Dynamic plant ecology: the spectrum of vegetational change in space and time

Quaternary ScienceReviews, Vol. 1, pp. 153-175, 1983.

0277-3791183/030153-23511.50/0

Printed in Great Britain. All rights reserved.

THE SPECTRUM

Copyright © 1983 Pergamon Press Ltd.

DYNAMIC PLANT ECOLOGY: OF VEGETATIONAL CHANGE IN SPACE AND TIME

H a z e l R. D e l c o u r t

Program for Quaternary Studies of the Southeastern United States, Department of Botany, and Graduate Program in Ecology, University of Tennessee, Knoxville, Tennessee 37996, U.S.A. Paul A. Delcourt

Program for Quaternary Studies of the Southeastern United States, Department of Geological Sciences, and Graduate Program in Ecology, University of Tennessee, Knoxville, Tennessee 37996, U.S.A. and Thompson

Webb III

Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, U.S.A.

Received 28 November 1982

Different environmental forcing functions influence vegetational patterns and processes over a wide range of spatial and temporal scales. On the micro-scale (1 year to 5 x 103 years, 1 m z to 106m 2) natural and anthropogenic disturbances affect establishment and succession of species populations. At the macro-scale (5 x 103 years to 106 years and 106m 2 to 1012m2) climatic changes influence regional vegetational processes that include migrations of species as well as displacement of ecosystems. Mega-scale phenomena such as plate tectonics, evolution of the biota and development of global patterns of vegetation occur on the time scale of ~ 106 years and over areas ~ 1012 m 2" Our knowledge of past vegetational changes resulting from Quaternary climatic change can be used to predict biotic responses to future climatic changes such as global warming that may be induced by increased carbon dioxide (COz) concentrations in the atmosphere. The time scale for future climatic warming may be much more rapid than that characterizing the early- to mid-Holocene, increasing the probability of rapid turnover in species composition, changes in local and regional dominance of important taxa, displacement of species ranges and local extinction of species. Integration of ecological and paleoecological perspectives on vegetational dynamics is fundamental to understanding and managing the biosphere. 153

154

H.R. Delcourt, P.A. Delcourt and T. Webb III

INTRODUCTION Appreciating the spectrum of spatial and temporal variation in the vegetation is an important prerequisite to understanding vegetational dynamics. Toward this end, it is imperative to recognize and interpret vegetational response to short-term events - such as fires, hurricanes, disease and clear c u t t i n g - within the context of longer-term changes in environmental disturbance regimes and the evolutionary development of the biota. Different physical and biological processes influence the vegetational patterns observed at each spatial-temporal scale. In order to organize and integrate this information, we present a hierarchical model illustrating distinctive combinations of perturbations, processes and patterns nested from scale to scale (Fig. 1). This model is based on the following principles: (1) the environment changes through space and time, (2) the changing environment provides the forcing functions for biological responses, and (3) these responses translate into observable vegetational patterns. These three principles are fundamental both to modern plant ecology and to Quaternary terrestrial paleoecology. We define three operational space-time domains: micro-scale, macro-scale and mega-scale (Fig. 1). Each represents a different range of area and time over which particular environmental factors are perceived as influencing vegetational processes and patterns. Levels of vegetational organization from individuals to populations, communities and formations can be viewed within a common context, and a framework exists for illustrating the whole spectrum of ecological and paleoecological data (Fig. 1). In the following sections we employ this hierarchical model in order to examine the appropriateness of particular ecological and paleoecological techniques for documenting vegetational change. We then review recent ecological and paleoecological studies that have explored ecological questions appropriate to the micro- and macro-scales. Finally, we use our understanding of vegetational dynamics in order to consider certain ecological questions that bear upon the future management of extant ecosystems within the context of anthropogenic (CO2-induced)climatic change.

