non-equilibrium coexistence of plants

Torrey Botanical Society

Non-Equilibrium Coexistence of PlantsAuthor(s): S. T. A. PickettSource: Bulletin of the Torrey Botanical Club, Vol. 107, No. 2 (Apr. - Jun., 1980), pp. 238-248Published by: Torrey Botanical SocietyStable URL: http://www.jstor.org/stable/2484227 .

Accessed: 16/09/2013 22:58

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

Torrey Botanical Society is collaborating with JSTOR to digitize, preserve and extend access to Bulletin of theTorrey Botanical Club.

http://www.jstor.org

This content downloaded from 147.8.31.43 on Mon, 16 Sep 2013 22:58:35 PMAll use subject to JSTOR Terms and Conditions

http://www.jstor.org/action/showPublisher?publisherCode=tbs

http://www.jstor.org/stable/2484227?origin=JSTOR-pdf

http://www.jstor.org/page/info/about/policies/terms.jsp


B U L L E T I N O F T H E T O R R E Y B O T A N I C A L C L U B

VOL. 107, No. 2, pp. 238-248 APRIL-JUNE 1980

Non-equilibrium coexistence of plants S. T. A. Pickett

Department of Botany, Rutgers University, P.O. Box 1059, Piscataway, NJ 08854

PICKETT, S. T. A. (Botany Dept., Rutgers Univ., New Brunswick, NJ 08903). Non- equilibrium coexistence in plants. Bull. Torrey Bot. Club 107: 238-248. 1980.-Equilib- rium plant communities are established by uninterrupted successions over several cen- turies. However, these equilibrium communities are often relatively poorer in species than some non-equilibrium community that preceded them. This, coupled with the paucity of confirmed within-community niche differentiation, suggests that factors preventing the establishment of a competitive equilibrium are critical to the coexistence of plants. Disturbance occurs frequently enough in many systems to destroy or disadvantage the competitive dominants of late successional communities and so allow the coexistence of species with many degrees of competitive ability. Without disturbance the persistence of certain species is threatened. The concept of patch dynamics is used to generalize and emphasize non-equilibrium coexistence. Key words: Patch dynamics; non-equilibrium coexistence; disturbance; competitive equilibrium; succession.

Conditions allowing coexistence of plants fall into two contrasting categories. In constant, uniform environments, species can coexist in competitive equilibrium if they differ in niche. A community in competitive equilibrium is one not undergoing compositional or structural change due to competition. If species do not possess some minimum degree of niche divergence then inferior competitors will be excluded from the equilibrium community. The equilibrium viewpoint is well summarized by Whittaker (1975: 78), who shows its rela- tionship to the competitive exclusion principle:

" (1) If two species occupy the same niche in the same stable community, one will become extinct. (2) No two species observed in a stable community are direct competitors limited by the same resources; the species differ in niche -in ways that reduce competition between them. (3) The community is a system of interacting, niche-differen- tiated populations that tend to comple- ment one -another, rather than directly competing, in their uses of the community 's space, time, resources, and possible kinds of interactions."

Most ecological research into species coexistence and community organization as well as the development of theory, has

sought out such situations or at least assumed competitive equilibrium to have been reached.

Alternatively, communities may be or- ganized under conditions which prevent the attainment of competitive equilibrium. Biotic events, such as predation, herbivory, and disease, as well as abiotic events, such as fire, or windstorm, may act to halt the process of exclusion by competition. Co- existence under the influence of such events occurs outside of local competitive equilibrium. Non-equilibrium coexistence thus occurs in situations where the hierarchy of competitive dominance that would exist in a community without disturbance is disrupted at intervals. Most species, but especially the poorer competitors, reproduce or -establish episodically at times when the control of the community by the competitive dominants is disrupted by disturbance. Although non-equilibrium coexistence has been recognized for a long time (Hutchin- son 1951, 1961, Raup 1957), it has only recently received the attention it deserves, except in marine work (Paine 1966, Con- nell 1971, Levin and Paine 1974, Dayton 1971, 1975, Menge and Sutherland 1976).

The purpose of this paper is to explore the possibility that coexistence of plants is generally a non-equilibrium phenomenon dependent on prevention of the attainment of competitive equilibrium. Non-equilibrium coexistence is not likely to be the only, Received for publication April 9, 1979.

238

0040-9618/80/02-0238-11$01.65/0 ? 1980 Torrey Botanical Club



1980] PICKETT: NON-EQUILIBRIUM COEXISTENCE 239

nor even the most important, organizing influence in all communities, although it may be in some. I wish merely to promote the recognition of non-equilibrium phenom- ena, based on disturbance and succession, as viable alternative mechanisms to equilibrium coexistenee.

This paper builds on an evolutionary and landscape-based interpretation of sue- cession (Pickett 1976). The phenomenon of non-equilibrium coexistence has been treated theoretically, in particular taxa, or in particular systems by a number of authors (Skellam 1951, Levins and Culver 1971, Horn and MacArthur 1972, Levin and Paine 1974, Slatkin 1974, Horn 1975, 1976, Woodin and Yorke 1975, Armstrong 1976, McClure and Price 1976, Levin 1976, Sale 1977, Grubb 1977, Connell 1978, Caswell 1978). The current paper differs in emphasizing the role of the external disturbance regime in structuring terrestrial plant communities, and in emphasizing the occurrence and basic similarity of such processes in many dissimilar environments. It further attempts to indicate how equilibrium is prevented by effects of distur;bance; to present a preliminary general framework for the consideration of disturbance effects; and to foster recognition of the extensive role of disturbance in plant community organization.

