Zamhezia (1983). XI (ii).THE DEATH OF TREES:AN ESSAY ON THE NATURAL ECONOMY OF THEFOREST*M.J. SWIFTDepartment of Biological Sciences, University of ZimbabweWHEN THE FACULTY of Science was founded in this university, twenty-five yearsago, two chairs in the Biological Sciences were established; that of Botany andthat of Zoology, a distinction which marks the traditional division of the livingworld into plant life and animal life. This dichotomy in life science goes back atleast to the days of Aristotle but is no longer regarded as defensible by modernbiologists who recognize not just two kingdoms of organisms but at least five:the unicellular bacteria andprotista, and the multicellular/?lants, animals andfungi. This change in the categorization of the living world derives in part froman increasing recognition of a fundamental duality in the study of Biology; thethrust on the one hand to account for the readily observable diversity of natureand on the other to explain life in terms of unity in function.UNITY AND DIVERSITY IN BIOLOGYThe diversity of living organisms is a feature of common observation and it isprobable that even now only a relatively small proportion of the living speciesof the earth has been scientifically described, although the lists for floweringplants and vertebrate animals, the organisms with which the layman is mostfamiliar, are relatively complete. The drive of modern biology has not been,however, to merely document in greater detail the diversity of form, functionand behaviour of life but to recognize an underlying organizational unity. Thischange in perception has come from the application of sophisticatedtechniques and concepts, developed mainly for the study of chemistry andphysics (Jacob, 1970; Judson, 1979).Among the most important of these revelations has been the demonstrationthat the totality of an organism's character is written in a single molecule, thenucleic acid known as D.N.A. (deoxyribose nucleic acid). Moreover thisgenetic code, the language of life, is the same for all organisms, from bacteriumto man. This insight is revolutionary in the sense that it gives a new perspectiveto man's view of his place in the universe (Monod, 1970; Jacob, 1982).Concomitant with this discovery is the accompanying realization that manyother characteristics of'life' Š muscular movement, reproductive behaviour,sensitivity to external stimuli, nerve transmissions, even thought itselfŠ cansimilarly be described in terms of the laws of physics and chemistry. 'Vitalism'seems to have been laid to rest at last.If life has such a unified basis, then how do we account for its enormousvariety? How (an4why) is the diversity derived from the unity? This dialectic* An inaugural lecture delivered as Professor of Botany before the University of Zimbabweon 13 October 1983.79is the driving force of much modern biological thought and discovery. Thelinking concept of course is the theory of evolution.It is just 101 years since the death of Charles Darwin; his exposition of thetheory of evolution, an exposition which in its day also altered man's conceptof his place in the scheme of things, continues to excite controversy. But thereis no part of biological study which is not in some way informed by the theoryor does not in some way impinge upon it. It is thus appropriate that the theme ofmy lecture is couched within an evolutionary framework and can begin fromDarwin himself.Charles Darwin was a polymath, his contribution to biology wasoutstandingly wide. The last book he wrote, published a year before his deathin 1882, was a volume entitled The Formation of Vegetable Mould throughthe Action of Worms with Observations on Their Habits (Darwin, 1881).This book was an attempt to explain the existence of a living organic soil as aconsequence of the activity of a particular group of animals, the earthworms(Satchell, 1983). That same story is an essential part of this lecture, althoughin this case the origin (and disappearance) of soil will be related in particular tothe interaction of two other types of organism Š forest trees and fungi.AN EVOLUTIONARY STORYThe earliest steps: Molecular biology has given us insight not only to thefunctioning of living organisms but also to the very origins of life itself. Theearliest evidence of cellular life Š similar in form to present-day bacteria Šdates life as far back as at least 3.4 X 109 years before present (B.P.). Interes-tingly, a significant component of this evidence comes from Zimbabwe whereindications of fossil microbes have been found in limestone sediments from theBelingwe area, dating back about 2.5Š2.8 X 109 years (Bickle, Martin andNisbet, 1975; Nagy, Nagy, Zumberge, Sklarew and Anderson, 1977). Itseems apparent, however, that even by this stage there was a considerabledegree of diversity and the precellular origins of life must be placed somewhatearlier, perhaps at the start of Archean aeon about 3.9 X 109 years B.P.It has been inferred that many of the simple organic molecules from whichall known living systems are constructed Š carbohydrates, amino acids,purines, pyrimidimes and lipids Š would have predated these fossils. Suchsubstances are indeed known to be present in interstellar space, to becomponents of meteorites and to be formed abiotically from simple inorganicprecursors. The evidence for the latter process has been providedunequivocally by experimentation; under anaerobic conditions in the presenceof an energy source such as ultra-violet radiation these compounds may beformed from precursors such as methane (CH-*), hydrogen cyanide (HCN),ammonia (NHs), and water (H2O) (Margulis, 1981). It is probable that this isa reasonable simulation of events that occurred early in the earth's history.What remains a mystery, however, is how these 'building blocks' becamepolymerized to form the macromolecules that are the chemical basis of livingsystems. Even more crucially there is presently little evidence to show howthese molecular constituents became organized into the self-perpetuating unitswhich we call 'cells'. Whatever the solution of this, surely the most intriguingof all biological mysteries, we know that by the start of the Proterozoic aeon,2.6 X 109 years B.P., there had already emerged cells that showed the form ofoxygenic photosynthesis characteristic of all modern-day plants.80All living systems are organic in nature; that is, they are built frommolecules containing carbon (C). There are fundamentally two ways in whichliving organisms obtain their carbon. The heterotrophs Š represented in thepresent by the animal and fungal kingdoms and by a large majority of thebacteria and protista Š obtain their carbon from preformed organiccompounds. The autotrophs Š the plants and a minority of bacteria andprotista Š utilize the inorganic compound carbon dioxide (CO2) as a source ofcarbon. To utilize C the organisms must also have a source of energy. The mostabundant source is that of sunlight and it is this energy which plants utilize;hence their mode of nutrition is known in fall as photo-autotrophy. Analternative