A HIERARCHY OF SPACE-TIME DOMAINS Vegetation is a complicated mosaic of life forms and plant species populations distributed across the landscape. Individual investigators cannot easily represent the full complexity of vegetation on single maps or graphs. Choices about sampling intervals influence what vegetational phenomena are recorded. Vegetation mapping at a variety of spatial scales, however, may help focus research on the ecological questions that are appropriate to each level of spatial resolution (Olson et al., 1976). At the micro-site scale (see Table 1 for definitions), individual plants are examined. An array of sample plots within a meso-site allows study of interacting species populations as they vary in space. Sampling a number of adjacent forest stands (meso-sites) enables community characteristics to be described within the larger mosaic of the vegetational subtype; these data also permit examination of changes in dominance of species across their ranges. Different types of vegetational pattern are recognized at successive spatial scales. Vegetational dynamics are also best understood in the context of a hierarchy of temporal scales. Physiological changes resulting in changes in growth rate and hence accumulation of biomass may be observed on a time scale of 10°-101 years. Measurements of wood density and tree-ring widths

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record the effects of such changes. Periods of 101 - 1 0 2 years may be required for differential rates of establishment, growth and mortality of individual plants to affect the population structure of the species within a forest stand. Over a period of 5 x 1 0 1 - 5 x 102 years, changes in population structures of species due to competitive interactions may lead to the replacement of certain species by others and thus lead to changes in the structural and compositional dominants in a forest stand. These processes are commonly recognized under the general heading o f ' f o r e s t succession' (Drury and Nisbet, 1973; Pickett, 1976; Mclntosh, 1980; West et al., 1981). Extrinsic factors such as climatic changes of the magnitude of the Little Ice Age (A.D. 1 4 5 0 - 1 8 5 0 ) can also affect forest composition at this time scale (Bemabo, 1981;Webb, 1981).

TABLE 1. Spatial hierarchies of vegetational units (modified from Olson et al., 1976). The typical range in spatial coverage for each vegetational unit is expressed in terms of orders of magnitude for area in square metres. Note that specific examples of vegetational units may partially overlap in area with units at adjacent spatial scales.

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Vegetational Change in Space and Time

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On time scales of 5 x 102 years-10 s years, continued replacement of species populations in response to climatic change, soil development and species migrations leads to longer-term vegetational changes over broad regions. One thousand to one million years may be required for global-scale extinctions of taxa to alter the pool of available species and for the evolution of new taxa (Valentine, 1973; Stanley, 1979). Fundamentally different processes are therefore observed at different time scales. Most importantly, there is a direct relationship between the space and time scales appropriate for observing different aspects of pattern and process. Interactions between individual plants may be observed on a yearly basis, but the dynamics of forest stands can only be perceived on a scale of tens to hundreds of years. Understanding of vegetational dynamics at the formation level requires a perspective of thousands of years (Fig. 1). Environmental changes, vegetational processes and vegetational patterns of primary concern to both plant ecologists and Quaternary paleoecologists are those operative on the micro- and macroscales of temporal and spatial resolution (Fig. 1). Mega-scale phenomena such as plate tectonics, evolution of the biota and the development of global vegetational patterns are operative on time scales greater than 10 6 years and over areas greater than 1012m2. Patterns and processes at the mega-scale are of primary concern to Phanerozoic paleoecologists and will not be discussed further here.

E F F E C T S O F W I L D F I R E IN S P A C E A N D T I M E The idea of a space-time hierarchy can be illustrated through the example of wildfire, an environmental disturbance that is effective over several spatial and temporal scales (Christensen, 1981). A lightning bolt influences a micro-site by striking one tree. Elimination of the tree opens a gap in the forest canopy into which adjacent understory trees rapidly grow. The lightning strike may also initiate a fire that sweeps across a small forested watershed, killing many trees. Within the watershed the fire is extinguished by rain or by fire breaks such as streams and roads. Seed from the adjacent, undisturbed vegetation is dispersed into the fire scarred area, and by 150 years following the fire, recolonization and plant succession return the vegetation to mature forest (Oliver, 1981; Bormann and Likens, 1979). Recurrence of such widespread fire events about every 60 years results in a vegetational mosaic consisting of early- and mid-successional vegetation (Swain, 1973). In an extreme case, one fire may burn more than 4 x 101 lm2 of vegetation in 24 hours (Heinselman, 1981). Where such a broad area is affected, the vegetation may continue to adjust over many hundreds of years. Spatial vegetational patterns at a particular moment may reflect proximity to the burned area as well as the adaptive responses of the species available. At the center of the burn, fire-tolerant plants may regrow immediately. In the North-central United States and adjacent portions of eastern Canada, for example, aspen (Populus grandidentata and P. tremuloides)resprouts readily after fires and fire also stimulates the opening of serotinous cones of jack pine (Pinus banksiana). The pattern of recolonization is different at the burn margin where additional, fire-intolerant plant species provide seed sources. The rate of recolonization is influenced by the mechanisms of seed dispersal and resprouting. One extreme fire may thus affect vegetational dynamics over the spectrum ranging from micro- to macro-scale domains (Heinselman, 1973). Consistent repetition of such disturbances constitutes a forcing function for vegetational variation on time scales from tens to thousands of years (Loucks, 1970; Wright and Heinselman, 1973).

