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Chapter 4: Plant Succession and Disturbances in the Urban Forest Ecosystem1

Eliana Kämpf Binelli, Henry L. Gholz, and Mary L. Duryea2

Welcome to Chapter 4 of the publication Restoring the Urban Forest Ecosystem. This publication consists of 10 chapters available only in PDF format. The chapters explain basic ecological principles for the urban forest's water, soil, plant and animal communities. They discuss problems common in the urban forest such as aquatic eutrophication, soil aeration, invasive plants and loss of biodiversity. Solutions, strategies, examples, and additional resources are presented to help make urban forest restoration projects successful.

Abstract

Ecosystems are dynamic. Disturbances lead to changes in ecosystems, collectively called succession. Disturbances can be natural and/or anthropogenic (human-caused). Natural disturbances, such as wildfire, play an important role in forest succession. Knowledge of natural disturbance regimes is important to maintaining biodiversity. In forest succession, species composition, ecosystem structure and ecosystem functioning all change gradually over time. In urban areas, the alterations of natural disturbance regimes, along with the introduction of invasive species have altered natural succession. Natural disturbances vary in spatial scale (from small to large areas) and temporal scale (from hours to eons). Variation in the temporal and spatial scales of disturbances leads to ecosystems spread over the landscape that are in different successional stages. This landscape diversity meets the needs of a variety of wildlife species. In order to restore more natural successional regimes, we have to learn about ecosystems: their natural disturbance regimes, their expected stages of succession, and how they fit into the overall landscape. Small scale urban forestry projects should incorporate the concepts of succession, while eliminating invasive species and re-introducing natural disturbances regimes. Large scale projects can also adopt these strategies, but have the additional opportunity to manage for several stages of succession across the landscape and to restore missing stages of succession.

Change

A common misperception is that nature is in an unchanging balance. However, natural scientists have found strong evidence against this idea and we now know that change is one of the most fundamental characteristics of natural ecosystems.

Since trees generally live much longer than humans, the forests they are in were also perceived as unchanging. But, in fact, forests are highly dynamic. In many forests, wildfires, floods, windstorms or insect infestations produce major, but infrequent changes. In other forests, change is more subtle: single trees die and are replaced while most trees remain alive. However, since individual trees can live a long time, it is difficult to see or measure changes in forests over short periods of time.

There are two related aspects of change over time in forests: disturbances and succession. Disturbances lead to subsequent changes in ecosystems, which are collectively called succession.

This chapter discusses the dynamic nature of forest ecosystems and why it is important to understand disturbances and succession in order to manage and restore urban forest ecosystems successfully.

Disturbances

What are disturbances?

Disturbances are any event, either natural or human-induced (anthropogenic), that changes the existing condition of an ecosystem. Disturbances in forest ecosystems affect resource levels, such as soil organic matter, water and nutrient availability, and interception of solar radiation. Changes in resource levels, in turn, affect plants and animals over time, leading to succession.

Disturbances occur in all ecosystems. We often think disturbances result only from human activity. However, the definition of disturbance should not carry a connotation of negative human impact; naturally occurring disturbances are part of every ecosystem on earth.

What types of disturbances affect forests?

All forests are subjected to both natural and anthropogenic disturbances. Examples of naturally occurring disturbances include wildfires, winds (hurricanes, tornadoes and windstorms), insect and disease epidemics, landslides, ice storms, floods and droughts (Figure 1).

Figure 1.1. 
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Figure 1.2. 

Historical fires (1.1) and natural hydroperiods (1.2) are examples of naturally occurring disturbances which have been virtually eliminated from urban forest ecosystems.


Credit:

Larry Korhnak


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Examples of anthropogenic disturbances include pollution, conversion of forests to nonforest areas, timber harvesting, prevention of wildfires, global warming, alteration of natural hydroperiods (flooding), application of herbicides, introduction of exotic species, litter raking, trampling and compaction, fertilization and irrigation (Figure 2).

Figure 2.1. 
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Figure 2.2. 

Conversion of forests to development (2.1) and raking of litter (2.2) are examples of anthropogenic disturbances in urban forest ecosystems.


Credit:

Larry Korhnak


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The urban forest ecosystem is also subjected to anthropogenic and natural disturbances. However, natural disturbances, such as wildfires and normal flooding periods, have been virtually eliminated from urban forest ecosystems (Table 1).

Table 1. 

Types of disturbances that occur or have been eliminated from urban forest ecosystems (UFE's)

TYPES OF DISTURBANCES THAT OCCUR

MOST OFTEN IN UFEs

• removal of topsoil and soil grading

• air and soil pollution

• litter raking

• introduction of invasive species

NATURAL DISTURBANCES THAT HAVE

BEEN ELIMINATED FROM UFEs

• natural fires

• normal periodic flooding

• nutrient cycle

The focus of this chapter will be on naturally occurring disturbances and their importance to ecosystems. Ideally, restoration should return a site to a condition that includes a natural disturbance regime, but it may also be aimed at minimizing those anthropogenic disturbances that are considered undesirable.

