vegetation and climatic change

vegetation and climatic change Climate is one of the dominant controls on vegetation; this domination is clearly illustrated by comparing maps of global climate with the world's major vegetation communities. Ten major vegetation communities, or biomes, are recognized on the terrestrial surface of the Earth: Arctic tundra; northern coniferous forest (taiga); temperate forest; tropical rainforest; tropical seasonal forest; temperate grassland; tropical savannah grassland and scrub; and chapparral, which corresponds to those regions of the world that possess a Mediterranean climate; and mountains. The distribution of these major biomes correlates extremely well with global patterns of climate based on a combination of factors such as temperature, precipitation, and the intensity of isolation. Hence it is concluded that the two are interrelated in some way.

This interrelationship can be shown by looking at the present geographical range of plant communities and their physiological variables, such as the length of the growing season, temperature range, and precipitation. In the late 1960s and early 1970s, Reid Bryson and co-workers demonstrated the close association between biomes and weather systems on a subcontinental scale. Their investigations revealed that there was a good statistical correlation between the mean frontal positions of meso-scale airmass boundaries and the modern distribution of biotic communities. In northern Canada, the position of the boreal forest coincides with the area over which continental and Arctic air masses dominate in winter and over which the Pacific and tropical airmasses are present during the summer months. Bryson found that similar patterns exist for the areas of the North American continent occupied by tundra, the prairies, the eastern deciduous woodlands, and the south-eastern evergreen forests, all of which appear to be strongly associated with different and areally well-defined air masses. Thus, the correlation between climatic variables such as air masses and vegetation communities occurs on both a subcontinental and a regional scale.

The clear correlation between climate and vegetation becomes more obscure at a local scale. At this scale, the distribution of plant communities and individual plant species can be controlled by a variety of environmental factors, which include non-climate variables such as soil type, topography, and factors related to the soil, as well at some climatic variables like precipitation and temperature. Many ecologists, such as Margaret Davis and John Birks, argue that individual plant species respond differently at a finer scale of resolution to non-climatic and climatic variables, and the effects of both combine to structure vegetational communities. However, temperature, rainfall, evaporation, water, and sunlight can be the dominant controls in explaining the geographical distribution of a plant, and this can also be demonstrated at a species level. For example, the grey hair grass (Corynephorus canescens) occurs throughout central and southern Europe, reaching its northern limit in the British Isles and southern Scandinavia. This northern limit corresponds to the 15 °C July mean isotherm. Marshall suggests that climatic factors restrict the distribution of the grey hair grass to the south of this isotherm because the grass cannot germinate or flower in areas affected by low temperatures. The study of modern ecology and environmental tolerances of plant communities and plant species enables ecologists and biogeographers to determine how far climate can influence geographical distribution.

Studying the development and distribution of past vegetation has proved valuable for understanding the relationship between climatic change and vegetation. Pollen analysis has been a productive technique for reconstructing past vegetational communities because every species of plant produces a morphologically distinctive pollen grain or spore. Once released by their host plant, pollen grains and spores can be transported over wide areas and are ultimately deposited and preserved in a variety of sediments. Sediments such as peat and lake deposits that are acidic and anaerobic preserve pollen and spores particularly well. By employing pollen analysis, scientists have been able to reconstruct past vegetational communities and analyse how those vegetation patterns have changed over time. Fossil pollen records have, however, been used extensively as a source of information for inferring past changes in climate. If a circular argument is to be avoided in assessing the role of climate in determining the structure of plant communities, it is therefore necessary to have a method of establishing the nature of past climates that is independent of the pollen record. Fortunately, we can also gain an idea of temperature change during the recent geological past by using a range of proxy sources (e.g. oxygen and deuterium isotope records from ice cores) to reconstruct past temperature curves. Another source of proxy data is that of fossil Coleopteran (beetle) remains, which, like pollen grains, are also preserved in sediments. Many species of Coleoptera have a very specific range of tolerance to temperature. Their occurrence and preservation in sediments deposited during the recent geological past thus provides an indication of the temperature conditions when the sediment was deposited. By looking at both the pollen and Coleoptera records, the relationship between past climate change and vegetational history can thus be established. A chronology of vegetation history and climate change has been established by radiocarbon dating of the sediments in which the pollen grains and Coleoptera are preserved.