158

H.R. Delcourt, P.A. Delcourt and T. Webb III

Climate controls the typical interval for fire recurrence and therefore imposes a constraint upon the time available for recolonization and secondary succession (Swain, 1978). Some vegetational types are 'fire-adapted' (e.g., prairie, savanna and certain conifer forests) whereas others persist only in regions where fires are rare (e.g., certain deciduous forest types) (McAndrews, 1966; Vogl, 1980). The individual lightning strike is instantaneous, but the frequency of strikes is a feature of a macroclimatic regime that can influence macro-scale vegetational patterns. Quaternary stratigraphic records of charcoal, pollen grains and elemental isotopes record changes in fire frequency and vegetational composition within changing climatic regimes (Swain, 1973; Cwynar, 1978). Thus, ecological and paleoecological studies provide data that document the limits within which environmental forcing functions vary in space and time.

T E C H N I Q U E S F O R A N A L Y S I S OF V E G E T A T I O N A L C H A N G E Vegetational change can be measured using any one of a number of techniques that overlap in temporal resolution. Available techniques include direct sampling of the vegetation and analyses of tree rings, pollen and plant macrofossils (Fig. 2). On the micro-scale, direct methods of vegetation sampling yield the most information about population structure, taxonomic and life-form composition, abundance and dominance of constituent species, diversity (including floristic richness), biomass and primary productivity of extant vegetation. Vegetation sampling is usually confined to measurement of only one or several of these variables, in order to economize on time and energy expended, or to focus on questions of immediate interest (Cain and Castro, 1959; Phillips, 1964; MueUer-Dombois and Ellenberg, 1974). The sampling techniques chosen often reflect an investigator's specific interests such as (1) understanding diversity and population dynamics of structural dominants (Peet, 1981) or of understory herbs (Rogers, 1980); (2) examining community dynamics (Harcombe and Marks, 1978); or (3) studying ecosystem function (Reichle et al., 1973). Few plant-ecological sampling programs have successfully monitored vegetational change over time periods of 50 years or more (McCormick and Platt, 1980; Peet and Christensen, 1980a, b).

Scale of Resolution MICRO

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Temporal Scale of Vegetational Change FIG. 2. Appropriatetechniques for measuringvegetationalchange.

Vegetational Change in Space and Time

159

Increments of tree-ring growth can be used to measure vegetational change in terms of physiological response of individual trees to changes in temperature, precipitation, competition, or disturbance regimes (Fritts, 1976). For certain species, tree-ring chronologies extend back over 8,000 years (Ferguson, 1980), and study sites are distributed across a wide geographic area (Blasing and Fritts, 1976; Brubaker, 1980). The size- and age-structure of entire stands can be surveyed on permanent plots over many decades to yield records of changing productivity, biomass and tree population dynamics through the history of the stand (Peet, 1981). Trees located on sites with evapotranspiration stress or limited nutrient availability show more pronounced responses of growth rate to climatic variation than those on mesic sites (Fritts, 1976). Ring-width studies of these stressed trees yield data on both changes in forest productivity and past climates, and the data may be summarized on either a micro-scale or macro-scale. Pollen and plant macrofossils provide a less precise vegetational record than can be obtained with direct vegetation sampling (Webb et al., 1981). However, these fossil data have the advantage of providing information on a time scale beyond that of direct sampling or of tree-ring analysis (Fig. 2). Plant macrofossils tend to be deposited close to the site at which they are produced and thus reflect local vegetation (Birks, 1980). Fossil fruits, seeds, leaves, cones and other plant fragments can often be used to identify species more readily than pollen grains (Delcourt et al., 1979). Quantification of vegetation based strictly upon plant-macrofossil remains is not yet feasible (Birks, 1980), but plant macrofossils provide useful taxonomic information, especially when combined with fossil pollen data from the same site. The spatial scale over which fossil pollen data are effective in recording vegetational change is dependent upon the pollen source area (forest stand to formation zone), which in turn is determined by size, morphometry and sediment type of the depositional site (Davis and Brubaker, 1973; Webb et al., 1978; Jacobson and Bradshaw, 1981). Lacustrine sites vary in their rate of sediment accumulation because of differences in geomorphic, hydrologic and climatic characteristics of their watersheds. In order to compensate for variability in sedimentation rates one may adjust sampling intervals in a given sediment core and thereby obtain the desired time resolution (Delcourt, 1979). Calibrations of pollen percentages with the dominance or areal coverage of corresponding tree taxa within forests are required in order to generate quantitative reconstructions of paleovegetation (Livingstone, 1968; Andersen, 1978, 1980; Parsons et al., 1980; Webb et al., 1981 ; Parsons and Prentice, 1981; Delcourt et al., 1982; Delcourt et al., 1983). Knowledge of the autecology of 'indicator' plant species also aids the interpretation of past community composition or environmental conditions (Birks and Birks, 1980).