source of energy is that built into organic compounds Š chemo-energy. The animals and the fungi and many other heterotrophs thus utilize thesame organic compounds as a source of both carbon and energy and aretherefore known as chemo-heterotrophs.The earliest living organisms were probably chemo-heterotrophs utilizingabiotically.formed organic compounds as sources of both carbon and energy.This was a wasting asset, however, and natural selection would have favouredorganisms that utilized alternative and less readily exhaustible sources ofenergy and/or carbon (Broda, 1975). The most abundant source of the formerwas light. A considerable number of such metabolisms are known but by far themost widespread is that of oxygenic photosynthesis.6CO2 + 6H2O+ 6O2In this process the energy of sunlight (hv) is used to split the water molecule,the resulting hydrogen (H) being used to reduce the CO2 to form carbohydrate(CeHnOe); the oxygen released from the water is evolved in molecular gaseousform (O2).The emergence and eventual dominating success of this particularbiosynthetic system (Stanier, 1974) changed the pathway of evolution and thecourse of earth history. The system is highly efficient in thermodynamic termsand made possible the synthesis of a large biomass of organic compounds ofgreat variety. At the same time the generation of oxygen began to change theatmosphere towards its present-day composition. The presence of an oxygen-containing atmosphere in its turn created a situation in which more efficientways of chemo-heterotrophic utilization of the new excess of organiccompounds as a source of energy (by various processes of oxidation) couldevolve.The decomposition, by oxidation, of organic molecules can take place inthe absence of oxygen. This indeed was the basis of heterotrophy before theappearance of oxygenic photosynthesis. Many bacteria living in anaerobicenvironments do this today, but it is essentially an inefficient method ofutilizing the chemical energy locked in organic molecules because it nevergoes to completion. Six-carbon sugars (e.g. CeHnOe-glucose), for instance,are commonly converted only as far as forming two three-carbon acids. In thepresence of oxygen this breakdown can be taken to completion (6CO2 +6H2O) Š a total reversal of the synthetic equation given above. This form ofnutrition is given the general term of'respiration' and is characteristic of thehigher plants, the animals and the fungi of today. In the plants it is just acomplement of their synthetic process Š a way of using the energy fixed into81the sugar molecules by photosynthesis. In the animals and fungi, and in manybacteria and protists, it is the major means of capturing both energy andcarbon.It was only following the emergence of these two highly efficient forms ofaerobic metabolism that the multicellular organisms evolved. At the start ofthe Phanerozoic era about 0.6 X 109 years B.P. the origin of these organismsŠthe ancestors of the plants, animals and fungi of today Š from the simplerbacteria-like life which had preceded it was almost as large a step in evolutionas the origin of cellular life itself and in its turn has provoked a deal of recentspeculation (Margulis, 1981). Whatever the nature of the early beginning, thesucceeding 200 million years produced a great diversity of aquatic plants, andit was probably from ancestors resembling today's green algae that the firstterrestrial plants evolved.Tree making: The first plants to emerge on to land were essentially aquatic inhabit and probably very simple in form. That is to say that they were flattened(dorsiventral), adopting a habit that pressed them closely to the surface of theearth. This form gives two advantages; the ability to spread laterally ensuresrapid cover of the land surface and gains maximum benefit from the sun's rays;at the same time the plant remains in contact with the earth so that it can absorbthe water which runs on the surface and with it take up the essential nutrientswhich are to be found in soil (nitrogen (N), phosphorous (P), calcium (Ca),potassium (K), magnesium (Mg), etc.). This lateral spread was soonsucceeded by vertical growth. The earliest form of vertical growth may wellhave been the raising of a spore-producing head above the ground, as is foundin the mosses and liverworts of today. This enables the plant to gain theadvantage of wind as a dispersal agent instead of relying on water'in themanner of its ancestors. But plants also gain other advantages in verticalgrowth. A plant taller than its neighbours retains maximum benefit from thesunlight but can also over-shade its neighbours. Vertical growth combined witha laterally spreading canopy has a strong competitive advantage. The greatimperative of plant evolution was thus the climb towards the sun; and itsculmination was the production of the tree.The tree is the optimum plant form for life on earth and before the interven-tion of man the major part of the earth's surface was naturally covered invegetation of a tree-like form. Trees have an essentially polar structure incontrast to aquatic plants. At one end they have a canopy Š a crown of leaveswhich is the photosynthetic system of the plant. At the other end is the rootsystem, the site of absorption of water and mineral nutrients. Linking these twoessential systems is the trunk which is a feature of plants dictated by the problemof life on land, for the upward growth of terrestrial plants brings three majorattendant difficulties which must be overcome.Firstly there is the problem of dessication. Life on land is dry and the plantsmust cover themselves with impermeable coats, the cuticle and the bark, toprevent excessive loss of water by evaporation.The second problem is that of transport; if the photosynthetic surface israised above the ground, then it loses direct contact with the source of waterand nutrients in the soil which are essential to its growth. It is, therefore,necessary for there to be a development of a transport system within the plant,conducting the water and the nutrient from the soil to the photosyntheticsystem above (Raven, 1977). Correspondingly, there is a need to transportsugars downwards to the roots which are an intrinsically heterotrophic part ofthe tree.82The third problem, attendant upon this growth towards the sun and thedevelopment of the internal transport system, is the need for rigidity in theplant Plants living in water or plants living simply on the surface of the soilhave no need for structural rigidity. But for a plant to grow tall it must be able tostand on its own and to withstand the compressive forces imposed on it bylateral winds. The plant cell walls of algae and of the simpler earth-boundplants are composed almost entirely of polysaccharides of which thecommonest component is cellulose. Cellulose is simply a chain of sugarmolecules linked together to form a very long unbranched macro-molecule;these molecular chains themselves aggregate to form fibrils and these fibrilswind round in the cell wall, and give it some strength (Fig. 