Ecological Basis of Uneven-Aged Silviculture*

Silviculture is an ecological art and science subject to economic, social, and legal constraints. It is practiced at the stand level, which is the basic management unit of the forest. Forest management more properly deals with the forest as a whole rather than with individual stands.

Broadly speaking, nature provides two patterns for silviculturists to follow. The first is called succession --the normal growth and development of an existing forest or stand (Kimmin 1987). The second is known as disturbance--the partial or complete destruction of an existing forest or stand through natural events (Spurr and Barnes 1980). Ecologically, succession and disturbance combine to determine the development of the forest or stand. Silviculture, however, does not precisely mimic nature, because nature's ways are far more random and sometimes more catastrophic than silviculturists or society finds acceptable.

A silvicultural prescription is more predictable and more regulated than a corresponding set of natural disturbances to better achieve society's expectations or needs. Silviculture evolved as a modem, applied science when 18thcentury foresters adapted natural patterns of succession and disturbance in managing forests to achieve the landowner's purpose. Today, foresters use silviculture to implement both disturbance and stand development in a stand in specific ways, which have varying degrees of "naturalness" about them. Unevenaged silviculture offers some advantages over evenaged silviculture by emulating different stages of succession and different scales of disturbance.

Succession and disturbance are opposing yet complementary forces. Simply stated, under succession, a community progresses from early rapid changes in vegetation to later stages characterized by slow changes. Under disturbance, some or all of the vegetation is killed, setting succession back to an earlier stage. The actual interplay between succession and disturbance, however, is much more complex.

In this chapter, an overview of the ecological basis of succession and disturbance is presented, including some elements more appropriate to evenaged stands. After this overview, the discussion will emphasize the dynamics of stand development and disturbance that apply specifically to unevenaged silviculture.

Succession

Succession-more specifically, secondary forest succession-is what we all think of as the normal growth and development of a forest stand. It begins immediately following disturbance when new trees start to grow and continues through distinct stages that, if unaffected by further largescale disturbance, extend to oldgrowth.

Succession can be explained in terms of an idealized time continuum. Assume that, at some starting point called year zero, a major "catastrophe," such as a forest fire or windstorm, eliminates all the vegetation on a site. Secondary forest succession begins with this devegetated condition at year zero, just after the disturbance has occurred. From this point on, assuming no more catastrophic disturbances, the stand passes through four distinct stages of development. Two excellent summaries of these stages are presented by Bormann and Likens (1979) and Oliver (1981); a more in-depth discussion of stand development is found in Oliver and Larson (1990). These four stages or phases of stand development can be summarized as follows.

The stand initiation stage (reorganization phase) begins immediately after the disturbance in year zero. Removing the vegetation makes more water, nutrients, and sunlight available on the site. New species (herbs, shrubs, and trees) become established because of the moist, fertile, and open conditions. Many different species with various degrees of shade tolerance and different regeneration habits become established during this period. The species that grow best are generally intolerant of shade (herbaceous plants, shrubs, and some trees like pines and sweetgum) and those that reproduce by sprouting from existing rootstocks (oaks, maples [Acer spp.], and hickories).

The stem exclusion stage (aggradation phase) takes over when one or more of the resources of the newly exposed site become limiting, preventing the further establishment of new plants. Some herbaceous species disappear, while others dominate for various periods; eventually the main canopy is dominated by trees. The new stand occupies the site completely, and competition for nutrients, moisture, and light begins in earnest.

As this stage proceeds, tree growth and competition lead to recognizable and predictable patterns of stand development. The early successional herbaceous plants can no longer persist because the main canopy forms a continuous, dense layer. Intense competition results in a stratified canopy of vegetation. Shade-intolerant species must be strong competitors to retain rapid height growth and remain in the upper canopy. Otherwise, they will lag behind and probably succumb to suppressing competition. Shade-tolerant species seldom remain in the upper canopy, but can persist in the shaded conditions found in the lower canopy. Over time, a prominent layer of shade-tolerant species, such as flowering dogwood (Cornus florida L.), maples, American hornbeam ( Carpinus caroliniana Walt.), and eastern hophornbeam (Ostrya virginiana [Mill.] K. Koch) may develop in the lower portions of the main canopy.

