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 12 Figure 14. American kestrels are widely distributed in North America. They feed on insects and small mammals, which are present in early stages of succession (grasses and forbs). However, the American kestrel also requires intermediate and late stages of succession, such as mixed woodlands (shrubs and trees) for nesting. Photo by David Sarkosi scrub vegetation has a profound effect on wildlife habitat availability in Florida (Figure 15). Figure 15.2 Photo by Anne Birch Figure 15.3 Photo by Paul Schmalzer Figure 15. The Florida scrub jay (15.1) is endemic to the scrub ecosystem in the southeastern U.S. It requires a low shrub layer, bare ground and a few scattered trees (15.2) avoiding canopied areas. The scrub ecosystem is maintained by periodic fires (15.3). Succession in More Detail Figure 15.1 Photo by Wayne Peterson The Florida scrub jay (Aphelocoma coerulescens coerulescens) (15.1), an endemic species in Central Florida, is restricted to the pine/oak scrub ecosystems (15.2). This bird requires a low shrub layer, bare ground and a few scattered trees, avoiding heavily canopied areas. The scrub ecosystem is maintained by periodic fires (15.3). In this case, if fire is excluded for long periods of time, a sand pine canopy develops and scrub jays abandon the site (Woolfenden and Fitzpatrick 1984) (15.4). Following a severe disturbance, sites are initially dominated by early successional plants, called pioneer species. Pioneers are usually prolific seeders (or sprouters), fast-growing and short-lived species, and generally intolerant of shade. Pioneer species are then followed by shrubs and early successional trees which, in turn, are eventually replaced by late-successional species. Later successional species are generally shade tolerant and may grow much more slowly. Their seedlings will survive and grow beneath an established canopy, and eventually they will overtop the shrubs and replace
Chapter 4: Plant Succession and Disturbances in the Urban Forest Ecosystem 13 early successional trees (Figure 16). Therefore, during succession, pioneers create conditions conducive to species that will form an intermediate or transitional community. This, in turn, creates conditions favorable to species that form late-successional communities. Figure 17. Bormann and Likens (1979) proposed four phases of secondary succession: reorganization, aggradation, transition and steady state (or climax). Figure 16. Following a disturbance, sites are initially dominated by early successional plants, called pioneer species (grasses and herbs). Pioneers are then followed by other shrubs and early successional trees which, in turn, are eventually replaced by late-successional species. The composition and relative dominance of various plant species changes over time because, in part, they have different life strategies (some plants grow best in full sun while others require shade, for example). Succession can be viewed as a biological race to make optimum use of available site resources, such as light, soil, nutrients and water. The pattern of vegetation found in a landscape results from the interactions among soil types, water availability, life history strategies of plants and natural disturbances, all of which vary at different spatial and temporal scales (Turner 1987). These interactions will result, over time, in patches of vegetation in different stages of succession across the landscape. Therefore, the dynamics of forests cannot be grasped by looking at only a single site, and individual forests' stands should not be managed in isolation from others in the landscape in which they are embedded (Perry 1994). Phases of secondary succession Although succession is a continuous process, it is useful to identify four main phases in secondary succession (after Bormann and Likens 1979): 1. Reorganization phase This is the period immediately following a disturbance, when pioneer species are establishing. There is usually a high availability of resources (light, nutrients and water) and plant competition is low. Because the quantity of leaves per unit of ground area is not yet high, loss of water from leaves is low and runoff of water is high. Consequently, there is also a high potential for nutrient losses from the soil and erosion, since nutrient uptake by plants is low and water runoff high. 2. Aggradation phase During this phase, plants rapidly accumulate biomass, especially in woody stems, while detritus also builds up on the ground. Restoration ecologists usually try to shorten the reorganization phase, and consequently hasten the aggradation phase, by planting trees and shrubs that will grow quickly, covering the site with leaf surface area. 3. Transition phase This phase is characterized by a first wave of tree mortality, caused by increased competition among the pioneer trees, accumulation of snags and logs, and the establishment of shade tolerant species in the understory. 4. Steady State (or Climax) phase The transition phase ends at a stage characterized by large accumulations of both living biomass and coarse woody debris (snags and logs). Forests that reach this phase usually have high structural diversity. Tree growth slows down in this phase, accompanied by increased tree mortality; any growth that does occur is offset by mortality. The period of time that different ecosystems stay in each of these successional phases depends on
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Chapter 4: Plant Succession and Disturbances in the Urban Forest ...

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