One of the most valuable ways of studying the relationship between vegetation and climate change is to analyse how vegetation patterns develop as the Earth moves from a period of glaciation (a glacial period) into warmer deglaciated conditions (an interglacial period). The last glacial–interglacial transition occurred between 14 000 and 9000 years before the present (BP), when the last glacial period ended and the present interglacial, known as the Holocene, commenced. As part of an international project, known as IGCP–253, a sub-project called the North Atlantic Seaboard Programme (NASP) was initiated to investigate the environmental changes that accompanied the termination of the last glaciation in north-west Europe. The preservation of deposits that contain microfossils such as pollen during this transition has provided a unique opportunity of studying how vegetation responded to what can be described as a relatively abrupt change in climate. Research in the UK, primarily by Professor Russell Coope of Birmingham University, has provided temperature curves, based on Coleoptera records, for the last glacial–interglacial transition. This makes it possible to compare the vegetation record, as depicted by pollen analysis, with a proxy climate record. A brief summary of the climate and vegetation changes during this time-period given below illustrates the strong interrelationships between them (see Fig. 2).

Between 14 000 and 13 000 years BP, glacial conditions prevailed in the British Isles. Vegetation was dominated by open ground communities containing Gramineae (grasses), Rumex (docks) and Artemisia (mugworts). The climate appears to have improved rapidly after this in the British Isles, and a period known as the Late Glacial Interstadial occurred between approximately 13 000 and 11 000 years BP. Initially, between 13 000 and 12 000 years BP, July mean sea-level temperature averaged 18 °C, before gradually falling to below 12°C by 11 000 years BP. The amelioriation in climate allowed a scrub vegetation to develop, and this was followed by Woodland in many areas of England, Wales, and Scotland. The scrub vegetation appears to have been characterized by pioneer trees such as Betula (birch) and Juniperus (juniper). In southern areas of the British Isles, conditions were suitable for the development of woodlands containing Pinus (pine) by about 11 500 years BP. The transition from glacial to interglacial conditions does not, however, appear to have been smooth. Every so often evidence for a sudden deterioration in climate can be identified in the fossil record. These sudden reversals in climatic conditions are known as ‘revertance’ episodes. One such revertance episode seems to have taken place throughout large areas of north-west Europe, including the British Isles, between approximately 11 000 and 10 000 years BP (the Younger Dryas). These revertance episodes were accompanied by a change in vegetation. During the Younger Dryas in the British Isles, vegetation was characterized by a cold-loving tundra and low alpine scrub, typically comprising species such as Gramineae, Cyperaceae (sedges), Caryophyllaceae, Cruciferae, Rumex, and, in upland areas, Selaginella and Lyocopodium (both clubmosses). These correspond with a phase of much lower temperatures; the mean July temperature between 11 000 and 10 000 years BP is estimated to have been below 10 °C. The Holocene period began at approximately 10 000 years BP, and it is marked a vegetational succession from tundra and low alpine scrub to closed mixed deciduous woodland. This vegetational succession correlates closely with the development of warmer climatic conditions. First, the open ground communities are gradually replaced by Betula and Juniper, which are thought to have formed an open woodland. This is then followed by the arrival, in sequence, of the tree species that characterize a mixed deciduous woodland. By about 9000 years BP, Pinus and Corylus (hazel) became established in the British Isles, closely followed by Quercus (oak) and Ulmus (elm). A more diverse woodland community developed as climate warmed, allowing more thermophilious taxa such as Alnus (alder), Tilia (lime), and Fraxinus (ash) to establish themselves between 8500 and 6000 years BP. By then, climatic conditions in the British Isles are thought to have reached their optimum, highlighted by the mean July temperature which had risen to over 16 °C by 9000 years BP. Thus, by using pollen and Coleoptera data a proxy record of vegetation and climate change can be inferred, and these data suggest that there was a strong degree of correlation between climatic change and vegetational development in the past.

One of the main controversies that exists in the study of the relationship between vegetation and climate concerns the degree to which plant communities are in equilibrium with the prevailing climatic conditions, and how quickly vegetation responds once a change in climate takes place. In theory, if the vegetation response rate occurs relatively slowly once climate has changed, it is plausible that for a certain length of time the relationship between climate and vegetation is out of equilibrium (or is in disequilibrium). The time it takes for the vegetation to readjust to new climatic conditions is known as the lag time. Margaret Davis suggests that the differential response time of plants to a climatic change that results in a lag time results from inherent differences in life-history characteristics, such as the lifespan of individual plants, the dispersal capabilities of a plant, and its intrinsic rate of population increase. Thompson Webb III and others have studied in detail the response time of vegetation to climate changes. Webb suggests that the rate at which vegetation responds to a change in climate varies over different time-scales. The time required for vegetation to adjust and then reach a new equilibrium in response to a climatic change on a timescale of 10⁴ years is so far unknown. On much shorter timescales of 500 to 1000 years duration, however, vegetation responses or lag times appear to be relatively short.