1 a). This form of cellwall is indeed quite tough; we know this because we use cotton fibre (which isalmost pure cellulose) to make textiles; we also use cellulose to make paperand cardboard. But this strength is a tensile strength, a strength which with-stands tearing or shearing stress; it does not have to withstand forces ofcompression. Cellulose walls unless stiffened by turgor pressure Š the pressureof cells filled with water Š do not have a great degree of rigidity. Cellulosemolecules and those of the other polysaccharides also have the property ofattracting and absorbing water. This renders them soft and pliable as we knowfrom the way in which cotton fibre or paper may be wetted. Cellulose walls arvthus not tough enough to build trees from. An important stage in the evolutionof the terrestrial plants came when another type of chemical component wa >added to the polysaccharide cell wall Š the molecule lignin.Lignin is an entirely different type of chemical from cellulose and the otherpolysaccharides (Fig. lb). First of all, it is not built from sugars; it is built froma series of aromatic compounds known as phenylpropanes, which havehydrocarbon side chains. Moreover these units are not linked4n a regularlinear form, as in cellulose, but in a highly branched fashion. Whereascellulose resembles a chain of equal-sized links, lignin is more like a piece ofwire-netting which is snipped in a number of places and then folded around in arather random fashion.Lignin has two properties which contribute to its importance in cell walls.First of all, its much more diffuse non-linear structure means that it can foldaround the flbrillar structure of the cellulose molecule like hardened cementround a metal scaffolding. In this way the lignin fills in the spaces between thefibres of the cellulose and other polysaccharides and this in-filling itself givesadded strength to the cell wall. Secondly the lignin molecule is hydrophobic,that is to say it repells water. Lignified cell walls, instead of having thetendency to absorb water with the attendant loss of strength, are waterrepellent and their rigidity is much enhanced by this property (seeWainwright, Biggs, Currey and Gosline, 1976, for a general discussion ofstrengthening in plants).Trees have been described as the apogee of plant evolution but of coursethere are many thousands of plant species that are not trees. At the edges of theforest and in the drier parts of the world, where tree building is not possiblebecause of limitations on the supply of water and the extreme dessication of theenvironment, the vegetation may be dominated by grasses or shrubs. Theseareas are, however, relatively uncommon. Of the entire area of the land surfacecapable of supporting vegetation over 70 per cent was originally occupied byforest and woodland. Within the forest itself there are many non-tree plants;plants which can tolerate the shade cast by the forest trees but benefit from the83stable and benign environment created by the forest Š the epiphytes andclimbers of the canopy and the herbs and shrubs of the floor._The forest also provides the habitat for a great diversity of heterotrophicorganisms Š the animals, the protists, the fungi and the bacteria. Theseorganisms have co-evolved with the forest. Their emergence in evolution isdependent upon, and the product of, the evolution of the plants. Among thesewe can focus on one group as of particular significance to the functioning of theforest Š the fungi.Evolution of the decomposer system: Most fungi obtain their food from themost direct and obvious of sources within the forest Š the plants. Access tothis food source may be attained by creation of an intimate association directly(a) Cellulose(b) LigninFigure 1: Structure of cell wall macromolecules.(a) Cellulose. The upper diagram shows part of a single molecule with theglucose units joirued together in a repetitive unbranching chain which may beseveral thousand units in length; the lower diagram shows the aggregation ofcellulose chains to form fibrils which form a 'scaffolding' for the cell wall.(b) Lignin. The upper configuration shows the basic phenyl-propane unit; thelower configuration illustrates a portion of a molecule showing the highlybranched non-repetitive structure (from Swift, Heal and Anderson, 1979).84between living fungus and living plant. In some instances this association maybe mutually beneficial; the fungus penetrates into the roots of the plant andabsorbs sugar produced by photosynthesis by direct transfer from the plantcells; in return the thread-like fungal hyphae, which ramify out into the soil, aidin the absorption of mineral nutrients such as N and P which are transferred tothe plants. Such associations are widespread in modern vegetation and ancientin origin, having been detected even in the primitive root systems of the earliestof vascular plants (Malloch, Pirozynski and Raven, 1980). Some authoritieshave even speculated that such mutualism was essential to the evolution ofefficient absorptive systems in plants (Pirozynski and Malloch, 1975),although others have seen this type of association as a later feature in fungalevolution (Lewis, 1974).A second group of fungi also associate with the living plant but on a shorter-term basis. They parasitize the plant by penetrating into the living tissue andsecreting poisons which kill it. The dead plant can then become food for thefungus. My first research work here in Zimbabwe was concerned with this typeof parasitic association between fungi and plants (Swift, 1964; 1972) but mypresent interest and the theme of this lecture relates to a third type of fungalbehaviour.Trees, like all living organisms are mortal. Parts of the tree, in the normalcourse of events, die and fall to the forest floor, creating a litter of twigs,branches, leaves, petals, bud scales. A huge variety of fungi use this deadorganic matter as their source of food. These fungi are the only group oforganisms in the living world that possess the enzymes capable of breakingdown not only the simpler sugars, amino acids and proteins of the plant cellsbut also the cellulose, other polysaccharides and in particular the lignin, cutinand suberin of the plant cell wall. In so doing they liberate energy and nutrientsfor their own growth. But during that breakdown process, which we call theprocess of decomposition, they bring about two other events which are of greatand indeed essential significance to the functioning of the forest These twoprocesses are the processes of mineralization and of humiflcation (soil organicmatter formation).Mineralization is that part of the decomposition process in which essentialelements such as N, P, S and others are converted from an organic form (that isin the form which they are found within leaves, twigs, roots or in the carcassesof animals) to an inorganic form (e.g. NH4+, NO3;, PO4;/, SO/). In the organicform, combined with carbon, these nutrients cannot be taken up by plant roots.Conversion from the organic to the inorganic form is thus essential for