MICRO-SCALE RESOLUTION OF VEGETATIONAL CHANGE On the micro-scale of resolution (1 year-5 x 103 years, 1 m2-106m 2) traditional ecological questions can be addressed for the space-time domain that is of primary interest to modern plant ecologists. Environmental forcing functions operative at this scale range from disturbances such as windstorms and fires to extreme variations in weather, outbreaks of pathogens, and man's activities such as logging, plowing, burning, and paving (Fig. 1). Effects of topographic position, aspect, substrate, as well as water and nutrient availability are among the most important for the establishment and succession of species populations at this micro-scale. Even on this space-time scale, local effects are influenced by oscillations in macroclimate.

160

H.R. Delcourt, P.A. Delcourt and T. Webb IIl

Several recent studies have examined vegetational processes and patterns on the micro-scale. In the Duke Forest on the North Carolina Piedmont, permanent 0.1 ha vegetation plots have been sampled 9 times in the past 50 years in order to determine successional trends on abandoned farm land. In a series of recent publications, Peet and Christensen (Peet and Christensen, 1980a, b; Peer, 1981) have analyzed 50-year trends in vegetational composition and size-class distributions, as well as patterns of establishment and differential mortality of plant populations. Loblolly pine (Pinus taeda) populations were established initially after cessation of severe disturbance. With time, natural thinning of mature pine stands opened gaps within the forest canopy, allowing the establishment of hardwood species. Forest succession in these stands is strongly influenced by disturbances that locally increase availability of resources, including light, soil nutrients, and water, through time. Brubaker (1975) initiated a study of the local effects of substrate type on species immigration and vegetation composition in the Upper Peninsula of Michigan. Using lake basins adjacent to different substrates (from outwash sands to clayey till) with differing soil textures and nutrient contents, Brubaker distinguished the regional pollen rain (common to all sites) from locally-derived pollen and thus documented local patterns of establishment and succession of forest communities during the past 10,000 years. Soils with intermediate porosity, permeability and nutrient content supported mixed coniferous-deciduous forest communities. On these sites, changes in the competitive balance between species occurred only during times of evapotranspiration stress induced by mid-Holocene climatic warming and drying (Brubaker, 1975). However, even with changing macroclimatic conditions, sites with soils having very different textures (e.g., outwash sands versus clayey till) were found to maintain different forest communities through time. In Minnesota, small paired basins were used to separate local from regional components of the pollen rain, and thus to trace the detailed pattern of immigration and establishment of white pine (Pinus strobus) as well as the competitive interactions between white pine and oaks (Quercus) (Jacobson, 1979). During the mid-postglacial climatic warming, oaks migrated northeastward across central Minnesota. This migration ceased when white pine entered the vegetation of Minnesota from the east about 7,000 BP. White pine first became established within oak openings and, subsequently, proved to be a superior competitor to the oaks. Readjustment of the ranges of oak species resulted in stabilization of the ecotone between oak- and white pine-dominated forests from 6,500 BP to 4,000 BP. Cool and moist climatic conditions after 4,000 BP allowed white pine to advance westward and resulted in the restriction of oak forest and savanna to a narrow zone in western Minnesota (Jacobson, 1979). Jacobson's study exemplifies the interface between the micro-scale and macro-scale of resolution. The phenomenon of plant competition within stands is a micro-scale process that operates within the context of Holocene climatic change and consequent species migration, two processes operating at the macro-scale.