Preliminary considerations. Under the prevailing equilibrium viewpoint, communities are assumed to be composed of species that differ in their resource use within that community. Many report that func- tional and microhabitat differentiation allow coexistence in animal communities (e.g., Schoener 1974), but there are few studies of niche differentiation in plants (Willson 1973), and many such studies actually examine the habitat level (e.g., MeNaughton and Wolf 1970, Platt 1975, Platt and Weis 1977). However, differentiation on habitat gradients, which are coextensive with topo- graphic or geographic gradients, cannot explain coexistence within a community or site (Whittaker et al. 1973). The studies which have demonstrated differentiation in plant structure and microhabitat (e.g., Beals and Cope 1964, Parrish and Bazzaz 1976, Bratton 1976, Yeaton et al. 1977) are of relatively simple communities or parts of more diverse assemblages.

Plant resources, such as nutrients, light, and water, have been proposed, but not yet

demonstrated, to be effectively partitioned among species within communities (a niche). In addition, it may be that the majority of plant resources are not readily partitionable except by habitat specialization, because they are not packaged in dis- crete, differently behaving units (Harper 1965, 1968). Together, these two points suggest that coexistenee of plants based on the degree of niche partitioning they have achieved is unlikely (see extensive review by Grubb 1977). Perhaps the lack of evidence for niche differentiation within plant communities indicates the short history of interest in niche relations in plants and consequently a failure to discover the scale and mechanisms of a-niche differentiation. Consistent association of structurally or functionally different species or guilds plus experiments demonstrating a resultant re- duction of competition (e.g., Putwain and Harper 1970), will be needed to clearly show a-niche differentiation. However, the lack of clearly demonstrated equilibrium coexistence by a-niche differentiation in diverse plant communities encourages both further work on niche differentiation in plants as well as the search for alternative mechanisms of plant community organization. The second possibility will be pursued here.

The process of competitive exclusion in plants, although important on geographic gradients (Whittaker 1969) is commonly expressed as species turnover and altered community structure in succession. Non- equilibrium conditions are thus best considered in the context of succession. Suc- cession is a diverse process, and is based on invasion, initial floristic composition, life span, tolerance, and interference. The mechanism of interest here is the assortment in successional time, of initially present and incoming propagules on the basis of their competitive abilities (Horn 1976, Wilson 1969) and life histories (Connell and Slatyer 1977). Assortment is the dominance of different species through time after a site has been opened. It is based on differential abilities of competition, growth, and invasion. As a result of competitive assortment through time, a site (i.e., an area in which individuals can actually interact) will eventually be dominated 'by maximally competitive species and this community will then be at compositional equilibrium. Such equilibrium, or climax,



240 BULLETIN OF THE TORREY BOTANICAL CLUB [VOL. 107

communities are the endpoint of the process of competitive exclusion (Loucks 1970, Leak 1972). In this paper, " climax " de- fines the equilibrium community at a site, and " climax species " are the taxa that compose that community.

This view of succession as a process of competitive exclusion is justified by direct observation of competitive displacements of early successional species in several systems (Keever 1950, Watt 1964, Webb et al. 1972, Leak 1972, Horn 1975, Raynal and Bazzaz 1975, Miceli et al. 1977), and knowledge of the contrasting tolerances of early and late successional species (Fowells 1965, Pickett 1976, Wells 1976). As a result of competitive exclusion, a variety of old-growth forests exhibit lower tree species diversity than younger forests or disturbed patches (Gysel 1951, Richards 1955, Loucks 1970, Auclair and Goff 1971, Leak 1973, Shafi and Yarranton 1973, Nicholson and Monk 1974). Maximum diversity in some mesic seres occurs prior to compositional equilibrium, before intolerant species have been excluded by tolerant ones (Loucks 1970, Horn 1974). Thus, one or two species of tree can potentially competi- tively exclude the vast majority of those capable of occupying the habitat. However, while succession can produce depauperate, equilibrium communities, natural events can prevent equilibration and provide a major role for non-equilibrium coexistence of plants. The following section presents empirical support.

Deflection of competitive assortment. The preceding discussion of climax or com- petitively equilibrated communities ne- glects periodic events or continuous processes that disrupt competitive assortment. Disturbance has been recognized to affect vegetation patterns, but has not been em- ployed in a general theory of community organization. Competitive assortment and the resultant succession toward equilibrium may be halted by events having three characteristics. 1) The effective disturbance event must create an environment that, at least temporarily, favors species with poorer competitive abilities than the competitive dominants in the closed community. Such environments will periodically appear as gaps or patches in the canopy of dominants and they may vary internally or grade into the dominant canopy. 2) The

event must create patches at time intervals shorter than those required for exclusion of poor competitors throughout a region. A "region" includes a number of sites that are potential sources of propagules of all species in the succession (cf. Pickett and Thompson 1978). 3) Patches must be spatially disturbed such that they are acces- sible to the poorer competitors. These three requirements of non-equilibrium coexistence appear to be provided by many of the agents creating patches in nature.

Abiotic events can generate patches of varying sizes in natural systems. Fire is one of the most common and well studied of these disturbance agents. Areas large enough to support poor competitors burn in many systems, especially boreal forest (Barney 1969, Johnson and Rowe 1975, Tande 1979), but also northern deciduous and coniferous forest (Lorimer 1977), drier temperate deciduous forest (Haines et al. 1975), dry tropical deciduous forest (Janzen 1967), and tundra (Wein 1976). Indigenous cultures are apparently respon- sible for fires in some situations (Day 1953). Lightning is a source of mortality in the absence of fire as well (Komarek 1968). Normally moist tropical and temperate forests do not burn, except perhaps very rarely (Lorimer 1977).