From our understanding of vegetation and climate interactions, it would appear that climate is the major driving force behind colonization and establishment, or removal, of vegetation within any area. However, on a local scale, site conditions must also be suitable if a plant is to establish itself. Physical factors such as soil development are important; the soil must provide nutrients in order for a plant to survive. Other factors, including parent material, topography, drainage, and the amount of disturbance, can contribute to the fertility of a soil and its suitability for plant growth. Furthermore, geomorphological changes can also influence the structure of plant communities. Suitable climatic conditions do not automatically mean that a plant will colonize and establish itself within an area. Natural barriers, such as a mountain range or an ocean, can prevent the dispersal and spread of a plant. Sometimes the distribution of a plant is dependent on a chance event or it is advertently or inadvertently modified by human activity. Biological interactions, such as competition or disease, can play a role in structuring plant communities. Thus, it is important to remember that vegetation, especially at a species level, can respond differently and vary over different timescales to climatic and non-climatic variables. Many of the non-climatic factors that can alter the vegetational structure of the landscape could also, in theory, contribute to the lag time in the response to a rapid climatic change. Non-climatic factors might also explain why the distribution of certain plant species does not always correspond to their potential geographical range as defined by climatic variables. Webb has argued that the influence of non-climatic factors to create a lag in vegetation is in fact dependent upon the time-scale over which a climatic change takes place. Many non-climatic processes can limit the total response of vegetation to rapid climate changes of very short duration, say between ten and a hundred years. Webb suggests that when the timescale is extended to 1000 years or more, only very slow rates of plant dispersal or soils with development rates of greater than 1000 years will hinder the readjustment of vegetation communities to new climatic conditions. Climate, however, might have some degree of control over some of the biological and physical factors that influence the structure of vegetation communities. For example, J. S. Clark has shown that the frequency of fire disturbance of the western margin of the Hemlock–White pine–northern hardwood forest region of north-western Minnesota can be associated with changes in climate. Clark's research shows that during periods of warmer, drier climate, the periodicity of the fire regime of the forest occurred over a much shorter time-cycle when compared with periods of cooler and moister climatic conditions. During the dry and warm fifteenth and sixteenth centuries ad, the recurrence interval of major forest fires was approximately 33 to 44 years. A change in climate to cool, moist conditions took place between 1600 and 1864 ad (a period known as the Little Ice Age) and correlates with a doubling of the fire recurrence interval to approximately 88 years.

The relatively fast rate of climate change compared to vegetational response can mean that in certain areas some plants become disjunct from their main areas of distribution. These isolated pockets of vegetation are known as climatic relicts (also refugia); their presence is simply explained by the former presence of more suitable climatic conditions. An example of a climatic relict is the strawberry tree (Arbutus unedo). The strawberry tree has a disjunct distribution in western Europe, having its main centre of distribution in the Mediterranean region and scattered pockets in western France and western Ireland. Cox and Moore have suggested that the occurrence of the strawberry tree in western Ireland resulted from the relatively warm climatic conditions which occurred after the retreat of glaciers at the end of the last ice age. This allowed plants that are adapted to a climate of more Mediterranean type to expand their range northwards along the coastlines of western Europe. Climatic conditions were suitable for the northward spread of the strawberry tree because close proximity to the sea and the influence of the warm Gulf Stream provides western Ireland with a wet, mild and frost-free climate. Since then, the population of strawberry trees in western Ireland has become isolated as a result of a rise in sea-level and a deterioration in climatic conditions but the tree population has so far continued to survive in an area that is outside its normal Mediterranean biome.

