Mid-
Michigan dirt roads. Thanks for help sampling for hydraulic conductivity, for digging a
lot of deep holes with small hand trowels, for sporting the orange uniform while braving
the woods with me during various hunting seasons, and especially for supporting me

emotionally while I raced to finish my thesis just days before our wedding.

iii

TABLE OF CONTENTS

INTRODUCTION ............................................................................................................... 1
MATERIALS AND METHODS ......................................................................................... 4
Plant Material ........................................................................................................... 4
Hydraulic Conductance ............................................................................................ 4
Anatomical Study ..................................................................................................... 6
Phenology ................................................................................................................ 7

Root Pressures .......................................................................................................... 8

Frost Depth ............................................................................................................... 9
RESULTS .......................................................................................................................... .9
Percent Embolism .................................................................................................. .9
Specific Conductivity (Ks) ..................................................................................... 12
Anatomical Study ................................................................................................... 14
Percent Conductive Lumen Area ............................................................... 14

Number of Conductive Rings .................................................................... l6

Vessel Diameters ....................................................................................... 22
Root Pressures ........................................................................................................ 28
Frost Depth ............................................................................................................. 28
DISCUSSION .................................................................................................................... 33
LITERATURE CITED ...................................................................................................... 43

iv

LIST OF TABLES

Table 1: Clone locations for hydraulic conductivity sampling for 1996-97 and
1997-98 ................................................................................................. 5

Table 2: Comparison of mean maximum frost depth (11 = number of frost gauges) to mean
sampled root depth (n = number of root samples) for 1996-1997. .. ................................. 31

LIST OF FIGURES

Figure 1: Percent loss in conductivity due to embolism (i SE) for Sassafras albidum for
1996 and 1997 (temperature data for both years from St. Johns, MI) ............................... 10

Figure 2: Percent loss in conductivity due to embolism (i SE) for Rhus typhina for 1996
and 1997 (temperature data for both years from East Lansing, MI) .................................. 11

Figure 3: Seasonal maximum specific conductivity (Ks max) 1 SE for Sassafras albidum
in 1996 and 1997 ................................................................................................................ 13

Figure 3: Seasonal maximum specific conductivity (Ks max) i SE for Rhus typhina in
1996 and 1997 .................................................................................................................... 15

Figure 5: Percent conductive lumen area (i SE) for Sassafias albidum for 1997, before
embolism removal (initial) (11: 16 for roots and 8 for stems; "' stem value at zero) .......... 17

Figure 6: Percent conductive lumen area (:t SE) for Rhus typhina for 1997, before
embolism removal (initial) (n=16 for roots and 8 for stems; * stem value at zero) ........... 18

Figure 7: Seasonal change in the number of conductive rings (1 SE) of Sassafias albidum
for the 1997 growing season, before (initial) and after (final) removal of embolism (n=16
for roots and 8 for stems; mean root age = 5.70 i 0.27; mean stem age = 4.87 i 0.28)....19

Figure 8: Seasonal change in the number of conductive rings (i SE) of Rhus typhz'na for
the 1997 growing season, before (initial) and after (final) removal of embolism (n=l6 for
roots and 8 for stems; mean root age = 5.30 i 0.20; mean stem age = 3.27 :I: 0.23) ......... 21

Figure 9: Sassafras albidum mean conductive vessel diameters (i SE) for July,
September, and October/November 1997, before (initial) and after (final) removal of
embolism (n=16 for roots and 8 for stems; * stem value at zero; (I) indicates significance
between initial and final at .05 level) ................................................................................ 23

Figure 10: Maximum conductive vessel diameters (t SE) for Sassafras albidum for 1997,
before (initial) and after (final) removal of embolism (n=l6 for roots and 8 for stems; *
stem value at zero; (D indicates significance between initial and final at .05 level) .......... 24

Figure 11: Rhus lyphina mean conductive vessel diameters (i SE) for July, September,
and October/November for 1997, before (initial) and after (final) removal of embolism
(n=l6 for roots and 8 for stems; "‘ stem value at zero) ...................................................... 26

vi

Figure 12: Maximum conductive vessel diameters (:1: SE) for Rhus typhina for 1997,
before (initial) and after (final) removal of embolism (n=l6 for roots and 8 for stems; *
stem value at zero) ............................................................................................................. 27

Figure 13: Mean frost depth (:1: SE) for each of five sites in 1996-97 (dotted lines indicate
mean depth of sampled roots of Sassafias albidum and Rhus typhina) ............................ 29

Figure 14: Mean frost depth (i SE) for each of five Sites in 1997-98 (dotted lines indicate
mean depth of sampled roots of Sassafias albidum and Rhus typhina) ............................ 30

Figure 15: The number of sampled Sassafias albidum roots at different soil depths in
1997 (dotted line indicates maximum frost depth for winter 1996) .................................. 32

vii

INTRODUCTION

Seasonal patterns of xylem embolism (air blockage) in temperate woody plant
stems are well documented. Such stems Show the greatest percent embolism in the winter
(Cochard and Tyree 1990; Sperry and Sullivan 1992; Cochard et al. 1994; Sperry et al.
1994; Tognetti and Borghetti 1994; Magnani and Borghetti 1995; Hacke and Sauter
1996). However, seasonal studies of xylem embolism are lacking for roots of woody
plants that experience severe winter freezing conditions.

Freezing-induced embolism occurs in stems when the xylem sap freezes and air
bubbles are forced out of solution. Upon thaw, and especially when the stem xylem sap
is under negative pressure (tension), the air bubbles expand, forming embolisms that
block water transport. The larger the xylem conduit, the more vulnerable it is to
cavitation during fi'eeze-thaw events (Zimmermann 1983; Ewers 1985; Wang et al. 1992;
Sperry and Sullivan 1992; Lo Gullo and Salleo 1993; Sperry et al. 1994; Tyree et al.
1994; Sperry 1995). Increases in percent embolism have been correlated with the number
of freeze-thaw cycles (Sperry et al. 1988; Cochard and Tyree 1990; Sperry et al. 1994;
Wang et a.l 1992; Sperry 1993, 1995). Faster thaw rates can result in cavitation occurring
at higher (less negative) pressure (Langan et a1. 1997). In stems of Larrea tridentata the
minimum sub-freezing temperature impacts the amount of embolism. With freezing at —
9°C there was no increase in embolism, whereas minimum temperatures below -19°C
resulted in 100% embolism.