The interval of recurrence of fire is often within the normal life cycle lengths of most species (Loucks 1970, Heinselman 1971, Leak 1972, Tande 1979, Wein and Moore 1978). Regional competitive exclusion is likely to require much longer time periods. For example, Henry and Swan (1974) found that the composition of a stand of northern hardwood forest was determined not by competition within the community between disturbances, but by establishment and release of suppressed individuals at the time of disturbance. Changes in canopy composition between disturbances were also very minor in the area studied by Oliver and Stephens (1977). The reconstruction studies of Henry and Swan (1974) and Oliver and Stephens (1977) deal with rather small areas, but it is just such areas that are important in determining whether a local community is in competitive equilibrium or whether disturbance may be expected to play a role in species coexistence. The fact that disturbance did play such a major role in the small areas studied by Henry




and Swan (1974) and Oliver and Stephens (1977) is significant.

Areas that have been disturbed by fire are more species rich than areas unburned for long periods (Loucks 1970, Heinselman 1971, 1973). Fire and other disturbance agents in areas of potential Thuja occi- dentalis dominance normally prevent this species from assuming monotonous dominance (Grigal and Ohmann 1975). Many other authors recognize the diversifying influence of fire in various systems (Mias- surow 1941, Komarek 1968, Vogl 1970, Thompson and Smith 1971, Heinselman 1973, Botkin and Sobel 1976, Pickett and Thompson 1978).

Wind damage involving one tree or large forest tracts is another major disturbance agent. Jones (1945) indicated the influence of wind and gap filling in maintaining the structure and diversity of temperate deciduous forest. Wind, which is the major disturbance agent in mesic deciduous forest (Wright 1974), has left extensive evidence in the form of downed timber and tip-up mounds in forest that have not been culturally cleared (Cain 1935, Lutz 1940, Goodlett 1954, Drury and Nisbet 1973, Henry and Swan 1974, Oliver and Stephens 1977). Pioneer trees often specialize on light gaps or the microtopography and debris created by windthrow (Bray 1956, Curtis 1959, Auclair and Cottam 1971, Marks 1974). The clumping of pioneer trees in beech-maple forest corresponds to the size of disturbance gaps (Williamson 1975).

Additional evidence for the effect of wind is found in presettlement forest sur- veys. In Indiana 21 % of the length of transects surveyed were recognizably disturbed by tornados (Lindsay 1972). In Maine 2.6% of the surveyed transects had been recently disturbed by wind (Lorimer 1977: 146), with an estimated interval between "light to moderate disturbances" of 250 yr. The small disturbances estimated to occur so frequently by Lorimer (1977) are the major cause of non-equilibrium coexistence. Ice can act analogously to wind in damaging deciduous forest canopies (Siecama et al. 1976, Harcombe and Marks 1977).

Tropical wet forests are profoundly af- fected by wind (Vaughn and Wiehe 1937, Webb 1958, Gomez-Pompa 1971, Whitmore 1975, Richards and Williamson 1975, Long-

man and Jenik 1974). Epiphyte overload may be an important contributing factor in wind damage (Strong 1977). The recurrence interval for gap formation at a site ranges from 45 to 90 yr in some tropical forests (Leigh 1975). Pioneer, shade intolerant species exist scattered throughout old-growth tropical forest (Richards 1955, Jones 1956, Knight 1975), and the clumping in some pioneer populations corresponds to mean gap size (Richards and Williamson 1975). Disturbance in wet tropical forest, which is composed of massive but shallowly rooted trees, may be even more effective in generating and maintaining diversity (Connell 1978) than in most temperate forests.

Landslides associated with earthquakes or torrential rains are common in moun- tainous regions (Hack and Goodlett 1960, O'Laughlin 1972, Veblen and Ashton 1978, Garwood et at. 1979). Slides may be in- vaded by either pioneer or late successional species depending on the nature of the slide and the colonization source (Flaccus 1959, Hack and Goodlett 1960). In general, landslides appear to be qualitatively different from disturbance by fire and especially by wind. Whereas wind- and fire-created gaps would be expected to cause a release of resources for revegetation, landslides may reduce the resource availability. Mud- flows in tundra provide areas exploitable by colonizing species (Langenheim 1956, Lambert 1975). Landform disturbance may occur during severe floods (Hack and Goodlett 1960, Leopold et al. 1964) in addition to direct physiological damage to dominants.

Biotic agents can deflect competitive interactions. Host-specific or frequency- dependent predation on seeds or seedlings may prevent otherwise dominant species from monopolizing resources and canopy space (Janzen 1970, Connell 1971). Her- bivory and grazing may act similarly to maintain diversity (Harper 1969, Kalk- stein 1976) and outbreaks of herbivores are distributed patchily in space and time (Morris 1963).. Similar organizing effects of predators and herbivores have been ex- tensively investigated in marine systems (Paine 1966, 1974, Connell 1971, 1975, Menge and Sutherland 1976), and provide a firm precedent. In terrestrial systems, hill prairies grazed by domestic herbivores have higher diversity than sites free of grazing




(Evers 1955), and animal burrowing opens herbaceous communities to invasion by fugitives and poorer competitors (Ross et al. 1968, McDonough 1974, Platt 1975, Platt and Weis 1977).

Cyclic changes in environment, gen- erated by a series of interacting species themselves, may be viewed as a case of non- equilibrium coexistence at a site. Watt (1947, 1964) described cases of cyclic coexistence in a number of communities and considered these to be "alternating equilibria. " I believe " cyclic coexistence " would be more appropriately labeled as a non-equilibrium situation because each patch is the actual arena of interaction within the system. Frequency-dependent coexistence may occur when juveniles of a species establish better beneath the canopies of a second than beneath adults of their own species. The theory for frequency- dependence has been given by de Wit (1960), and has been proposed to occur in several forest types (Fox 1977). Although the mechanisms underlying the cycles are unknown, the seedlings of each of the two dominant species are more common under pure canopies of the second dominant. 'Such pure stands, which were rare in even un- exploited forests, might be expected to be rare under most of the natural disturbance regimes so far studied. Forcier (1975) sug- gested that coexistence of even beech and maple in old forests depended on gap formation and a subsequent microsuccession. Although cyclic coexistence or alternation of species might not always depend on disturbance, it seems likely that species alternation will often result from differential strategies for responding to disturbance.