The understory reinitiation stage (transition phase) begins when trees in the main canopy begin to succumb, either singly or in small groups, to natural mortality caused by lightning, windthrow, or insects and disease. The gap in the canopy that results is ecologically significant in that resources formerly used by the dead tree are reallocated to the surviving vegetation. If there is a closed mid-canopy beneath the overstory, shade-tolerant species may respond to release and ascend to the overstory. If there is not a closed mid-canopy, enough sunlight may penetrate to the forest floor to support the establishment of new trees. As overstory trees next to the opening also succumb, the opening expands, allowing even more light, nutrients, and moisture to be used by the new plants. These regeneration processes, called "gap-phase regeneration dynamics" (Bray 1956, Pickett and White 1985, Runkle 1982, Runkle and Yetter 1987), are similar to those that occurred during stand initiation, only at a smaller scale. This difference in scale, however, is of ecological significance because the gaps are small enough to be influenced by adjacent overstory vegetation.

The old-growth stage (steady-state phase) is distinguished from the understory reinitiation and stem exclusion stages in that the old-growth stage tends toward greater stability in biomass and productivity. Regeneration dynamics continue through the gap-phase regeneration process described in the transition period. Theoretically, a series of gaps created over time will result in a stable balance of trees of different species, sizes, and ages. An observer in the stand during this stage has difficulty seeing for great distances because of the overlapped screening effect of foliage from overstory, midstory, and understory trees.

Again, under ideal circumstances, this period is indeterminate. The coexistence of young and old trees, the dynamics of development within gaps and between gaps, and the relatively stable biomass and productivity levels continue over the long term. Vegetation changes slowly in this stage because changes in soil, weather or climate, competition, diseases, and insect pests also occur gradually.

Disturbance

Natural disturbance is the ecological counterpoint to succession. Through succession, plant communities develop; through disturbance, that development is altered. Some disturbances are severe enough to set a plant community back to the beginning of the stand initiation stage. Others are so minor that only one tree is affected, thereby advancing development during the stem exclusion, understory reinitiation, and old-growth stages.

When judged by ecological time scales, the proportion of time that disturbances affect a stand is infinitesimal. But these periods are extremely important ecologically because they allow new generations of vegetation to become established and develop. By their very rarity, disturbances are of keen interest to ecologists and foresters as physical phenomena; by their occurrence, they establish ecological conditions within which new plant communities are created.

Disturbances vary according to three dimensions (White 1979). Frequency is the rate at which existing disturbances recur or is the mean number of recurrences over time. Frequent disturbances occur every few years, whereas infrequent disturbances occur once every few centuries. Predictability describes the regularity of occurrence of the disturbances; i.e., how predictable the recurrence of a disturbance is. Magnitude is the duration of the disturbance and varies from a few seconds or minutes (such as during a wildfire) to several years (such as a drought). Reckoned by human time scales, disturbances vary from being somewhat to exceedingly rare; however, this does not make them any less essential to a holistic view of forest ecology.

Disturbance frequency is generally inversely correlated with severity. In nature, a catastrophic disturbance that sets succession back to the stand initiation stage is exceedingly rare. The concept of disturbance cannot be restricted to large, catastrophic events, such as the volcanic eruption of Mt. St. Helens in 1980, the wildfires in Yellowstone National Park in 1988, or Hurricane Hugo in 1989, spectacular though they may be.

The severe, partial disturbance in which most of a stand is destroyed but part of the overstory and midstory survives is more common, but still rare. Stand development after such a disturbance is affected by competition not only within the newly developing reproduction but also between the reproduction and the scattered overstory trees that survived the disturbance. The resulting stand is much more variable in structure and species composition than a stand that follows a complete disturbance

Small disturbances that affect one to a few trees are the most common, occurring far more frequently than stand-replacing disturbances. This solitary mortality is a basic element of stand development throughout succession and influences the formation of canopy layers, depending on the size of the tree that succumbs. During the early stages of stand development, the death of an individual tree usually improves conditions for its neighbors--in silvicultural terms, density-dependent mortality similar to thinning.