In general, stems of temperate woody plants are able to overcome the potentially

debilitating effects of freezing-induced embolisms. The two most well known
mechanisms for embolism recovery are replacement of embolized xylem by new xylem
production and refilling of embolized xylem by positive root pressure. In temperate ring-
porous trees the vessels of stems generally remain conductive for only one growing
season. Such trees can tolerate the loss of function in previous year’s growth rings
because of the production of new, wide diameter, earlywood vessels in the early spring,
prior to leaf maturation (Zimmerman 1983; Ellmore and Ewers 1986; Cochard and Tyree
1990; Sperry et al. 1994; Hacke and Sauter 1996). Xylem refilling by positive root
pressure has been reported for many temperate deciduous diffuse-porous trees and for
temperate species of Vitis, (Scholander et al. 1955; Sperry et al. 1987; Sperry et al. 1994)
but not for temperate ring-porous trees. Root pressures, which are able to dissolve
embolisms by raising the pressure within the xylem conduits to atmospheric pressures or
above, originate from water absorption into the roots. Positive root pressures have been
detected before dawn, during rainstorms, and in the spring before bud burst when
transpiration levels are minimal (Sperry et al. 1987; Cochard et al. 1994; Ewers et al.
1997; Fisher et al. 1997).

Some studies suggest that roots of diffuse-porous trees are potentially more
vulnerable to drought-induced embolism than stems. This is based upon vulnerability
curves of roots versus stems in Betula occidentalis (Sperry and Saliendra 1994) and Acer
grandidentatum (Alder et al. 1996). Because tree roots tend to have wider vessel
diameters than stems (Zimmermann and Potter 1982; Gasson 1985; Pate et al. 1995;
Gartner 1995; Ewers et al. 1997), roots could also be more vulnerable to freezing-induced

embolism.

Results suggested that in roots of the ring-porous tree Sassafi'as albidum, the wide
vessels remained conductive for many years, versus just one year for stems (Bosela and
Ewers, in preparation). The present study investigated the seasonal patterns of xylem
conductivity and embolism in the roots versus stems of two clonal tree species, Sassafias
albidum and Rhus typhina. My hypothesis was that the roots, but not the stems, would be
able to reverse winter, freezing-induced embolism.

Both species are ring-porous temperate deciduous trees that spread clonally from
root buds that arise from horizontally growing parent roots. The clonal growth habit of
the two species allowed for genetically identical samples to be taken monthly from clones
throughout 1996 and 1997. Frost gauges were used to determine if the soil water
surrounding the horizontal parent roots froze. In addition to monitoring the seasonal
change in conductivity and embolism, various phenological events were recorded
including bud burst, cessation of leaf growth, cessation of twig elongation, peak
flowering, appearance of fruit (Rhus only), leaf color change, and leaf drop. The
mechanism for possible recovery from winter embolism was also investigated using
bubble manometers to measure root pressure. In addition, crystal violet dye was used to
mark the conducting xylem elements in roots and stems throughout the study period, to

see which elements were more prone to embolism.

MATERIALS AND METHODS

Plant Material

The Sassfras albidum (Nutt) Nees and Rhus typhina L. clones sampled in this
study were located within Clinton, Ingharn, Gratiot, and Shiawassee counties in southern
lower Michigan (Table 1). Each stem arising fiom the horizontal root system was
considered a ramet. The smallest sampled Sassfias and Rhus clones had 119 and 161
ramets respectively. Four clones of each species were monitored in 1996-97 and eight in
1997-98 (Table 1). In 1996, 32 Sassfi'as and 34 Rhus ramets were sampled, versus 56
ramets of each species in 1997. Different clones of a species were separated from each
other by more than 160 m in each case, so each clone was assumed to be a separate genet.
All the sampled clones had many stems that arose from horizontally Spreading roots.

Hydraulic Conductance

Measurements were made as described by Sperry et al. (1988) and Tyree and
Ewers (1991). A single ramet from each of the clones was sampled per month throughout
the monitoring period. The horizontal root systems of the sampled ramets were
excavated to approximately 25 cm both proximally and distally. Roots were cut in the
field and ramets were transported to the lab in water. Stem, junction, and root segments
were then cut under water to approximately 14 cm in the lab and hydraulic conductance
per unit pressure gradient (K) and percent embolism were measured using a Sperry
Apparatus (Sperry et al. 1988). The cut ends of the segments were re-cut with a razor
blade and connected to sections of vinyl tubing. A solution of lOmol m'3 citric acid (pH

= 3) was used in order to discourage microbial growth. The solution was run through a

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0.2 pm Gelman filter. A pressure head of 2.5 kPa was used for measuring the
conductivity in the native state (Ki. initial) and a pressure of 173 kPa was used to flush out
any embolism. Volumetric flow rate was determined with a 0.1m] pipette and a
stopwatch. This process was repeated until a maximum value (K.I m) was achieved. To
determine which were the conductive xylem elements, segments were perfused for 15
minutes with 0.5% crystal violet to mark conductive vessels. Dyes were used both on
control, to represent the native or initial state, and on segments with the embolisms
removed (final). Specific conductivity (K,), (Kn) divided by the xylem area, was
calculated for all segments measured in 1996 and 1997. The xylem area was calculated
from the xylem diameter, with the pith area then subtracted.

Anatomical Study

To determine the number of conductive xylem growth rings, all segments were
cut approximately 3 cm from the flow end on a band saw and re-cut with a fresh razor.
The smooth end was then examined under a dissecting microscope so that the number of
xylem growth rings and the number of active rings could be determined. A growth ring
was not counted as conductive unless it had more than 20 conductive vessel elements in
transverse view.