thevegetational productivity of any ecosystem (Swift, Heal and Anderson, 1979).Decomposition is never a complete process, however. There is always aresidue of organic material left from the activitiy of the decomposer organisms.The material which is left is quite different from the starting material. And ofthese residues the most important are the group of chemical molecules knownas humus. The humus components (humic and fulvic acids and bitumens) havemolecular structures quite different from those of the sugars, cellulose,proteins, amino acids and other molecules found in plant or microbial cells,although these molecules, particularly lignin and certain microbial pigments,form the precursors for their synthesis. It may indeed be hypothesized that theappearance of abundant humus is a product of the incorporation of lignin inplants and the appearance during evolution of lignin-degrading fungi. Likecellulose and lignin the humic molecules are polymeric and like lignin they lack85a linear repetitive structure. The molecular core of the humus molecules is ineach case aromatic but non-aromatic moieties resembling carbohydrates,peptides and organic acids are also detectable within the molecules; the exactstructure, however, remains unresolved. The humus molecules also containsignificant contents of nitrogen, phosphorous and sulphur, which is one of themain reasons why the presence of humus in soil adds to soil fertility. Thehumus molecules are highly resistant to decomposition but they do decay at avery slow rate, releasing their nutrients to the soil solution. Where a large massof humus has accumulated this contributes a very important and stablecomponent to the nutrient cycle. Humus is negatively charged and retains highlevels of cations on the soil. The presence of humus also improves the soil byforming complexes with the mineral components such as clay, giving a morestable physical structure and improving its ability to hold moisture and towithstand compaction.I have emphasized the role of fungi in decomposition but they do not do thisalone. Many of the invertebrate animals, protists and bacteria are alsoinvolved in decomposition. Indeed, evolution has produced some very closeand intimate relations between various representatives of these groups ofhetrorophic organisms. It is perhaps better to think not just of the evolutionof organisms but of the evolution of systems Š on the one hand of thevegetation system and on the other of the decomposer system. These twosystems are co-ordinated, and have co-evolved to produce the major biologicalcomponents of the ecosystem.THE NATURAL ECONOMY OF THE FORESTTo talk of the evolution of the decomposer system is tantamount to talking ofthe evolution of the soil. The substratum colonized by the first plants inhabitingthe land was largely bare rock covered in a fine dust of mineral particles. Assuch it had little capacity to retain a reservoir of the soluble mineral nutrientsthat are essential to plants. It was only with the addition of organic componentsto this substratum that a soil began to develop that was able to accumulate alevel of fertility capable of supporting a high level of vegetation al productivity.Thus there has developed, through the evolution of plant life on land, aninextricable relationship between five elements which comprise the essentialcomponents of any ecosystem (Fig. 2). The climate and parent rock of the areaare predetermining factors which establish the overall potential of thevegetation system. This latter system is the source, through autotrophicsynthesis, of the organic content of the ecosystem. The heterotrophic animalsand micro-organisms of the decomposer system feed upon the organic matterand the product of this activity, interacting with the minerals derived from theparent rock, is the soil. This soil is the reservoir of the fertility of the ecosystem,a fertility evolved over many centuries by the interactions of the biotic andabiotic components. These inter-relations are so intimate that ecologists withknowledge of the climate and parent rock of an area of the world can predict thetypes of vegetation and soil that it will sustain.The decomposer system thus comprises an essential component of the'natural economy of the forest'. This economy is based on a currency ofnutrients, such as the elements of nitrogen, phosphorous, potassium, sulphurand calcium. The pattern of the economy is a cycle of these nutrients betweenthe organic tissues of the living trees and the organic and inorganic componentsof soil (Fig. 2). The continued fertility of the forest system depends on thiseconomy being in balance and that is achieved by the exact but antitheticcorrespondence of the synthetic productivity of the vegetation and thedecomposer activity of the soil organisms.It is widely believed that the climactic ecosystems of the world are in steadystate with regard to their productivity and nutrient cycling (Bormann andLikens, 1979). Every patch of vegetation is a mosaic of the youthful, themature and the senescent. The forest contains, for instance, a mixture ofseedlings, saplings and mature and dying trees. In balance the biomass (theliving weight of the forest) remains constant. It is added to annually by plantproduction; it is depleted by death and decomposition. The balance betweenthese two is essentially an equilibrium between the autotrophic andheterotrophic forms of metabolism that we saw emerging so early in the historyof life, an equilibrium between the two anti-parallel processes, plantproduction and decomposition. The sustainability of a forest, or any other typeof vegetation, depends on the maintenance of function of both of theseprocesses, for as we have seen they are inter-dependent components of thenatural economy. Estimates of the natural extent of vegetation on the world'ssurface suggest that about 70 per cent of the land surface is productive^, theremainder (the hot and cold deserts) being virtually incapable of supportingvegetation. Abouttwo thirds of this productive area(thatis about7.4 X 109 ha)was covered in vegetation dominated by trees (including the coniferous forestsof the boreal zones, the temperate forests of the mid-latitudes, the woodlandsand woodland-savannas of the subtropics and the equatorial rain forests). Incontrast, grasslands (including the open savannas) probably originallycomprised only about 18 per cent of the productive area (Whittaker, 1975).This distribution has, however, dramatically changed in the last ten thousandyears as a result of man's activity. The present estimate of the extent of tree-dominated vegetation is now about only 2.6 X 109 ha, that is about one third ofthe original area (O.E.A., 1978).CLIMATEVEGETATIONDECOMPOSERSPARENT ROCKFigure 2; The relationship between the fundamental components thatdetermine the functioning of a natural ecosystem (see text for details).87The clearance of the forest The forested area stood at about 4.8 X 109 ha in1950. Thus the extent of clearance in the last thirty years has been almost asgreat as in the whole previous ten