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1.1 GLOBAL CLIMATE AND PLANT DISTRIBUTION Climatic conditions are controlled by the amount of solar energy that is intercepted by the Earth and its atmosphere. Some energy is used in photosynthesis and becomes temporarily stored in the biosphere, but mostly it is absorbed and converted to heat. In tropical regions there is a net surplus of energy, but higher latitudes experience a net negative radiation balance because more energy is lost through re-radiation than is received. The energy surplus in lower latitudes is redistributed through the circulation of the atmosphere and oceans and this ultimately determines global temperature and precipitation patterns. In general terms the Earth's climates are warm and moist in the tropics and become colder and drier towards the poles. To this must be added the effects of mountain barriers and continentality, so that many different thermal regimes and seasonal precipitation patterns interact to form the diverse climates of the world. In this way some 25 climate types are defined in the Koppen classification scheme, each being designated by a 2- or 3-letter code (Figure 1.1). The Koppen system is an empirical classification in which climates are grouped according to predefined limits and the correspondence between the climate classes and large-scale vegetation patterns is essentially coincidental. The predominant influence of temperature and precipitation on world vegetation also forms the basis of the Holdridge method of classifying plant formations (Figure 1.2). In this scheme temperature and precipitation interact through potential evapotranspiration to define humidity provinces and so establish a functional relationship between vegetation and climate. The general nature of each plant formation is determined by the structural character of the vegetation, and so reduces the great variation in floristic diversity to relatively few classes. The growth form of a plant is an adaptive response to its environment and provides an ecological classification that may be indicative of habitat conditions. Physiognomic characteristics such as plant height, woody or herbaceous growth and leaf type are often used in this way. The most widely used system is that proposed by Raunkiaer (1934) and is based on the arrangement of the perennating tissues of the plants growing under different climatic conditions: five principal life forms are distinguished (Figure 1.3). Phanerophytes, represented by trees and tall shrubs, carry their buds on the tips of their branches where they are exposed to climatic extremes: it is the predominant life form in mild, moist environments where the plants are not subject to frost and drought. Chamaephytes include small shrubs and herbs that grow close to the ground: they occur most frequently in regions where snow cover affords some protection during cold winter months. Hemicryptophytes are characteristic of moist temperate regions. These plants die back at the end of the growing season, and the buds are protected by the withered leaves and soil. Cryptophytes are well adapted to extreme conditions of cold or drought and persist because they regenerate from buds, bulbs and rhizomes that are completely buried in the soil. Therophytes are annual plants that regenerate from seed each year; they occur abundantly in desert areas. Raunkiaer's system is actually a floristic approach in that the 'biological spectrum' of a region is calculated from the number of species in each category. Different plant formations are distinguished by the proportional contribution of each category (Figure 1.4). 1.2 CLIMATE AND COMMUNITY STRUCTURE Plant formations, such as tropical forest or tundra, are the largest and most complex units of vegetation and represent the level at which most world maps are compiled. Traditionally the global distribution is referred to as the 'formation-type', with 'formation' reserved for the regional subdivisions- for example, the American, European and East Asian formations of the temperate deciduous forest formation-type. The distribution of these complex units is generally determined by climate, although some, such as heaths, may be influenced by soil conditions. A formation is structurally similar throughout its range, even though pronounced floristic differences may occur between regions. Consequently, the formation is subdivided into associations on the basis of the dominant species that grow under uniform habitat conditions. For example, the clay vales of Europe support stands of pedunculate oak (Quercus robur) whereas beech (Fagus sylvatica) is the dominant species on the chalk uplands. Faciations are recognized where some associational dominants are locally absent, and lociations may be defined according to important subdominant species. Societies are established within these higher classes through the inclusion of subordinate species (Polunin, 1960). In this hierarchical system it is assumed that different plant species are habitually associated with each other over wide geographic areas. This has led to the assumption that the classes are rigidly defined and thus represent discrete units in the natural landscape which have developed under the controlling influence of climate. Such ideas are encompassed in the 'organismic' approach developed by Clements (1916) in which communities were thought to consist of species