To determine diameters of conductive vessel elements, all segments from July,
September, October, and November of 1997 were cut transversely with a sliding
microtome set at a thickness of 40 um. Sections were taken through an ethanol-xylene
dehydration series and mounted in permount. Vessel lumen areas were measured using a

light microscope interfaced with a high-resolution CCD video camera and multi-scan

analog monitor (Dage-MTI, Inc) and a computer running NIH Image 1.5. All the vessels
in a pie-shaped wedge, from the center to the outer edge of a transverse section and
bordered by rays were sampled for each slide. A minimum of 100 vessel lumens were
measured per stem, root, or junction. In some of the smaller roots all the vessels in the
transverse section were measured. This method allows all vessels above a set minimum
to be measured within a designated area. Vessel diameters less than 25 um were excluded
in order to avoid including fibers within the measurements. Vessel diameters were
calculated from lumen areas.

Macerations were done to determine vessel diameter for comparison with the
image analysis technique. Wood fiom the November 1997 collection was slivered from
the vascular cambium to the pith using a fresh razor. Material from all the segments was
separated into three categories, root, junction, and stem. Slivered wood was placed in
Jeffery’s solution (10% chromic acid: 10% nitric acid) and kept at 60°C for 36 hours.
The macerated shavings were washed with water and pelleted by centrifugation up to
three times. The pellet was transferred into a vial of 70% ethanol. Macerated samples
were stained with 0.1% methylene blue. Vessel members were sampled randomly in the
fashion described by Ewers and Fisher (1989). A total of 100 vessels were sampled for
roots, junctions and stems of each species. Vessels diameters were measured with an
optical micrometer.

Phenology
One sample ramet from each of the 16 clones was monitored throughout the 1997

growing season for recording phenological events and results were averaged for each

species. Methods were adapted from Gilbert (1961), Lechowicz (1984, 1995), McGee
(1986) and Wang et al. (1992). Ramets chosen for phenological observation were similar
in size to ramets sampled for hydraulic conductivity. Five marked buds per tagged ramet
were monitored for bud burst beginning in early May 1997. Twenty-two marked leaves
in Sassafras and 16 in Rhus were measured weekly throughout the growing season in
1997, to generate the mean date of cessation of leaf growth. The same method was used
to generate the mean date of cessation of twig elongation. Eight twigs were tagged and
monitored per species. Peak flowering was described as mean date at which greater than
50% of the individual flowers on an inflorescence were open. Appearance of fruit was
noted only in Rhus. Mean date of fruit visibility was the date by which all monitored
female ramets had visible fruit. Mean date of leaf color change was calculated by
averaging the dates that colored leaves appeared in each clone. Leaf drop was defined as
the mean date by which all leaves on monitored ramets had fallen. Data was recorded
every week through September and then every other week through leaf drop.
Root Pressures

The possibility of positive root pressure was investigated using bubble
manometers (Fisher et al. 1997). For both species in 1996 and 1997, xylem sap pressures
were measured on both junctions and horizontal roots in the early spring following soil
thaw but before bud burst. Measurements for Sassafras began the first week and
continued through the third week of May. For Rhus, measurements were taken in the first
week of May and June. Root pressures were measured on at least one individual per
clone for both species in 1996 and 1997. Manometers were booked up in the evening and

checked the following morning before dawn. The root or junction was then re-cut with a

fresh razor and the vinyl tubing reattached. The same root or junction was checked for
three consecutive days after which a new root or junction was selected.
Frost Depth

The depth of soil frost was monitored for the winters of 1996-97 and 1997-98
with frost gauges (Rickard and Brown 1972; McCool and Mohnau 1984; and Saul and
Potts, 1996). Five sites were monitored each winter, two of which remained the same for
both years (Table 1). Each site had five gauges that were placed throughout the clone.
Gauges were constructed of 1.5m lengths of 1.9cm ID PVC with rubber stoppers at each
end. The interior tubes were constructed of 1.2m lengths of 1.6cm OD clear vinyl tubing,
stoppered at both ends and filled to within 25.4cm of the top with distilled water dyed
with 5 drops of 0.1% methylene blue. The inner vinyl tubing was connected by nylon
string to an eye on the inner side of the top stopper of the outer PVC tube to facilitate
removal for measurrnent. Depth of frost was measured on the vinyl tube from ground
level down. As the ground froze, the water in the tube would freeze and the blue dye
would settle out below the frost line. Gauges were inserted 1.0m into the ground. Frost
depth was measured every other week from the first frost through the last fi'ost and on any

date when the air temperature was colder than had previously been recorded that season.

RESULTS

Percent Embolism
In 1996, Sassafras stem segments became conductive in late May and became

fully embolized by November (Figure 1). Roots remained conductive from the start of

% Loss in Conductivity due to Embolism

 

 

 

 

 

 

 

 

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Figure 1: Percent loss in conductivity due to embolism (1 SE) for Sassafras albidum for
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11

sampling in July through November. In 1997, Sassafi'as stems and junctions remained
fully embolized through May (Figure 1). Stems and junctions became conductive by late
June to early July, as the new current year’s earlywood (EW) became conductively active.
This was well after bud burst and peak flowering. Embolism was lowest in August but
began to increase in September as leaves changed color and then became fully embolized
by October following leaf drop but before the first hard freeze. Roots of Sassafras
remained conductive throughout the entire monitoring period. Root embolism increased
from 25% in April to around 70% as junctions and stems became conductive in late June
and early July.

In 1996, Rhus stem segments remained fully embolized until July and became
completely embolized again by November (Figure 2). Junctions also became fully
embolized by November. Roots remained conductive throughout the monitoring period.
Rhus stems and junctions remained fully embolized through May in 1997 (Figure 2).
Stems and junctions became conductive in J unc following bud burst, as new earlywood
became conductive. Stems became fully embolized by October and junctions by
November following the first hard freeze. Roots remained conductive throughout the
monitoring period.

Specific Conductivity (K,)

In both years, for both species, K, mu values were always greater in the roots than
in the stems. In 1996, Sassafias stems and junctions had peak IQ m, values in August of
4.5 :1: SE 1.1 kg s"MPa'l mm'2 and 3.3 :i: 2.0, respectively, and the root values fluctuated

around an overall mean of 28.5 i 5.1 (Figure 3). In 1997, Km“ peaked in July at 11.0 i

12

Specific Conductivity (Ks max)

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April May June July August Sept. Oct. Nov.
100 -. 1997
j + ROOI
I -O— Junc.
‘ + Stem
1O 1 T .
O
J O
1 .4 . 1r-
: O
. O
O
r
0.1 '5

 

 

 

I I I I I I I I

April May June July August Sept. Oct. Nov.