thousand years of man's social history.Present signs show that whilst the rate of clearing may have levelled off in thetemperate zone, in the tropics it is actually still accelerating (O.E.A., 1978).The main sites of deforestation in the last two decades have been the TropicalRain Forests. In some areas Š Indonesia, Malaysia, the PMllipines Š there isvery little left of the original forest and West Africa and Central America arefast approaching the same condition (Ranjitsinh, 1979; Melillo et at, 1983).What are the reasons for the removal of forest? Is this a good and necessarything, essential for man's survival, or is it short-sighted and self-destructive?Trees are cut largely for one or more of three reasons. Firstly they may becut for commercial utilization, as timber or pulpwood; this is the majorincentive for the present clearance of the tropical rainforest but it should benoted that the utilization of this timber and pulp is largely outside the tropicalzone. Secondly trees are cut to provide fuel, the major source of local (asopposed to export) utilization. It has been estimated that on the Africancontinent over 85 per cent of the wood that is cut annually is used for fuelwhereas in North America only 4 per cent of utilization is for this purpose(F.A.O., 1979). The third purpose for forest clearance is to provide land foragricultural use.The effects of clearance: When wood is removed from a forest, whether fortimber or fuel, the natural economy of the forest is disrupted. Countries whichsell their trees are exporting more than the timber: they are also selling thenutrients previously locked into the biomass of the forest. When the forest isused for fuel, then the nutrients may also be lost unless efforts are made toreturn the ash to site. In both cases it is important that new stocks of trees beintroduced, for ecological as well as economic reasons. Harvest need not becritical in itself if efforts are made to ensure that the forest is replenished Š byplanting new trees or by encouraging natural regrowth Š because vegetation isresilient and capable of regenerating new nutrient stocks if given time. By thismeans ecosystem stability may be fairly readily regained. But if these properprecautions are not. taken the harvest of the timber may also be a harvest of thesoil. There is now well documented evidence from many parts of the world ofthe effects of clear-felling forest cover. An outstanding example of studies ofthis kind was done in the U.S.A. at the Hubbard Brook Forest in NewHampshire, a deciduous forest of temperate region trees such as birch, beechand maple (Likens etal, 1977). Studies of the streamwaters flowing out of thisforest showed them to be remarkably stable in their nutrient content eventhough the rainfall (and therefore streamflow) varied markedly from year toyear (see controls in Fig. 3). Clear-felling a catchment of its trees had a markedeffect on water output from the land to stream, increasing the output by 30 to40 per cent. The concentration of nutrient ions in that outflow increased evenmore dramatically (Fig. 3). The most significant increase was in NOs-N whichshowed 46 times higher loss from the dear-felled catchment than from acontrol forested catchment. The amount of N lost in three years followingfelling was roughly equivalent to the total N previously held in the vegetation.Similar effects could be noted for other nutrients. The main reason for theselosses is the uncoupling of vegetation from decomposers. The decomposerscarry on with their activity, re-generating inorganic nutrients, but there are noplants to take them up; It is as though a thick black line is drawn across Fig. 2,separating vegetation from soil.burning. Dimrii:c. rvorteo to .n<The Hubbard Brook experiment is an extreme case because re-growth wasprevented by use of herbicides; but the lessons are quite dear. Massive lossesof nutrients will occur unless steps are taken to encourage rapid regrowth to re-stabilize the system. The dear-felling operation should also be so structuredthat there is always a buffer of standing vegetation in the catchment to stabilizethe hydrology and prevent irreversible loss of nutrient.The procesc of loss m.°.v be further aggravated by other practices such asa it- ':<.'< V'vn.c **\Ktet i*» oxidized and many elements aref'u% s xii\ *«-ŁŁ Ł ., n: re.v.rded as a bypassing of the,',.-.Ł{ v p.iv".-«_!Ł, Lj,s .s. — ,,t .*: )S rapid'fertilization'may bev- !'i't .:< ct ; u? -iLi. >. . - .Ł.,-Ł>.* .»-.i ny run-off and leaching at the.\ir.cu.'I|;1<"'Ai'-.:i^ ., . .: . w. Ł «. ttir absence of established plant root>\°.!-ras It j> n _,^< -J-cs,' .ŁŁki..i.>s "iices lack of plants and natural littercover Š that soil oiosioh wiii occui. Mucti has been written and publicized inZimbabwe on this topic. Whitlow (1980) has shown the extent of treeclearance that ha:; :;:L:n y\u : h< the last twenty years and his writings andothers iFJweii an; St^i^.-.ni:. ;;>"5i have demonstrated the correlationbetween this -uv IU. J^ ..- -r TKJU of massive areas of soil erosion.Tk .'{>ŁŁ :- ŁŁ , u'e local disruptions of the integratedfuncTionit;^ < ŁŁ *:^- Ł : .. . Ł, o>> >/ Ł'->: most of these effects can be controlledby sensible manage c... - Ł Ł - -.iems that are more economic, social andFigure 3: Nutrient losses from a forested catchment at Hubbard BrookExperimental Forest, U.S.A. The graphs show variation with time in theconcentration of ions in the streamwater draining two catchments. The brokenline is for a catchment which retained its tree cover throughout the experiment(= control); the solid line is for a catchment which was clear-felled inDecember 1965. Note the change of scale for concentration of nitrate ions(from Swift Heal and Anderson, 1979).89political than of an intractable biological nature. But the massive extent ofclearance has had effects which are global as well as local. The structure of theecosystem described earlier postulates a balance between production (byplants) and decomposition (by fungi and their allies) (Fig. 2). This balance ofthe economy was described in terms of a currency of the essential elements.One such element is carbon; and a balance of production and decompositionimplies a balance between the uptake of CO2 from the atmosphere by theplants and its release back to the atmosphere by the decomposers. Thecircumstances described above, in which massive areas of forest are burned ordecomposed without replacement by equally productive vegetation, suggestthat an imbalance may develop in the carbon budget. This can indeed beverified; the CO2 content of the atmosphere has risen steadily for the last twodecades and it is predicted that by the year 2030 it will have reached double theconcentration present at the start of this century (Houghton et at, 1983).Forest harvest and decay is only one component of this increase (the largest isthe burning of fossil-fuels) but it is nonetheless a significant factor. Theconsequences of doubling the CO2 content of the atmosphere can be debated(e.g. see Clark, 1982; Idso, 1982), but there can