Chapter 4: Plant Succession and Disturbances in the Urban Forest ...

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Chapter 4: Plant Succession and Disturbances in the Urban Forest Ecosystem 6 there before. Severe erosion, for instance, may lead to a degraded ecosystem that will never fully recover to the prior condition without extremely costly intervention, such as importing soil. In urban areas the challenge is to determine the appropriate natural disturbance regime to mimic and/or reinstate. Succession Figure 6.3 Figure 6. Longleaf pine ecosystems are dependent on frequent, low intensity ground fires. Fires maintain an open canopy (6.1) and an extremely diverse flora in the ground layer (6.2). In the absence of fires, other species, such as vines and shrubs, are favored resulting in the loss of this ecosystem's natural diversity (6.3). Figure 7. The intermediate disturbance hypothesis indicates that species diversity is highest at intermediate frequencies or intensities of disturbance. What is succession The changes in an ecosystem that follow a disturbance are collectively called succession. Succession is a dynamic and continuous process, often occurring gradually over time. Forest succession is the change in species composition, age and size, and ecosystem structure and function over time. Let's consider the development of an abandoned farm field in the Piedmont of the Southeastern U.S. over time to demonstrate succession (Figure 8). This farm field is surrounded by pine-hardwood forests, typical of this part of the country (8.1). During the first year or two, annual forbs cover the field (8.2). Plants such as goldenrod and asters follow the second and third year (Perry 1994). In this early stage of succession, if we walk in this field, we can hear birds such as grasshopper sparrows and meadowlarks (Meyers and Ewel, 1990). related. For example, volcanic eruptions or large meteor impacts (high intensity) fortunately only occur rarely (at a low frequency). Some anthropogenic disturbances, such as global climate change, occur only at a very low intensity. However, these disturbances may be directional and may cause large cumulative effects over a long period of time. Because short-term effects are small, they are very difficult to detect. If a disturbance is very intense, ecosystems can be totally destroyed, as when a forest is converted to a parking lot. The more intense the disturbance, the more difficult and costly it is to restore what was Figure 8.1 The grass-forb stage would be gradually replaced by a shrub-pine-seedling community that will last perhaps 15 to 20 years (without further
Chapter 4: Plant Succession and Disturbances in the Urban Forest Ecosystem 7 disturbances) (8.3). Birds such as the yellowthroat and field sparrow will be common. Pine seedlings continue to grow in the abundant sunlight and, from about year 25 to year 100, a pine forest may dominate the site, providing habitat for birds such as the pine warbler (Meyers and Ewel 1990). Figure 8.2 Photo by Natural Area Teaching Laboratory at University of Florida Pine seedlings do not grow in the shade of taller pines, but shade-tolerant oaks and hickories do. In about 150 to 200 years, in the absence of fire, an oak-hickory forest may replace the pine stand (8.4). Birds such as the red-eyed vireo will thrive in the deciduous forest (Meyers and Ewel 1990). The seedlings of oak and hickory, capable of growing in the shade of the older trees, will thrive and thus replace the older oaks and hickories that die of disease, old age or other causes. However, if fire does occur again, or the trees are harvested, pine forests can be maintained in the landscape for hundred of years. Natural disturbances can keep an ecosystem in a certain successional stage for long periods of time. This issue will be discussed further in the section The Role of Disturbances in Succession. Figure 8.3 Photo by Natural Area Teaching Laboratory at University of Florida Figure 8.4 Photo by USDA Forest Service Figure 8. Sequence of successional stages in an abandoned farm field in the southeastern U.S. over time. During the first years (8.1) the area is colonized by a mixture of pioneer species (8.2). This stage is gradually replaced by a shrub-pine community (8.3). In about 150-200 years, without further disturbances, an oak-hickory forest may replace the pine forest (8.4). Why is succession important Urban trees are often managed as individuals instead of as parts of ecosystems. Individual urban trees and other vegetation may well provide many benefits such as energy conservation, beauty, recreation and climate amelioration. Yet, by managing them as part of an ecosystem, additional benefits can be achieved, such as increased animal biodiversity, reduced storm-water runoff and erosion, and significantly reduced maintenance costs. Ecosystems that proceed through natural succession may be managed with much less costly intervention (Figure 9). Urbanization and its associated activities have a profound impact on natural succession, with the end result that little natural succession occurs in most metropolitan areas. For example, a widespread practice in urban forests is to clean out the understory by raking leaves, branches, seeds and seedlings on the forest floor. Logs and snags are also often removed. Such a loss of the understory, along with logs and snags may have negative consequences for many wildlife
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