This article provides an overview of types of plant communities and the process of succession in those communities.
In each type of habitat, certain species group together as a community. Fossil records indicate that some of these groups (or very closely related precursors) have lived together for thousands or even millions of years. During that time, it is possible that an intricate balance has been fashioned. Community members share incoming solar radiation, soil water, and nutrients to produce a constant biomass; they recycle nutrients from the soil to living tissue and back again; and they alternate with each other in time and space. Synecologists attempt to determine what is involved in this balance between all the species of a community and their environment [Barbour 1987: 155].
Community concepts and attributes
A plant community is an identifiable stand of plants growing together in a certain spot. Clusters of species, called associations, are often found growing together in several different places within a larger region. “An association is a particular type of community, which has been described sufficiently and repeatedly in several locations such that we can conclude that it has: (a) a relatively consistent floristic composition, (b) a uniform physiognomy [appearance], and (c) a distribution that is characteristic of a particular habitat” [Barbour 1987: 156].
There are opposing views about why particular plant species are often found growing together in a plant community. The continuum view posits that species distribution is driven individualistically by each species’ particular tolerance to various environmental conditions. By contrast, the association view suggests that a plant community is an integrated whole, whose component species are interdependent.
Whatever the reasons that particular species tend to grow together in stands, however, such stands “exhibit collective or emergent attributes beyond those of the individual populations” [Barbour 1987: 159]. Examples of such community attributes include its vertical structure, canopy cover, species composition and diversity, biomass, productivity, stability, and nutrient cycling, for example.
Succession
Ecological succession is an important concept that helps explain the particular assemblage of plants growing in a given location.
“Plant succession is a directional, cumulative change in the species that occupy a given area through time” [Barbour 1987: 230]. This does not refer to cyclical changes that occur over seasons, nor to changes occurring in response to climate shifts over extremely long time spans like thousands or millions of years. Rather, succession is when the composition of plants at a particular site changes over a period of decades to centuries.
Succession begins when pioneer species colonize bare ground. These first arrivals tend to be opportunists that grow fast, reproduce quickly, and do not live long. The early successional plants start to improve the habitat conditions for other, more competitive plants to then take over, displacing the pioneers. “One of the driving forces behind succession is the effect plants may have on their habitat. Plants cast shade, add to the litter, dampen temperature oscillations, and increase the humidity, and their roots change the soil structure and chemistry. … Both the environment and the community change, and this metamorphosis is due to the activities of the organisms themselves.” [Barbour 1987: 233]
Overtime, slower-growing, larger, longer-living plant species outcompete the earlier successional species, eventually forming a climax community, which is not subsequently replaced by any other community. “Succession often leads to communities with greater and greater complexity and biomass and to habitats that are progressively more and more mesic (moist)” [Barbour 1987: 233]. Such changes result in climax communities tending to be self-sustaining due to efficient nutrient cycling and internal moderation of external fluctuations in temperature and humidity.
The particular plant composition of a climax community depends on the regional climate, as well as local soil conditions and topography, meaning that several climax communities can exist in a given landscape.
Typically, many plant communities coexist in a complex mosaic pattern. That is, one climax community does not cover an entire region. … In [some] cases, the mosaic reflects topographic differences, such as south-facing versus north-facing slopes, basins with poor drainage and fine-textured soil versus upland slopes with good drainage and coarser soil, or different distances from a stress such as salt spray. In such cases, the communities within the mosaic do not bear a successional relationship to one another; they constitute a toposequence. Each community in a toposequence may, in fact, be a climax community [Barbour 1987: 238].
Understanding ecological succession can help us to predict the future vegetation of a site by observing its current vegetation. “It is often possible to estimate a community’s future composition by extrapolation from changes measured in a short time, by comparing other communities that have plants of different ages, or by noting differences between overstory plants and understory seedlings” [Barbour 1987: 231] In some cases, the understory seedlings will later become the canopy, provided the localized conditions support this succession.
Barbour, Michael G., Jack H. Burk, & Wanna D. Pitts, 1987, Terrestrial Plant Ecology, Menlo Park: The Benjamin/Cummings Publishing Company, Inc.