But during the last two stages of stand development, the death of a tree in the overstory provides growing space for reproduction and development of the understory. At this stage, if a tree in the main canopy succumbs, the crowns of the neighboring overstory trees may not expand fast enough to occupy the resulting canopy opening. Unless there is a closed mid-canopy, this disturbance fosters new reproduction or releases advance growth.

Thus, in late successional stages, the regeneration process depends on whether overstory crown expansion closes the gap before the understory trees fill the gap from below. Understory trees are more likely to make it to the overstory if-- (1) the gap is large, (2) the trees adjacent to the gap do not respond vigorously in lateral crown growth, and (3) the gap is enlarged by the death of trees that border it.

Applying Ecological Principles to Silvicultural Practice

Foresters use silviculture to impose disturbance and modify successional development in a stand. Some trees (or other plants) are removed so others can develop better. The degree to which these prescribed actions "imitate" nature depends on how they are implemented.

The first alternative of the forester is that of no treatment. But subsequent alternatives involve removing increasing proportions of the vegetation. The choice of alternatives must be consistent with the ecology of the species that comprise the stand, the existing condition of the stand, and the future condition desired by the forest owner.

Silviculture and Stand Development

The stages of stand development and gradients of disturbance reflect the ecological basis within which silviculturists operate. The early stages of stand development set the stage for even-aged silviculture. By imposing disturbances severe enough to promote regeneration across the entire stand, the forester can encourage the development of intolerant and mid-tolerant species as one or two age classes distributed uniformly across the stand.

The later stages of succession, primarily the understory reinitiation stage, provide the ecological basis for uneven-aged silviculture. A silvicultural prescription that imitates scattered natural mortality in the upper crown classes can promote development of reproduction continuously over time. The goal of uneven-aged silviculture is to stabilize stand structure and biomass (volume) over the long term, thus emulating the old-growth stage. But other desirable ecological attributes of the old-growth stage (such as downed woody debris and low net growth) are less likely to be achieved in uneven-aged silviculture.

Silviculture and Disturbance

Ecologically, silviculture is simply an effort by the forester to imitate succession and disturbance. Reproduction cutting imitates disturbance; stand management after reproduction cutting imitates stand development. The decision to practice evenaged or uneven-aged silviculture depends upon many things, of which the most important is the landowner's objectives. The next step is to evaluate the ecological situation considering those objectives and regarding the silvicultural options.

Even-aged reproduction cutting methods imitate disturbances that affect an entire stand; uneven-aged reproduction cutting methods mutate disturbances that affect only part of a stand. By choosing to pursue unevenaged silviculture, the forester is opting to work with various small-scale disturbances that disturb only part of the stand and promote the later stages of development, especially the understory reinitiation stage.

The most intensive small-scale disturbances do not affect an entire stand, but can create large openings within a stand. Natural examples include a localized insect infestation such as southern pine beetle (Dendroctonus frontalis Zimmermann), a locally severe wind, or the flareup of a surface fire. Such a disturbance creates a gap in the canopy of the stand; reproduction becomes established and develops within this opening. Ecological conditions within the gap are affected by bordering trees, depending on opening size and shape. Group selection is used by foresters to approximate these conditions.

The least intensive, small-scale disturbance likely to occur in a stand is a single tree falling, or dying while standing, in the woods. Causes of such individual tree mortality include disease, insects, lightning, windthrow, or some combination of these. If the dying tree had a large crown, reproduction will become established in the gap created in the canopy. In the smallest gaps, the opening may close before the reproduction can grow into the main canopy, and the reproduction may then persist without further growth or may even become suppressed and die. The occurrence of multiple gaps (where trees next to a recently created gap succumb due to some cause linked to their proximity to the gap) or the concurrent creation of several small gaps within the same area can tip this ecological balance in favor of reproduction survival and development. Single-tree selection is used by foresters to approximate these conditions.