The Importance of Natural Disturbances: Yellowstone and the Suppression of Wildfires

Fire may be the most widespread natural disturbance in the world's forest ecosystems. In fact, many forest and wildlife species persist because of periodic fire disturbance. However, the perspective that all disturbances are abnormal led to the Smokey the Bear syndrome where all forest fires were perceived as bad.

A classical example of the consequences of fire suppression is the 1988 catastrophic fire that swept through Yellowstone National Park, killing much of its vegetation. The natural cycle of fire disturbance in the park had been interrupted for more than one hundred years by intentional fire suppression. This led to a dense invasion by shade-tolerant trees and understory vegetation, and excessive accumulation of litter and woody debris in the forest, which eventually caused rampant, intense and impossible to control wildfires (Figure 3).

Figure 3. 

Suppression of natural cycles of fire disturbance in the Yellowstone National Park caused fires of destructive dimensions in 1988.


Credit:

Jeff Henry


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Why are disturbances important?

Disturbances are the norm for forest ecosystems. Completely undisturbed forests are extremely rare or even nonexistent.

The role that natural disturbances play in forests is one of renewal. Whether the disturbance is big or small, mild or intense, it plays an important role in determining a forest's succession (Figure 4). Disturbances initiate succession in ecosystems by killing some or all individuals (depending on its intensity), as well as disrupting litter/detrital (dead organic matter) pools.

Figure 4.1. 
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Figure 4.2. 
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Figure 4.3. 

Fires play an important role in forest renewal and succession. Figures 4.1, 4.2, and 4.3 sequentially show the regrowth of vegetation following the 1988 Yellowstone National Park catastrophic fires.


Credit:

Jeff Henry


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Fires initiate succession by reducing the number of plants on a site and creating openings in the canopy and near the ground, allowing understory plant species and tree seedlings to grow. For example, in the longleaf pine ecosystem in the southern U.S., frequent low intensity fires keep the ground clear of underbrush. These fires kill many saplings of trees and a few larger trees, while allowing sufficient seedlings to become established and maintaining an open tree stand of low density. In the absence of fire, the forest eventually loses the

Fires revitalize the soil by allowing some nutrients that are bound in the leaf and branch litter to be returned to the soil. Trees and branches that fall in forest fires create habitat for ground-nesting birds, reptiles and amphibians (Figure 5). Thus, fires can provide conditions for a wide variety of plant and animal species, and maintain biodiversity in forests. Disturbances, such as fire, are therefore a major diversifying force in forest ecosystems.

Figure 5.1. 
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Figure 5.2. 

Fires (5.1) release nutrients that were bound in the leaves, branches and organic matter and make them available for plant uptake (5.2). Burned logs and snags are also habitats for a variety of mammals, reptiles and amphibians.


Credit:

Larry Korhnak


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However, it is important to note that not all disturbances renew and invigorate ecosystems. Some disturbances are damaging and result in destabilization of the ecosystem. One example of such a disturbance is chronic pollution, which may cause long-term cumulative impacts that may not be easy or possible to reverse.

Disturbances and Biodiversity

Prairies, oak savannas, and long-leaf pine ecosystems of the Southern U.S. are examples of ecosystems that are dependent on frequent, low-intensity ground fires. These fires have occurred historically at intervals of 1 to 25 years. The life histories of the dominant species in these communities have been shaped evolutionarily by fire (Platt et al. 1988). Without fire, these ecosystems gradually change to other vegetation types (Figure 6). A knowledge of natural disturbance regimes is essential for maintaining regional biodiversity.

Figure 6.1. 

Figure 6.2. 

Figure 6.3. 

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).


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Ecologists have evidence that species diversity will be highest at some intermediate frequency or intensity of disturbance (Connell 1978, Pickett and White 1985). Frequent disturbance allows only species that colonize rapidly to persist, whereas long periods without disturbance may exclude desirable dominant plant species from the ecosystem (Figure 7).

Figure 7. 

The intermediate disturbance hypothesis indicates that species diversity is highest at intermediate frequencies or intensities of disturbance.


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Land managers should realize that species in any region have adapted, through evolution, to a particular disturbance regime. If we radically alter that regime, many species will be unable to cope with the change and will be eliminated.

How often do disturbances occur?

The disturbance regime is a combination of how often the forest is disturbed (frequency), how severe the disturbance is (intensity), and how large the affected area is (extent). In general, the frequency and intensity of natural disturbances are inversely 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 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

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).

Figure 8.1. 

Figure 8.2. 
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The grass-forb stage would be gradually replaced by a shrub-pine-seedling community that will last perhaps 15 to 20 years (without further 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 8.3. 
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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.