that were so coadapted that they were most successful when they occurred together. Thus, distinct assemblages developed because all species replaced each other at discrete points along an environmental gradient (Figure 1.5). The alternative viewpoint proposed by Gleason (1926) and others emphasized the species as the essential determinant of vegetation structure and composition. This 'individualistic' approach considers that each species responds in its own way to the physical and biotic environment. Analytical studies of community structure suggest that species are arranged so that each has an optimum environment that does not coincide with that of a potential competitor (Whittaker, 1965). The development of welldefined communities is therefore precluded because species are distributed rather randomly and discontinuities will occur only where environmental conditions change abruptly. However, it has also been argued that there are points along environmental gradients where pronounced changes in species composition do occur, and that the appearance or disappearance of dominant species at their critical limits of tolerance can be accompanied by changes in subordinate species. For example, in the western United States the loss of shrubby sagebrush (Artemisia tridentata) distinguishes the Artemisia~Festuca association from the Agropyron~Festuca association dominated by bunch-grasses (Daubenmire, 1966). The loss of sagebrush not only alters the composition of the plant cover, but shrub-dependent birds also disappear, and thus the overall composition of the ecosystem is changed. This functional relationship between members of a community is an important extension of the early ideas and demonstrates the complexity of environmental interactions. Thus, whereas global vegetation patterns are related to climatic conditions, other factors such as soils, herbivory and competition account for much of the variation at a local scale. 1.3 SOIL AS AN ENVIRONMENTAL FACTOR Soil is an integral part of the biosphere. It consists of organic and inorganic matter in the form of solids, liquids and gases, but in addition to these inert materials the soil provides a habitat for many organisms which are essential for the maintenance of the overlying plant communities. Soil is the principal source of nutrients and water for most terrestrial plants, and they in turn provide the bulk of the organic litter necessary for the maintenance of the living soil community. Differences in soil properties produced by the interaction of parent materials, climate, topography and vegetation over time have a profound effect on the biological systems that they support. The inorganic mineral fraction of a soil influences physical characteristics such as texture and structure, and thus affects soil moisture, aeration and other properties. Similarly, soil chemistry is partly determined by the breakdown of primary minerals through weathering processes, although the supply of plant-available nutrients is principally controlled by the amount and type of organic matter that is present. Additions of organic matter and other materials to the soil and losses from leaching, seepage and erosion, coupled with transfers and transformations within the soil itself, result in the gradual development of different soil types (Simonson, 1959). The unique combination of factors operating in different parts of the world has resulted in many distinctive soils and this has necessitated some method of classification. One of the most widely used soil classifications is the Soil Taxonomy developed by the US Department of Agriculture (Soil Survey Staff, 1975). It uses diagnostic horizons to classify soils into 10 Orders which represent the highest class in the system. Six of these diagnostic horizons develop at the soil surface and include, for example, the dark, humus-rich mollie epipedons associated with temperate grasslands. Diagnostic subsurface horizons develop at depth within the soil: 18 are recognized including, for example, the reddish-brown, illuviated spodic horizon associated with coniferous forest and the highly weathered oxic horizon that is characteristic of many tropical soils. The presence or absence of such diagnostic horizons reflects differences in the degree and type of dominant soil-forming processes that have occurred. The soil Orders that are differentiated in this way are classified into suborders and lower classes mainly on the basis of soil moisture and temperature regimes or distinctive physical and chemical properties associated with differences in parent materials and topography. Although the Soil Taxonomy is a comprehensive empirical method of soil classification, it has not been adopted universally. A comparison of the 10 Orders in this scheme and the more familiar Great Soil Groups used in the earlier classification of Baldwin eta!. (1938), and the equivalent categories used on the Soil Map of the World (FAO-Unesco, 1974-78) is therefore presented in Table 1.2. 