In addition to direet evidence for the role of disturbance in structuring plant communities, the evolutionary response of plants to non-equilibrium conditions is indicated by differential adaptations of species to successional environments (Pickett 1976). There are many species which have evolved specializations to use disturbed or open environments having resource release and reduced competitive pressures (Fow- ells 1965, Loucks 1970, Wells 1976). The mutually exclusive nature of pioneer and climax strategies indicates not only that evolutionary adaptation to non-equilibrium conditions is selectively feasible, but that such selection pressures for fugitivity are common. Some elements of the fugitive

strategy are found in all plant species (Harper 1965), allowing successional position (Whittaker 1969) or dispersal ability (Platt 1975, Platt and Weis 1977) to be used as ecotope dimensions.

In summary, the process of competitive exclusion seldom proceeds to completion (i.e., climax as defined earlier) at a site and even more rarely over a region. Patches are created by a variety of disturbance agents, and are often occupied by species that would not otherwise coexist at a site. Even so-called climax species may require disturbed patches for reproduction and dominance.

Patch dynamics. The general role of disturbance in plant community organization can best be recognized by using the concept of patch dynamics (Thompson 1977, 1978, Pickett and Thompson 1978). The components of patch dynamics have been introduced and modeled by others (Levin 1970, 1974, Levin and Paine 1974, 1975, Woodin and Yorke 1975). Patch dynamics as used here consists of 1) the pattern of patch creation in time and space, 2) patch size and structure, and 3) the changes in individual patches of a cohort and size class due to species availabilities, adaptations, and interactions.

This approach also has value because it can serve as a more comprehensive replacement for traditional concepts of succession and climax which are becoming increasingly less useful (Drury and Nisbet 1973, Horn 1974). A patch dynamic view of plant community structure readily incorporates the unpredictability in rate of change and species composition that is so common in succession (Horn 1976, Frye 1978). Thus rather than viewing succession as an order- ly, predictable, and linear progression to an idealized climax, plant communities are seen as being strongly conditioned 'by disturbance and the opportunistic response of species to disturbance. Patch dynamics of vegetation and non-equilibrium coexistence are closely linked.

On the scale of patch dynamics of interest here, disturbance creates gaps or patches in which resources are often re- leased (Marks 1974, Vitousek 1977) and environmental conditions change (Marquis 1966, Skeen and March 1977). The size of the gap and severity of the disturbance determine the degree of resource release and thus the intrinsic suitability of the gap



19801 PICKETT: NON-EQUILIBRIUM COEXISTENCE 243

for various species. The requirements of the potential gap occupants for seedbed, moisture, light and other conditions for establishment and persistence differ and adapt them to different sizes of gaps (Baker 1942, Fowells 1965). The time of creation of a gap in relation to season, weather and propagule availability also affects the suc- cess of the potential gap fillers.

These features of patch dynamics en- sure that reproductive individuals can ex- ploit suitable and unpreempted patches within their dispersal ranges and life times. Because of such certainty in ecological time, the coexistence of species may depend on patch dynamics.

The maintenance of diversity (Grubb 1977, Connell 1978) is not the only important outcome of non-equilibrium coexistence. The predictable occurrence of patch dynamics over long times and large areas permits all species to evolve specialization for specific gap sizes and associated features just as they may evolve along any continuum of resources and regulating factors. This important point has not been emphasized in previous discussions of dis- turbanee. Fast-growing species which require high levels of resources for suecess are adapted to large gaps while slow-growing species which tolerate low levels of resources are adapted to small gaps.

The patch dynamic approach presents plant communities as largely non-equilibrium assemblages in which gap filling, based on evolved differences in reproductive strategy, as well as some degree of opportunism play major roles. The traditional linear concept of succession, with its emphasis on the attainment of an equilibrium community, fails to account for the probabilistic nature of species replacement (Horn 1974, Waggoner and Stephens 1970), the general importance of disturbance (Pickett 1976, Pickett and Thomp- son 1978), and the ability of all but ex- treme colonists to preempt and hold resources (Connell and Slatyer 1977). Patch dynamics and non-equilibrium coexistence take these often observed features of plant communities into account and offer a par- simonious, mechanistic view of the plant community.

The definition of scale is extremely important in framing the non-equilibrium viewpoint. It is the scale I have chosen to use that differentiates my argument from

that of Grubb (1977), which is mechanis- tically very similar. I define a community as an actually interacting group of plants which co-occur in space and time. It is thus a very localized and potentially competitive assemblage. Grubb (1977) follows a different approach to this problem. In a sense, he expands the boundaries of the plant community in time and space, such that responses to disturbance and gaps are a part of the plant niche. Grubb's "regeneration niche" may be considered to be evolved and partitioned in equilibrium over large areas (at least somewhat larger than the single, interactive community) and over long times. Other authors antici- pated this expanded scale of plant interaction and coexistence (e.g. Levin 1970, Horn and MacArthur 1972). Patch dynamics preserves the plant community as an intimate, interactive assemblage which is a temporally and spatially eircum- scribed unit, but also connects it with the larger time-space framework. A region can of course come to equilibrium on the proper scale (Jones 1945). Furthermore, coexistence based on division of resources which are permanently heterogeneous in space, would appear to be qualitatively different from coexistence based on periodic and ecologically unpredictable disturbance. Consequently, it seems useful to separate mechanisms of community organization which rely on the two processes. I hope this treatment will encourage further conceptual development, as well as the development of hypotheses to examine the alternative modes of plant community organization.

