Plant succession
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Plant succession
Ecology TheoryIntroduction
Figure 1. Purple colored Fireweed, Epilobium angustifolium) is a common pioneer plant species that quickly colonizes temperate forest habitats that have been disturbed by fire or logging. Source: Wikimedia
Plant succession is a directional non-seasonal cumulative change in the taxa of flora species that occupy a given area through time. This process takes place on a time scale longer than a single growing season. Succession involves the processes of colonization, establishment, and extinction that act on the participating plant species. Most successions contain a number of stages that can be recognized by the collection of species that dominate at that point of time in the succession. Most successions begin when an area is made partially or completely devoid of vegetation because of a disturbance. Shortly after the disturbance, the first wave of colonizing plants, or pioneer community, establish themselves (Figure 1). The pioneer community usually consists of various short-lived, small, but fast growing plant species. This event is then followed by successive stages of plant re-growth on the disturbed site, with a general trend of species becoming long-lived, larger, and slower growing. From a hypothetical standpoint succession stops when species composition changes no longer occur with time. This stage is known as the climax community
The concept of a climax community assumes that the plants colonizing and establishing themselves in a given region can achieve stable equilibrium over time. The idea that succession ends in the development of a climax community has had a long history in the fields of biogeography and ecology. One of the earliest proponents of this theory was Frederic Clements who studied succession at the beginning of the 20th century. Clements posited that most plant communities undergoing succession reach the climax state. Beginning in the 1920s scientists began questioning the notion of a climax state after mounting field evidence. By 1950, many scientists began viewing succession as a process that rarely attains a stable equilibrium or climax community. The reason why equilibrium is rarely reached is related to the nature of disturbance. Clements’ theory of succession was based on the assumption that plant communities were rarely disturbed. (Clements, 1916) Yet, numerous studies of natural vegetation that occurred after he published his idea indicated that disturbance was instead a frequent phenomenon in most plant communities. Because of its high frequency, most plant communities rarely reach a stage in their development where species composition reaches a steady-state climax community. Any intermediate stage plant association that is part of an overall succession is known as a seral community. (Clewell and Aronson. 2007) Studies of succession also revealed that disturbance acts on plant communities at a variety of spatial and temporal scales. Further, the effect of disturbance is not always 100 percent; most disturbance events remove only a part of the previous plant community. As a result of these new ideas, plant communities are now generally seen as being composed of numerous patches of various sizes at different levels of successional development. A particular patch in a larger community may undergo repeated disturbance events that do not allow it to proceed past the pioneer stage. However, the general characteristic of the larger plant community around the patch may be late successional.
Types of plant succession
Figure 4 : Plant life recovering on a forest site that was burned one year earlier. Most of the establishing plants came from seeds stored in the soil that were not destroyed by the fire., Copyright: Michael Pidwirny
Figure 5. Red Alder (Alnus rubra) has the ability to symbiotically fix nitrogen with the help of certain species of actinomycetes. This relationship causes an increase in the nitrogen content of the soil with time. Copyright: Martin Krzywinski
Figure 6. Shade tolerant Douglas Fir (Pseudotsuga spp.) seedlings are most successful in establishing themselves when the nitrogen concentration in the soil becomes plentiful. Once established, the Douglas Fir trees grow until they tower over and shade the Red Alder. This shading reduces the amount of light available for photosynthesis and eventually causes the Red Alder to die out. Copyright: Michael Pidwirny
At the most basic level we can classify plant successions as being of two types: primary succession and secondary succession. Primary succession occurs when plants establish on land that was not previously vegetated or in habitats where disturbance has killed all of the seeds stored in the soil surface layers. Examples of habitats undergoing primary successions include the vegetation that develops on volcanic deposits, rock outcrops, sand dunes, newly created river sandbars, new oceanic islands, and exposed ground surfaces de-vegetated by glaciers. Secondary succession involves the invasion of a site by plants on ground that was previously vegetated (Figure 4). The establishing plants often come from seeds stored in the site’s soil. Removal of past vegetation may be caused by natural or human mediated disturbances such as fire, logging, cultivation, or weather disturbances like strong winds, tornadoes, and hurricanes. Successions can also be defined according to the factors causing the change of species composition. An allogenic succession is caused by a change in environmental conditions that in turn influences the composition of the plant community. In Cornwall England, observations on the estuary of the Fal River suggest that the deposition of silt, perhaps causing an allogenic succession from salt marsh to woodland. Measurements indicate sedimentation rates of about 1 cm per year on the mudflats that are found 15 kilometers into the estuary. Over the last 100 years, this salt marsh has increased its elevation and has extended itself seaward by 800 meters. The adjacent woodland has followed the salt marsh by invading its landward limit.