Figure 8.4. 

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).


Credit:

USDA Forest Service


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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.

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 species dependent on these forest structures. In the long term, such practices will lead to loss and degradation of the forest itself, since nutrients are not efficiently stored and recycled. As trees die, there are no replacements, since the seed bank and seedlings were removed, and natural succession is severed. As a consequence, erosion increases and fertilizers and soil amendments must be used to bring nutrients back to the system.

Figure 9.1. 

Figure 9.2. 
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Figure 9.3. 

Ecosystems that are able to follow natural succession, such as naturally landscaped backyards (9.1), may be managed without costly intervention. Such backyards will require less mowing, irrigation, fertilizers, herbicides and pesticides (9.2) when compared to backyards that use lawns extensively with only a few scattered trees (9.3).


Credit:

Larry Korhnak


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Likewise, the extensive use of ornamental invasive species and "weed-free" lawn areas have similar impacts. Herbicides, fertilizers, pesticides, irrigation, and frequent mowing and raking are often required to maintain such areas, representing extra maintenance costs for urban managers. On the other hand, natural ecosystems that are able to follow succession can be managed without these additional costs (Figure 10).

Figure 10.1. 
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Figure 10.2. 

Extensive use of ornamental invasive species will affect succession. This English ivy (10.1), for example, displaced and killed a native pine species. Control of invasive species, whether mechanical or chemical, is a costly and time consuming operation (10.2).


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To successfully manage urban forest ecosystems, managers need to understand how living and dead vegetation, wildlife and various disturbances interact. The ecological and economic advantages of maintaining and/or restoring natural succession need to be identified and incorporated into the management of the urban forest ecosystem.

Types of succession

There are two types of succession, primary and secondary.

Primary succession

Primary succession occurs in environments that lack organic matter and which have not yet been altered in any way by living organisms. Primary succession includes the development over time of the original substrate into a soil, and occurs over centuries or even eons.

The 1981 eruption of Mount Saint Helens in Washington provided an example of primary succession (Figure 11). This eruption wiped out most or all traces of life in a substantial area to the northeastern part of the mountain, leaving barren areas of deep ash deposits (11.1). A set of organisms adapted to survive and reproduce in these conditions has since become established (11.2). Some plants were able to extract nitrogen directly from the atmosphere (nitrogen-fixing species) and most were also dependent on the formation of fungal associationwith the roots (mycorrhizae) for extracting nutrients from the ash (Perry 1994).

Figure 11.1. 
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Figure 11.2. 
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Because of these characteristics, such organisms began to modify the site by accumulating nutrients and building up soil organic matter. As these organisms modify the site further, they will eventually be replaced by other organisms better adapted to the new conditions. For example, plants that required abundant light to grow will be replaced by more shade tolerant species.

As trees become established, there may be relatively long periods of this successional stage (e.g., Douglas-fir forests), which may persist only until the next eruption (11.3). In areas protected from future eruptions, a relatively persistent ecosystem may eventually occupy the site (e.g., Western hemlock forest) (Perry 1994) (11.4).

Figure 11.3. 
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Figure 11.4. 

The eruption of Mt. Saint Helens is an example of primary succession. It eliminated most traces of life in a substantial area of the northeastern part of the mountain (11.1). Less than a decade later, pioneer and early successional plants have colonized the area (11.2). Eventually, Douglas-fir forests will become established (11.3) and, without further disturbance, over several hundred years a Western hemlock forest may eventually occupy the area (11.4).


Credit:

National Park Service


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Another example of primary succession occurs on rock or subsoil surfaces exposed by landslides. Primary succession can occur in urban forests where, for example, surface soil and organic matter have been completely removed from a site. In this case,

Secondary succession

Secondary succession occurs in an environment that has supported mature vegetation in the past, and where, after the disturbance, the substrate (i.e., soil) remains relatively intact.

Secondary succession also occurs in urban areas. Suppose you decide to give up the fight with weeds in your backyard and no longer mow your lawn. The changes that take place will be typical of "old-field" secondary succession. First, your backyard would be colonized by a variety of plants, mostly annuals. Within a few years, these plants would be joined by perennials and smaller shrubs and the grass would start to disappear. Later, a mix of taller shrubs and tree species would seed in. Then, maybe 50 years from now, you would have a successional forest in your backyard.

Additional examples of secondary succession include the changes in vegetation and ecosystem characteristics in abandoned agricultural fields and in forests after clear-cuts, windstorms or fires.

The Role of Disturbances in Succession

Let's consider again the previous succession example of an abandoned farm field in the Southeastern U.S. (Figure 8). Natural disturbances may occur at any time during the development of the abandoned farm field into the pine or oak-hickory forest. Natural disturbances can keep an ecosystem in a certain successional stage for long periods of time. Fire of any type, for example, may prevent hardwood regeneration and maintain pine forests in the landscape for hundred of years.

