Plant Community Dynamics

Plant communities are assemblages of species studied extensively by ecologists. The organization, development, and repeatability of plant communities drew considerable discussion and controversy among plant ecologists during the twentieth century. Disagreements regarding plant communities resulted from attempts to define the composition, stability, and boundaries of communities. Recognition of the role of disturbance to plant communities led to the current views. The modern synthesis of the plant community is an assemblage of species' populations aggregated in a region resulting from dispersal, tolerances to local site characteristics, and response to disturbance. The phenomena of succession and climax and the disturbances that initiate succession are integral to understanding plant community dynamics.

Plant succession is described as the change in composition of vegetation in one place over time. Succession is viewed as one of two related processes: primary succession or secondary succession. Primary succession is the development of vegetation on substrate previously lacking plants (Figure 2). Examples include the development of vegetation on sand dunes, glacially exposed substrate, filling in of lakes, etc. The earliest ecological descriptions of primary succession were by H. C. Cowles on the sand dunes of Lake Michigan and by W. S. Cooper on the development of vegetation following retreat of glaciers in Glacier Bay, Alaska. In both, the development of vegetation was viewed as a chronosequence characterized by the predominance of conspicuous plants. In both, early invaders were conspicuous only to be replaced in time by species that were inconspicuous at the onset or arrived at a later time. An example of such a chronosequence at Glacier Bay is the presence of avens (Dryas drummondii), followed by alder (Alnus tenuifolia), and subsequently the abundance of spruce (Picea sitchensis). The process was estimated to take several hundred years.

Secondary succession is the development of vegetation following a disturbance to the original plant community. Disturbances leading to secondary succession include fire, windstorms, and anthropogenic disturbances such as logging and farming (old field succession). These successions occur on substrate that is wholly or partially intact, and consequently occur more rapidly. The pattern of secondary succession is greatly dependent upon the type of the disturbance. For example, in a study of vegetational development at one location over several hundred years, the outcome was dependent upon whether the disturbance initiating the succession was fire or windstorm.

Figure 2 Primary succession of forest development following retreat of the Mendenhall Glacier, Alaska, USA. (a). Initial stages of vegetation on newly exposed glacial till in the foreground. The site was exposed within the last 20 years, and the vegetation consists of willows (Salix sp.), fireweed (Epilobium sp.), lupines (Lupinus sp.), alders (Alnus sp.), and scattered Sitka spruces (Picea sitchensis) and western hemlocks (Tsuga heterophyla). (b). Forest development on glacial till adjacent to Mendenhall Lake. Ice occupied these sites in the mid-1700s and is currently retreating at a rate of approximately 40 m per year. The vegetation consists largely of alders, Sitka spruces, and western hemlocks.

Figure 2 Primary succession of forest development following retreat of the Mendenhall Glacier, Alaska, USA. (a). Initial stages of vegetation on newly exposed glacial till in the foreground. The site was exposed within the last 20 years, and the vegetation consists of willows (Salix sp.), fireweed (Epilobium sp.), lupines (Lupinus sp.), alders (Alnus sp.), and scattered Sitka spruces (Picea sitchensis) and western hemlocks (Tsuga heterophyla). (b). Forest development on glacial till adjacent to Mendenhall Lake. Ice occupied these sites in the mid-1700s and is currently retreating at a rate of approximately 40 m per year. The vegetation consists largely of alders, Sitka spruces, and western hemlocks.

In a classic study of old field succession, C. Keever found that the time of year the stand was released from farming affected the sequence of development of early plants. From this arose a body of evidence supporting the view that species' characteristics such as seed dispersal, photosynthetic light requirements, growth rate, and longevity control much of succession. Accordingly, early-successional plants are often rapidly dispersed, require high light, have high photosynthetic rates and growth rates, but are relatively short lived. These plants give way in time to species with slower dispersal rates, greater shade tolerance, slower growth rates, but are long lived. This concept of tradeoffs of suites of traits between early-and late-successional species frames much of the current discussion of the mechanisms of succession.