MACRO-SCALE RESOLUTION OF VEGETATIONAL CHANGE Regional vegetational patterns are perceived at the macro-scale resolution of 5 x 103-10 6 years and 106-10 ~2m2 (Fig. 1). Climatic changes have caused major changes in regional vegetational processes and patterns during the Quaternary. Systematic fluctuations in orbital geometry between the sun and the earth have resulted in long-term variations in the solar radiation intercepted by the earth. These fluctuations have triggered global climatic cycles of glacial cooling and interglacial warming with discrete periodicities of 100,000, 41,000 and 23,000 years (Hays et al., 1976;

Vegetational Change in Space and Time

161

Imbrie and Imbrie, 1979) (Fig. 3). Variation in macroclimate determines to a large degree both disturbance regimes and processes of soil development within a macro-region. At the macro-scale, biotic responses to climatic change include migration, extinction and speciation of plant taxa as well as changes in composition, structure, areal extent and distribution of ecosystems (Fig. 1).

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PAST V A R I A T I O N A N D FUTURE CHANGES IN CLIMATE FIG. 3. Past variation in global climate for the last glacial-interglacial cycle and future changes in climate projected for the next 25,000 years (modified from Imbrie and Imbrie, 1979). For the purpose of illustrating macro-scale vegetational processes and patterns during the past 20,000 years, eastern North America provides a well-documented region from which to draw examples. Across that region, biotic interactions and predominant vegetational patterns have been strongly influenced by changing environments resulting from shifting climatic regions (Bryson and Wendland, 1967; Delcourt and Delcourt, 1983). Several complementary approaches have been taken to summarize available pollen data on this temporal and spatial scale. Each analysis spotlights a different aspect of vegetational dynamics. A series of migration maps (Davis, 1976, 1981) depicts changes in the leading edges of species ranges as they moved northward during the Holocene, following the retreat of the Laurentide Ice Sheet. Isopoll, difference and isochrone maps (Bernabo and Webb, 1977; Webb, 1981) illustrate broad-scale patterns of changing vegetational composition as reflected in changing pollen assemblages through the Holocene (Fig. 4). A mapclassification approach (Delcourt and Delcourt, 1981) uses modern pollen samples obtained from known vegetation types as analogs for interpreting and mapping changes in distribution and areal extent of major vegetation types over the past 20,000 years (Fig. 5). In combination, these maps depicting species migrational patterns, changing percent composition of species assemblages, and shifts in location and areal extent of paleovegetation types portray several important aspects of the broad-scale vegetational response to changing climates over the past 20,000 years. On the macro-scale, migrations of tree taxa are primarily independent of one another, differing in both rate and direction of movement from southern refuge areas

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FIG. 5. Paleovegetation maps for 18,000 BP, 10,000 BP, 5,000 BP, and 200 BP. The black dots represent locations of sites from which pollen data were used to reconstruct the past vegetation (Delcourt and Delcourt, 1981).

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H.R. Delcourt, P.A. Delcourt and T. Webb III

(Davis, 1976, 1981). Interspeciflc differences in migration rates probably result from the different responses and adaptations of individual species to changes in climate, disturbance regimes, soils, competition and photoperiod. Among these factors, climate predominates, because it influences disturbance regimes, soil development and competitive balance (Fig. 1). Changes in relative abundances of pollen from major trees through time reveal changing patterns of species composition and relative dominance (Bernabo and Webb, 1977; Webb, 1981). Certain species assemblages have been ephemeral. For example, the northern hardwood forests of the North-central United States (Fig. 4) became dominant only in the late Holocene. Other vegetational types have varied greatly in species abundances, composition and areal extent over the past glacial-interglacial cycle. At the level of formation and large vegetation type, however, the physiognomic characteristics of many vegetation units (e.g. forest, savanna, prairie and tundra) have remained dominant over large areas (mesoregion to macroregion) during the past 20,000 years (Delcourt and Delcourt 1981). Despite changes in distribution, areal extent and fine-scale community composition, the majority of vegetation types today characteristic of boreal and temperate regions of eastern North America have persisted since the last full-glacial period (Delcourt and Delcourt, 1981, 1983) (Fig. 5). Thus, spatial continuity and temporal persistence has been maintained for vegetational formations defined primarily by physiognomy and for many major forest types defined by dominant taxa, including boreal forests, deciduous forests and southeastern evergreen forests. At the level of forest stand, vegetational subtype, and some vegetational types, plant communities defined principally by explicit species composition have changed through the postglacial primarily as a result of differential migration and establishment of species.