Conclusion. This paper discusses the hypothesis that, in general, coexistence of many plant species in a given area occurs, not because resources are effectively partitioned in equilibrium communities, but rather because attainment of competitive equilibrium at the site is prevented. Plants of diverse communities have not been shown, and may be unable, to partition resources effectively. However, because most perennial plant species generally live longer than the intervals between disturbances, they may coexist because of inter- ruptions of competitive assortment which would otherwise establish a stable, but depauperate equilibrium community. Dis- turbances prevent the few maximally com-




petitive species capable of high dominance in closed communities from controlling resources continuously. This allows the com- petitively inferior species, and those which require resource release, to attain transient dominance and thus persist in an area. Such species probably comprise the majority of forest floras. The unpredictability of disturbance in ecological time provides a mechanism for the maintenance of regional diversity. The predictability of disturbance over evolutionary time reduces seleetion pressure for resource partitioning in plants and allows for augmentation of diversity through specialization on different aspects of the disturbance regime. Evidence for non-equilibrium coexistence is found in forest and herbaceous communities from the tropics to the arctic.

The temporal and spatial pattern of disturbance and succession are called patch dynamics. This draws attention to the generality of disequilibrating processes in many systems at various scales. The ac- cumulating knowledge about the frequency and generality of disturbance, when coupled with information on succession, suggests a theory of coexistence that is sensitive to the nature of plants and the time scale of competitive assortment and disturbance in plant communities. Theories of animal coexistence have not addressed these problems. At present, non-equilibrium coexistence through patch dynamics appears to be a better explana- tion of plant community organization than does equilibrium niche theory.

Acknowledgments. I am grateful to John N. Thompson and Glenn R. Brown for countless stim- ulating discussions and insights into patch dynamics. Judy Parrish, Fakhri Bazzaz, Orie Loucks and Peter Vitousek freely discussed various aspects of plant community organization. Many of these persons have reviewed prior drafts of the paper. I am also grateful to Henry Horn, E. W. gtiles and Kevin Dobelbower for critical review. Input from participants in OTS 77-4 and the Rutgers Ecology Program has also been helpful.

Literature Cited ARMSTRONG, R. R. 1976. Fugitive species: Ex-

periments with fungi and some theoretical considerations. Ecology 57: 953-963.

AUCLAIR, A. N. and G. COTTAM. 1971. Dynamics of black cherry (Prunus serotina Erhr.) in southern Wisconsin forests. Ecol. Monogr. 41: 153-177.

- and F. G. GoFF. 1971. Diversity relations of upland forests in the western

Great Lakes area. Amer. Nat. 105: 499- 528.

BAKER, F. S. 1949. A revised tolerance table. Jour. For. 47: 179-181.

BARNEY, R. J. 1969. Interior Alaska wildfires. 1956-1965. Pacific Northwest Forest and Range Experiment Sta., USDA For. Serv. Inst. of Northern Forestry, Juneau, AK.

BEALS, E. W. aiid J. B. COPE. 1964. Vegetation and soils in eastern Indiana woods. Ecol- ogy45: 777-792.

BOTKIN, D. B. and M. J. SOBEL. 1975. Stability in time varying ecosystems. Amer. Nat. 109: 625-646.

BRATTON, S. P. 1976. Resource division in an understory herb community: Responses to temporal and microtopographic gradients. Amer. Nat. 110: 679-693.

BRAY, J. R. 1956. Gap phase replacement in maple-basswood forest. Ecology 37: 598- 600.

CAIN, S. A. 1935. Studies on virgin hardwood forest. III. Warren 's Woods, a beech- maple climax forest in Berrien Co., Michi- gan. Ecology 16: 500-513.

CASWELL, H. 1978. Predator-mediated coexistence: A non-equilibrium model. Amer. Nat. 112: 127-154.

CONNELL, J. H. 1971. On the role of natural enemies in preventing competitive exclusion in some marine animals and rain forest trees. Pages 298-312 in P. J. DenBoer and G. R. Gradwell, eds. Dynamics of Popula- tions. Center for Agricultural Publication and Documentation, Wageningen.

CONNELL, J. H. 1975. Some mechanisms producing structure in natural communities: A model and evidence from field experiments. Pages 460-490 in M. L. Cody and J. M. Diamond, eds. The Ecology and Evolution of Com- munities. Belknap Press of Harvard Univ. Press, Cambridge.

. 1978. Diversity in tropical rain forest and coral reefs. Science 199: 1302- 1310.

and R. 0. SLATYER. 1977. Mecha- nisms of succession in natural communities and their role in community stability and organization. Amer. Nat. 111: 1119-1144.

CURTIS, J. T. 1959. The Vegetation of Wisconsin. Univ. Wisconsin Press, Madison.

DAY, G. M. 1953. The Indian as an ecological factor in the northeastern forest. Ecology 34: 329-346.

DAYTON, P. K. 1971. Competition, disturbance, and community organization: The provi- sion and subsequent utilization of space in a rocky intertidal community. Ecol. Mongr. 41: 351-389.

. 1975. Experimental evaluation of ecological dominance in a rocky intertidal community. Ecol. Mongr. 45: 137-159.




DRURY, W. H. and I. C. T. NISBET. 1973. Succes- sion. Jour. Arn. Arb. 54: 331-368.

EVERS, R. A. 1955. Hill prairies of Illinois. Ill. Nat. Hist. Surv. Bull. 26: 367-446.

FLACCUS, E,. 1959. Revegetation of landslides in the White Mountains of New Hampshire. Ecology 40: 692-703.

FORCIER, L. K. 1975. Reproductive strategies and the co-occurrence of climax tree species. Science 189: 808-810.

Fox, J. F. 1977. Alternation and coexistence of tree species. Amer. Nat. 111: 69-89.