Natural disturbances vary in spatial scale (they may occur in small, medium or large areas) and temporal scale (they occur at different time periods). For instance, individual trees or a group of trees may die and fall, forming small gaps in the forest, while wildfires may kill trees over thousands of acres (Figure 12). Consequently, in many forested ecosystems, disturbance leads to a condition where local successional patches are continuously formed, leading to a "shifting mosaic" across the landscape (Bormann and Likens 1979).

Figure 12. 

In many forested ecosystems, disturbances such as fires, promote areas with burned and unburned vegetation. Small successional patches are formed. Eventually, across broad stretches of forest, there will be patches of vegetation in several successional stages.


Credit:

Paul Schmalzer


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Different wildlife species are adapted to different successional stages (Figure 13). In "old-field" succession, for instance, pine warblers would be common to the pine forest successional stage, while red-eyed vireos and wood thrushes would be found in oak-hickory forests.

Figure 13. 

These bird species require different successional stages as habitats. Adapted from Smith 1990


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Some mature forests (such as old-growth forests in the northwestern US) take many hundreds of years to reach a late successional stage. Some species associated with these forests, such as the northern spotted owl (Strix occidentalis), may not survive if only earlier stages of succession are present (Eckert 1974). It is a major challenge is to determine and maintain an appropriate mix of successional stages within a landscape.

Different Stages of Succession Provide Habitat for Different Wildlife Species

The American kestrel (Falco sparverius) needs several stages of succession to meet its requirements for food and cover (Figure 14). This bird feeds primarily on insects and small mammals, which are present in early successional stages that contain annual and perennial forbs and grasses. However, it also requires intermediate and late stages of succession, such as mixed woodlands (shrubs and trees) and more mature forests, for nesting (Neilson and Benson 1991).

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.


Credit:

David Sarkosi


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American kestrels are widely distributed in North America. However, the number of southeastern American kestrels (Falco sparverius paulus) has decreased over 80% in the last 50 years (Wood et al. 1990). The main cause for the decline has been the destruction of longleaf pine ecosystems, the preferred nesting habitat for this species.

Other animals are also highly dependent on a certain stage of successional development. For instance, the structure and stage of development of scrub vegetation has a profound effect on wildlife habitat availability in Florida (Figure 15).

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).

Figure 15.1. 
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Figure 15.2. 
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Figure 15.3. 

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).


Credit:

Paul Schmalzer


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Succession in More Detail

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 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 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.


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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): Figure 17. Bormann and Likens (1979) proposed four phases of secondary succession: reorganization, aggradation, transition and steady state (or climax).

Figure 17. 

Bormann and Likens (1979) proposed four phases of secondary succession: reorganization, aggradation, transition and steady state (or climax).


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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 environmental conditions and the nature of disturbance regimes. For example, the reorganization phase usually passes quickly but after severe disturbances or in harsh climates it can be greatly prolonged. Likewise, the aggradation phase varies widely from one forest type to another, and is much more rapid in favorable environments and where denser, more even-aged stands develop.

Changes in ecosystem function, structure and composition through succession

In addition to species composition, the structure and functioning of ecosystems also change during succession (Table 2). For example, most forest ecosystems only have abundant logs and snags (structure) later during succession or after a disturbance, such as a severe windstorm. In other ecosystems, a low intensity, frequent disturbance such as ground fire, burns low vegetation and some trees, releasing nutrients and competition, which changes both the pattern of nutrient cycling (function) and the vertical layering of vegetation (structure).

Table 2. 

Changes in ecosystem function, structure and composition that occur during succession.

ECOSYSTEM ATTRIBUTE

ASSOCIATED CHANGES

Function

high rainfall interception, efficient nutrient cycling, cooler environment (evapotranspiration cooling), high filtration of air pollutants, lower runoff.

Composition

number of plant, wildlife and microorganism species.

Structure

presence of logs and snags, layering of live vegetation, litter accumulation.

Species composition, ecosystem structure and ecosystem function all change during succession and are linked. By changing one component, such as composition, there will be changes in the ecosystem's function and structure. Invasive plants, for example, can modify the functioning of ecosystems (such as nutrient cycling and productivity) as well as their species composition (Figure 18).

Figure 18.1. 
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Figure 18.2. 

Several invasive plants, when introduced to natural areas can modify the ecosystem's function and alter natural succession. For instance, Chinese tallowtree (Sapium sebiferum) (18.1 tree, 18.2 inflorescence), can alter nutrient cycling and productivity by displacing native vegetation in natural areas.


Credit:

Edward Gilman


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For example, Myrica faya has invaded young volcanic areas in Hawaii. These areas are extremely nitrogen-deficient, and no native nitrogen-fixing plants exist. Because Myrica faya actively fixes nitrogen, it can form dense stands which out-compete and may replace native vegetation. Its invasion completely alters nutrient cycling and the rate and direction of primary succession (Vitousek 1986).