Succession can also be driven by the species in the plant community. In this type of a succession the plant community causes abiotic environment change because of its presence. This environmental modification then facilitates a change species composition. This type of succession can be called an autogenic succession. In the coniferous forests of coastal Oregon, Washington, and British Columbia, Red Alder (Alnus rubra) is a fast growing and short-lived early colonizer on disturbed sites where vegetation has been removed (Figure 5). Red Alder also has the ability to form symbiotic relationships with nitrogen fixing bacteria. This association causes levels of soil nitrogen to increase dramatically with time. With higher soil nitrogen levels Douglas Fir (Pseudotsuga spp.) seedlings become more successful in colonizing and establishing themselves on these sites. Eventually, the Douglas Fir become more dense and grow tall enough to cause the competitive demise of red alder by limiting the transmission of light to the forest floor (Figure 6). Some recent research has illustrated that autogenic factors in hydorsere succession may be much more important than climatic change during the Holocene. (Delcourt and Delcourt. 1991)
Analyzing the research work on numerous cases of succession in a variety of habitat types reveals some interesting patterns. These patterns are fascinating because they provide some insights into the possible processes operating. In terms of biotic and abiotic factors, the following differences can be identified when comparing early with late successional stages.
- Pioneer species tend to be small, short-lived taxa that have the ability to grow rapidly. (Kricher and Morrison)
- Late successional species are adapted to long life, have a large biomass, and grow slowly.
- General morphology and physiology of late succession species tends to be more complex. They have many more adaptations required for survival.
- Late successional species, especially their seedlings and young plants, tend to have high photosynthetic efficiency at low light levels. This requirement requires a heavy investment in nitrogen to produce very efficient chloroplasts.
- Early successional species tend to produce small seeds with dispersal mechanisms that move the seeds great distances. However, to be successful these seeds must colonize sites free of competitors.
- Late successional species normally have large poorly dispersed seeds. These seeds contain enough resources to produce seedlings that are very competitive upon germination especially for light.
- Early successional species tend to be robust competitors for soil nutrients and water but poor competitors for light.
- Decomposition of organic matter in early stages tends to be rapid with considerable loss out of the community. Late stages have slow decomposition rates with most of the released nutrients being returned to the plants.
- Site characteristics tend to be quite extreme in early stages when compared to late stages of succession. The developing plant community causes abiotic conditions to become more moderate.
- Organisms involved in the decomposition process become more abundant and more varied with time.
- Plant species diversity becomes greater with time.
- In terms of plant adaptive strategies, early colonizers are ruderals (plants that grow in poor soils), while late successional species belong to the competitive strategy.
Succession mechanisms
In the late 1970s, ecologists J.H. Connell and R.O. Slatyer published a thorough overview of the state of plant succession research. Based on their review of numerous past studies, it was suggested that the processes involved in succession can be grouped into three general models: Facilitation, Tolerance, and Inhibition. The review also suggested that most successions seem to involve processes from more than one model.
Facilitation model
The defining feature of the facilitation model of succession is that the developing plant community changes the abiotic environment. These changes make the environment more favorable for plant species that were previously not able to colonize the site. Once colonized, the new species displace the early site modifying species through resource competition. An apt example of this form of succession was documented at Glacier Bay, Alaska. In 1916, William Cooper set up some long-term study plots in areas that were recently deglaciated. At this site, lichens, cyanobacteria, fungi, and mosses were the first organisms to colonize the bare ground and glacial till. Together these organisms create thin mats that cover the ground surface. Under these mats the glacial deposits begin developing into proper soils because of the accumulation of organic matter, retention of soil moisture, and the reduction of soil erosion. By about 20 to 50 years, the developing soils have a variety of grasses, herbaceous plants, and small shrubs growing on their surface. Two common colonizing shrubs at this stage in the succession are the Yellow Mountain-Avens (Dryas drummondii) and Mountain Alder (Alnus crispa) (Figure 7). Both of these plant species facilitate the succession process by modifying the nutrient status of the developing soil. In both of these shrubs, nitrogen-fixing bacteria are found living symbiotically in root nodules. These bacteria provide the plants with nitrogen (nitrate) in exchange for photosynthetically produced carbohydrates. The presence of the bacteria also greatly increases the quantity of growth-limiting nitrogen in the soil. This increase in soil nitrogen allows for the successful colonization of trees like Cottonwood (Populus trichocarpa) and Sitka Spruce (Picea sitchensis). Both of these trees can only establish themselves on sites that have sufficient quantities of soil nitrogen required for growth. The establishment of trees on the developing site also reduces the quantity of light available at the ground surface. As a consequence, this reduction in light causes the competitive exclusion of many grasses, herbaceous plants, and small shrubs found growing beneath the tree canopy.
Figure 7. Yellow Mountain-Avens (Dryas drummondii) is a pioneer species associated with plant succession on glacial deposits. This nitrogen fixing plant species plays a facilitative role in this succession by enriching the soil with nitrogen. This enrichment allows for the subsequent colonization of Cottonwood (Populus trichocarpa) and Sitka Spruce (Picea sitchensis) trees. Source: Mary Clay Stensvold @ USDA-NRCS PLANTS Database
Tolerance model
The tolerance model of succession suggests that a predictable sequence is produced because different species have different strategies for exploiting resources. In this model, all colonizing plant species can survive as an adult under the environmental conditions that exist at the beginning of the succession. As the succession progresses, soil and light resources become increasingly limited as the plant community develops and competition becomes more intense. Species dominant in the early stages of the succession tend to be those that are small in size, easily dispersed, and grow rapidly. Overtime these species are replaced in dominance by larger, long-lived, slow growing plants. These late succession species are able to tolerate lower resource levels due to competition and can grow to maturity in the presence of early species, eventually out competing them. This model is typical of the succession dynamics seen in some forested communities.