FOWELLS, H. A. 1965. Silvies of the Forest Trees of the United States. USDA Handbk. 271.

FRYE, R. J. 1978. Structural dynamies of early oldfield succession oln the New Jersey Pied- mont: A comparative approach. Ph.D. Thesis, Rutgers Univ., New Brunswick.

GARWOOD, N. C., D. P. JANOS, and N. BROKAW. 1979. Earthquake caused landslides: A major disturbance to tropical forests. Science 205: 997-999.

GOMEZ-POMPA, A. 1971. Posible papel de la vege- tacion secundaria en la evolucion de la flora tropical. B3iotropica 3: 125-135.

GOODLETT, J. C. 1954. Vegetation adjacent to the border of the Wisconsin drift in Potter County, Pennsylvania. Harv. For. Bull. 25.

GRIGAL, D. F. anid L. F. OHMANN. 1975. Classi- fication, description and dynamics of upland plant communities within a Minne- sota wilderness area. Ecol. Monogr. 45: 380-407.

GRUBB, P. J. 1977. The maintenance of species richness in plant communities: The importance of the regeneration niche. Biol. Rev. 52: 107-145.

GYsEL, L. W. 1951. Borders and openings of beech-maple woodlands in southern Michi- gan. Jour. For. 49: 13-19.

HACK, J. T. and J. C. GOODLETT. 1960. Geomor- phology and forest ecology of a mountain region in the central Appalachians. U.S. Geol. Surv. Prof. Pap. 347.

HAINES, D. A., V. J. JOHNSON, and W. A. MAIN. 1975. Wildfire atlas of the North central states. USDA For. Serv. Gen. Tech. Rep. NC-16.

HARCOMBE, P. A., and P. L. MARKS. 1977. Unider- story structure of a mesic forest in south- east Texas. Ecology 58: 1144-1151.

HARPER, J. L. 1965. Establishment, aggression and cohabitation in weedy species. Pages 243-265 in H. G. Baker and G. L. Steb- bins, eds. The Genetics of Colonizing Spe- cies. Academic Press, New York.

. 1968. The regulation of numbers and mass in plant populations. Pages 139- 158 in R. C. Lewontin, ed. Population Bi- ology and Evolution. Columbia Univ. Press, New York.

. 1969. The role predation in vegetational diversity. Brookhaven Symp. Biol. 22: 48-61.

HEINSELMAN, M. L. 1971. The natural role of fire, in northern conifer forests. Pages 61- 72 in Fire in the Northern Environment. Pacific Northwest Forest and Range Exp. Sta., USDA Forest Serv. Portland, OR.

. 1973. Fire in the virgin forests of the Boundary Waters Canoe Area, Min- nesota. Jour. Quart. Res. 3: 329-382.

HENRY, J. D. and J. M. A. SWAN. 1974. Recon- structing forest history from live and dead plant material-An approach to the study of forest succession in southwest New Hampshire. Ecology 55: 772-783.

HORN, H. S. 1974. The ecology of secondary succession. Ann. Rev. Ecol. Syst. 5: 25-37.

. 1975. Markovian properties of forest succession. Pages 196-211 in M. L. Cody and J. M. Diamond, eds. Ecology and Evolution of Communities. Belknap Press of Harvard Univ. Press., Cambridge.

. 1976. Succession. Pages 187-204 in R. M. May, ed. Theoretical Ecology- Principles and Applications. Saunders, Philadelphia.

and R. H. MACARTHUR. 1972. Com- petition among fugitive species in a harle- quin environment. Ecology 53: 749-752.

HUTCHINSON, G. E. 1951. Copepodology for the ornithologist. Ecology 32: 571-577.

. 1961. The paradox of the plank- ton. Amer. Nat. 95: 137-145.

JANZEN, D. H. 1967. Fire, vegetation structure and the ant x acacia interaction in Central America. Ecology 48: 26-35.

1970. Herbivores and the number of tree species in tropical forests. Amer. Nat. 104: 501-528.

JOHNSON, E. A. and J. S. ROWE. 1975. Fire in the subarctic wintering ground of the Beverley caribou herd. Amer. Midl. Nat. 94: 1-4.

JONES, E. W. 1945. The structure and reproduction of the virgin forests of the north temperate zone. New Phytol. 44: 130- 148.

. 1956. Ecological studies of the rain forest of southern Nigeria. IV. The plateau f orest of the Okumu Forest Re- serve. Part II. Jour. Ecol. 44: 83-117.

KALKSTEIN, L. S. 1976. Effects of climatic stress upon outbreaks of the southern pine beetle. Environ. Entom. 5: 653-658.

KEEVER, C. 1950. Causes of succession on old fields of the Piedmont, North Carolina. Ecol. Monogr. 20: 229-250.

KOMAREK, E. V., SR. 1968. Lightning and liglht- ning fires as ecological forces. Proc. Ann. Tall Timbers Fire Ecol. Conf. 8: 169-197.

KNIGHT, D. H. 1975. Analysis of late secondary succession in species-rich tropical f orest. Ecol. Studies 11: 53-59.




LAMBERT, J. D. H. 1976. Plant succession on an active tundra mud slump Garry Island, Mackenzie River delta, Northwest Terri- tories. Canad. Jour. Bot. 54: 1750-1758.

LANGENHEIM, J. H. 1956. Plant succession on a subalpine earthiflow in Colorado. Ecology 37: 301-317.

LEAK, W. B. 1972. Competitive exclusioin in forest trees. Nature 236: 461-463.

. 1973. Species and hardwood structure of a virgin northern hardwood stand in New Hampshire. Northeastern For. Exp. Sta. Res. Note NE-184.

LEIGH, E. G., JR. 1975. Structure and climate in tropical rainforest. Ann. Rev. Ecol. Syst. 6: 67-86.