Changing Natural Succession: The Casuarina Example

Casuarina species are nitrogen-fixing, fast-growing species which are tolerant of infertile soils. As a result, they would seem to be an excellent choice for restoration projects, growing very fast, shortening the reorganization and aggradation phases and, consequently, reducing water runoff and nutrient losses.

However, Casuarinas are also highly aggressive invasive species (Figure 19). By planting them, nitrogen is added to soils, altering the nutrient cycle. A thick litter layer is also produced, reducing germination of native plant species (Ewel 1986), and altering the composition of plant species in the next successional stage. Wildlife species are also affected, since food sources and cover have been modified (see also Chapter 9 - Invasive Plants).

Figure 19.1. 

Figure 19.2. 

Australian pine (Casuarina spp.), an aggressive invasive species, alters composition, structure and function of ecosystems. These fast-growing species form monospecific stands (19.1) that displace native vegetation. They are seen here growing above the original ecosystem's canopy (19.2).


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Managing Disturbances and Succession

Natural disturbance regimes and succession have often been altered by humans, such as through the introduction of exotic species and the suppression of natural fires. To restore ecosystems it is necessary to actively manage succession.

Goals for restoring ecological succession could be economic (e.g., reducing maintenance costs of an urban park), ecological (e.g., restoring the normal hydrological period of an urban wetland) or aesthetic or recreational (e.g., bringing birds and watchable wildlife back to a neighborhood greenspace). These goals are not mutually exclusive. For example, the Patuxent Wildlife Research Center, near Laurel, Maryland integrates both ecological and economic goals in the management of succession. In 1960, the U.S. Fish and Wildlife Service and Potomac Electric Power Company agreed to implement a management program that would develop a shrubland community on a newly constructed right-of-way. Mowing was halted and selective herbicides were periodically applied to undesirable tree species. After 30 years, the right-of-way was dominated by a shrub community with high diversity and heavy use by wildlife (Obrecht et al. 1991). Additionally, the economic goal of reducing the number of trees growing too close to powerlines has also been achieved.

A restored site (an urban park, for instance) may contain one or more types of ecosystems or remnants of ecosystem. It is important then, to understand historical patterns of succession in these ecosystems.

Information should be regularly collected to document patterns and effects of management, including current and historical site conditions, such as soils, vegetation and disturbances. A site inventory should be conducted to determine the potential of the site (see also Chapter 7 - Soil and Site Factors). If a location is too degraded (due to pollution, nutrient loading, or heavy pesticide use), it may not be possible to restore it to a desired historical successional stage. Realistic and feasible restoration goals will ultimately determine a project's success.

A particular stage, or a mosaic of different successional stages, may be chosen as the objective of restoration, based on the information collected from the site inventory. The plant species to be established should be those characteristic of the corresponding natural successional stages. For instance, planting trees and shrubs to attract as many bird species as possible, many of which are not typical of the desired successional stage, may not lead to a sustainable objective.

Incorporating disturbances and succession into small scale projects

Restoration projects in small areas may include ecosystem(s) in which succession can be effectively managed. These situations may include the restoration of a bare site, elimination of invasive species or re-introduction of more natural disturbances.

Restoring bare sites

On a bare site, one stage of succession could be chosen and a first effort to restore it could be by planting a mix of all species typical of that successional stage. However, it may take decades for the trees to become mature, and litterfall and logs may need to be imported if a late successional stage is to be approximated. Introduction of natural disturbance regimes, such as frequent ground fire, may be desirable or necessary in some cases.

The Greening the Great River Park Program, established in 1995, seeks to restore native ecosystems along the Mississippi River in St. Paul, MN. The project involves the landscaping of industrial lands with four native plant ecosystems, including forests and prairies. For example, a 35-acre project will restore a natural prairie ecosystem close to downtown St. Paul (Figure 20). Prairies will be maintained in a grassy successional stage by using frequent low intensity fires. "Prescribed fire" and/or shrub/tree cutting will be used to maintain this grass-like stage and keep weeds under control. Such strategy will provide, in the long run, an important successional stage that was missing from this urbanized landscape.

Figure 20.1. 
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Figure 20.2. 

Prairies are maintained in a grassy successional stage by frequent low intensity fires (20.1). The Greening the Great River Park initiative (20.2), uses prescribed fire and/or cutting to maintain the grass successional stage of prairies in a 35-acre project in downtown St. Paul, MN.


Credit:

Greening the Great River Park


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Eliminating invasive species

In some sites, removal of invasive plants may be sufficient to release native species from competition and restore natural succession. In the Ivy Removal Project in Forest Park, Portland, removal of English ivy (Hedera helix) has renewed the health of the existing vegetation (Figure 21). English ivy is an aggressive exotic vine, extensively planted in the surrounding neighborhoods, that has invaded the park and suppressed its native vegetation. Regular removal of ivy has allowed native plant species to follow natural succession by eliminating plant competition.

