Plant Community Processes

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Plant Community Processes

Ecosystems are formed from a mingling of nonliving abiotic components and the biotic community, which is composed of assemblages of living organisms. Many individuals in the biotic community are capable of capturing energy from sunlight through photosynthesis and, as a subset, form the plant community. The most prominent plants in the landscape are those with xylem and phloem forming vascular systems. While they are often the focus of plant community descriptions, green algae, mosses, and less-conspicuous plants also play a functional role in this ecosystem component. Heterotrophic organisms (including animals, bacteria, and fungi) feed on plants and form other subsets of the biotic community. These organisms are frequently examined along with plants in contemporary community studies. Understanding plant-plant, plant-animal, and animal-animal interactions has become a highly productive, community-level research area.

Community Concept

It is possible to use the term plant community in two different but intertwined ways. Frequently it refers to a description of what is growing at a specific location in the landscape, such as the plant community making up the woods behind your house or the vegetation in a marshy area beside a pond. These communities are real and you can walk out into them and touch the trees or pick the flowers. Foresters refer to these real communities as stands and the term can be extended to all types of vegetation. The other reference to a plant community is more abstract. The term can be used to describe the properties of a particular assemblage of plants that appears repeatedly in many different places. People living in the eastern United States immediately draw up a picture in their mind when the phrase "oak forest" is mentioned, while those in the Midwest and Southwest know what someone is referring to when the phrases "tall prairie grassland" or "hot desert" are used respectively. Ecologists expand this familiarity into lists of probable plants and predictable appearances such the shapes of the trees, leaf types, and height of the vegetation in various types of communities. They cannot predict exactly what will be in a stand at a specific location, but they can statistically describe what would most likely be found. These descriptions expand the understanding of the meaning of the abstract community so that scientists know what someone is describing, even if they have never been there themselves.

Community types are characteristically found in geographic locations with similar climate patterns and habitat characteristics. These large-scale segments of the terrestrial landscape are referred to as biomes. The description of the overall climatic conditions, appearance, and composition of the biomes is studied in the field of biogeography . Many different types of plant communities exist within each biome since there are many combinations of variation in the slope, moisture availability, soil type, exposure, elevation, and other habitat characteristics within these biogeographic regions.

Plant communities of different types form carpets of vegetation that cover smaller segments of geographic regions, such as the drainage basin of a river or the hillsides of a mountain. This patchwork of communities, and the corridors that connect them, is referred to as the landscape mosaic. This level of organization is intermediate in scale between the biome and individual communities. Landscape elements are not only interconnected spatially, but also by functional interactions. There are properties of the landscape, which emerge from these interdependencies, that cannot be predicted from community-level studies alone, as described by Richard Forman (1995).

Study of Communities

Plant ecologists over the years have developed many different techniques for gathering both descriptive and quantitative data from real stands, which can be used to characterize the abstract community types. Terrestrial Plant Ecology (1998), edited by Michael Barbour, includes an introduction to plant community sampling methodology and data analysis. John Kricher (1988 and 1993) uses a field guide approach to the understanding of the natural history of plant communities. Chapters covering community structure and function can be found in the references by Timothy Allen (1998), Manuel Molles (1999), and Robert Leo Smith and Thomas M. Smith (1998). The American roots of this discipline can be traced in The Study of Plant Communities (1956) by Henry J. Oosting, Plant Communities: A Textbook of Plant Synecology (1968) by Rexford Daubenmire, and Plant Ecology (1938) by John Weaver and Frederic E. Clements.

Plant community ecology can be traced back to the nineteenth century, when the Prussian biogeographer Fredrich Heinrich Alexander von Hum-bolt began to view vegetation as associations of plants and Johannes Eugenius Warming described various characteristics of different community types. Many other Europeans followed this line of research, notably Josias Braun-Blanquet, a central figure at the beginning of the twentieth century in what became known as the Zurich-Montpellier School of Phytosociology, where synecology (another name for community ecology) flourished.

