Chapter 11 - Vegetation Dynamics

Transcription

1 Chapter 11 - Vegetation Dynamics CHAPTER 11 - VEGETATION DYNAMICS INTRODUCTION POPULATION DYNAMICS SUCCESSION Lake Michigan sand dunes Glacier Bay, Alaska Old-field succession in the North Carolina Piedmont Northern hardwood forests Boreal forests of interior Alaska Mechanisms of succession Pattern and process in plant communities BIOGEOGRAPHY AND CLIMATE CHANGE VEGETATION DYNAMICS MODELS TABLES FIGURE LEGENDS... 34

2 Ecological Climatology 11.1 Introduction Vegetation is in a state of perpetual flux, changing with time in response to the life history patterns of species, competition among plants for light, water, and nutrients, and disturbances that create clearings in the landscape. Some species grow fast and are short lived. These plants are ephemeral features in the natural landscape, using a life history that allows them to rapidly colonize and dominate recently disturbed patches within the landscape. These denuded patches are created through processes endogenous to the landscape such as the death and uprooting of a large tree that creates a gap in the canopy or through exogenous disturbances such as the eruption of volcanoes, forest fires, and hurricanes. Over time, these early dominants give way to slower growing, longer lived species. The rise and fall of taxa is part of the natural life cycle of communities and ecosystems a process ecologists call succession. It creates pattern to the arrangement of vegetation across the landscape related to disturbance history. Long-term changes in climate superimpose additional changes on the successional cycle. A forest may slowly give way to savanna or grassland with a drier climate. Trees may invade tundra with a warmer climate. In this chapter, the ecological principles governing vegetation dynamics and plant responses to changing environments are reviewed (Bazzaz 1996; Shugart 1998). Much of the patterns of succession can be understood in terms of the dynamics of plant populations. Consequently, population dynamics is reviewed first. Then patterns of succession in different environments are illustrated along with the mechanisms controlling succession. The pattern of vegetation in a landscape is seen as the result of dynamics at two timescales. Succession over periods of decades to centuries in response to recurring disturbances creates a mosaic of patches in various stages of development. Climate change over periods of centuries to millennia alters the biogeography of species Population dynamics Population density changes over time through the birth, growth, and death of its members (Harper 1977; Silvertown 1982). The classic description of population growth is the logistic growth equation dn / dt = rn(1 N / K) where N is density at some time t, r is an intrinsic rate of population growth determined as the difference between birth and death rates, and K is the carrying capacity. The first term in this equation ( rn ) describes 2

3 Chapter 11 Vegetation Dynamics exponential population growth in the absence of limiting resources. However, resources are rarely unlimited, and the resources available to individuals become scarcer as a population density increases. At high density, competition among individuals for available resources limits population growth. The second term (1 N / K ) reduces the rate of growth as population density approaches the maximum number of individuals the environment can support (i.e., the carrying capacity of the environment). This equation has an S-shape or logistic form, with exponential growth at low density declining to zero growth (i.e., constant density) at high density. This depiction of population growth has lead to numerous theoretical insights to the study of populations and communities. For example, it provides the basis for r- and K-selected species. Species that are K-selected maintain populations typically at or near carrying capacity. They survive and reproduce under conditions of strong competition and resource depletion. In contrast, r-selected species, with their capacity for rapid colonization, are favored in open environments. However, the equation does not accurately describe plant populations, which adjust the sizes of individuals in relation to density. In addition, plants are sedentary and do not compete for space and resources with all members of a population but rather only with their immediate neighbors. Plant population dynamics is best understood in terms of demographic processes such as growth, mortality, reproduction, and dispersal and the influence of the environment and plants on these processes (Figure 11.1). Dispersed seeds are stored in a dormant state in soil, forming a seed bank from which seedlings are recruited. The environment acts as a sieve that filters seeds from the bank. Recruitment of seedlings from the seed bank depends on safe-sites that provide the necessary conditions for germination and establishment. Only those seeds receiving suitable light, moisture, and temperature germinate, and only some of these become established as seedlings. As seedlings grow, their roots spread out to acquire water and nutrients; leaves form to absorb light and CO 2 for photosynthesis; stems develop to support leaves. When large enough, individuals interfere with the growth of neighboring plants, competing with them for light, water, nutrients, and space. Plants that gain resources first pre-empt their use by others and are at a competitive advantage. Success goes to the largest plant that captures space and resources at the expense of smaller neighbors. Many plants die in this process. Others can survive in the resource-depleted understory, waiting for a taller plant to die and create a gap in the canopy. Growth and survival culminates in seed 3