Figure 21. 

The Ivy Removal Project, removes English ivy (Hedera helix) that has invaded Forest Park in Portland, OR, suppressing its native vegetation. In this case, removal is sufficient to release native species from competition and bring back natural succession.


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However, in cases where the site has been invaded by aggressive invasives and native vegetation has been seriously damaged, removal of invasives may have to be followed by planting. A mix of native plant species typical of the desired successional stage can be planted (as in the bare site situation). An example occurred at Bill Baggs, a heavily used urban park in Miami FL, where a hurricane destroyed the monoculture of Australian pines (Casuarina equisitifolia) that previously dominated the park's vegetation (Figure 22).

Figure 22.1. 

Figure 22.2. 

Australian pine (Casuarina equisitifolia), a highly invasive species, covered major areas of this urban park, Bill Baggs (22.1, beyond buildings) and suppressed native vegetation. After hurricane Andrew struck (22.2) natural removal of Australian pines allowed managers to restore the park's natural ecosystems.


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Australian pines covered major areas of the park and suppressed the native vegetation prior to the hurricane. The "clean slate" that resulted from this natural removal of Australian pines allowed managers to reestablish the ecosystems that existed before by planting native species typical of that area. For more information on invasive species see Chapter 9 - Invasive Plants.

Re-introducing natural disturbances

When re-introducing disturbances, ecosystem characteristics and site conditions should be carefully considered. In the Southern U.S., for example, upland ecosystems are adapted to frequent (every 1 to 15 years) low intensity fires. In the case where fire has been absent for long periods of time, thinning of trees and/or manual removal of excessive fuel loads may be necessary prior to application of prescribed fire. Such management practice would prevent damage (and other associated risks) by a high intensity fire to which this ecosystem is not adapted.

On the other hand, where high intensity disturbances have been excluded for excessively long periods, other strategies may need to be pursued. For instance, the sand pine scrub ecosystems, also in the Southern U.S., are adapted to infrequent (every 15 to 100 years) high intensity fires. Historically, after a lengthy fire-free period, an intense fire occurs. If fires become too frequent, sand pines disappear, and the association becomes oak shrub or changes to other pines. If fires become too infrequent, a xeric hardwood forest develops. Most scrubs naturally depend on fires, but these fires need to be applied in such a way that various stages of development are maintained within isolated fragments. Without these fragments, species with special habitat requirements (such as the endemic Florida mouse, Podomys floridanus, the Florida scrub lizard, Scelopors woodi, the gopher tortoise (Gopherus polyphemus) and the sand skunk, Neoseps reynoldsi) might be eliminated (Figure 23). Although preliminary steps have been taken to develop techniques to burn the scrub, reintroduction of fires in scrub ecosystems within urban areas may not be feasible (due to liability, fire control considerations and public reaction). In such areas, patches of the scrub ecosystem could be maintained by cutting, scraping and chopping to simulate fires (Meyers and Ewel, 1990). Implementation of either burning or mechanical techniques will require careful attention to public education.

Figure 23.1. 
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Figure 23.2. 

In the scrub ecosystem of the southern U.S., the correct frequency and intensity of fire is critical. If fires become infrequent and too intense, a sand pine ecosystem develops, excluding the endangered scrub lizard (Sceloporus woodi) (23.1) and gopher tortoise (Gopherus polyphemus) (23.2).


Credit:

Dave Rich


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In other ecosystems, small or large gaps may need to be cut to stimulate further succession. Such a practice is becoming common for restoration of longleaf pine ecosystems in the Southeastern U.S., where dense hardwood thickets now dominate many sites. Gaps are cut and regenerated (Figure 24), and prescribed fire is used to keep hardwoods from re-invading.

Figure 24.1. 

Figure 24.2. 

In the longleaf pine ecosystems in the southeastern U.S., gaps are cut to stimulate succession (24.1). Such practice allows regeneration (24.2) and the return of a missing stage of succession to the landscape.


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Re-instating several different stages of succession in one area can only be achieved on very large land areas. Small sites may prove not to be functional, although a small mosaic of semi-natural successional stages may, nevertheless, be effective in schoolyards for educational purposes. The Schoolyard Ecosystems for Northeast Florida initiative, for example, teaches students about different animals that utilize a combination of small patches of mowed areas, early succession and more mature areas (Figure 25). Some important structural elements, such as logs, snags, brush piles and plants with different heights, are constructed to simulate a more mature area and to promote wildlife.

Figure 25.1. 

Figure 25.2. 

The Schoolyard Ecosystems for the Northeast Florida initiative (25.1) encourages the establishment of successional stages in school areas. The objective is to teach students about different animals that utilize a mowed area, an early successional patch and a more mature area (25.2).