Climax was described historically as the endpoint of succession and was envisioned as a self-sustaining community. In the current view climax represents oscillation of composition of community organization around a midpoint. The difficulty of describing climax is one of scale. Vegetational change is observed differently as the size of the unit of measurement increases from a single stand of a few individuals to regional scales incorporating hundreds of stands. Small-scale disturbance of the order of loss of single individuals may cause a microsuccesional sequence, if that individual is predictably replaced by another species with other ecological attributes. This may create a patchwork of communities, which on one scale appear to be in different stages of succession while on a larger scale appear to represent a dynamic mosaic of regional vegetation.

Disturbance is characteristic of most plant communities. Natural disturbance includes fire, wind damage, grazing, insect damage, frost heaving, flooding, disease, and other causes. The spatial extent of disturbance may differ from an individual to large regional effects. Disturbance has two attributes affecting vegetation: frequency (or return interval) and intensity. Fire is a disturbance that illustrates these attributes (Figure 3).

Fire has an important influence on community structure in most semiarid regions including biomes such as grasslands, savanna, and Mediterranean scrub. Fire also is suggested to be important in boreal forests, coniferous forests, and even deciduous forests. Fires may reoccur at frequencies of 1-5 years in grasslands, 25-100 years in Mediterranean scrub communities and eucalypt forests, and 100-500 years in coniferous and deciduous forest communities. Each of these frequencies results in different modifications to the species and communities that persist in the region. In the absence of frequent fires communities may change in composition and structure. For example, as a result of anthropogenic decreases in fire frequencies, the vegetation of the prairie peninsula of midwestern USA was converted from grasslands to deciduous forest.

The intensity of fire is gauged by its destruction. Fires may be classified as high, moderate, or low intensity. Highly destructive fires kill all the aboveground living matter, and only seeds, rootstocks, and rhizomes remain to repopulate the community. Such is the case in grassland fires, Mediterranean scrub, and some coniferous fires. Moderately destructive fires kill the photosynthetic structures and some meristematic tissues. Standing vegetation may resprout and produce new photosynthetic structures. An example of this would be the Pine Barrens of New Jersey, USA, where pitch pine (Pinus rigida) produces new branches from epicormic buds beneath the bark. Finally, low-intensity fires affect the ground layer of vegetation without a dramatic effect on the canopy. The effect of this

Figure 3 Unburned and burned chaparral in Santa Barbara County, CA, USA. Both photographs were taken of the same area but separated by 32 years. (a) Typical unburned chaparral in 2003 consisting of chamise (Adenostoma fasciculatum), California lilacs (Ceanothus spp.), and toyon (Heteromeles arbutifolia). (b) Burned chaparral following an intense fire in 1971. The utility pole serves as a reference point. The photographs indicate the rapid recovery of the vegetation following fire.

Figure 3 Unburned and burned chaparral in Santa Barbara County, CA, USA. Both photographs were taken of the same area but separated by 32 years. (a) Typical unburned chaparral in 2003 consisting of chamise (Adenostoma fasciculatum), California lilacs (Ceanothus spp.), and toyon (Heteromeles arbutifolia). (b) Burned chaparral following an intense fire in 1971. The utility pole serves as a reference point. The photographs indicate the rapid recovery of the vegetation following fire.

type of fire is to control the composition and reproduction of the community. Examples of these communities include the ponderosa pine forests of western North America and the pine-wire grass communities of the southeastern coastal plane of North America. In both cases, frequent fires maintain the dominant overstory trees (Pinusponderosa or P. palustris, respectively) by reducing the successful establishment of broad-leaved competitors.

Each type ofdisturbance produces different ecological and evolutionary constraints on the vegetation. Frequent, low-intensity disturbance results in a distinctively different vegetation than infrequent, high-intensity disturbance. Each of these vegetations will have a different response to the disturbance. With frequent, low-intensity disturbance there may be little compositional change following the disturbance. The plants of this vegetation have adaptations that allow rapid recovery following the disturbance. Conversely, with infrequent, high-intensity disturbance a successional sequence may occur because the adaptations of initial invaders that favor response to disturbance may not convey adaptive advantage in competition over time.

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