Community Organization

The American ecologist Frederic Clements extended the community concept to the point where obligatory plant community composition and the resulting functional interactions were thought of as unique superorganisms, with individual species being as essential to their identity as the organs are to an animal. This idea prevailed from the 1920s until after the middle of the century, when Robert Whittaker carried out several studies in mountainous regions of the United States. He clearly demonstrated that a wide variety of intermediate community compositions existed in these complex environments and that those communities functioned perfectly well. What appeared to be a superorganism, with obligatory development patterns and species composition, just happened to exist over wide areas with similar habitat conditions. This was not a completely original idea. In 1926 Henry Gleason proposed that the appearance of obligatory groupings resulted from the success of individual species having similar environmental needs occurring together by chance. This was only shortly after the super-organism concept gained its foothold on scientific thought. However, until the evidence from Whittaker's methodical study was available to support Gleason's idea, many held that interactions between individuals produced community evolution similar to that proposed for species.

The community is now seen as a many-dimensioned gradient of possible combinations of plant species. Readily identifiable community types exist because certain groupings that are successful under conditions occurring repeatedly in the landscape are more likely to be encountered than others.

Succession in Communities

One aspect of community organization accepted by ecologists is that the plants, animals, and microorganisms are very interconnected in function. Trees shade the forest floor and make it cooler than adjacent fields. Leaves from those trees decompose when they fall and provide nutrients for a variety of plants through their roots, which they may even reabsorb themselves. The same leaves could provide food for browsing animals while on the tree or for decomposing organisms as part of the litter on the forest floor. Fires, floods, volcanic eruptions, or human activities such as farming and forestry disrupt these interactions, but are not as disastrous to the long-term survival of the natural community as they might first appear, particularly if they do not occur with great frequency. This is because communities have self-repairing capabilities through the process of directional succession.

If the disruption to the community is limited primarily to the biological matter above the ground and at least some of the soil remains intact, as is the case with an abandoned agricultural field, pasture, or recently burnt forest, the process is called secondary succession. This is a replacement process that is facilitated by a variety of mechanisms for the replacement of vegetation. In many cases, seeds are already present in the soil as part of a seed bank; sometimes wind or animals transport them in. Often, if the disruption has not been too severe, or if the regrowth is due to a change in land use, some vegetation, including weeds, will already be growing. It will become the basis for the early stages of successional development. In other cases, such as lumbering, where the tree trunks have been harvested, or where the aboveground parts were killed by certain types of fire, branches will sprout up from living roots. This produces what is called coppice growth , and one or more stems will produce a new tree trunk. Because of this process, the age of forest trees determined by counting rings in trunk wood may be a gross underestimate of the actual age of the organism as defined by the root tissue. Frequently more than one of these mechanisms will play a role in reestablishing plants in a disturbed area.

However, if there is no soil left at all, as is the case following a rock-slide, the retreat of a glacier, or the development of vegetation on lava deposited from volcanic flows, then the process takes much longer. This is because at least some soil development is required before plants can become established in this process of primary succession. This sequential replacement on dry habitat sites is called xerarch succession, but can also occur when previously aquatic sites fill in through sedimentation resulting in the production of terrestrial communities called hydrarch succession. Changes under intermediate soil moisture conditions, including those for most secondary succession, occur in mesarch environments.

Different functional models exist to explain how succession proceeds. One model proposes that early species alter the environmental conditions and facilitate, or prepare the way, for species that occur in later stages. The second model suggests that some species become established early on in the process and inhibit the successful invasion by others. The third model does not involve facilitation or inhibition, but essentially holds that species that can tolerate the conditions that exist are successful in becoming established. Most likely all three processes can occur depending on conditions and timing.

Self-generating or autogenic succession leads to changes in community structure and ecosystem function. In the late 1960s, Eugene Odum described this as an overall strategy for ecosystem development. Even though general patterns of change appear to emerge, exceptions sometimes occur. There are, however, tendencies toward increases in biodiversity as succession progresses with slight declines as systems mature. Similarly, complexity and structure increase as succession proceeds, and increased proportional amounts of energy flow are needed to support increasing living community biomass ; there can be a tightening of nutrient cycling as the systems age. Thirty years later, Odum (1997) updated his thoughts in light of extensive research stimulated by the original model. In addition to systemic changes such as these, there are also plant life cycle strategies such as high seed number production, aggressive seed dispersal, high sunlight preferences, and rapid growth amongst invasive species that appear early in succession. These are in contrast with the shade-tolerant, slower-growing, longer-lived species that play a larger role as the system matures. Fundamentally, as succession progresses, the organisms change the environment and in turn, the environment alters the relative success of individuals within the communities.