1.4 THE INFLUENCE OF LIVING ORGANISMS Plants seldom grow in isolation except in very extreme environments such as polar deserts or sea cliffs where the number of suitable sites are limited. Elsewhere plants grow together as members of a community and must compete for resources. The most successful competitors become common members of the community, others are less prominent, and some may be eliminated. Competition is most intense between organisms with similar ecological requirements. Intraspecific competition may occur between members of the same species when the supply of water, nutrients or other resources is limited. Because of this some individuals become stunted and die, and the population is ultimately thinned to its optimum density. Similar interaction between different species is termed interspecific competition, and the outcome depends on how effectively each competitor utilizes the scarce resources. Species with identical resource requirements cannot coexist in a stable community. The floristic diversity of a community is thus a reflection of the degree of resource specialization that has occurred between species in that particular habitat. The specialized functional interaction between an organism and its environment defines that species niche. Competition occurs where species niches overlap, and because of this many species are necessarily restricted to particular microhabitats for which they are best adapted. Competition can reduce a plant population through death of established individuals or by altering the reproductive success of a species. Intraspecific competition typically operates as a density-dependent process in which mortality is regulated by the size of the population. In this way a larger proportion of the population is killed as population density increases. The greatest losses normally occur at the seedling stage, but stands are naturally thinned as the plants mature through the suppression of smaller individuals. Conversely, density-dependent fecundity may regulate population size by the number of seeds that are produced by plants at different population densities (Silvertown, 1982). Interspecific competition operates through the imposition of stress, and successful competitors are characteristically adapted to avoid this. Competitive species are usually perennial plants with well-developed capacities for resource capture, and exhibit rapid growth of roots and shoots to ensure their advantage (Grime, 1979). Other plants tolerate the stresses imposed by competitors or other unfavourable environmental conditions such as drought or cold. Stress-tolerant species are also perennials, but their conservative growth strategy enables them to survive for long periods with little growth or reproduction. Such is the case for many tree species that must remain in the understory of a forest until a suitable gap occurs in the canopy. A third ecological strategy is associated with areas subject to disturbance from factors such as fire and grazing. Under these conditions ruderals are at an advantage: mostly they are annual plants with rapid growth rates and short life spans. Reproductive allocation is typically high in ruderal species, and the production of large numbers of readily dispersed seeds with prolonged dormancy is the norm. Many ruderals are fugitive weedy herbs, but shade-intolerant trees, such as jack pine (Pinus banksiana), also depend on disturbance for continued establishment. Plants comprise the resource that sustains higher trophic levels in an ecosystem, and in addition to various competitive interactions they must tolerate the impact of herbivores. Each herbivorous species has a preferred diet and selects plants which are the most palatable. In natural ecosystems the palatable species are damaged and depleted through loss of photosynthetic tissue and reduced assimilation, and this may render them less competitive or limit their reproductive capacity. However, they are never eliminated by overgrazing, because complex mechanisms have evolved to ensure the optimum balance between plants and herbivores. Plants can be classified according to the probability that they will be found by grazing animals (Feeny, 1976). Apparent species tend to occur abundantly in specific habitats, and because they are conspicuous to herbivores they depend on non-selective, dose-dependent mechanisms to reduce grazing intensities. Unapparent species exploit a number of habitats, and so their distribution is patchy and unpredictable. Further protection is afforded by potent secondary metabolites which deter specific predators. Alternatively, the different methods used by plants to reduce herbivory has been attributed to resource availability and growth rates (Table 1.3). The functional life span of leaves and other organs is longer in slower growing species of plants: this favours increased investment in defensive mechanisms and makes them less palatable to herbivores. With increasing community diversity herbivore preferences become more specific so that some insects, for example, may be restricted to a single plant species. Host-specific herbivore populations are generally limited because the plants are widely dispersed: this in turn offers some protection to the plants themselves. 