Month

Figure 3: Seasonal maximum specific conductivity (Ks max) 1 SE for Sassafras albldum
in 1996 and 1997.

13

7.6 for stems and at 2.5 i 0.4 for junctions. For the junctions, values of Kum were
associated with greater heartwood formation in the lower stem. Stems and junctions of
Sassafias always had Ks mu values above zero even when Ks in,“ values reached zero in
the early spring and fall of both years. Ks“. values for the roots fluctuated very little
seasonally and remained around an overall mean of 27.3 i 2.7.

In 1996, the stems and junctions of Rhus had peak Km“ values in August at 5.3 i
1.5 kg s"MPa‘l mm’2 and 2.6 :1: 0.4 respectively (Figure 4). The overall K“mm mean for the
roots was 6.6 i 1.1. In 1997, initial and maximum specific conductivity values were at
zero for both stems and junctions in April and May. The Km“ values peaked in July for
junctions, 1.6 :l: 0.4, and in late July early August for stems, 1. 8 i 0.5. Stem Ksinitinl
values reached zero in October and junctions in November as they became fully
embolized. In November 1997, 50% of stems and 50% of junctions sampled were at a Ks
mm value of zero. Maximum specific conductivity values of zero were associated with the
formation of tyloses in the current year’s growth ring. The Ksm. values of the roots
fluctuated around an overall mean of 6.1 i 0.5 for 1997.

Anatomical Study
Percent Conductive Lumen Area

In the stems of both species, the percent conductive lumen area prior to embolism
removal declined fiom July to October/ November. However, the seasonal trend that was
seen in the stems was not present in the roots of either species. In Sassafras, the percent
conductive vessel lumen area for stems in their native condition declined from a value of

31.3% :I: 11.4 in July to 0% by October and November (Figure 5). The values for the

14

Maximum Specific Conductivity (ks max)

12

10..

 

1996

—0— Root
-0— Junc.
+ Stem

 

 

 

 

 

 

I r I I

April May June July

I l l 1

August Sept. Oct.

Nov.

 

1997

—0— Root
‘ —O— Junc.
+ Stem

 

 

 

 

 

 

 

I I I I

 

 

April May June July August Sept. Oct.

Month

Figure 4: Seasonal maximum specific conductivity (Ks max) 1 SE for Rhus typhlna
in 1996 and 1997.

15

roots in their native condition did not change significantly and remained around an
overall mean of 42. 1% i 1.9. In Rhus the percent conductive lumen area for the stems in
their native condition decreased from a value of 32.3% i 6.6 in July to 0% in October and
November (Figure 6). Rhus roots did not show a significant change in percent conductive
lumen area seasonally and fluctuated around an overall mean of 39.6 :I: 3.2. The
junctions, not shown in Figures 5 and 6, displayed a pattern very similar to the stems in
both species.

Number of Conductive Rings

In both species, the number of conductive growth rings was far greater in roots
than in stems. In Sassafias, the number of conductive xylem growth rings in stems
(initial) prior to embolism removal varied seasonally (Figure 7). From April, the number
of conductive xylem growth rings increased from zero to 2.1 i 0.9 rings in August (the
August sample contained a stem which had 7 out of 7 xylem growth rings conductive)
and then decreased to zero conductive rings again by October. Roots (initial) fluctuated
around an overall mean of 5.1 i 0.3 rings, and did not demonstrate a clear seasonal
pattern. Sassafi'as roots (initial) had a significantly greater number of conductive rings
than stems (initial) (paired t-test, P<0.000001).

Sassafi'as stems (final) following embolism removal had a significantly greater
number of conductive rings than native stems (initial) (paired t-test, P< 0.000005). The
number of conductive xylem growth rings in stems (final) fluctuated around an overall
mean of 3.6 i 0.2 rings throughout the monitoring period but were always above zero.

The average ratio of conductive rings to total rings was 0.9 i 0.03, which indicated that

16

‘II Conductive Lumen Area

 

60

- Root (initial)

5° ‘ :1 Stem (Initial)

 

40‘

30~

 

20‘

10a

 

 

 

 

 

 

 

 

0 " I I I
July September Oct/Nov.

Month

Figure 5: Percent conductive lumen area (_+_ SE) for Sassafras albldum for 1997, before
embolism removal (initial) (n I 16 for roots and 8 for stems; * stem value at zero).

17

% Conductive Lumen Area

 

60

 

 

 

 

 

 

 

 

- Root (initial)
1: Stem (initial)
50 * ~
40 - _
30 4 —
20 -r i-
10 - _
o - . . I *
July September Oct/Nov.

Month

Figure 6: Percent conductive lumen area ( 1 SE) for Rhus typhina for 1997, before
embolism removal (initial) (n = 16 for roots and 8 for stems; *stem value at zero).

18

 

+ Roots (final)
—O— Roots (initial) _
+ Stems (final)
—v— Stems (initial) _

6 ~ . ’.—. . c
: \.‘...“ ‘ _

I I I I I I I I

APRIL MAY JUNE JULY AUGUST SEPT. OCT. NOV.

I

II Conductive Rings

 

 

 

Month

Figure 7: Seasonal change in the number of conductive rings (1 SE) of Sassafras albldum
for the 1997 growing season, before (initial) and after (final) removal of embolism (n = 16
for roots and 6 for stems; mean root age = 5.70 -_I-_ 0.27; mean stem age = 4.67 1 0.28).

most embolisms could be removed in this species. Junctions followed the same seasonal
trend as stems. In roots, removal of embolisms also resulted in a significantly greater
number of conductive xylem growth rings (paired t-test, P<0.01). Roots of Sassafias had
a significantly greater number of conductive rings than stems (paired t-test, P<0.0005).

Stems (initial) of Rhus had no conductive rings until June in 1997 (Figure 8). For
June, July and August, stems (initial) had 1.0 :l: 0.0 conductive rings and then dropped
back down to zero again by October. In the roots (initial) of Rhus, the number of
conductive rings did not show a seasonal pattern. The number of conductive rings was
significantly greater in the roots (initial) than in the stems (initial) (paired t-test,
P<0.000005).