4 Ecological Climatology production and dispersal by mature plants, which forms the seed rain for the next generation. Annuals complete this life cycle over the course of a year. Woody shrubs and trees complete this cycle over many years. This intense competition among plants for resources is manifest in typically high rates of juvenile mortality. This is particularly true in forests, where the mortality of tree seedlings is high (Harcombe (1987). Only a few of the many thousands of seedlings produced by a mature tree become established in the understory. Even fewer survive to reach the canopy. Most die as a result of failure to successfully acquire light, water, and nutrients in the presence of strong competition from neighboring individuals. Risk of mortality typically declines with increasing age, giving a concave shape to survivorship curves (Figure 11.2). The importance of crowding and competition in regulating the size of plants and the total biomass of a population was demonstrated by a series of studies in the 1950s and early 1960s (Kira et al. 1953, 1954, 1956; Ikusima et al. 1955; Hozumi et al. 1955, 1956; Shinozaki and Kira 1956, 1961; Koyama and Kira 1956; Yoda et al. 1957, 1963). One experiment studied temporal changes in biomass for five populations of soybean grown at different densities (Kira et al. 1953). Initially, yield increased with higher density (Figure 11.3). Mean plant biomass was independent of density so that more plants yielded higher total biomass. After about 45 days, during which there was no mortality, total biomass of each population was relatively constant and independent of density. Differences in density were compensated for by density-dependent changes in mean plant mass so that total biomass was constant. This study shows that initially when plants are small they do not interfere with the growth of neighbors. Yield increases linearly with density. Over time, as plants grow larger, they compete for resources and interfere with the growth of neighbors. Interference occurs sooner and is greater for plants growing at high density, where plants are crowded, than at low density, where plants are widely spaced. Once competition begins, mean plant mass decreases with higher density. A subsequent study showed that the reduction in mean plant mass is manifested in the frequency distribution of plant sizes (Koyama and Kira 1956). As plants increase in size, competition for light, water, nutrients, canopy space, and root space intensify. Some individuals gain more available resources than others and grow faster. Others are suppressed and grow little. The result is a hierarchy of plant sizes with 4

5 Chapter 11 Vegetation Dynamics many small individuals and fewer large individuals. The inequality of plant sizes becomes progressively greater over time as a few individuals achieve dominance and the rest are suppressed. Size inequality is greatest in dense stands, where competition and crowding is most intense (Mohler et al. 1978; Weiner 1985; Weiner and Thomas 1986; Miller and Weiner 1989; Knox et al. 1989; Bonan 1988, 1991c). The final study in this series of experiments showed that as large plants usurp the majority of light, water, nutrients, and space smaller plants die and density progressively declines in a process known as selfthinning (Yoda et al. 1963). In a self-thinning population, mean plant mass ( m ) and total biomass (M) are inversely related to density (N) as m= cn M = cn 3/2 1/2 where c is a constant that varies among species and with site conditions. This relationship is derived from the geometry of space occupied by plants (White 1981; Lonsdale and Watkinson 1983; Weller 1987b, 1989; Osawa and Allen 1993). It has been found in a wide variety of plants ranging from annual herbs to trees (White and Harper 1970; Mohler et al. 1978; Westoby 1981, 1984; Weller 1987a; Peet and Christensen 1987; Lonsdale 1990; Bi et al. 2000). Loblolly pine stands in North Carolina illustrate the dynamics of self-thinning (Peet and Christensen 1980, 1987; Christensen and Peet 1981; Knox et al. 1989). One study found that despite large variation in initial density from about 600 to almost stems per hectare, the number of surviving trees converged on a narrow range of densities of about ha -1 over a 50-year period (Figure 11.4). Mortality was greatest in the densest stand and progressively declined with lower initial density. Graphs of mean tree volume in relation to density of survivors reveal the temporal trajectory of stand dynamics. Initially, mean tree volume increased without a corresponding decrease in density. As the canopy closed and resources became limiting, mean tree volume and density were related by a line with a slope of 3/2. Self-thinning represents a temporal trajectory of population dynamics, and the self-thinning line describes a boundary towards which plant populations grow. It is a density-dependent upper limit to mean plant mass or total stand biomass in a given environment. In the early stages of stand development or at low density, when plants do not interfere with the growth of neighbors, biomass increases toward the boundary line without a corresponding decrease in density. Once at the boundary line, further increases in biomass 5

6 Ecological Climatology are achieved through decreases in density. Mean plant mass increases faster than density decreases so that total biomass increases. How soon plants experience competition and mortality depends on density. High density stands reach the boundary line faster than low density populations. However, regardless of starting density, populations eventually converge on a common trajectory of biomass versus density Succession Plant succession refers to temporal change in community composition and ecosystem structure following disturbance. It is a progressive change in dominance by particular groups of species, biomass accumulation, and nutrient cycling over periods of several decades to centuries. Colonization of new land not previously vegetated is known as primary succession. This occurs in response to volcanic eruptions, alluvial deposits along floodplains, formation of sand dunes, and melting of glaciers. In contrast, secondary succession is the regrowth of vegetation on previously vegetated land following a disturbance such as fire, windthrow, logging, or farm abandonment. Succession is an integral part of forest stand dynamics and the response of forests to environmental change, but also occurs in grasslands and other herbaceous communities (Bormann and Likens 1979; West et al. 1981; Shugart 1984, 1998; Botkin 1993; Oliver and Larson 1996). The change in community composition is generally accompanied by changes in the environment caused by plants themselves. For example, during forest succession the development of a dense canopy shades the understory, litter accumulates on the forest floor, and nutrient cycling is altered based on the production of and chemical quality of detritus. These changes can facilitate the establishment of other species, abetting the directional change in dominance to other species, or inhibit the establishment of new species. Succession driven by environmental change caused by plants themselves is known as autogenic succession. In contrast, allogenic succession is driven by external factors (e.g., climate change) that cause the environment to change. Temporal changes in community composition and ecosystem structure alter ecosystem functions. This is particularly evident in forest succession following disturbance, where biomass accumulates over time in wood and soil. For example, Figure 11.5 illustrates biomass accumulation in commercial stands of slash pine in southeastern United States following harvesting. Total biomass is initially lost during site 6