CLIMATIC CHANGE, BIOTIC RESPONSE AND V E G E T A T I O N A L P A T T E R N S Biotic processes and patterns are inherently tied to environmental forcing functions (e.g., Watt, 1947; Grime, 1979; White, 1979). Ecologists, however, have tended to focus on vegetational succession as it would occur under the ideal conditions of a constant climatic regime, a stable flora and minimal rates of evolution (Wright, 1974; Oliver, 1981). When described in such a context, succession is viewed as a biological process that is independent of environmental changes. Plant succession within macro-sites may be envisioned as proceeding along a trajectory [Fig. 6(A)] of minimal change in vegetational composition as species populations replace themselves within micro- and meso-sites through time (Loucks, 1970; Botkin and Sobel, 1975; Vogl, 1980). Under these conditions dynamic vegetational equilibrium, expressed as a patchwork of successional stages distributed on meso-sites across the landscape, is attained in response to the prevailing disturbance regime (Pickett, 1976; Romme, 1982; Romme and Knight, 1982). Late-successional vegetation may develop in the absence of disturbance, and, according to the classical model, the trajectory from early to late successional stages is determined essentially by biotic interactions (autogenic succession of Tansley, 1935). In temperate forested regions, with sufficient time for biotic response to altered conditions following a severe perturbation, plant succession will typically result in development from pioneer herbs, through shrubs, immature forest, to late-successional mature forest [as depicted by the shaded curve in Fig. 6(A)]. The length of time required for plant succession to result in the development of a mature forest in part depends upon the longevity of the dominant tree species involved. For example, in the eastern United States, the average maximum life span for late-successional tree species is between 300 and 400 years (Shugart and West, 1977), but is as much as 1000 years in the Pacific Northwest (Franklin and Hemstrom,

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1981). Even with relatively constant climatic conditions, plant succession will be directed by local environment conditions (aUogenic succession of Tansley, 1935) if the disturbance regime is characterized by frequent, drastic events. In a changing climatic regime there are many possible trajectories for plant succession after disturbance. For example, given a climatic sequence with rapidly changing climate for the first 400 years, stable climate for the next 200 years, and changing climate for the subsequent 400 years, vegetational composition might follow trajectory B in Fig. 6, with an episode of rapid vegetational change, then equilibration, followed by another episode of rapid change. With rapid and major climatic change, a complete turnover in species composition could occur within several hundred years and within only two plant-successional sequences (trajectory C in Fig. 6). Unstable environmental conditions may result in immediate ecotypic differentiation at the population level, differential rates of migration, or even local extinction of species (Axelrod, 1981). Rapid climatic change results in changes in the species available for establishment, species dominances, composition of assemblages and physiognomy of vegetation. Perpetuation of such disequilibrium conditions results in long-term vegetational change, which continues as long as (1) climate continues to change, (2) the nature of the disturbance regime changes, or (3) species availability is substantially altered through immigration, local extinction and ecotypic divergence of populations through natural selection. Climatic change acts on the macro-scale, eliciting biotic responses including widespread migrations and compositional change. Climate dictates broad boundary constraints for the disturbance regime, which in turn influences the overall patchwork of successional stages and average vegetation composition on the macro-site. During episodes of rapid, major climatic change, compositional change in meso-sites may be directly reflected in the shifts in species composition in subsequent successional stages as well as the broader vegetational trajectory. Climatic changes during the Quaternary have resulted in major shifts in climatic regions and disturbance regimes (National Research Council, 1975). On the basis of the Quaternary record of oscillations in global climate (Fig. 3), we suggest that vegetational change, when resolved on the macro- and mega-scales of space and time, is predominantly controlled by allogenic factors. Our knowledge of past vegetational changes resulting from Quaternary climatic change can be extrapolated to place constraints on the likely future biotic response to man-induced environmental perturbations. Present-day interglacial climatic conditions are unusual in Quaternary history (Emiliani, 1972; van Donk, 1976; Imbrie and Imbrie, 1979). Within the past 150,000 years (Fig. 3), mean global temperature has reached or exceeded that of today only during two periods, each of approximately 10,000 years' duration. Yet continued input of CO2 into the atmosphere from burning of fossil fuels is estimated to result in a global warming trend of unprecedented magnitude commencing within the next 50 years. This CO2-induced 'super-interglacial' period may persist as long as 2,000 years into the future (Baes et al., 1977; Mitchell, 1977; Imbrie and Imbrie, 1979; Fig. 3). Given an understanding of the analytical techniques available for examining vegetational change under changing climatic conditions, and an appreciation of scale, we may examine past analogs in order to evaluate the nature of the expected biotic response anticipated from future CO2-induced warming. One of the problems in predicting the consequences of this proposed climatic change is to find the right analog for a rapid change of the magnitude predicted. Our model of space-time domains places the time-interval of change on the micro-scale, but the past analogs for this magnitude of climate change occur on a macro-scale time period. During the present interglacial period, the midHolocene interval from 8,000 to 4,000 BP was the time when the peak temperatures were most