Following laboratory removal of embolisms in April and May, 25% and 50%,
respectively, of Rhus stems (final) showed conductive earlywood in the current year’s
xylem growth ring and some conductive latewood from the previous year’s growth ring.
Like the stems (initial), stems (final) had 1.0 i 0.0 conductive rings for the months of
June, July and August. By October, 75% of the stems (final) had one conductive xylem
growth ring following embolism removal. In November, Rhus stems (final) had zero
conductive xylem growth rings. Stems of Rhus never had more than one conductive
xylem growth ring, which was always the current year’s growth ring. Embolisms in
previous years’ xylem growth rings were usually irreversible by the technique used. The
difference in number of conductive rings between stems (initial) and stems (final) of Rhus
was not significant. Junctions followed the same seasonal trend as stems. Flushing did

not have a significant effect in the roots, as there was no significant difference between

20

 

 

8
+ Root (final)
7 _ —0- Root (initial)
+ Stem (final)
—v— Stem (initial)
6 . _
5 d r-
n:
o 4 a -
.2.
fl
‘5’
'g 3 .4 _.
o
0
a: 2 a _
1 - _
0 . _

 

 

 

 

7 I I I I I I I

APRIL MAY JUNE JULY AUGUST SEPT. OCT. NOV.

Month

Figure 8: Seasonal change in the number of conductive rings ( 1 SE) of Rhus typhlna
for the 1997 growing season, before (initial) and after (final) removal of embolism (n =
16 for roots and 8 for stems; mean root age = 5.30 1 0.20; mean stem age = 3.27 1 0.23).

21

roots (initial) and roots (final).

Comparing the two species, roots (final) of Sassafias had a significantly greater
number of conductive rings than roots (final) of Rhus (unpaired t-test, P<0.005), but roots
(initial) were not significantly different between species. Stems (final) of Sassafras had a
significantly greater number of conductive rings than stems (final) of Rhus (unpaired t-
test, P<0.00000005). Stems (initial) of both Sassafras and Rhus were not significantly
different as they showed the same seasonal pattern.

Vessel Diameters

In Sassfias, the mean conductive vessel diameters of the stems and roots did not
show a clear seasonal pattern in the difference between initial and final conditions (Figure
9). For stems of Sassfras, a significant difference between initial and final conditions
was present in July (paired t-test, P<0.0005) and in October/November (paired t-test,
P<0.0000005) there was a significant difference between conditions because there were
no conductive vessels in stems (initial). The difference between initial and final
conditions in stems sampled in September could not be tested because several individuals
were already 100% embolized. There was no significant difference in the mean
conductive vessel diameters of the initial versus the final conditions in the roots for the
months of July and September (Figure 9), but there was a small difference between
conditions in October/November (paired t-test, P<0.05). The mean conductive vessel
diameters in Sassafras roots were significantly greater than in the stems (paired t-test,
P<0.01).

In Sassafras, clear seasonal trends between initial and final conditions for

maximum conductive vessel diameters were also not present (Figure 10). In the stems, a

22

Vessel Diameter (Ilm)

 

        

 

  

    

 

 

 

 

 

 

 

 

 

[:1 Root (initial)
100 4 Root (final) -
Stem (initial)
m Stem (final)
1
80 “ Z ‘
7 r ’y
’°“ ? Z)... Z
gm; Z§ Z i
/\;I;I; /\I:Z:i / i’:
40 é§$$z Z§2323 ? :3:
‘ /\§ /\§ / é
¢§Sziz ¢§$zi / :;:
/§§ /\% ¢ g
/\:1:3: / \123 / t;:
/\fl /\% / 5
/\:1:1: /\:1:3: / :;:
20 ‘ /\:3:1: /\:1:3 / 5:
/\:1:3: /\:3:1 / :3:
¢\fl /\% / 3
\;I;I; /\;I;I; / ;.;
Z§a Z§s é g
0 é§kifi I é§3$zi f é i2},
July September Oct/Nov.

Month

Figure 9: Sassafras albldum mean conductive vessel diameters (1 SE) for July,
September, and October/November1997, before (initial) and after (final)
removal of embolism (n = 16 for roots and 6 for stems; " stem value at zero;

I) indicates significance between initial and final at .05 level).

23

Mean Maximum Conductive Vessel Diameters (um)

 

    
         

      

   

 

 

 

 

 

 

     

 

 

 

220 - [:1 Root (initial) 1
Root (final)
zoo - Stem (initial) .
N Stern (final)
180 .
1... g
140 - 7 g
/ /
/ /
_ / /
120 ¢ ¢
100 - Z é
/ . % ~13?
_ A: ¢ -2222
80 / \.;.;. /\I°Z'I
A22: A525:
60- Z§§ Z§fi
4.- as; Z§§
A; A32;
20 - A A
0 4M; . A3232? .
July September Oct/Nov.
Month

Figure 10: Maximum conductive vessel diameters (1 SE) for Sassafras albldum for
1997, before (initial) and after (final) removal of embolism ( n = 16 for roots and 6
for stems; * no stem value at zero; 4) indicates significance between initial and
final at .05 level).

24

significant difference was present between initial and final conditions in July and
October/November. There were no significant differences between initial and final
conditions of roots during any of the sampling periods. The roots, in both the initial and
final conditions, had significantly greater maximum vessel diameters than the stems
(paired t-test, P<0.05).

In Rhus, there was no significant difference in mean conductive vessel diameter
between stems in initial and final conditions in July or in October/November when both
initial and final conditions had zero conductive vessels (Figure 11). The difference
between initial and final conditions in stems sampled in September could not be tested
because several individuals were already 100% embolized. There were no significant
differences in mean conductive vessel diameters between initial and final conditions in
the roots. There were no significant differences in maximum vessel diameters between
initial and final conditions for roots or stems of Rhus during July, September, or
October/November (Figure 12). In October/November, both initial and final conditions
for stems had no conductive vessels.