1.5 ENERGY AND NUTRIENT FLOW THROUGH ECOSYSTEMS Herbivores do not consume all of the plant material that is available to them. Similarly, most organisms at higher levels in the food web escape predation. The energy and nutrients stored in their tissues is eventually released through death and decomposition. Decomposer organisms are therefore essential for the maintenance of ecosystem structure and function and complement the assimilative activities of other plants and animals. The assimilation of energy by plants in photosynthesis transforms carbon dioxide into carbohydrates such as glucose and starch: further synthesis produces other organic compounds such as fats, oils and proteins. The rate of photosynthesis varies with light intensity, temperature and moisture availability: growth may be further limited by soil nutrient levels. Consequently, there are pronounced regional differences in plant productivity which are primarily related to latitudinal changes in climate (Figure 1.6). Productivity is highest in the humid tropics where the combination of strong radiation, warm temperatures and abundant rainfall provides favourable growing conditions all year round. At higher latitudes plant growth is limited by cooler temperatures and shorter growing seasons, and in desert areas productivity is reduced by the lack of water. Productivity in oceans and lakes is primarily limited by nutrient availability: it is generally highest in the temperate regions where seasonal overturn brings nutrient-rich water to the surface, where light intensity is sufficient for phytoplankton photosynthesis. The amount of energy that passes to higher trophic levels in an ecosystem is determined by net primary production and the efficiency with which this plant material is transformed into animal biomass. Even under the most favourable conditions, it is estimated that as little as 1-5% of the incident radiant energy is used in photosynthesis. Some of the energy assimilated by plants is used in respiration and cellular maintenance and is therefore unavailable to consumers. Net production efficiency, defined as the ratio of net to gross production, is as high as 75-80% for rapidly growing plants in temperate regions, but drops to 40--60% in the tropics (Ricklefs, 1990). Once the food is eaten the energy it contains is dissipated in various ways. Most of the ingested energy is used in metabolic activity and is lost as heat through respiration; some is excreted in waste products or is egested because it could not be digested, and the remainder is used for growth and reproduction. The nutritional value of vegetation depends on the amount of lignin and other indigestible materials that the foodstuffs contain. Herbivores assimilate as much as 80% of the energy in seeds and 60-70% of the energy in young vegetation: this declines to 30-40% for animals that browse on shrubby material and is only 15% for millipedes and other organisms that feed on wood. The rate at which energy is transferred from plants to herbivores is calculated as the ratio of the energy stored in biomass to that added each year in net production. In this way the average residence times of energy in living plant biomass varies from only 1 day in marine continental shelf ecosystems to 25 years in temperate and boreal forests (Table 1.4). Solar energy assimilated by plants passes through the food web, and is dissipated as heat at each trophic level. Organic debris is similarly broken down by the activity of detritus feeders and decomposer organisms, and the energy transformed into biomass through this pathway may again pass temporarily into the food web by way of predation. All life ultimately depends on the energy that continually enters the biosphere through photosynthesis. However, nutrients are circulated within the biosphere, and the amounts available to plants depends on various gains and losses which occur within the ecosystem. Although the interchange of nutrients between the biotic and abiotic components of an ecosystem is unique for each element, two classes of biogeochemical cycles are generally recognized. In gaseous cycles elements such as nitrogen and carbon can exist as gases under normal atmospheric conditions. Alternatively, in sedimentary cycles elements such as phosphorus and potassium remain as solids or pass into solution. Gases enter the biosphere through biological fixation: in sedimentary cycles natural inputs originate from weathering of rock materials. Circulation occurs through abiotic processes such as precipitation, particulate fallout from the atmosphere and streamflow, but in many ecosystems the greatest supply of nutrients is held in organic matter and is made available to plants through decomposition. Many organisms consume detritus, but ultimately the fragmented material passes to the micro-organisms and is rendered to its inorganic components by the activities of fungi and bacteria. The residence time of dead organic matter in the litter varies from a few months in tropical areas where conditions are optimal for decomposer organisms to more than 100 years in the cool boreal forests, but here, as in most biomes, the process of mineralization is accelerated by fire. Fire not only reduces the accumulated litter, but also releases nutrients from any live plants that are consumed. 