Overstory #165 - Species Succession and Tolerance
Species succession and tolerance
A thorough understanding of local patterns of forest succession is fundamental to the practice of positive impact forestry. Succession is the orderly and predictable replacement of plant and animal communities as conditions change over time. The key phrase is "orderly and predictable." If not for a manager's ability to predict changes in successional patterns brought about by disturbances--especially those caused by human interventions--there would be no scientific basis for managing forest ecosystems to obtain benefits.
The patterns and processes of forest succession have been the subject of many studies, and much has been written to describe how it plays out in different forest types. But for every answer that science uncovers, new questions emerge, generating many different theories about how forests change, and why. In western forests there are ecosystems that have never seen human disturbances, and studies of these areas are enormously valuable in our quest to understand succession elsewhere. In eastern forests, such is not the case; virtually every acre has suffered the impacts of human exploitation. So much so that we can only speculate about pattern and process absent human interventions. Unfortunately, forest science arrived on the scene well after most presettlement forests had been cut hard for products or cleared for agriculture, and so in the eastern forest we lack critical baseline information about how forests change when humans are not around.
The engine of forest succession is fueled by the relative tolerance of trees to competitive conditions. Complements of species are adapted to changing forest environments, but the growth and maintenance of these species lead to further changes (Horn 1971). Each succeeding complement tends to persist longer than the species it replaced until the site is occupied by a complement of species that will persist in the absence of disturbance. Trees that are capable of fulfilling their life cycle--reproduction, establishment, growth, and maintenance--in extremely competitive circumstances are said to be tolerant. These species are often found in the understory of older stands where timber has been lightly harvested in the past, or in the overstory as old-growth sentinels--and in the understory--of very mature stands that have avoided disturbances for many, many years. Only species that are capable of establishing offspring in their own shade will persist when a successional pattern reaches a climax. Environmental change in an undisturbed climax association is very slow and mostly associated with replacement of old-growth trees that succumb to a combination of stresses, causing them to relinquish their positions in the canopy usually to offspring (or close relatives).
Tolerance is a relative indication of a species' ability to survive competition for light, nutrients, moisture, and space. On most sites, the one environmental factor that is more determinative than others is a species' relative tolerance to low light. Tolerant species are capable of growing in shaded conditions. The more shaded the conditions under which a species can reproduce, establish, grow, and maintain itself, the more tolerant the tree, and the more likely it is to persist in a climax forest. It is this ability more than anything else that allows tolerant species to dominate sites when severe disturbances are lacking. In most forests, however, experiencing the effects of a severe disturbance is not a question of if rather, it is question of when. In these areas, a climax forest is a temporal phenomenon; given time, even the most stable forest will experience a disturbance severe enough to set succession back, creating an environment favorable to species that are less tolerant.
An intolerant species, as the name implies, is the opposite of tolerant. In other words, tree species that are intolerant of highly competitive and low-light conditions are incapable of reproducing, establishing, growing, and maintaining themselves under their own canopies. The intolerants, or pioneers, since they are the first species to invade severely disturbed sites, have developed strategies that allow them to take advantage of catastrophic conditions. For example, aspen--or popple--reproduces with prolific quantities of light, wind-borne seed that can easily germinate on bare soil, exactly the sort of reproduction strategy one would expect of a species that is incapable of germinating in its own shade. But this same species will also sprout from dormant buds on the root system that are triggered to develop when the tree detects that the top is missing and soils are warmed by sunlight.
In northern areas, from Maine to Washington where aspen stands are extensive, managers mostly rely on this spouting ability to reproduce stands of aspen. The hitch is that the disturbance must be severe and properly timed. Clearcutting is the only method that will successfully reproduce aspen. In other words, species that are adapted to catastrophic events require catastrophic conditions to reproduce successfully.
Other examples of intolerant reproduction strategies:
- The jack pine of northern Michigan is adapted to fire. It produces a cone sealed with pitch that must be softened by bright, hot sunlight, or fire. Cones open explosively, ejecting seeds usually onto a recently burned surface where seed will geminate even in extremely harsh conditions. Jack pine is the principal species of one of a few forest types in the United States that are adapted to fire. Also known as fire-dependent communities, some other examples are chaparral forests of the southwestern United States and lodgepole pine in the intermountain areas of the West, where aspen is also considered a fire-dependent community.
- Pin cherry, a noncommercial species of the northeastern United States, is attractive to birds and other animals for the pulp of its fruit. When the seed passes, it is capable of living in the soil for periods in excess of 120 years, waiting for a catastrophe to remove the overstory and allow the warmth of sunlight to trigger germination. Although not really considered a fire-dependent community, when pin cherry seed is in the soil, a fire will promote conditions that favor germination.
- The Ohia tree, native to the Hawaiian Islands, has a wind-borne seed that germinates on relatively recent, grainy lava flows. It is a very slow-growing tree, which is uncharacteristic of intolerant species as a whole, but it also develops into pure stands with a very dense canopy. Although the Ohia is intolerant of shaded conditions, and thus qualifies as a pioneer species, it also forms a climax type on sites where it grows. This is known as an edaphic climax because soil conditions are the primary controlling factor.