be little doubt that they mustbe taken seriously.Another major component of the increase in global CO2 derives from theconversion of forest to agricultural use. This, the third of our categories ofdeforestation, also results in disruption of the natural economy; in this event itis replaced by an agricultural economy,THE MANAGED ECONOMY OF CULTIVATED LANDThe domestication of plants: I have already referred to the clearance of forestfor agricultural land. It has been estimated that there are about 1.2 X 109hectares of land suited to arable agriculture, most of it in the forest biomes. Atpresent, only 1.4 X 109 hectares are so used, i.e. about 44 per cent of theconvertible area. Nonethless, on our previous estimate that represents about20 per cent of the original forest reserve and 30 per cent of what has alreadybeen cleared Š a substantial extent of conversion. It is, therefore, appropriateto ask at this point how efficient the conversion has been.During the ten-thousand year history of man as a cultivator he has utilizedseveral thousand species of plants as crops. Of these, less than two hundred areeconomically or nutritionally significant in the world today and the major partof the world's nutritional requirement is supplied by only fifteen species. Mostof these species were domesticated at the very onset of agricultural history inone of a number of centres of origin recognized around the world (Harlan,1975). In broad terms two types of plant domestication can be recognized;vegeculture is the growing of root or tuber crops (such as yams, sweet and irishpotatoes, cassava), a practice which probably originated in the tropical'lowlands at the margin of forest and savanna; and granoculture, thepropagation of plants by seed, which largely originated in the sub-tropical beltsof Central America and the Middle East. Most of the seed plants of primaryimportance are derived from two families, the grasses and the legumes. One ofthe interesting features of these plants is that they fall into the group that thebotanist would call ruderals. In common parlance such plants are termedweeds; the farmer's definition of a weed is a plant growing in the wrong place,that is competing with crop plants. To a botanist or an geologist a weed or90mderal species is a plant with a certain set of characteristics: rapid growth,rapid setting of seed, efficient dispersal, an anneal life cycle, relatively largeseeds with good reserves, and an efficient seed dormancy physiology (Grime,1979), All these characteristics adapt plants to competitively colonizedisturbed areas. The natural distribution of such plants is thus in clearings ofnatural vegetation, at forest margins, and on unstable hillsides Š situationswhich are mimicked by man when he clears land for agriculture, settlement orroads. It has indeed been postulated that it is just because of these 'weedy'characteristics that these cereal grasses and legumes were domesticated byman (Hawkes, 1969). During the phase of man's evolution when he wassedentary, relying for Ms food on fishing, small-range hunting and thegathering of plants, such organisms might well have become the weeds of Msrubbish heaps and of the cleared spaces round Ms village. It was an easy butimportant step when man irst gathered the seed and planted it deliberately inground cleared for the purpose. It was an even more significant one when hestarted selecting seed for planting from among the diverse groups of'weeds'which were available as food plants.Over the centuries this selection has operated to produce the handful ofprimary arable species that currently occupy the attention of the world'sagriculturalists. The most recent, most sophisticated and most productivephase of this ten-thousand year evolution has been the Green Revolution Šthe breeding of ultra-high yielding varieties of wheat and rice and more latterlyhybrid maize (Borlaug, 1968; Wellhausen, 1978). The consequence of thisprocess is that cereal grasses now occupy over 50 per cent of the arablehectarage of the world and supply an even higher proportion of the world'sbasic nutritional requirement. This conversion of natural forest to cultivated'grassland* clearly represents a major ecological change and we must nowexamine its consequences.The economy of cultivated land: The practice of agriculture introduces a neweconomy into an area in place of the aatural economy of the forest The harvestof 5 metric tons of grain from a hectare of land will remove something like100kg of N, ,20kg of P and 25kg of K. In an area previously covered withTemperate Deciduous Forest this might be equivalent to between 2 and 5 percent of the total reserves of these nutrients in the soil. In tropical regions wherethe reservoir of soil nutrients is generally lower this harvest may be equivalentto 10 or even 15 per cent of the stock. Clearly such a level of depletion cannotbe maintained indefinitely.Cultivation also affects the natural economy in other ways. IJhave alreadydescribed how the soil is a product of the equilibrium between the vegetationsystem and the decomposer system. Changing the vegetation system from adiverse forest to a cereal monoculture alters this equilibrium in a drastic way.The litter input to the soil may be less than half that of the forest andadditionally contains a much lower proportion of lignin. This results in adecrease in the extent of humus formation. At the same time the rate of humusbreakdown is markedly accelerated by tillage practices which tend to break upsoil particles, increase the soil temperature and expose it to more aerobicconditions. As a result the introduction of'conventional' agricultural practicesinvariably results in a decrease in the content of soil organic matter. Theinevitable consequences are a further depletion of soil-nutrient reservoirs (Fig,4), loss in the quality of the soil structure and a decreased moisture-holdingcapacity.91Natural ecosystems have a vegetational cover the whole year round; cropecosystems often remain bare of vegetation during the intervals betweengrowing seasons. In some cases, because of practices such as burning, they arealso devoid of any cover of litter. These conditions are those, as we havealready seen, which promote soil erosion and nutrient loss by leaching and run-off. Even under relatively careful management of the crop residues (litter) theproportion of nutrient retum to the plant from decomposition may not be asgreat as in a natural ecosystem. Both plant growth and decompositon arecommonly triggered by the arrival of favourable seasonal conditions (anincrease in temperature and day-length in temperate zones, the onset of rain inthe tropics). In natural ecosystems the vegetation and the decomposers areboth adapted to benefit from this. In a crop ecosystem the two systems are atthe mercy of man. If the farmer does not manipulate both vegetation (plantingat the correct time) and the soil (introducing crop residues at the correct time)then the tight 'coupling' between the two systems may be disrupted. Oneconsequence of this may be, for instance, that