7 Chapter 11 Vegetation Dynamics preparation. Thereafter, total biomass increases as biomass accumulates in trees and shrubs. Forest floor biomass increases at a rate of 122 g m -2 yr -1 through 34 years although total detritus (forest floor and soil organic matter) is relatively constant. Maximum leaf area index is attained in 14-year-old stands. Aboveground net primary production increases to maximum at 26 years, after which it declines. A study of transpiration in Scots pine forests of central Siberia illustrates the effect of succession on energy fluxes (Figure 11.6). The stands range in age from 28 to 383 years old and regenerated naturally following fire. A canopy height of 4-5 m is attained after 28 years. Stand density is high until about 67 years, when mortality during self-thinning begins to reduce the number of trees. Heights of m are attained after about 200 years. In these forests, canopy water loss is a function of sapwood area and stand density, which are modified by stand age. Maximum sapwood area and transpiration are achieved in the 67- year-old stand prior to self-thinning. Thereafter, sapwood area and canopy transpiration decline Lake Michigan sand dunes The growth of vegetation on sand dunes along the Indiana shoreline of Lake Michigan near Chicago is a classic example of primary succession (Cowles 1899; Olson 1958). The changing shoreline of Lake Michigan during and after glacial retreat from the last ice age left several distinct beach and dune systems exposed above lake level. Dunes are progressively older with distance from shoreline, and the vegetation of these dune systems provides a record of succession. In general, young sand dunes are covered with herbaceous annuals and perennials and woody shrubs. Pine trees and then broadleaf deciduous trees become progressively more abundant on older dunes. This vegetation change from exposed sand dune to forest occurs relatively rapidly over a few hundred years. The growth of trees results in rapid accumulation of fresh and decomposed litter accompanied by an increase in soil nitrogen and cation exchange capacity. Successional change in community composition depends on site conditions. The most prominent pathway is a change from grasses to pines to black oak (Figure 11.7). Marram, sand reed, and little bluestem grasses are the most abundant pioneer vegetation on newly exposed dunes. These occur in pure stands or in mixtures. Woody plants such as cottonwood trees and sand cherry shrubs may become established depending on seed availability. This primary vegetation builds up and then stabilizes the dunes, allowing for subsequent invasion of jack pine and white pine trees. Various types of black oak forests that differ in understory and ground cover eventually replace these short-lived pine trees. Pioneer herbs and 7

8 Ecological Climatology shrubs rapidly decline in abundance with the growth of trees several decades following dune formation. Pine is reduced to a minor species after the first generation or two, being replaced by black oak, which remains dominant on even the oldest dunes. Alternative successional pathways are possible depending on soil moisture. Wet depressions may develop into a tall grass prairie or red maple swamp forest. Succession from basswood to northern red oak to sugar maple without usually passing through pine and black oak stages occurs on leeward slopes and depressions, where the microclimate is more moderate and moisture accumulates Glacier Bay, Alaska The establishment of vegetation following deglaciation at Glacier Bay in southwestern Alaska is another classic example of primary succession (Cooper 1923a,b,c, 1931, 1939; Chapin et al. 1994; Fastie 1995). Since about 1750, a large glacier that once covered the inlet has rapidly retreated at a rate averaging 400 m per year. Some 2500 km 2 of ice has melted, exposing several hundred square kilometers of glacial till and outwash along the shoreline of Glacier Bay to plant colonization. Four major successional stages characterize the growth of vegetation following deglaciation (Figure 11.8). During the first 20 years or so after glacial retreat, the exposed till is gradually colonized by pioneer vegetation consisting of blue-green algae, lichens, liverworts and forbs. Scattered woody shrubs such as Dryas, a mat-forming shrub that fixes nitrogen, willows, and Sitka alder, another nitrogen-fixing shrub, and tree seedlings (black cottonwood, Sitka spruce) may be present depending on proximity to seed sources. This pioneer community gradually transitions to a Dryas stage after about 30 years following deglaciation. The ground is covered with a continuous mat of Dryas, with scattered willow, alder, cottonwood, and spruce. About 50 years following deglaciation, alder increases in abundance to form dense thickets. The shorter Dryas, shaded and buried under deciduous leaf litter, does not grow well and disappears. Sitka spruce trees eventually overtop the alder, forming a needleleaf evergreen forest at about 100 years since glacial retreat. Soils change markedly during this succession (Figure 11.8). Soil depth increases from 5 cm to 15 cm as a result of rapid weathering of glacial till. Soils in the pioneer stage have low soil carbon and nitrogen, high bulk density, low cation exchange capacity, and high ph. With the growth of woody vegetation and the deposition of litter, the soil becomes progressively enriched in carbon and nitrogen. Soils in spruce forests have high organic matter content, high nitrogen content, decreased bulk density, and 8