LEOPOLD, L. B., M. G. WOLMAN, and P. J. MILLER. 1964. Fluvial Processes in Geomorphology. Freeman, Sani Francisco.

LEVIN, S. A. 1970. Community equilibria and stability and an extension of the competitive exclusion principle. Amer. Nat. 104: 413-423.

. 1974. Dispersion and population interactions. Amer. Nat. 108: 207-228.

. 1976. Population dynamic models in heterogeneous environments. Ann. Rev. Ecol. Syst. 7: 287-310.

and R. T. PAINE. 1974. Disturbance, patch formation, and community structure. Proc. Natl. Acad. Sci. USA 71: 2744-2747.

and . 1975. The role of disturbance in models of community structure. Pages 56-63 in S. A. Levin, ed. Ecosystem Analysis and Prediction. Soc. Industrial Appl. Math., Philadelphia.

LEVINS, R. and D. C. CULVER. 1971. Regional coexistence of species and competition between rare species. Proc. Natl. Acad. Sci. USA 68: 1246-1248.

LINDSAY, A. A. 1972. Tornado tracks in the presettlement forests of Indiana. Proc. In- diana Acad. Sci. 82: 181.

LONGMAN, K. A. and J. JENIK. 1974. Tropical Forest and its Environment. Longmans, London.

LORIMER, C. G. 1977. The presettlemeiit forest and natural disturbance cycle of northeastern Maine. Ecology 58: 139-148.

LouCKs, 0. L. 1970. Evolution of diversity, ef- ficiency, and community stability. Amer. Zool. 10: 17-25.

LUTZ, H. J. 1940. Disturbance of forest soil resulting from the uprooting of trees. Yale Sch. For. Bull. 45.

MARKS, P. L. 1974. The role of pin cherry (Prunus pensylvanica L.) in the maintenance of the stability in northern hardwood ecosystems. Ecol. Monogr. 44: 73-- 88.

MARQUIS, D. A. 1966. Soil moisture-soil temperature interrelationships on a sandy-loam

soil exposed to full sunlight. Northeastern For. Exp. Sta. Res. Note NE-64.

MAISSUROW, D. K. 1941. The role of fire in the perpetuation of virgin forests of northern Wisconsin. Jour. For. 32: 201-207.

MCCLURE, M. S. and P. W. PRICE. 1976. Ecotope characteristics of coexisting Erythroneura leafhoppers (Homoptera: Cicidellidae) on sycamore. Ecology 57: 928-940.

McDONOUGH, W. T. 1974. Revegetation of gopher mounds on aspen range in Utah. Gt. Basin Nat. 34: 267-275.

McNAUGHTON, S. J. and L. L. WOLF. 1970. Domi- nance and the niche in ecological systems. Science 167: 131-139.

MENGE, B. A. and J. P. SUTHERLAND. 1976. Spe- cies diversity gradients: Synthesis of the roles of predation, competition, and temporal heterogeniety. Amer. Nat. 110: 351- 369.

MICELI, J. C., G. L. ROLFE, D. R. PELZ, and J. M, EDGINGTON. 1977. Brownfield Woods Illi- nois: Woody vegetation and changes since 1960. Amer. Midl. Nat. 98: 469-476.

MORRIS, R. F. (ed.) 1963. The dynamics of epi- demic spruce budworm populations. Mem. Entom. Soc. Can. 31: 1-332.

NICHOLSON, S. A. and C. D. MONK. 1974. Plant species diversity in oldfield succession on tlhe Georgia Piedmont. Ecology 55: 1075- 1085.

O 'LOUGHLIN, C. L. 1972. A preliminary study of landslides in the coast mountains of south- westerli British Columbia. Pages 101-111 in H. 0. Slaymaker alid H. J. McPherson, eds., Mountain Geomorphology: Geomor- phical Processes in the Canadian Cordil- lera. British Columbia Geographical Series No. 14. Tantalus Research Ltd., Vancouver.

OLIVER, C. D. anid E. P. STEPHENS. 1977. Re- construction of mixed-species forest in central New Englalnd. Ecology 58: 562-572.

PAINE, R. T. 1966. Food web complexity and species diversity. Amer. Nat. 100: 65-75.

. 1974. Intertidal community structure. Oceologia 15: 93-120.

PARRISH, J. A. D. anid F. A. BAZZAZ. 1976. Un- derground niche separation in successional plants. Ecology 57: 1281-1288.

PICKETT, S. T. A. 1976. Succession: An evolutionary interpretation. Amer. Nat. 110:107- 119.

and J. N. THOMPSON. 1978. Patch dynamics and the design of nature re- serves. Biol. Conserv. 13: 27-37.

PLATT, W. J. 1975. The coloinization and formation of equilibrium plant species associa- tions on badger disturbances in a tall grass prairie. Ecol. Monogr. 45: 285-305.

and M. I. WEIS. 1977. Resources partitioning and competition within a guild of fugitive prairie plants. Amer. Nat. 111: 479-513.




PUTWAIN, P. D. anid J. L. HARPER. 1970. Studies in the dynamics of plant populations. III. The influence of associated populations on populations of Rumex acetosa L. and B. acetosella L. in grassland. Jour. Ecol. 58: 251-284.

RAUP, H. M. 1957. Vegetational adjustment to the instability of the site. Proc. Pap. 6th Tech. Meet. Intl. Union Conserv. Nature and Natural Resources. pp. 36-48.

RAYNAL, D. J. and F. A. BAZZAZ. 1975. Interfer- ence of winter annuals with Ambrosia ar- temisiifolia in early successional fields. Ecology 56: 35-49.

RICHARDS, P. anid G. B. WILLIAMSON. 1975. Tree- falls and patterns of understory species in wet lowland tropical forest. Ecology 56: 1226-1229.