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Incorporating disturbances and succession into large scale projects

Parts of larger project areas (greater than about 20 acres) may present situations similar to small scale projects (with some bare sites, sites invaded by exotic invasive species and sites where disturbances could be re-introduced). But in larger areas, there is also the opportunity to manage for several stages of succession at the same time, if a mixed successional landscape is typical of the ecosystem in question or could be used for educational purposes. Learning about the ecosystem, its stages of succession and how they fit into the overall landscape becomes critically important. The Chicago region, for example, contains prairies, savannas, woodlands and forests. The absence of fire has impacted these ecosystems and their stages of succession in the landscape. Oak savannas have been almost totally excluded in the Chicago area and prairies have been invaded by woody species. Historically, the frequency and intensity of fire determined the successional stage of these ecosystems, that is, whether a given piece of land would be an open grove or a dense forest (Figure 26). Restoration efforts in this case are based on re-introducing fires. To date, fire has been reintroduced in several areas and native species typical of the region's ecosystems are being planted. In some areas, native trees have been cut to allow more light to reach the ground (Figure 27). Such practices allow the landscape to support several stages of succession, ranging from open prairies to forests.

Figure 26. 

Historically, the frequency and intensity of fire determined the successional stage of ecosystems (whether a given piece of land would be an open grove or a dense forest) in the Chicago area. Photo courtesy of Chicago Wilderness


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Figure 27.1. 
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Figure 27.2. 

Due to suppression of fires, the once open savannas in the Chicago area (27.1) developed into thickets of vegetation deprived of sunlight (27.2). Oak savannas began losing their vast diversity of plants and animals and were almost excluded from the landscape.


Credit:

Chicago Wilderness


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Some continuous or intermittent form of management may be needed to create disturbances in situations where human activity has severely modified natural disturbances cycles. Efforts to restore historical flooding cycles in the South Platte River watershed illustrate the need for an integrated restoration plan for a whole region. The floodplains along the South Platte river in Nebraska consist of a mosaic of different vegetation types. The presence of wooded or open vegetation was historically determined by natural periodic floods. Forests were confined to drier sites, since native woody species, such as willows (Salix spp.) and cottonwoods (Populus spp.), would not survive flooding. Grasses, on the other hand, could tolerate flooding, allowing for open areas along the river.

Channelization and upstream development reduced the water flow and, consequently altered flooding periods. As a result, previously open areas of the floodplain are nowdrier and invaded with adjacent native forest species. Before channelization and development, migratory birds, such as the endangered whooping crane (Grus americana) and the sandhill crane (Grus canadensis) (Figure 28), used the open grassy floodplains for feeding and avoided roosting in areas with abundant woody species. Because of these changes in natural succession, the whooping crane population decreased 80% over 30 years.

Figure 28. 

Sandhill crane (Grus canadensis) populations have decreased as a consequence of successional changes in ecosystems along the South Platte River.


Credit:

Larry Korhnak


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Current restoration efforts include selective clearing of trees along some parts of the river. However, restoration of historical patterns of succession in the region will ultimately depend on the reinstatement of normal flood periods. An integrated upstream restoration effort along all the South Platte River extension will be required to achieve such a goal (U.S. Fish and Wildlife Service1981).

In another example from Central Florida, scrub vegetation without fire grows very tall and thick with very little open space for the endangered gopher tortoise (Gopherus polyphemus) to nest and feed (Figure 29). Little sunlight can reach the ground and herbs, which are a food source for this tortoise, can no longer grow (Smith 1997). Conservationists are using prescribed fires to restore the open nature of the historic scrub ecosystem. A number of other animals with wide ranges, such as black bear, white-tailed deer, bobcat, gray fox and spotted skunk, also utilize the scrub and should benefit from the efforts as well (Meyers and Ewel 1990).

Figure 29. 

Without fire the scrub ecosystem grows very tall and thick with very little open space for the endangered gopher tortoise (Gopherus polyphemus) to nest and feed.


Credit:

Ben Coffin (with the Friends of the Enchanted Forest in Titusville, FL)


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Conclusions

Disturbances and succession occur virtually inevery place on earth. To successfully manage the urban forest ecosystem, managers need to understand natural disturbance regimes and how species composition, ecosystem structure and wildlife interact over time within these regimes. There are many opportunities to incorporate the concepts of disturbance and succession in either small or large scale urban restoration projects:

  • Learn about the historical disturbance regimes that occur in the ecosystems in your region. Remember that disturbances have a variable spatial and temporal scale. If appropriate, propose re-introducing some disturbances back to these ecosystems.

  • Understand the successional stages of the ecosystem(s) you are managing.

  • Take advantage of any research conducted that relates to historical site conditions, including soils, climate, vegetation and disturbances. Conduct a site analysis and decide whether your restoration plans should include disturbances and succession management.