9 Chapter 11 Vegetation Dynamics low ph. The transition from Dryas to alder, an important nitrogen-fixing shrub, marks a fourfold increase in soil nitrogen and a decline in soil C:N ratio of 245:1 to 133:1. The soil C:N ratio increases in the spruce forest because of the low litter quality of needles. Succession and soil development at Glacier Bay is rapid and comparable to rates observed during secondary succession. This might be because of its maritime climate, with moderate temperatures and high annual rainfall Old-field succession in the North Carolina Piedmont The regrowth of forests on abandoned farmland in the North Carolina Piedmont of Southeast United States is a classic example of old-field succession (Billings 1938; Oosting 1942; Keever 1950, 1983; Bormann 1953; Christensen 1977; Peet and Christensen 1980, 1987; Christensen and Peet 1981, 1984). Old-growth forests in this region are composed primarily of oak and hickory trees in the canopy with numerous smaller broadleaf deciduous tree and shrub species in the understory. Much of these oldgrowth forests were cleared in the 1700s and 1800s for agriculture. As the farms were abandoned, vegetation invaded the cleared fields, passing through distinct stages of herbaceous field and pine forest before becoming a mature oak-hickory forest (Table 11.1, Figure 11.9). One-year-old fields are covered with annual and perennial herbaceous species. Crabgrass and horseweed are most abundant. In the second year following abandonment, the flora is similar but aster and ragweed, minor components of first-year fields, dominate. The abundance of species declines in the third year as broomsedge, a perennial grass, increases in dominance. At about this time, pine seedlings (shortleaf pine or loblolly pine) become established. Broomsedge maintains dominance for a few years until the rapidly growing pine seedlings overtop the shorter grasses, typically by the fifth year following abandonment. Closed stands of relatively even-aged pine trees form 10 to 15 years following abandonment. Germination and survivorship of pine seedlings declines in the low light under the closed canopy, maintaining the even age-structure of the stand. Following canopy closure, stand dynamics is largely a result of growth and mortality of pine trees. By 40 years, a distinct broadleaf deciduous tree understory forms. By 70 to 80 years, the short-lived pine trees begin to die and the longer-lived oaks and hickories replace them in the canopy. Between about 150 and 200 years following abandonment, oak and hickory trees gain dominance and pines remain as scattered relics. The forest is uneven aged, with old, large oak and hickory trees in the canopy, abundant oak and hickory trees in understory, and a large number of smaller subordinate trees in the understory. 9

10 Ecological Climatology Northern hardwood forests The Hubbard Brook study provides a detailed description of changes in community composition, biomass, and biogeochemical cycles during succession in northern hardwood forests of New England (Likens et al. 1977; Bormann and Likens 1979). During the first two to three years following clear-cutting, raspberry and blackberry (Rubus species) flourish, complete their life cycles, and then decline in importance (Figure 11.10). These are replaced by pin cherry, a common early successional tree species restricted to recently disturbed forests. These trees grow fast and are short lived. Canopy closure occurs rapidly when large amounts of buried, viable seed form dense stands. Pin cherry dominates the canopy for the next several years. After about 25 to 35 years, sugar maple and beech, which are tolerant of shade and able to survive beneath the dense pin cherry canopy, become dominant. Where pin cherry is less dense, it may be co-dominant with other fast-growing species such as yellow birch and quaking aspen. These longer-lived trees establish within the first few years following disturbance and dominate the canopy for several decades before the slower growing maple and beech trees reach canopy status. Biomass accumulation following clear-cutting generally has four phases (Figure 11.11). Reorganization is a period lasting about 10 to 20 years during which total biomass (living and detritus) decreases despite regrowth of vegetation. Loss of biomass from the forest floor and dead wood exceeds accumulation of biomass in plants. The aggradation phase is characterized by accumulation of biomass and nutrients in living plants, dead wood, and the forest floor. It begins about 15 years after clear-cutting and lasts for more than a century, culminating in peak accumulation of biomass. Accumulation of organic matter on forest floor increases available water-holding capacity and cation exchange capacity. The transition phase is a period during which total biomass declines. At steady state, total biomass fluctuates around a mean value. The reorganization phase begins immediately following clear-cutting. It is a period during which biotic regulation of hydrologic and biogeochemical cycles is lost. Deforestation eliminates or alters transpiration, nutrient uptake by plants, decomposition, and mineralization, which closely regulate the hydrology and biogeochemistry of an aggrading forest ecosystem. Loss of regulation is seen in increased export of water, nutrients, and sediments from the ecosystem. Comparison of forested and deforested watersheds at Hubbard Brook demonstrates the regulation of the hydrologic and biogeochemical cycles by 10