Vegetational Change in Space and Time

171

analogous to the proposed future warming in eastern Northern America (Wright, 1968, 1976; Delcourt and Delcourt, 1981; Fig. 3). In temperate regions, mean annual temperatures during the period from 6,000 to 4,000 years ago were as much as 1-2°C warmer than today (Wright, 1976; Imbrie and Imbrie, 1979). During the mid-Holocene period, pronounced changes in floristic composition, dominance of important taxa and physiognomy of vegetation are well documented from the Midwestern United States (Wright, 1976; Bernabo and Webb, 1977; Delcourt and Delcourt, 1981 ; Fig. 5). Climatic change has been relatively gradual throughout the present interglacial, and in mid-latitudes it has been relatively minor when compared with the climatic changes that occur over a full glacial-interglacial cycle (Fig. 3). The mid-Holocene episode of warming, followed by stabilization and cooling (Fig. 3) affected vegetational change in eastern North America on both the micro-scale and the macro-scale. This clhnatic change may have altered vegetational trajectories in a manner similar to that illustrated in Fig. 6(B), through shifts in species ranges and through changes in dominance of species within vegetation types. The time scale for future climatic warming is predicted to be much more rapid than that of the mid-Holocene warming (Kellogg and Schware, 1981 ). An accelerated time-scale for climatic change could result in a trajectory for vegetational change comparable to that in Fig. 6(C). Such circumstances could increase the probability of rapid turnover in species composition, resulting in local and regional changes in dominance, displacement of species ranges and local extinction of species. In addition to anthropogenic climatic change, fragmentation of forests that were once contiguous into discontinuous woodlots is occurring across eastern North America because of land clearance, cultivation and urbanization (Burgess and Sharpe, 1981). Man's use of the land is rapidly removing the migration corridors that existed for forest species during earlier periods of climatic change. Unless counteracted by the establishment of a network of nature preserves, this factor may increase the probability of species extinctions accompanying CO2-induced climatic warming.

CONCLUSIONS The Quaternary paleoecological record demonstrates the importance of climatic change as the dominant influence upon vegetational processes and patterns at the macro-scale of spatialtemporal resolution. Even at the micro-scale, prevailing disturbance regimes that affect plant succession must be viewed within the context of their prevailing macroclimate. Species have evolved in a fluctuating, not constant, environment; the future persistence of communities and ecosystems depends upon their resilience in the face of impending, rapid climatic change. An appreciation of spatial and temporal scale is necessary in order to use past analogs effectively in the prediction of future trends in vegetational change. In order for ecologists to build a comprehensive framework for understanding the evolution and dynamics of species and communities, the empirical record of past vegetational and environmental change must be used to constrain theoretical models. Integration of ecological and paleoecological perspectives on vegetational dynamics is essential if we are to understand and manage the biosphere on a scientifically sound basis.

ACKNOWLEDGEMENTS We wish to thank Drs I. Colin Prentice and Robert K. Peet for stimulating discussions and constructive critiques of this manuscript. This research was sponsored by the National Science Found-

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ation through Ecology Program Grant No. DEB-80-04168 to the University of Tennessee, through Ecosystem Studies Program under Interagency Agreement No. DEB-77-26722 with the Office of Health and Environmental Research, U.S. Department of Energy, under contract W-7405-eng-26 with Union Carbide Corporation, and by the Department of Energy through contract DE-AC0279EV 10097 to Brown University. Contribution No. 25, Program for Quaternary Studies of the Southeastern United States, University of Tennessee, Knoxville, Tennessee 37996. Publication No. 2082, Environmental Sciences Division, Oak Ridge National Laboratory.

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