nutrients are released from thecrop residues before the crop roots are sufficiently grown to benefit from theirpresence. In these circumstances ifiuch of the released nutrient may be lostthrough leaching (Anderson and Swift, 1983; Swift 1983).The agricultural ecosystem is thence a different and intrinsically lessproductive ecosystem than the one it replaces. The intervention of man in theevolution of his crops is equally an intervention in the evolution of the soil. Thesoil selected in this way is less fertile and less productive than the natural soil.It should be stressed that this is an inevitable and unavoidable consequence ofthe practice of agriculture, in contrast to the excessive and catastrophic effectsof deforestation for timber and fuelwood described earlier which can beprevented by sensible management. And most agriculture is, of course,practised under conditions of sensible management. Man's ingenuity isenormous and he has evolved many ways in which he can practise productivearable agriculture against this inevitable decline in soil fertility.35 40 45Figure 4: Depletion of the nutrient reserves in soil following cultivation. Thegraph shows the nitrogen content of four prairie soils from the U.S.A. over theforty-year period following conversion from natural grassland to cerealcultivation. The depletion in N is associated with a parallel decrease in thehumus content (from Swift, Heal and Anderson, 1979).92Broadly speaking there have been two types of solution; that of theintensive farming characteristic of the higher latitudes in which farming issedentary in nature and the impoverishment of the soil is subsidized by theimport of synthetic inorganic fertilizer; and secondly that of the extensivefarming characteristic of much of the tropics where a shifting cultivation of onekind or another is practised (Ruthenberg, 1980). In this latter case soilimpoverishment is reversed by allowing the return of the forest in the form of afallow, which alternates with periods of cultivation in any given area. Thus inshifting agriculture there is also a subsidy Š the holding of a large area of landout of cultivation at any one time.These two types of fanning may be respectively characterized, at theirextremes, as technological and ecological, in terms of the resources oo whichthey depend. The high-yielding technological farm of North America utilizesintensively bred, genetically specialized varieties of crop plants grown asmonocultures at very high densities. Soils are prepared by deep mechanizedploughing and are fed with water, with macro and micronutrients and withchemicals to combat weeds, pests and diseases. The supply of all thesematerials is dependent on an energy subsidy largely petrochemical in origin,and on industrial processing. The soil in this type of agriculture is treatedsimply as a rooting medium and the biological processes are bypassed (Fig. 5);in some cases the soil may even be sterilized before planting. The crop mayalso be harvested by mechanical means.In contrast the gardener of a food plot in the Tropical Rain Forest of PapuaNew Guinea clears an area from the forest and prepares the soil by hand orwith a stick. He plants possibly twenty or more species of crop. Each crop isgenetically variable as the seed or roots are gathered from the previous cropwithout any particular selection. The garden may have a complex structurewith a tree layer, a shrub layer and a variety of ground plants. All the resourcesutilized are indigenous and the farmer is totally dependent on the naturalfertility of the soil for the level of yield he obtains. In terms of the ratio of energyoutput to energy input such a method of farming may be more efficient than onerelying on technological subsidy (Black, 1971; Rapaport, 1971); in terms ofFigure 5: The intervention of agricultural management into the functioning ofan ecosystem (compare with Fig, 2).93the absolute yield per unit area, however, the technological farm is many timesmore productive.The world food crisis: The predicted rate of global population increasenecessitates a doubling in world food output in the next twenty years in orderthat we may stand still, let alone make up the enormous nutritional deficitwhich currently exists. This requirement is most acute in the tropical regionswhere the major part of the population growth will occur. The demand forincreased food production can be met in one of two ways. Firstly, by bringingmore of the potentially arable land into effective production; secondly byincreasing yields per unit area of land. The figures quoted earlier show thatthere is plenty of suitable land still available Š 56 per cent'of the 3.2 X 109hectares of potentially arable land remain unconverted and most of this is in thetropics. But the cost of clearance on this scale in environmental terms may bevery high, and a good deal of the land is probably only marginally suitable inany case. The real key to the food deficit surely lies in increasing theeffectiveness of conversion of the land, that is in finding means of increasingthe yield.TOWARDS A MIXED ECONOMYThe obvious method of increasing yield would seem to lie in introducingproductive technological farming on a much wider scale than is currentlypractised. Attempts at this were made with some measure of success during thelast decade; the Green Revolution in Mexico and India produced spectacularincrease in yield under some conditions. In other instances, however, theintroduction of the high-yielding varieties has not had the predicted successand indeed has brought unforeseen problems of both an agricultural and asociological nature (Borgstrom, 1973; Pearse, 1980). There are, however, twobroad categories of constraint that place limits on the expansion oftechnological farming. Firstly there are economic limits; for this type offarming requires a very high subsidy from the other sectors of the economy,much of it petroleum-based, in order to provide fertilizer, fuel and pesticides.Whilst grain yields have gone up by factors of almost two in the last twodecades, the cost of producing the same amount of grain has more than tripled.If this type of farming is to be expanded its energetic efficiency must beimproved.The second constraint is potentially even more serious; it is biological.Many soils in the tropics do not seem to be able to bear the intensity ofcultivation that is required to produce this type of yield from monoculturefarming. There is perhaps an evolutionary lesson to be learned from the factthat the cultivation most characteristic of tropical areas is of the ecologicaltype where conservation of the soil is at a premium Š a lesson that has beenlearned by a painful process of trial and error over the centuries.But shifting cultivation is an equally unviable solution for the modernworld. In many parts of the tropics the population pressures are such that thelength of fallow period has already been drastically shortened to levels wherenatural regeneration of soil fertility is no longer possible. In many places it hasbeen abandoned in