11 Chapter 11 Vegetation Dynamics vegetation. Prior to deforestation, both watersheds had similar evapotranspiration and streamflow (Figure 11.12). After clearing in , evapotranspiration decreased and streamflow increased in the deforested watershed compared with the forested watershed. Similar results have been found in other forested areas (Dunne 1979; Bosch and Hewlett 1982; Hornbeck et al. 1993; Bowling et al. 2000). Likewise, the loss of vegetation resulted in a rapid increase in the concentration of dissolved nutrients and sediments in stream water (Figure 11.13). Elimination of nutrient uptake by plants and increased rates of decomposition in the warmer, deforested site caused nutrients to be flushed from the system. Recovery of biotic regulation is rapid following regrowth of vegetation. Much of this is related to the rapid colonization and growth of early successional species such as pin cherry. For example, within two years net primary production averages 38% of a 55-year-old forest (Bormann and Likens 1979, p. 140). Between 4 and 5 years, aboveground net primary production ( g m -2 yr -1 ) is comparable to that of a 55-year-old forest. Likewise, the leaf area index of a 3-year-old stand (5-6 m 2 m -2 ) is comparable to that of older forests (Aber 1979). The Hubbard Brook study (Figure 11.11) and the slash pine study (Figure 11.5) illustrate another pattern typical of forests: the decline in net primary production in old stands. The Hubbard Brook forest accumulated biomass at a rate of 485 g m -2 yr -1 between 1965 and Accumulation declined to 46 g m -2 yr -1 between 1982 and Forests typically reach peak growth relatively early in stand development, followed by substantial decline as the forest matures (Ryan and Yoder 1997; Ryan et al. 1997). Peak growth is usually attained as leaf area reaches its maximum. Thereafter, while photosynthetic uptake no longer increases, respiration loss increases as woody biomass accumulates. In addition, the hydraulics of water movement in large trees may limit photosynthesis. The increase in hydraulic resistance with greater height increases water stress in the leaves, closing stomata and reducing carbon gain during photosynthesis. The lower photosynthetic capacity may account for the decline in forest productivity with age (Friend 1993; Ryan and Yoder 1997) Boreal forests of interior Alaska In the boreal forests of interior Alaska, forest communities are a successional mosaic of broadleaf deciduous and needleleaf evergreen trees that reflects recovery from recurring floods and fires (Van Cleve et al. 1983a,b, 1986, 1991; Bonan 1989, 1990a,b; Bonan and Van Cleve 1992). Topographic location has a large role in determining community composition and the pathways of succession. Floodplains along rivers 11

12 Ecological Climatology are a successional mix of willow and alder shrubs and highly productive balsam poplar and white spruce forest. Warm, well-drained south-facing slopes are dominated by productive quaking aspen, paper birch, and white spruce. Black spruce forms open stands with low productivity on cold, wet soils underlain with permafrost. Along river floodplains, recurring floods initiate succession (Figure 11.14). For the first one or two years, large floods deposit sediments and form terraces along river banks. As the terraces rise, flooding is less frequent and willow, alder, and balsam poplar invade the exposed sites. Shrubs increase rapidly in cover before being overtopped by trees about 20 to 30 years after establishment. With the formation of a closed balsam poplar canopy, the pioneer shrubs decline in abundance. Further buildup of the terrace from sediment deposits and litterfall prevents additional flooding. This allows for the invasion of white spruce seedlings. The shift in dominance from balsam poplar to white spruce is gradual and takes about 100 years. The buildup of feathermosses and a thick layer of organic matter on the ground is a prominent feature of this change. On upland sites, recurring fires initiate succession (Figure 11.14). Warm, dry sites are initially colonized by herbaceous plants, shrubs, and trees. Shrubs, primarily willow, and tree saplings dominate until about 25 years following fire, when the saplings grow into a dense stand of birch and aspen. These deciduous trees dominate for the next 50 years, though white spruce seedlings and saplings grow in the understory. After about 100 years, white spruce grows into the canopy and is the dominant tree. Similar to floodplain succession, invasion of feathermosses and the accumulation of a thick forest floor accompanies the development of a white spruce stand. Post-fire succession on poorly drained upland sites and north slopes involves the recovery of black spruce (Figure 11.14). During the first few years following fire, herbaceous plants and black spruce seedlings colonize the site. Many shrubs regrow from root sprouts. These shrubs grow rapidly and dominate the vegetation. A tree canopy begins to form about 25 to 30 years after fire. Mosses invade at about this time, and a thick organic layer develops on the forest floor. This organic layer has a low thermal conductivity, insulating the soil and cooling soil temperatures. As a result, productivity declines and the stand thins. 12

13 Chapter 11 Vegetation Dynamics The general trend in community composition during succession is from broadleaf deciduous to needleleaf evergreen. This shift in dominance is accompanied by accumulation of organic material on the forest floor. Soil temperatures decline, slowing the rate of decomposition and further promoting the accumulation of organic material. Soil moisture increases with buildup of the forest floor. This promotes the establishment of mosses. In addition, the chemical quality of the forest floor declines with the shift from deciduous trees to spruce, further reducing decomposition rates. Nutrients accumulate in the undecomposed material on the forest floor. Tree growth declines. Fires and floods reset the system to earlier stages of succession. Forests older than 200 to 250 years are rare in the uplands. Fires act as a rapid decomposer, consuming all or portions of the forest floor and replenishing the supply of some nutrients. This is especially important in black spruce succession (Figure 11.15). Fires open the canopy and consume much of the thick organic layer on the ground. As a result, the soil warms. Over time, however, as mosses invade and the thick organic layer accumulates, soil temperature declines. Nutrient availability declines as mineralization rates decrease. Tree growth also declines, and in some old black spruce stands the net primary production of mosses exceeds that of trees Mechanisms of succession Much of the debate among ecologists about succession has centered on whether it is a directional, predictable process of community change. This debate has its origins in the two contrasting views of plant communities and ecosystems developed by ecologists in the early 1900s that saw either the community and ecosystem as the essential ecological unit of study or individuals and species as the essential units. The holistic school held that a community or ecosystem is an integrated entity equivalent to a superorganism. An alternative view was that plant communities and ecosystems are not superorganisms with emergent properties but rather assemblages of individual organisms and species. Attributes such as community structure and succession are merely the result of the ecology of individual plants and species interacting with one another. Much of the history of ecology has been an evaluation of these two competing theories (McIntosh 1981, 1985; Golley 1993). In the holistic view, developed by F.E. Clements and culminating with the ideas of E.P. Odum, succession is an orderly, predictable process consisting of discrete stages of development driven by vegetation itself (Clements 1916, 1928; Odum 1953, 1969, 1971). Succession culminates in a stable 13