1.6 SUCCESSIONAL DEVELOPMENT IN PLANT COMMUNITIES Disturbance from fire and other natural factors, such as disease and hurricanes, periodically alters the nature of the plant cover in an area. Habitat conditions are also changed thereby favouring the establishment of different species of plants. In classical successions the species in these new communities are in time replaced by others until the climax associations are once more established. Disturbance of a previously existing plant cover initiates the process of secondary succession. Alternatively, plant colonization may begin on newly formed habitats, such as river sand bars or moraine exposed by glacial retreat: this is termed primary succession. Primary succession begins when new sites become available. Pioneer species arrive and become established at the site, and with time alter the environment so that other species can survive. The floristic and structural complexity of these communities increases with time until environmental conditions become stabilized and a self-perpetuating climax association is formed (Clements, 1916). Secondary succession proceeds in a similar manner, except that the process is generally much faster because some of the previous plant cover as well as buried seeds and rootstocks will normally persist at the site, and residual soil conditions will also be more suitable. The nature of the secondary plant cover therefore reflects the degree and intensity of the disturbance and the amount time that has elapsed since it occurred (Kellman, 1970). The appearance of a species during succession depends on the time when its propagules arrive at the site and its tolerance of existing environmental conditions. Subsequent development of the plant cover is controlled by the competitive interaction of the species and their effect on the environment. Three alternative models of successional change are suggested on this basis (Figure 1.7). The facilitation model incorporates the classical view of succession proposed by Clements in which the established plant cover modifies the environment so that it becomes more favourable for late successional species. Alternatively, in the inhibition model the initial plant cover modifies the environment so that it is less favourable for subsequent recruitment by other species, and succession proceeds only when the inhibitory species die and are replaced. In the tolerance model successional development is determined by the competitive abilities and life spans of the established plant species, so that the longer-lived species characteristically associated with later stages will persist in the plant cover. Consequently, the predictable and sequential changes in vegetation and environment envisioned in classical succession theory may not occur. The development from pioneer communities through distinctive sera! stages to the climax vegetation as a result of autogenic environmental modification has been termed relay floristics (Egler, 1954). Alternatively, viable propagules of most species may be present initially, and apparent sequential changes in the plant cover occurs because of different germination requirements and growth rates. Interspecific differences in seed dispersal and longevity are important to these concepts. Pioneer species are characteristically short-lived plants which produce abundant, readily dispersed seeds with comparatively long viability (Table 1.5) and consequently are more likely to be represented in the early plant cover in primary successions. Long-term storage of propagules only occurs beneath an established plant cover and so is more appropriately associated with secondary succession. Thus, the combination of seed availability and suitability of conditions for establishment and growth makes successional change more of a probabilistic process than originally envisioned. The process of succession is accompanied by changes in many community attributes (Table 1.6). Early successional communities occur in newly created habitats or areas of recent disturbance in which the physical environment is characterized by extreme conditions and variable resource availability. For example, in the absence of a mature plant cover soil moisture levels may fluctuate because of direct exposure to sun, wind and rain. The nutrient capital available to pioneer plants in primary successions is typically very low and in inorganic form, although availability is generally higher in disturbed secondary sites. Characteristically the pioneer communities are composed of few species, but population densities are normally high, because of their efficient dispersal mechanisms, wide ecological tolerance and relatively unspecialized patterns of resource utilization. The rate of primary production per unit of biomass is high because the community is mainly comprised of relatively small, fast-growing species. Overall performance is greatly influenced by environmental perturbations, and overall stability is low. Later stages in succession are characterized by an increase in species diversity and biomass, more complex community structure and greater utilization of resources. Efficiency in resource use is achieved through increased specialization. However, poorly adapted species may eventually be lost from the community and consequently species diversity can decline in climax communities. Because of their intricate organization, homeostatic regulation is characteristically high in mature communities. Structure and function become increasingly self-regulated with maturity, and interaction with the external environment declines. However, the mature plant communities still exhibit seasonal rhythms as growing conditions fluctuate during the year. They must also evolve in response to long-term environmental change. 