Predicting future climate change and how it will affect the pattern of vegetation is an extremely difficult task. However, the need to understand the relationship between climate change and vegetation is growing in importance as human intervention in the environment appears to be upsetting the natural balance of gases in the atmosphere. In particular, there is a strong body of data to suggest that the atmospheric concentrations of the greenhouse gases—those gases that are responsible for absorbing longwave radiation in the troposphere—are increasing and therefore raising the temperature of the troposphere and the Earth's surface. The International Panel for Climate Change (IPCC) published data in 1990 and 1992 which suggests that human activity is primarily responsible either for increasing the processes that release otherwise inert forms of greenhouse gases (e.g. the burning of fossil fuels) or for destroying the sinks (e.g. the destruction of tropical rainforests). They confidently predicted that over the next century the result of increasing greenhouse gas concentrations in the troposphere will result in a change in climate, and especially an increase in global temperature. By using general circulation models, climatologists predict that surface air temperature will increase globally between 1.5 and 4.5°C if atmospheric levels of carbon dioxide double. A rise in temperature of this kind would have a significant effect on the distribution of vegetation.

There is still, however, a great deal of uncertainty about the extent to which climate, and hence vegetation, will change on a regional and local scale. Most climatologists believe that climatic variables such as the distribution and levels of precipitation and soil moisture will change as a result of increased climatic warming. One example where ecosystem development and vegetation patterns will be altered if global temperatures increase is the Arctic tundra. W. Dwight Billings and Kim Moreau Peterson describe the Arctic tundra ecosystem as one which is dominated by plants that are unique in being able to grow and reproduce at temperatures just above freezing. This means that plants that occupy the tundra can complete successfully with more thermophilious plant species. If, however, the IPCC's predictions prove to be correct, and over the next century the Earth experiences global warming, this could have a disastrous effect on the tundra ecosystem. First, a rise in temperature in the Arctic would cause some permafrost to melt and allow more woody and thermophilious species to invade and displace the present plant communities. Kullman has demonstrated that birch (Betula pubescens) expanded into areas of the Swedish tundra in response to warming during the first half of the twentieth century, illustrating how the diversity, range, and composition of plant communities may change if our present climate continues to become warmer. Secondly, a warmer climate could also alter the global carbon balance by melting frozen peat. Carbon compounds (either carbon dioxide or methane) could then be released and further enhance the concentration of these greenhouse gases in the atmosphere. Research into the effects of global climatic change on forests by Jerry F. Franklin and his colleagues also suggests that the pattern of forest composition and distribution will change as temperature increases. At present, the dense coniferous forests of the Pacific coast of north-western North America are dominated by species like the Douglas fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), western red cedar (Thuja plicata), and the Pacific silver fir (Abies amabilis). The structure and distribution of these forests is controlled by different moisture regimes and natural disturbances, especially fire and windstorms, which are also thought to be climatically driven. Results from models of climate change for this region suggest that the climate will become significantly warmer and drier. It is thought that the main impact on climatic conditions will be a change to the present moisture regime: an increase in mean temperature by up to 4 °C, while precipitation levels remain unaltered. This would result in increased potential evapo-transpiration and could lead to shift in the major forest vegetation zones in this area. Sites currently supporting communities characterized by the mountain hemlock are expected to be replaced by a forest zone dominated by the western hemlock. Elevational shifts of certain forest zones are also predicted. For example, it is predicted that on the western slopes of the central Oregon Cascade mountain range that those forests tolerant of drier conditions, such as the area of forest dominated by Douglas Fir, would increase dramatically with a temperature rise of +2.5 °C, while the western hemlock forest zone would contract. However, Franklin and his co-workers suggest that it could be the indirect effects of climate change, altering natural disturbance regimes like the periodicity of fire, storms, and disease, that could force many of the changes to the forest communities in this region of the United States. The results of climate models do, however, illustrate the potential effect of changing climate on future vegetation patterns. Notwithstanding the uncertainties and possible errors contained within these climate models (especially the gaps in our knowledge of the mechanics of the climate system and how it interacts with the oceans and land), we can expect major changes in the pattern of global vegetation to take place in response to future changes in climate.

T. Mighall

Bibliography

Peters, R. L. and Lovejoy, T. E. (eds) (1992) Global warming and biological diversity. Yale University Press, New Haven.
Cox, C. B. and and Moore, P. D. (1985) Biogeography: an ecological and evolutionary approach. Blackwell Scientific Publications, Oxford.
Delcourt, H. R. and and Delcourt, P. A. (1991) Quaternary ecology: a paleoecological perspective. Chapman and Hall, London.
Walker, M. J. C.,, Bohncke, S. J. P.,, Coope, G. R.,, O'Connell, M.,, Usinger, H.,, and and Verbruggen, C. (1994) The Devensian/Weichselian Late-glacial in northwest Europe (Ireland, Britain, north Belgium, The Netherlands, northwest Germany). Journal of Quaternary Science, 9 (2), 109–18.