14 Ecological Climatology community composition, called the climax community, that is a predictable endpoint, uniform for a region, and determined by regional climate. This view of succession came to be known as relay floristics or facilitation because groups of plants relay dominance to others by facilitating their entry into the community (Egler 1954; Connell and Slatyer 1977). Succession is characterized by the successive appearance and disappearance of groups of species (Figure 11.16). Each group invades the site at a certain stage during succession, making conditions unsuited for their own regeneration but paving the way for invasion by the next group of species. With the advent of ecosystem ecology, the superorganism concept was extended to see succession in terms of ecosystem functions independent of the underlying plant populations. The climax ecosystem represents an equilibrium balance of functions, and succession is the process by which equilibrium is achieved. In the alternative view, championed by H.C. Cowles and H.A. Gleason, succession is seen as a population process that emphasizes individual organisms and the individualistic response of species, through their various life history patterns, to environmental change (Cowles 1899; Gleason 1917, 1926, 1927, 1939). Succession is an expression of the preemption of resources and canopy and root space and the differential growth, longevity, and colonizing ability of plants. It is neither fixed nor predictable. Chance, especially the presence of seeds or sprouts to colonize a site, plays a large role in determining community composition. This view of succession is epitomized by Egler s initial floristic composition model of succession following farm abandonment (Figure 11.16). Succession unfolds from the initial flora present in the seed bank at the time of abandonment. Seeds or vegetative parts of future dominant species are present at the time of abandonment. Progressive development is related to particular life histories. As each successive group dies out, another group of species, present from the start, assumes dominance. These two contrasting theories have grown into a broader conceptualization of succession. No one single theory sufficiently explains all the patterns of succession found in nature (Connell and Slatyer 1977). Clemenstian concepts such as facilitation can be found in various successions. Others fit an overall pattern in which inhibition of new species until early dominants die or tolerance of low levels of resources in later stages of succession determine community dynamics. However, the prevailing opinion emphasizes succession as a population process (Egler 1954; Drury and Nisbet 1973; Horn 1974; Pickett 1976; Connell and Slatyer 1977; Peet and Christensen 1980, 1987; West et al. 1981; Shugart 1984, 1998; Huston and 14

15 Chapter 11 Vegetation Dynamics Smith 1987; Smith and Huston 1989; Bazzaz 1996). It is a result of the different life histories of species operating in a gradient of environmental change. Succession is a consequence of differential growth, survival, and colonizing ability of species adapted to different points in an environmental gradient. Individual species colonize where conditions are favorable, die out of the community when the environment is no longer favorable, and grow in company with other species with similar environmental requirements. Seed rain and the chance that seeds or sprouts encounter a suitable safe site play a large role in vegetation development. The prevailing view of succession emphasizes species attributes their physiology, morphology, and life history as adaptations to different environmental conditions encountered during community development. Autogenic changes during succession in the availability of light, water, and nutrients create shifting patterns of dominance as a result of tradeoffs in the ability to effectively compete for above- and belowground resources. These tradeoffs result in a suite of traits that represents a complex adaptation to a particular environment. Moreover, there is a tradeoff between tolerance to low resource conditions and maximum potential growth rate under high resource conditions. These tradeoffs and correlations among traits limit the ability of any one species to dominate all environmental conditions (Figure 10.5). There is an inevitable shift in the competitive ability of species over the course of succession due to these tradeoffs. One trait that has received much attention is the different physiology of early and late successional plants (Bazzaz 1979, 1996; Huston and Smith 1987). This is related in part to shade tolerance. Shade tolerant trees photosynthesize at low light levels found in the understory of old-growth forests. Photosynthetic rates of shade intolerant species are not as high at low light. These species need full sunlight to maintain positive carbon gain. This physiological distinction between shade tolerant and intolerant species is part of a larger overall life history difference between early and late successional species (Table 9.7). In addition, the arrangement of foliage on branches influences the effectiveness with which trees grow in full sunlight or shade (Horn 1971, 1975). Monolayer trees produce dense foliage at the tip of branches, forming a single layer of leaves around the tree. They are favored in low light environments because self-shading of leaves is minimized and are at a disadvantage in high light environments because of their low leaf area. In contrast, multilayer trees distribute foliage in several layers throughout the canopy. 15