RICHARDS, P. W. 1955. The secondary succession in the tropical rainforest. Sci. Progr. 43: 45-57.

Rossi B. A., J. R. TESTER, and W. J. BRECKEN- RIDGE. 1968. Ecology of mima-type mounds in northwestern Minnesota. Ecology 49: 172-177.

SALE, P. F. 1977. Maintenance of high diversity in coral reef fish communities. Amer. Nat. 111: 337-359.

SCHOENER, T. W. 1974. Resource partitioning in ecological communities. Science 185: 27- 38.

SHArI, M. I. and G. A. YARRANTON. 1973. Di- versity, floristic richness and species even- ness during a secondary (post-fire) succession. Ecology 54: 897-902.

SICCAMA, T. G., G. WEIR, and K. WALLACE. 1976. Ice damage in a mixed hardwood forest in Connecticut in relation to Vitis infestation. Bull. Torrey Bot. Club 103: 180-183.

SKEEN, J. N. and W. J. March. 1977. The effect of sunimer thunderstorms oIn the near- ground temperature regimen within a sub- urban forest. Jour. Tenn. Acad. Sci. 52: 95-99.

SKELLAM, J. G. 1951. Random dispersal in thleo- retical populations. Biometrica 38: 196- 218.

SLATKIN, M. 1974. Competition and regional coexistence. Ecology 55: 128-134.

STRONG, D. R. 1977. Epiphyte loads, tree falls and perennial disruption: A mechanism for maintaining higher tree species richness in the tropics without animals. Jour. Bio- geogr. 4: 215-218.

TANDE, G. F. 1979. Fire history and vegetation pattern of coniferous forests in Jasper National Park, Alberta. Canad. Jour. Bot. 57: 1912-1931.

THOMPSON, D. Q. and R. H. SMITH. 1970. The forest primeval in the northeast-A great myth? Proc. Ann. Tall Timbers Fire Ecol. Conf. 10: 255-265.

THOMPSON, J. N. 1977. Patch dyiiamics in a bi-

ennial herb and resource availability to a specialized herbivore. Bull. Ecol. Soc. Amer. 58(2): 9.

. 1978. Within-patch structure and dynamics in Pastinaca sativa and resource availability to a specialized herbivore. Ecology 59: 443-448.

VAUGHN, R. E. and P. 0. WIEHE. 1937. Studies on the vegetation of Mauritius I. A preliminary survey of the plant communities. Jour. Ecol. 25: 301-343.

VEBLEN, T. T., and D. H. ASHTON. 1978. Catas- trophic influences on the vegetation of the Valdivian Andes, Chile. Yegetatio 36: 149-167.

VITOUSEK, P. M. 1977. The regulation of element concentrations in mountain streams in the northeastern United States. Ecol. Monogr. 47: 65-87.

VOGL, R. J. 1970. Fire and plant successloil. Pages 65-75 in The Role of Fire in the Intermountain West. Intermountain Fire Research Council, Missoula.

WAGGONER, P. E. anid G. R. STEPHENS. 197(0. Transitioin probabilities for a forest. Na- ture 225: 1160-61.

WATT, A. S. 1947. Pattern and process in the plant community. Jour. Ecol. 35: 1-22.

. 1964. The commuinity aind the individual. Jour. Ecol. (suppl.) 52: 203- 211.

WEBB, L. J. 1958. Cyclones as an ecological factor in tropical lowland rain forest, North Queensland. Austr. Jour. Bot. 6: 220- 228.

, J. G. TRACEY, and W. T. WIL- LIAMS. 1972. Regeneration and pattern in the subtropical rain forest. Jour. Ecol. 60: 675-695.

WEIN, R. W. 1976. Frequency and characteristics of arctic tundra fires. Arctic 26: 213- 222.

and J. M. MOORE. 1977. Fire history and rotations in the Acadian Forest of New Brunswick. Canad. Jour. For. Res. 7: 285-294.

WELLS, P. V. 1976. A climax of inidex for broad- leaf forest: Ail n-dimensional ecomorpho- logical model of succession. pp. 131-76 in J. S. Fralish, G. T. Weaver, and R. C. Schlesinger, eds. Central Hardwood Forest Conference.

WHITMORE, T. C. 1975. Tropical Rainforests of the Far East. Clarendon Press. Oxford.

WHITTAKER, R. H. 1969. Evolutioni of diversity in plant communities. Brookhaven Symp. Biol. 22: 178-195.

. 1975. Communities and Ecosystems. MacMillan, N.Y.

, S. A. LEVIN, aInd R. B. ROOT. 1973. Niche, habitat and ecotope. Amer. Nat. 107: 321-338.




WIT, C. T. DE. 1960. On competition. Versl. Landbouwk. Onderz. 66: 1-82.

WILLIAMSON, G. B. 1975. Pattern and seral composition in ain old-growth beech-maple forest. Ecology 56: 727-731.

WILLSON, M. F. 1973. Evolutionary ecology of plants: A review: Part IV. Competition and the niche in plants. The Biologist 53: 74-82.

WILSON, E. 0. 1969. The species equilibrium. Brookhaven Symp. Biol. 22:38-47.

WOODIN, S. A. aiid J. A. YORKE. 1975. Disturb-

ance, fluctuating rates of resource recruit- meiit, and increased diversity. Pages 38-41 in S. A. Levin, ed. Ecosystem Analysis and Prediction. Soc. Industrial Appl. Math., Philadelphia.

WRIGHT, H. E. 1974. Landscape development, forest fires, and wilderness management. Science 186: 487-495.

YEATON, R. I., J. TRAVIS, and E. GILINSKY. 1977. Competition and spacing in plant communities. The Arizona Upland Association. Jour. Ecol. 65: 587-595.



non-equilibrium coexistence of plants

Documents