1.7 HISTORICAL PERSPECTIVES IN PLANT GEOGRAPHY The present distribution of plant species is the product of past events, and the changing patterns which have occurred through time are preserved in the geological record. Plant microfossils extracted from lake deposits and peat bogs are most commonly used to interpret events in the Quaternary (Figure 1.8), especially the dramatic changes in plant distributions caused by the Pleistocene glaciations. Plant microfossils are mainly comprised of pollen grains and spores. They are exceptionally resistant to decay in anaerobic sediments and their unique morphologies facilitate identification. Plant macrofossils such as wood fragments, leaves and fruits may also be present, but their usefulness in palaeobotany is sometimes limited by problems of identification. It is also difficult to establish if the plants were actually growing at the fossil site (West, 1977b). The changing pattern of world vegetation is thereby projected back through time. Vascular terrestrial plants evolved in the Silurian period some 400 million years ago, and rapidly increased in diversity during the Devonian period with the appearance of the first seed plants, then declined during the period of mass extinction at the end of the Permian. These early floras are distinguished by the adaptive radiation of the pteridophytes. The gymnosperms increased in diversity during the Carboniferous and remained the dominant plant group until the appearance of angiosperms in the mid-Cretaceous (Signor, 1990). Today the number of ferns and their allies totals about 12 000 species, most of which are native to the moist tropics. The gymnosperms include some 500 species of conifer, 100 species of cycad and a few other ancient plants such as the maidenhair tree (Ginkgo biloba), which is native to China, and Welwitschia mirabilis, which is restricted to the coastal fog-belt of the Namib Desert. The greatest diversity is exhibited by the angiosperms with some 250 000 species. Biogeographic interpretation of the early fossil record has been greatly advanced by the general acceptance of the theory of continental drift. Thus, the fern-like Glossopteris is widely distributed in Permian rocks in India, Australia, southern Africa, South America and Antarctica, and presumably spread to these locations at a time when the southern continents formed part of Pangaea. The 'Glossopteris flora' consists of plants with deciduous leaves and pronounced growth rings in their wood, which suggests that they were adapted to a temperate climate (Schopf, 1970). The break up of Pangaea into the northern and southern landmasses of Laurasia and Gondwana occurred about 180 million years ago in the lateTriassic, and this early separation is still reflected in the present flora. Thus the proportion of plants with entire leaves is higher in the southern hemisphere floras than in the northern hemisphere. Similarly, the forests of high latitudes are dominated by conifers in the north but are mostly composed of angiosperms in the south, with an admixture of ancient and distinctive gymnosperms (Pielou, 1979). Rotational movements resulted in the separation of Africa, India and Madagascar from the other southern continents in the mid-Cretaceous about 100 million years ago (Figure 1.9). Australia, South America and Antarctica became separated during the Eocene, about 49 million years ago. These events are reflected in the distribution of the southern beeches (Nothofagus spp.) which now grow in temperate regions in South America, Australia, New Zealand and New Guinea, with fossil sites known from Antarctica. The absence of Nothofagus from Africa is generally attributed to the early isolation of this continent. Alternatively, the genus may have become extinct in Africa as the continent drifted northwards into tropical latitudes, or perhaps it was unable to compete with plants coming from Eurasia. The distribution of a plant species can thus be attributed to historical events as well as its migrational ability and adaptability to present environmental conditions. Many different types of distribution are recognized (Polunin, 1960). For example, cosmopolitan species are found in most parts of the world: they are mostly weedy species which have been distributed unwittingly by humans through cultivation, or are represented by mosses and other cryptogams. More restricted are the circumpolar plants which are found only at higher latitudes, and pantropical species which occur throughout the tropics. Disjunct distributions occur when locations are widely separated as is the case with some arctic-alpine species. In contrast endemic species are very restricted in their range, and may occur only in a peculiar environment, such as those associated with serpentine soils, or perhaps are limited to a single island. Some, like the redwoods (Sequoia spp.) may be relic species that are remnants of an earlier flora that was previously much more extensive. The geographic ranges of most species are comparatively small. Consequently, it is possible to subdivide the world into regional floras on the basis of their distinctive taxa. In this way the world is divided into 37 floristic regions (Good, 1964). These can be combined into eight biogeographic realms which account for the regional differences in phytogeography and zoogeography (Figure 1.10). Even though the biogeographic realms are taxonomically distinct because of the restricted range of most plant and animal species, adaptation to similar ecological niches has given rise to many equivalent forms. In this way the major biomes of the world are distinguished on the basis of their structural similarity and functional relationship with the environment.