16 Ecological Climatology They are favored in high light environments, where the light penetrates deeply into the canopy, but are at a disadvantage in low light conditions because interior leaves are shaded. One theory of community structure and dynamics the resource-ratio theory of succession is based on resource acquisition and tradeoffs in characteristics that allow plants to effectively compete for above- and belowground resources (Tilman 1985, 1988). Species with high allocation to production of leaves and stems are effective competitors for light, an aboveground resource. Species with high allocation to root production are good competitors for belowground resources such as nitrogen. Resource supply and acquisition determine which morphologies are viable in different environments. Root specialists are favored where aboveground resources are abundant but belowground resources are limiting. Shoot specialists are increasingly favored as light becomes more limiting. Succession is a result of autogenic changes in the availability of above- and belowground resources over time that favor different allocation patterns (Figure 11.17). During the initial stages of succession, light is readily available but nitrogen availability is low. Short, leafy species with high leaf allocation, low stem allocation, and moderate root allocation grow best. As the community develops, the canopy closes and light availability declines. Nitrogen supply increases with the introduction of nitrogen-fixing species, production of leaf litter, and accumulation of detritus. This environment favors tall species with increasingly greater allocation to stem and less to foliage and roots. The actual sequence of dominant morphologies differs depending on soil fertility, though in this example all three sites converge on a similar dominant morphology because they are assumed to attain the same final nitrogen availability. An alternative view is a classification of species into overall life history patterns of competitor, ruderal, and stress tolerator (Grime 1979) (Figure 9.29). This classification arises from tradeoffs among a broad range of traits including reproductive effort, dispersal, ability to capture resources, and ability to tolerate stress. It invokes disturbance, stress, and competition as opposing forces selecting for particular traits. Succession is a result of the interplay between these different life history patterns and changing environmental conditions (Figure 11.17). Ruderal species, with their rapid ability to colonize disturbed sites, are favored in open sites with abundant resources (light, space). As vegetation develops and biomass accumulates, competition for resources increases and selects for competitive species. Depletion of resources in late stages of succession leads to replacement by stress tolerators. This view of succession is 16

17 Chapter 11 Vegetation Dynamics driven by autogenic decreases in resource availability as succession progresses. On less productive sites, earlier onset of resource depletion limits the appearance of highly competitive species. On unproductive sites, succession moves directly from ruderal to stress tolerator. The concept of vital attributes combines life history patterns, resource utilization, and response to disturbance (Noble and Slatyer 1980). These vital attributes are: method of arrival or persistence at a site during and after disturbance; ability to establish and grow to maturity in the developing community; and time required for an individual to reach critical life stages such as seed production. These three attributes combine to produce numerous life histories. In general, life history patterns are related to a tradeoff between an ability to rapidly colonize and dominate a site versus slow growth and ability to compete in a resource limited environment. There is a fundamental physiological tradeoff between characteristics that allow either high photosynthesis and rapid growth under high light or the ability to grow and survive in shade. This dichotomy is reflected in the broad classification of plants into categories such as r- and K-selected species (MacArthur and Wilson 1967; Gadgil and Solbrig 1972), mono- or multilayered (Horn 1971, 1975), shade intolerant and tolerant (Huston and Smith 1987), early and late successional (Bazzaz 1979, 1996; Huston and Smith 1987), ruderal and competitor (Grime 1979), exploitive and conservative (Bormann and Likens 1979), and gap and non-gap (Shugart 1984, 1987, 1998). Regardless of what they are called, species that dominate communities in the early stages of succession produce prolific crops of small, widely dispersed seeds, grow rapidly to maturity, but are short lived and unable to survive in the light-depleted understory of a dense canopy. These species have traits that take advantage of the temporary nature of disturbance. They maintain their abundance in the landscape through rapid colonization and dominance of disturbed sites enriched in light, water, and nutrients. In contrast, species that dominate during the late stages of succession have lower rates of dispersal and colonization, slow growth, and are long lived. They maintain their presence by surviving intense competition for limited resources. Life history patterns such as seed size, maximum height, age of reproduction, and longevity explain succession at Glacier Bay (Table 11.2). For example, the order in which species colonize newly exposed glacial till is closely correlated with seed size and dispersability. Seeds of early successional species are lighter than those of later stages of succession. Early successional species also have a shorter 17

18 Ecological Climatology lifespan and mature earlier than mid- and late successional species. These reproductive traits are critical to successional changes in community composition. Long-distance seed dispersal and short generations are necessary to establish and maintain plants in early successional communities far removed from seed sources. The low density of alder and spruce in pioneer communities reflects low seed rain rather than inability to germinate and establish on newly exposed sites. Alder and spruce are slower to arrive on recently deglaciated terrain than pioneer species because their heavier seeds and greater age of reproduction limits seed sources. Interactions among species are also important. The change from Dryas to alder involves competitive displacement as the taller alder shrubs shade the shorter Dryas. Similarly, slowgrowing, long-lived spruce trees eventually overtop and displace alder. Hence, height and longevity, which increase throughout succession, are major factors determining community change. Aspects of facilitation and inhibition among species can be seen in the succession (Chapin et al. 1994). The progressive addition of organic matter and soil nitrogen facilitates rapid community development. Conversely, Dryas and alder inhibit the establishment of alder and spruce seedlings, respectively. Alder and spruce must disperse to the site before the prior colonizers modify the environment to inhibit seedling establishment. Inhibition of seedling establishment and growth by other species prevents late successional species from dominating more quickly. In old-fields in the North Carolina Piedmont, sequential change in the dominance of herbaceous annuals and perennials to pine trees and then oaks and hickories is related to differential life history patterns such as time of seed dispersal and germination, environmental requirements for germination, and type of seed dispersal. Pine trees gain early dominance because their seeds are light and widely dispersed by wind and because seedlings germinate and grow rapidly in the high light environment of old-fields. Variation among stands in the initial stocking of pine seedlings is related to the proximity of seed trees and the quality of seed. Pines lose dominance to broadleaf deciduous trees because they are relatively shortlived and seedlings germinate and establish poorly in shade. Broadleaf deciduous trees are tolerant of shade and can survive and grow better in low light. Among broadleaf deciduous trees, seed size and dispersal accounts for much of the temporal variation in species abundance. Tree species with small, wind-dispersed seeds occur in stands of all ages, though with greatest frequency in young stands (Table 11.3). In contrast, oak and hickory, with large nuts, are relatively infrequent in young pine stands and increase in frequency 18