- Douglas-fir is considered a midtolerant species of the northwestern forest that is adapted to periodic catastrophic fires, which prepare an ideal seed bed for Douglas-fir seed. Once established, it is fairly resistant to fire (except when fire gets into tree crowns), and it is a relatively long-lived species. In the absence of disturbance, Douglas-fir stands are gradually replaced by more tolerant trees like western hemlock, noble, silver, and grand fir, and western red cedar, depending on elevation and latitude. Although clearcutting extensive areas of Douglas-fir has been highly controversial, some form of disturbance that exposes mineral soil is necessary to reproduce this species by natural means. Harvesting single trees or small groups will mimic a pattern that causes succession to move forward. More tolerant understory species will fill in the gaps and eventually reach the main canopy.
Forest succession is usually represented as a unidirectional process that proceeds from a stand composed of pioneer species--which require open, bare-soil conditions for seeds to germinate and which are often short lived (at least in a relative sense), fast growing, sexually precocious, and somewhat smaller in proportion at maturity--to a stand composed of climax trees--which are generally long lived and slow growing but highly tolerant of competitive conditions and which will prevail in the absence of disturbance. Between the two extremes are those that sometimes act like pioneer species and sometimes like climax species. Mid-successional species, also known as subclimax species, are the schizophrenics of forest trees because, depending on the circumstances, they can assume the characteristics of either pioneer or climax species. Propagating midsuccessional species requires careful site interpretation in order to ascertain the role they have assumed. This topic is discussed in greater detail in chapter three.
Generally, as succession progresses from pioneer to climax species, ecosystem complexity tends to increase, as does biodiversity and tolerance. With each step of succession, stability tends to increase so that each succeeding change will last longer then the preceding stand, but the ability of the stand to recover from disturbance decreases. For example, if an aspen stand is clearcut or burned or blown down by a hurricane, it comes back as aspen. But if a climax sugar maple forest is clearcut (and the soils disturbed), succession is set back to an earlier stage, although how much earlier depends on the severity of the disturbance. Slight disturbances tend to move succession forward.
Forest succession is often portrayed as a unidirectional process that progresses--in the absence of disturbance--from pioneer species that are adapted to catastrophic disturbances to climax species that will remain in the stand until a disturbance that favors pioneers. Complexity, stability, diversity, and tolerance all increase as succession proceeds from pioneer to climax species, but a community's ability to recover from disturbance decreases the more severe the disturbance, the less chances are for recovering the predisturbance community.
Forest management practices are planned disturbances that are intended to speed up succession, reverse it, or slow it down depending on the manager's objectives. This is a fundamental concept of forestry and is the foundation of silvicultural practice.
The preceding brief discussion about forest succession is a gross oversimplification of a complex process. Although our understanding of the process is incomplete, there are three generalizations about forest succession that are important to keep in mind. First, climax communities are stable, but only in the absence of disturbance. Yet disturbance is the rule in many forest areas, not the exception. For example, pine forests of the inter-mountain regions of the western United States (and the chaparral forests in the hills of California) are periodically disturbed by dry conditions and fire; the same is true of the pine forests in the Southeast. In New England, forests are periodically disturbed by rain-wind events, usually the remnants of hurricanes. Although there are some forests of the world where severe disturbance regimes are lacking, in areas where forest management for wood production is feasible there are usually one or more natural sources of disturbance that drive forest succession. In these areas, a climax forest is a temporal phenomenon; it is not a question of if a forest will be disturbed, but when it will happen. Positive impact forestry attempts to mimic natural disturbances so as to provide benefits.
Second, drastic disturbances tend to favor intolerant species (remember, a drastic disturbance is mimicking a catastrophic event). The formation of intolerant complements of species is nature's way of dealing with change. Pioneer species tend to stabilize exposed soils and promote the formation of topsoil; the humic layers, rich in carbon, support organisms that use carbon for energy and bank essential mineral nutrients used by species that follow. Forest succession is a process of accumulation, of carbon and nutrients, to support communities that are eventually able to reproduce, establish, grow, and maintain themselves in the absence of disturbance.
Finally, natural catastrophes--tornadoes, hurricanes, fire, insects, and disease--tend to topple climax communities, seriously, which calls into question the concept of a climax forest. In some forest types, especially in the Pacific Northwest, stands composed of pioneer and early-successional species can grow to immense proportions. These stands may look like ancient forests but are actually far from a climax state. Although there are many exceptions to this rule, generally speaking if the species in the under-story of an undisturbed forest are different from the species in the canopy, the forest will continue to move toward a climax association. Light, periodic thinning in these stands will tend to move succession forward, mimicking the circumstances in which old, sentinel trees gradually drop out of the canopy due to insect or disease depredations, or for other reasons.
Horn, H.S. 1971. The Adaptive Geometry of Trees. Princeton University Press, Princeton, New Jersey.
This article was excerpted with the kind permission of the author and publisher from
Positive Impact Forestry by Thom J. McEvoy. Copyright © 2004 by the author. Reproduced by permission of Island Press, Washington, D.C.
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About the author
Thorn J. McEvoy is associate professor and extension forester in the Rubenstein School of Environment and Natural Resources at the University of Vermont, and author of Legal Aspects of Owning and Managing Woodlands (Island Press, 1998).
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