favour of continuous cropping regimes with eithernegligible or no fertilizer inputs at all, with the consequence of ever-decreasingyields (Ruthenberg, 1980). It seems clear that for these fragile tropical soils adifferent and more radical type of solution is required (Sanchez and Salinas,941981) Š one which Is reliant upon not the technological nor the ecologicalapproach alone, but one which benefits from insight into both. This type ofapproach might be described as a 'mixed economy' Š partly that ofagriculture, partly that of the forest.The return of the forest The absolute necessity for tree-planting is realized asa matter of high priority in this country and in many others throughout Africaand the tropics. This perception is based very largely on the high demand forfuelwood in an increasingly energy starved world (Eckholm, 1976). But it hasalso been recognized that the re-introduction of trees to the agriculturaleconomy can have other benefits. This has led to a developing interest in thepractice of agroforestry.Agroforestry is a term which embraces a wide range of different practicesbut basically means a land-use system that combines the production of annualcrops and/or animals with the growth of trees. Its fundamental feature is thatthe combination of trees with other types of land-use produces a viable andsustainable farming system (King, 1978; Lundgren and.Rajntree, 1983).Agroforestry systems have a large variety of possible structures. Thechoice of combination for trees and agriculture practice depends not only oneconomic but also on ecological and social considerations. In economic termsthe tree of choice may be primarily for fuelwood, for timber or for the yield of afood or cash crop. Whatever the decision in this respect, careful planning andmanagement can also provide indirect ecological benefits to the agriculturalsystem with which it is combined, whether that is arable or involves livestockproduction. The trees can be used to ameliorate soil degradation (Mongi andHuxley, 1979). The permanent cover of litter and roots at key positions on thecatchment area stabilizes the soil and prevents erosion and nutrient losses byleaching and run-off. The presence of the tree crop may indeed improve thehydrological status of the farm area, as Whitlow (1983), for example, hasrecently pointed out. The trees can also be used to improve the fertility of thesoil Their roots reach deeper into soil and nutrient capture into thevegetational zone is much greater than with shallow-rooting species. Thepresence of mutualistic associations between, fungi and the roots of the trees(see above) can further enhance the^extent of nutrient capture. The productionof abundant litter including components with high lignin content improves theorganic-matter status of the soil as well as stabilizing the soil surface and -enhancing infiltration. By ingenious choice of tree species litter productionmay be maintained through a large partof the year. The litter from the treesmay be used as mulch and compost for arable crops. In addition to their effects.on soil, blocks of trees may act as reservoirs of biological control agents againstthe pest and diseases of the crop plants. The trees provide shade and increasethe humidity of the area and can be used to nurture the crop plants by providing,a buffer against extreme environmental conditions.Whatever the economic benefits sought from the tree the choice should bemade on an ecological basis. The trees selected should be adapted to growth inthe climatic conditions of the area concerned and should be shown to have abeneficial relationship with the soil type in which they are grown. There is, a-huge wealth of species available and there is no" need to confine choice to.Central American pine species or Australian gum trees (N.A.S., 1975;Okigbo, 1977). Much research on suitable provenances is already being donein various parts of the tropical zone including Tanzania, India and CentralAmerica.95The yields of farm-produce from agroforestry systems are in some cases ashigh as can be obtained in the short term from more intensive farming. But inmany cases they fall short. The over-riding advantage lies, however, in thesustamability of this yield. Conversion to agroforestry, however, is complex; ittakes a considerable period of time to establish a system at its sustainable level,that is where all parts of the cycle are yielding on a repetitive pattern. Duringthis period of establishment, therefore, some degree of subsidy may berequired. Finally the planning must include a sociological component.Agroforestry must be planned on an ecological scale. The boundaries foragroforestry units must be determined by the hydrological character of thearea; that is to say they are the same boundaries as those of naturalecosystems, those of the catchment area. These boundaries do not coincidewith the boundaries of man's social organizations. So the introduction ofagroforestry means land-use planning which cuts across land-tenure systems.For all these reasons it is essential that the biological scientist, theagricultural scientist and the social scientist work together in planning theresearch and implementation criteria for rural agricultural development(Hoekstra and Kugura, 1983). This university has made significantcontributions to developing collaboration of this kind already but I believe thatit has more to make. One of the steps in this must be to turn its attention to trees,which means that at some stage forestry must be a part of its curriculum. Butthis does not just mean forestry in the production sense but forestry as a meansof re-introducing the tree into our way of life (F. A.O., 1978). Forestry must beintegrated with agriculture, with ecology and with all other aspects of naturalresources utilization and planning.EPILOGUE: MAN AND THE TREEIn the earliest stages of his own evolution man was an animal of the Africansavanna. He roamed areas similar to those still found in the untouched parts ofZimbabwe today. He lived in some type of equilibrium with that environment.At times its harshness defeated him but in general he used it to his advantage.In any event the trees were one of his benefits Š as a source of shade, of food,of firewood, of tools and eventually of material to build his home. But man haskilled the trees and now he has to live without them. And with the death of treesso also the soil has died. It is time now for a new stage in man's evolution, astage in which he learns to live again with the trees. As a master of them, usingthem to his own advantage, using them to supply his wants, using them aboveall to rebuild the soils he has destroyed. But using them with respect and withunderstanding. It is after all possible that, if man does not learn to do this, thetree will finally re-inherit the earth after mankind has become extinct by hisown self-destructive action.BibliographyANDERSON, J.M. and SWIFT, MJ. 1983 'Decomposition in tropical forests'. In S.L.Sutton, T.C. Whitmore and A.C. 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