Inhibition model
The inhibition model of succession also suggests that all species can survive as adults in the site during the early stages of the succession. (Krebs. 2008) It also suggests that the various species are unaffected by competitive interactions for soil nutrients or light. However, the species involved in the succession do compete for space. Further, these spaces only open up to a potential colonist when the present occupant is removed because of death. Death can occur from physical factors or biological interactions like disease or predation. In most successions, long-lived species become dominant with time because they gradually replace short-lived early colonists when they die.
Abandoned field to oak forest
Figure 2. Succession of plant species on abandoned fields in North Carolina. Pioneer species consist of a variety of annual plants. This successional stage is then followed by communities of perennials and grasses, shrubs, softwood trees and shrubs, and finally hardwood trees and shrubs. This succession takes about 120 years to go from the pioneer stage to the climax community., Copyright: Michael Pidwirny
One of the earliest studies of plant succession was done by Dwight Billings in the 1930s. In this investigation, Billings examined the succession of plant species that occurred on abandoned agricultural fields in North Carolina. (Barbour and Billings. 2000) A number of fields were studied that had been deserted from just a few years to a maximum of about 150 years. From observations of the plant communities that existed in these sites, Billings was able to construct a detailed successional sequence. The first stage of succession was characterized by the pioneering colonization of annual plant species on bare ground and nutrient poor soils (Figure 2). These annual species had short life spans (one growing season), rapid maturity, and produce numerous small easily dispersed seeds. The annuals were then quickly replaced in dominance in the next year by biennial plants and grasses. After about three to four years, the biennial and grass species gave way to perennial herbs and shrubs. These plants live for many years and have the ability to reproduce several times over their life spans. After about five to 15 years, the sites were then colonized by a number of different softwood tree species including Loblolly Pine (Pinus taeda), Shortleaf Pine (Pinus echinata), Virginia Pine (Pinus virginiana), and Sweetgum (Liquidambar spp.). As the softwoods increased in their numbers and grew in height, they began forming a forest canopy. This canopy reduces the amount of light reaching the forest floor. The resulting shaded understory conditions caused the exclusion of many light loving perennial herb and shrub species. Low light conditions also inhibited the germination of pine seedlings. Perennial herb and shrub species that were adapted to low light conditions now began to take over the ground cover. The canopy also changed the microclimate of habitat near ground level. It was now more humid, has moderated temperatures, and less wind. These conditions, plus the development of a soil litter layer, allowed for the germination of hardwood species, like Oak (Quercus spp.) and various species of Hickory (Carya spp.). The seedlings of these tree species also tolerate low light levels. By about 50 to 75 years after the initial colonization of the pioneer species, the hardwoods started to replace the softwood species in the developing forest. At this stage in the succession, the pines had maximum heights of about 25 meters (75 feet), while the oaks and hickories were on average about 10 meters (30 feet) tall. Because of their shorter life spans (50 years), many of the softwood species were beginning to die out. Subdominant hardwood trees would then fill the gaps produced by the softwood species deaths. Hardwood species, like Oak and Hickory, can live for more than 100 years. Sites more than 100 years old were found to be dominated by mature oak forests (Figure 3).
Figure 3. Very little light passes through the canopy of a mature oak dominated forest. This condition causes the death of softwood species that are adapted to grow best under intense sunlight., Source: Wikimedia Commons
References
- Andre F. Clewell and James Aronson. 2007. Ecological restoration, 216 pages
- Haxel R. Delcourt and Paul A. Delcourt. 1991. Quaternary ecology: a paleoecological perspective. 242 pages
- Frederic Edward Clements. 1916. Plant succession: an analysis of the development of vegetation, 512 pages
- John C. Kricher and Gordon Morrison. 1998. A field guide to eastern forests, North America
- J.H. Connell and R.O. Slatyer. 1977. Mechanisms of succession in natural communities and their role in community stability and organization. American Naturalist 111: 1119-44
- Charles J. Krebs. 2008. The ecological world view. 574 pages
- Michael G. Barbour and William Dwight Billings. 2000. North American terrestrial vegetation. 708 pages
Citation
Michael Pidwirny (Lead Author);C Michael Hogan (Topic Editor) "Plant succession". In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth May 1, 2010; Last revised Date September 14, 2011; Retrieved February 10, 2012 <http://www.eoearth.org/article/Plant_succession>
The Author
Michael Pidwirny studied Physical Geography at the University of Winnipeg and the University of Manitoba. He received his PhD from the Simon Fraser University in Burnaby, British Columbia in 1994. He currently is an Associate Professor of Physical Geography at the University of British Columbia, Okanagan Campus. Pidwirny’s research interests include climate change, the influence of land-use change on biodiversity, and the use of technology in education. He publishes regularly in encyclo ... (Full Bio)
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