Competition Within Communities

Because plant communities are composed of organisms with similar overall climatic requirements, and because resources such as nutrients, light, and water are present in finite amounts, there is a continuing interaction between individuals that determines their success in capturing and utilizing these resources. This interaction takes various forms and is referred to as competition. Competition is one of several different types of individual interactions that plants can be involved in and includes forms of exploitation, such as seed predation , herbivory, and parasitism; cooperation, such as mutualism , which may or may not be obligatory; and other specialized relationships. When the individuals are of the same type, the competition is said to be intraspecific, and when they are different, the interaction is interspecific. The term symbiosis is used to describe interspecific interactions involving close and continual physical contact and may be either deleterious, as in parasitism, or highly beneficial, as in the case of obligate mutualism.

Competition is somewhat unique in comparison to most other relationships, where at least one of the interacting individuals benefits from the interaction when it occurs. When competition is occurring, both partners to the interaction are most likely adversely affected. The most intense competition occurs between individuals with very similar needs. Consequently, intraspecific competition generally has a greater impact on the success of a particular plant in the community than interspecific competition. However, if most individuals of one species are more successful than most of another, then there will be more of them present. Since they lack the social organization of animals, complex coordinated group competition is unlikely to be an important aspect of competition in plants; in the case of plant competition, the interaction between individuals is more likely to be significant.

Competitive Exclusion and the Ecological Niche

The result of the interaction can affect the relative success of populations of a species and ultimately the community composition. In the 1930s, the Russian microbiologist G. F. Gause performed laboratory experiments that led to the conclusion that when populations of two different species are directly competing for a common resource in a limited environment, only one will ultimately be sustained in that space. The other will die out. This idea has come to be known as Gause's competitive exclusion principle. If this principle were to be valid in natural environments, then the number of surviving species would be greatly limited. However, this is not the case, particularly in complex plant communities.

The solution to this perplexing puzzle has been found in a process known as resource partitioning. Even some very obvious situations—where resource demand overlap between individuals clearly exists—demonstrate subtle differences in the way that the resource is exploited when examined in detail. Roots of one individual or species may penetrate to slightly different depths in the soil from another, or flowering times might be a few days different, thereby reducing competition for the services of a particular pollinator.

The entire complex of resource, habitat, physical, and other requirements that define the role of an organism within its community is called its ecological niche. The more similar the niches of two individuals or species, the greater the niche overlap. The greater the niche overlap, the greater the competition. The species composition of a community is a result of the way that individuals of different species with different niches can be packed together. Increases in the number of species within a community are accomplished by specialization, the reduction of the sizes of the niches, and efficient packing to reduce overlap. The number of niches that exist in a community is directly related to species diversity, the number of different types of organisms that can be supported.

Competitive Interactions

The plant community is a dynamic, competitive environment. Community composition exists in steady state, a status of apparent equilibrium, for varying periods of time. A pulse disturbance such as a fire, or more chronic stresses such as disease, evolutionary change, or global warming may alter the status quo. Increased global mobility of plant seeds and fragments of tissue, as well as various pathogens and disease vectors such as insects, have increased the chances of incursion by invasive species or the introduction of new competitors, which may lead to significant alterations in community composition. Because of this flexibility and inherent resilience, communities persist over time, even though the presence of specific organisms varies.

The intense defense of resources and the aggressive forays to acquire the essentials for survival by plants are not necessarily obvious. Competition between animals can be physical combat, and plants analogously can physically grow into the space occupied by another individual and crowd it out. However, the adaptations that make plants successful as competitors are generally more indirect. Examples of this include plants with more vigorous canopy growth that intercept the available light, or the individual with the healthier and more extensive root system that is more efficient at obtaining nutrients from the soil. Sometimes, just being able to grow faster is sufficient to give a competitive advantage. Many species have evolved to produce toxins that inhibit the growth of other plants, a condition known as allelopathy, and this can give them a competitive edge, particularly in the case of interspecific competition where self-inhibition is limited.

see also Allelopathy; Biogeography; Biome; Clements, Frederic; Interactions, Plant-Plant; Odum, Eugene; Symbiosis.

W. Dean Cocking