Comparing stems of the two species, the mean and maximum conductive vessel
diameters were not significantly different in either the initial or the final conditions. In
contrast, mean and maximum conductive vessel diameters of Sassafras roots, in both the
initial and final conditions, were significantly greater than in Rhus roots (unpaired t-test,
P=<0.005).

Macerations were done to quantify the proportion of vessel diameters below

25 pm excluded from the image analysis results. In Sassafras albidum roots, 4% of

25

Vessel Diameters (14m)

90

 

80-

70‘

60-

50‘

40~

30-

20-

10~

 

[:1 Root (initial)
Root (final)

Stem (initial)
m Stem (final)

     

     

      

  

 

 

 

 

 

WW

 

W

. o
C C
.0 .0
C. O.
[\Io- roe

I. ..

 

/

 

 

I

 

 

 

     

July September Oct/Nov.
Month

Figure 11: Rhus typhlna mean conductive vessel diameters (1 SE) for July,
September, and October/November for 1997, before (initial) and after (final)
removal of embolism (n I 16 for roots and 6 stems; “stern value at zero).

26

Mean Maximum Conductive Vessel Diameters (um)

 

 

       

 

 

 

      

 

 

 

 

 

 

160 - [2 Root (initial) -
Root (final)
140 _ Stem (initial) _
m Stem (final)
120 — . _
7
7 Z
100 - é ¢ -
ZN Z
so - /N % _
ZNfi é
/\Esis§ /
40 - §§ é _
/\§ /
20- ZNE Z -
We; a
o éNsisiz * * 6
July September Oct/Nov.

Month

Figure 12: Maximum conductive vessel diameters (1 SE) for Rhus typhlna for
1997,before (initial) and after (final) removal of embolism (n = 16 for roots and
6 for stems; * stem value at zero).

27

vessels had diameters below 25 pm. In the stems, 2% of vessel diameters measured were
below 25pm. In Rhus typhina, the roots had 5% and the stems had 6% below 25pm. The
proportion of the theoretical conductance excluded by disregarding vessels with
diameters less than 25 pm was less than .01 % for roots and stems of both species.
Root Pressures

Bubble manometers were attached to both species in early spring of 1996 and
1997 before bud burst or significant leaf expansion had occurred. Manometers were
attached to both the horizontally spreading roots (12 Rhus and 3 Sassafi'as) near the
junction and to stems (11 Sassafias) cut off just above the root stem junction. No
positive pressures were detected in either species in either year.

Frost Depth

The depth of soil freezing was variable between sites and between years (Figure
13 and 14). The deepest freezing at a site in 1996 was 315mm, versus 242m in 1997.
For 1996 the mean maximum depth of frost was 204mm versus 149mm for 1997 (Table
2). Sampled Sassafras roots were deeper on average than the mean depth of frost for
both years at 225mm and 299mm respectively (Figures 13 and 14). However, many
sampled ramets of Sassafras were shallower than the mean depth of fi'ost and could have
froze (Figure 15). Sampled roots of Rhus were much shallower (71mm in 1996 and
57mm in 1997) than the mean depth of frost recorded for both years, thus in both years,

all sampled roots of Rhus were in soil that had frozen (Figures 13 and 14).

28

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23
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29

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31

Number of Sampled Roots

16

 

 

 

 

eessssssssse

eeegeglleegh

mom '0 Inc-no

FFNNNM'VDID
Depth(mm)

Figure 15: The number of sampled Sassafias albldum roots at different soil depths
in 1997 (dotted line indicates maximum frost depth for winter 1996).

32

DISCUSSION

The seasonal patterns in hydraulic conductivity and embolism in the stems of
Sassafias and Rhus are consistent with previous studies of temperate deciduous ring-
porous tree stems, but the patterns in the roots are novel because such studies on roots
versus stems have not been reported. Roots of both species were conductive throughout
the entire monitoring period in both 1996 and 1997, with many conductive growth rings.
Percent embolism in the roots was lower than in the junctions and stems of both
Sassafias and Rhus. The difference in percent embolism of the roots between years may
have been influenced by environmental conditions.

The stems of both species were completely embolized in the early spring before
the new earlywood became conductive. This is in agreement with previous studies on the
temperate deciduous ring-porous trees, Quercus rubra and Q. aIba (Cochard and Tyree
1990), Ulmus americana and Fraxinus americana (Wang et al. 1992), Quercus gambelii
(Sperry and Sullivan 1992; Sperry et al. 1994), and Fraxinus excelsior (Hacke and Sauter
1996). All of these ring-porous species reach high rates of embolism by late winter. In
the present study Sassafras and Rhus stems were 100% embolized by November in both
1996 and 1997. This was before the minimum temperatures had dropped to -5°C in 1996
and 0°C in 1997.

Embolism in the stems of Sassafias was reversible by flushing even in growth
rings that were several years old and apparently no longer conductive. In contrast, by

November, embolism in the stems of Rhus could not be restored following embolism

33

removal by flushing, suggesting permanent blockage had occurred following embolism.
Blockage by tyloses has been studied in the stems of Quercus rubra and Q. alba
(Cochard and Tyree 1990) and Ulmus americana (Ellmore and Ewers 1986). In these
species tyloses do not block embolized earlywood vessels completely until the next
growing season. However, in stems of black locust, Robinia pseudoacacia, tylose growth
blocked the earlywood of the outer growth ring by December (Reichenbach, H. von.
(Anonymous) 1846; Fujita et al. 1978; Zimmermann 1979).

Percent embolism values for Sassafias and Rhus were high even during the
summer, and were never at 0%, perhaps because of the method used to measure
maximum hydraulic conductivity (Kh max.) (Sperry et al. 1988). In Sassafras vessels from
previous years’ xylem growth rings, earlywood and latewood, are apparently not blocked
by tyloses. Thus they contribute to the high maximum Kh values following removal of
embolisms. It is unclear whether these vessels in nature may ever become conductive
again.

In Sassafi'as stems, the new earlywood was visible before June in 1997, but based
upon crystal violet dyes used in the isolated segments, was non-conductive until late
June. Prior to the new earlywood of the stems becoming functional, embolism could be
removed in nearly all xylem growth rings. In Rhus stems, the new earlywood was formed
by late April in 1997, but remained non-conductive until June. ’Prior to June, the new
earlywood and a few latewood vessels of the previous year’s xylem growth ring did stain
following removal of embolisms by flushing. Both species were consistent with the idea
that cambial activity begins well before leafing out in temperate deciduous ring-porous

trees (Wang et al. 1992).