  • Manage site-specifically but remember that the site you are managing belongs to a larger landscape that may contain other successional stages.

  • Remember that species composition, ecosystem structure and ecosystem function are linked and change during succession. Invasive plants, for example, can modify the functioning and structure of ecosystems as well as their species composition.

  • Start with small demonstration projects.

Remember that succession and natural disturbances do not always follow our human-made geographical boundaries. Integrated efforts may be needed to better achieve restoration goals at the landscape level.

It is also important to involve the local community in every step of the restoration process. Successful urban forest restoration projects often include an educational and outreach component. Educate people about the benefits of succession and the benefits of re-introducing natural disturbances

Suggested Readings

Adams, L. and L.E. Dove. 1989. Wildlife reserves and corridors in the urban environment: A guide to ecological landscape planning and resource conservation. Columbia, MD: National Institute for Urban Wildlife.

Adams, L. W. 1994. Urban wildlife habitats: A landscape perspective. Minneapolis, MN: University of Minnesota Press.

Franklin, J.F. 1993. Preserving biodiversity: Species, ecosystems or landscapes? Ecological Applications 3(2): 202-205.

Smith, D.S. and P.C. Hellmund. 1993. Ecology of greenways: Design and function of linear conservation areas. Minneapolis, MN: University of Minnesota.

Cited Literature

Bormann, F.H. and G.E. Likens. 1979. Pattern and process in a forested ecosystem. New York, NY: Springer-Verlag.

Connell, J.H. 1978. Diversity in tropical rain forests and coral reefs. Science 199: 1302-1310.

Eckert, A.W. 1974. The owls of North America. New York: Doubleday and Co.

Ewel, J.J. 1986. Invasibility: Lessons from South Florida. In Ecology of biological invasions of North America and Hawaii, edited by H.A. Mooney and J.A. Drake. Berlin, Germany: Springer-Verlag.

Meyers, L. and J.J. Ewel. 1990. Ecosystems of Florida. Gainesville, FL: University of Central Florida Press.

Neilson, E.L. Jr. and D.E. Benson. 1991. Wildlife Habitat Evaluation Handbook. Colorado State University: Department of Fishery and Wildlife Biology.

Obrecht, H.H.III, W.J. Fleming and J.H. Parsons. 1991. Management of powerline rights-of-way for botanical and wildlife value in metropolitan areas. In Wildlife conservation in metropolitan environments, edited by L.W. Adams and D.L. Leedy. Columbia, MD: National Institute for Urban Wildlife.

Perry, D.A. 1994. Forest ecosystems. London: The Johns Hopkins University Press.

Pickett, S.T.A. and P.S. White. 1985. The ecology of natural disturbance and patch dynamics. New York, NY: Academic Press, Inc.

Platt, W J., G.W. Adams and S.L. Rathbun. 1988. The population dynamics of a long-lived conifer (Pinus palustris). American Naturalist 131:491-525.

Smith, R.B. 1997. Gopher tortoises (Gopher polyphemus). Kennedy Space Center and Enchanted Forest Nature Sanctuary, October 16 1997 [cited 1997]. Available from http://www.nbbd.com/godo/ef/gtortoise/index.html.

Turner, M.G. 1987. Landscape heterogeneity and disturbances. New York: Springer-Verlag.

U.S. Fish and Wildlife Service. 1981. The Platte River ecology study special research report. U.S. Fish and Wildlife Service, Jamestown, ND. Jamestown, ND: Northern Prairie Wildlife Research Center Home Page [cited July 16 1997]. available from http://www.npwrc.usgs.gov/resource/othrdata/platteco/platteco.htm.

Vitousek, P.M. 1986. Biological invasions and ecosystem properties: Can species make a difference? In Ecology of biological invasions of North America and Hawaii, edited by H.A. Mooney and J.A. Drake. Berlin, Germany: Springer-Verlag.

Wood, P.B., J. Schaefer and M.L. Hoffman. 1990. Helping our smallest falcon: The Southeastern American kestrel SS-WIS-16. Gainesville, FL: Florida Cooperative Extension Service, University of Florida.

Woolfenden, G.E. and J.W. Fitzpatrick. 1984. The Florida scrub jay: Demography of a cooperative-breeding bird. Monogr. Populat. Biol. no. 20. Princeton, New Jersey: Princeton University Press.

Footnotes

1.

This document is FOR93, one of a series of the School of Forest Resources and Conservation Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date August 2001. Revised February 2008. Reviewed November 2012. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Eliana Kämpf Binelli, former Extension Forester, Henry L. Gholz, Professor, and Mary L. Duryea, Professor and Extension Forester, School of Forest Resources and Conservation, Institute of Food and Agricultural Sciences, University of Florida, PO Box 110410, Gainesville, FL 32611.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.