19 Chapter 11 Vegetation Dynamics throughout succession. Presence of these species in young stands reflects proximity to seed trees while the abundance in late stages of succession is ensured through the growth of large canopy dominants. Pin cherry, which contributes greatly to the rapid recovery of northern hardwood forests following clear-cutting, is an example of an exploitive or fugitive life history (Marks 1974). Its life history is suited for efficient colonization of large gaps in the canopy. The combination of buried seeds that are viable in the soil for 15 years or so and dispersal by birds ensures a large seed bank well after disappearance from a particular site. Once established, pin cherry grows rapidly. By age 25 to 30, when pin cherry trees rapidly die, sufficient seeds have been produced and disseminated to ensure renewal after the next major disturbance Pattern and process in plant communities Many ecologists long viewed natural systems as being in balance and in equilibrium with the environment. The Clementsian view of plant succession held that succession is orderly, predictable change that culminates in a stable, climax ecosystem that represents an equilibrium balance of functions. However, this equilibrium view of nature is flawed. Fires, floods, hurricanes, herbivores, and insects regularly disturb plant communities, initiating the successional cycle. In this non-equilibrium view, the landscape is a mosaic of successional communities whose composition and structure is determined by the type and severity of disturbance. Disturbance and the resulting successional development of communities and ecosystems are the processes giving rise to the mosaic pattern of vegetation in the landscape (Watt 1947; Pickett 1976; Whittaker and Levin 1977; Bormann and Likens 1979; Shugart 1984, 1998; Pickett and White 1985). These disturbances can be as small as a 100 m 2 gap in the canopy created by the death of a large dominant tree or as large as hundreds of square kilometers following fire. Disturbance removes existing vegetation and creates an environment with high sunlight and reduced competition for resources conditions that favor the germination, establishment, and growth of early successional species. These plants are maintained in the landscape by a fugitive life history and recurring disturbance. Their abundance is limited by the availability of a site to colonize and availability of seed, which must be dispersed onto the site from other stands or be stored in the soil from prior colonization. In contrast, the abundance of late successional species is promoted by long intervals between 19

20 Ecological Climatology disturbance. As a result, the frequency and intensity of disturbance is often linked to the diversity of species (Connell 1978; Huston 1994). The smallest scale of disturbance is the death of a large tree, which creates small-scale cyclic dynamics within a community associated with gaps in the canopy (Figure 11.18) (Watt 1947; Bray 1956; Bormann and Likens 1979; Shugart 1984, 1998). Treefalls are common in forests (Runkle 1981, 1982, 2000; Runkle and Yetter 1987). The death and uprooting of a tree creates a gap in the canopy related to the size of the tree. If large enough for sufficient light to reach the ground and conditions are favorable, the site is colonized by numerous seedlings. Many of these seedlings die. Some grow into saplings, which thin over time into a mature stand of a few large trees. Eventually these trees die, initiating a new cycle of growth. The forest landscape is a mosaic of individual patches in different stages of development (Watt 1947; Bormann and Likens 1979; Shugart 1984, 1998; Pickett and White 1985; Botkin 1993). Within each patch, the process of establishment, thinning, and gap formation is cyclically repeated. Although the state of an individual patch changes from year to year, the proportion of patches in the landscape in various developmental stages is relatively constant. The result is a shifting mosaic steady state. Gap dynamics has been observed in prairie, desert, and tundra communities, but is most evident in forests. The concept of a gap dynamics and shifting mosaic steady state can be illustrated with a simple model of tree growth. The accumulation of tree biomass in a small patch on the order of 100 m 2 can be described by the equation dm / dt = a bm where M is biomass at some time t. With annual net primary production of a = 1000 g m -2 yr -1 and annual loss of biomass due to mortality of b = yr -1, a maximum biomass of g m -2 accumulates in the patch. This accumulation is achieved after about 200 to 250 years of growth. If the trees lived forever, biomass accumulation would saturate at g m -2 and remain constant over time. However, the trees in a patch have an annual probability of mortality. If mortality is independent of age, the probability a tree will survive to age n is p= ( 1 ) c n where c is the annual probability of mortality. For long-lived trees such as oak or hickory, which have a maximum age of about 300 years, c = means that 1% of the trees survive to an age of 300 years. 20