34

Studies such as those of Lechowicz (1984) and Wang et al. (1992) on the timing
of foliar phenology in temperate deciduous trees assert that there is a ftmctional linkage
between foliar phenology and the loss of hydraulic conductivity due to freezing-induced
embolism. A strong correlation exists between timing of spring bud burst and fall leaf
drop and high rates of embolism in the winter. Thus ring-porous trees, which have the
largest diameter xylem conduits and are the most vulnerable to freezing-induced
embolism, leaf out significantly later than diffuse-porous trees at the same location
(Wang et al., 1992). The present study is consistent with dates of bud burst previously
recorded for Sassafias and Rhus. These studies indicate that both species do leaf out later
than diffuse-porous trees at the same location (McGee 1986; Lechowicz 1995). Perhaps
by leafing out later, ring-porous trees are more likely to avoid late spring frosts. The
advantage which is incurred from later bud burst is very important for ring-porous trees,
which generally rely on the current year’s xylem growth ring for supplying water to the
leaves. The later bud burst is associated with later initiation of cambial activity. The new
xylem in ring-porous trees is thus generally not mature until danger of frost is past. The
advantage of earlier leaf drop in the autumn seems less clear, because most ring-porous
temperate deciduous trees become fully embolized following winter freeze. However,
perhaps this early leaf drop allows better salvaging of resources from senescent leaves
before the first solid fieeze.

Specific conductivity (K,), which is the hydraulic conductivity per unit pressure
gradient divided by sapwood cross-sectional area, is a measure of the porosity of the stern
segment (Ewers 1985; Tyree and Ewers 1991). Values for K, reported in this study were

calculated using total xylem cross sectional area, minus the pith, rather than sapwood

35

area. I used total xylem area since the sapwood area of the stems was often very narrow
and difficult to measure accurately. Thus, the K, m_ values reported are lower than they
would have been if they had been calculated using sapwood area.

For both species, unlike for stems and junctions, for roots there was very little
variation between the overall K, m, values in 1996 versus 1997. Specific conductivity
values for stems and junctions of ring-porous trees vary more from year to year compared
to roots. This is because the K, values for stems and junctions are based on the width of
the current year’s xylem growth ring, which is influenced by precipitation levels and
extremes in temperature (F ritts 1976). Years which have lower than average precipitation
rates or lower than average temperatures, result in growth rings which are narrower than
those of average years (F ritts 1976).

Stems of ring-porous trees have distinct earlywood and latewood regions.
Earlywood vessels are produced in the early spring at the beginning of the growth ring,
and tend to have wider diameter vessels than latewood vessels, which are narrower in
diameter and found towards the end of the growth ring. Latewood vessels, which have
lower K, values due to narrower diameters, are generally believed to have longer
conductive spans than wider earlywood vessels (Ellmore and Ewers 1986; Salleo and
LoGullo 1986; Gartner 1995). In Salleo and LoGullo’s (1986) study, narrower diameter
conduits experienced less cavitation than adjacent wider diameter conduits and thus had a
longer conductive span. Similarly, Ellmore and Ewers (1986) reported that the latewood
vessels of Ulmus americana remain conductive for up to six years, versus one season for
the earlywood vessels. This pattern of latewood vessels remaining conductive for many

years while earlywood vessels are conductive for only one year has also been observed in

36

other ring-porous tree genera such as Castanea (Zimmermann 1983), Quercus (Cochard
and Tyree 1990), and Fraxinus (Ewers and Cruiziat 1991).

That the latewood of the xylem growth rings of Rhus, and generally Sassafias as
well, does not remain conductive for more than one growing season, as observed in other
ring-porous species, is remarkable. The latewood of Sassafras and Rhus became
embolized by November in both 1996 and 1997. In Rhus all but 25% of earlywood
vessels in sampled stems became irreversibly embolized by October, while embolism in
most latewood vessels was still reversible by flushing. All earlywood and nearly all
latewood vessels in the stems and junctions of Rhus were irreversibly embolized by
November in both 1996 and 1997. Embolism could only be removed in less than 20
vessels/stem in the latewood of Rhus and thus were not counted as a conductive ring.
However it is interesting to note that such vessels, although small in number, generally
did not become permanently blocked until the late spring of the following year as the new
earlywood became conductive.

It is an open question as to why even narrow vessels of Sassafi'as and Rhus stems
have such a short conductive span. The mean and maximum conductive vessel diameter
data for roots and stems of both species suggest that wider vessels were not necessarily
more prone to embolism than narrow vessels. This is contrary to other studies, which
suggest that the narrow diameter latewood vessels have longer conductive spans than the
wider diameter earlywood vessels (Zimmermann 1983; Ellmore and Ewers 1986;
Cochard and Tyree 1990). Perhaps the freezing process damages their pit membranes in
winter.

The pattern of higher K, values for roots versus stems of Sassafi'as and Rhus are

37

consistent with patterns seen in the three Australian shrubs, Banksia prionotes, Banksia
ilicr'folia, and Adenanthos cygnorum (Pate et al. 1995). The greater difference between K,
m, values of roots versus stems of Sassafias, as compared to Rhus, may be due in part to
the greater difference in conductive vessel diameters of roots versus stems of Sassafi-as.
Although there was not much intraspecific variation for overall K, m. means of roots
between years, there was considerable intergeneric variation between the roots of the two
species. This may be explained, at least partially by comparing conductive vessel
diameters of the roots of the two species. Sassafi'as had significantly larger mean and
maximum conductive vessel diameters than Rhus. Specific conductivity is known to
increase as the mean conduit diameter increases (Tyree and Ewers 1991). The number of
vessels per cross-sectional area may also influence the intergeneric difference in K, W,
values. The different rates of K, between roots versus the stems of the two species reflect
the variation in mean and maximum vessel diameter and the number of conductive rings.

Sassafras stem segments with embolisms removed had a significantly greater
number of conductive rings than stems prior to embolism removal. This indicated that
many of the embolisms in the stems were reversible. Embolism in the stems of Sassafras
could be reversed in the early spring and late fall even when stems (initial) were 100%
embolized, suggesting the vessels remain free of blockage by gums, resins, or tyolses for
several years even though they may be non-conductive. The stems of Rhus never had
greater than one conductive growth ring even following embolism removal. The current
year’s growth became irreversibly embolized by November. Stems did not become
conductive in the spring until new earlywood became functional.

Stems of both species with embolisms removed by flushing showed a similar

38

pattern in the number of conductive rings. The number of conductive rings increased
from zero in early spring, increasing to exactly one conductive ring for Rhus and one ring
on average for Sassafras. The number of conductive rings dropped back down to zero
again by November in both species. The number of conductive rings in stems in the
native state of Sassafras were more variable than in Rhus, and ranged from all xylem
growth rings conductive to the more common condition with only the current year’s
growth ring conductive. Thus both Sassafias and Rhus stems appear to rely on the
formation of new xylem growth rings in order to overcome winter embolism. This
mechanism for recovery from winter embolism is consistent with previous studies of
ring-porous stems.

Roots of both species had more conductive xylem growth rings than stems.
Following embolism removal, all the growth rings in roots were conductive, and even
prior to embolism removal, most if not all of the growth rings were conductive. The
growth rings in the roots remain conductive for many years and do not show a dramatic
seasonal pattern. Older growth rings in the roots do not become completely embolized as
they do in the stems. Embolism in the roots may increase later in the winter as depth of
frost increases and soil temperatures drop. Unfortunately, the difficulty of obtaining
“natural” samples from the frozen ground prevented sampling throughout the winter.

The percent conductive lumen area in the stems of both species did suggest a
seasonal pattern. The percent conductive lumen area was greatest in July and was at zero
for both species by October/November. The decrease in percent conductive lumen area
from July to September for both species was due to increased overall embolism, with

some sampled individuals 100% embolized by September. The overall decrease in %

39

conductive lumen area seasonally may have been a result of water stress-induced
embolism, or embolism from other causes such as pathogens, but I did not determine this.

Most studies on susceptibility of xylem conduits to water-stress-induced-
embolism have found no interspecific relation between susceptibility and conduit
diameter (Alder et al. 1996; Sperry and Saliendra 1994; Sperry et al. 1995). However
several studies found that within a plant the wider diameter vessels were more susceptible
to water-stress-induced embolism than narrow diameter vessels (Hargrave at al. 1994;
Sperry and Tyree 1988; Salleo and LoGullo 1986, 1989; LoGullo and Salleo 1991). The
susceptibility of the wider diameter vessels was most likely due to the connection
between conduit diameter and the number and size of pit pores within an individual
(Sperry and Tyree 1988; Jarbeau et al. 1995). In addition, the age of vessels (i.e. vessels
in different growth rings and vessels produced at different times within one growth ring)
may also affect their vulnerability to embolism (Sperry, Perry, and Sullivan 1991). It is
the degradation of the pit membranes, which causes air-seeding resulting in embolism.
This may have contributed to the decrease in percent conductive lumen area from July to
September in 1997 in both Sassafias and Rhus.

Percent conductive lumen area in the roots was always higher than in the stems.
The percent conductive lumen area for roots of Sassafias and Rhus did not vary
considerably from July through November in 1997. When put into perspective with the
number of conductive growth rings in the roots, it appears that conductive lumens in the
roots, although never above 50%, were scattered throughout all the growth rings rather
than localized in the current year’s growth ring as they are in the stems.

Frost depth in soil is highly variable. A strong relationship exists between frost

40

depth and snow cover, deeper snow results in shallower frost penetration (Saul and Potts
1986; Isard and Schaetzl 1995). Other factors that are recognized in their influence on
depth of fi'ost are canopy cover by evergreen trees (Hardy and Albert 1995), elevation
(Saul and Potts 1986), and litter depth (MacKinney 1929; Sartz 1973). Due to the
difficulty in predicting frost depth because of the many factors that influence it, frost
gauges were used to measure frost depth around monitored Sassafias and Rhus clones in
the winters of 1996-97 and 1997-98.

The soil surrounding the parent roots of all sampled roots of Rhus and a
significant portion of sampled Sassafras roots fioze, but whether the root xylem
themselves froze is unknown. Perhaps the roots of both species employ some mechanism
for avoiding fi'eezing. It is also possible that more embolism would occur at sufficiently
low subfreezing soil temperatures, as in the stems of Larrea tridentata (Langan et al.
1997). Further research might investigate the xylem root temperatures throughout the
winter in order to determine whether the xylem sap, like the soil surrounding the roots
freezes.

The roots of both species never became 100% embolized during the 1996 and
1997 monitoring periods. Since sampling was not carried out between November and
April of both years, due to the difficulty of sampling in frozen ground, it is unknown
whether the roots remained conductive or became fully embolized. No positive root
pressures were detected in the roots of either species during the time monitored by bubble
manometers in either year. It is possible that positive root pressures occurred prior to the
time when root pressures were monitored or that they did not occur at all. If freezing of

the roots did occur, as frost depth indicated they may have, then roots apparently reversed

41

embolism by a mechanism other than positive root pressures. Salleo et. al. (1996)
observed reversal of embolism in one-year-old stems of Laurus nobilis with no positive
root pressures associated. The authors suggest that the reversal of embolism occurs by a
phloem-mediated embolism reversal mechanism caused by an increase in phloem loading
of solutes triggered by the plant growth regulator, Indol-3-Acetic Acid (1AA). Although
this mechanism has been suggested to reverse embolism diumally rather than seasonally,
the possibility of embolism reversal in the roots of both species and the stems of
Sassafias, a relative of Laurus, should be investigated.

In roots of plants in general, perhaps neither positive root pressures nor phloem
loading are needed to reverse embolisms. When xylem pressures are at atmospheric or
only very slightly negative, given sufficient time, embolisms will dissolve according to
Henry’s Law (Tyree and Yang 1992; Ewers et a1. 1997). Such pressure conditions may
occur when the soil is saturated with water, such as in the early spring in Michigan or

following heavy rainfall throughout the year.

42

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