Key Ecological Elements of Uneven-Aged Silviculture

Gap-phase regeneration dynamics. Opening size greatly influences understory development. It affects species composition and growth of reproduction. Understanding the interplay involved is critical to the forester's ability to apply the concepts silviculturally.

The primary factor to consider is gap size relative to the shade tolerance of the desired species or mixture of species. Generally, large gaps favor shadeintolerant species, and small gaps favor shadetolerant species. As gap size decreases, the adjacent trees increasingly constrain the development of reproduction within the gap until, in the smallest gaps, the reproduction is suppressed. In addition, advance reproduction in place before the gap occurs also influences the species composition of reproduction after the gap has been created.

The foregoing applies to circular gaps in flat terrain. The situation becomes more complicated where gaps are irregular in shape and where terrain is hilly. The less direct the exposure to sunlight and the less circular the gap, the greater the influence of adjacent vegetation within the gap.

However, gaps may not be necessary for adequate reproduction. Simply reducing the overstory stocking to levels that would be considered "understocked" under evenaged management can allow sufficient diffuse sunlight to penetrate to the forest floor, if the midstory is also understocked. Repeated disturbances to maintain this understocked condition can successfully regenerate a stand over time.

Thus, the interplay of ecological condition, gap size, competition, and physical resources is the key to understanding regeneration dynamics in the last two stages of succession. These patterns can provide an ecological basis for practicing either group selection or single-tree selection, although the ecological distinctions between these two methods of selection are often fuzzy.

Canopy dynamics and shade management. The silviculturist applies these concepts of small-scale disturbances and gap dynamics by means of shade management. The position and shape of a tree's crown determine how much solar radiation it can intercept. The decision of whether to cut a tree can be linked to the size and vigor of a tree's crown, which reflect its demands on soil nutrient and moisture resources. As this decision is made for each tree in the stand, the degree of shade retained across the entire stand is determined. Thus, by managing the shade in the stand, the forester directly affects the future development of both overstory and reproduction.

The result of shade management properly applied in uneven-aged silviculture is easily seen by the forester in the canopy profile. The desired species will be present in all expected levels of the canopy profile, depending on the number and frequency of cutting-cycle harvests. For stands that have been under regular cutting-cycle harvests for an extended period, the desired species will exist in all levels of the canopy. For stands under uneven-aged silviculture for only one or two cutting- cycles, the desired species will be apparent in only the upper and lower layers of the canopy.

One should expect each cutting-cycle harvest to contribute some reproduction to the stand, and this should be apparent from both a range of tree sizes and the presence of the desired species in the appropriate canopy levels. In stands with frequent cutting-cycles and prolific reproduction, new reproduction need not be obtained after every harvest. Even so, reproduction should follow at least every other harvest. As the cutting-cycle lengthens or as the desired species becomes more difficult to regenerate, reproduction should be obtained after each cutting-cycle harvest. In both examples, the presence of different size classes of desired species should be clearly seen, not only in the diameter distribution but also in the canopy profile.

Gap-phase dynamics and shade management can also sustain the development of intolerant species in uneven-aged stands. The key is to ensure that there is always adequate light for desired species to persist and, if possible, to grow into the understory and lower layers of the canopy. This is done by controlling average stand stocking (basal area) and average gap size. An estimate for the desired basal area for a given species is the overstory basal area that marginally suppresses height growth of the reproduction if overstory trees are distributed uniformly across the stand. The goal should be to attain this basal area at the end of the cutting-cycle. Then, the gaps normally created by "cutting the worst and leaving the best" are where the establishment and development of reproduction will be most vigorous. The stand as a whole, both overstory and the sparsely stocked understory, will develop throughout the cutting-cycle, but the more densely stocked portions of the stand will support only overstory development. Essentially, this process ensures a continuous development of new gaps and expansion of old gaps.

However, once this balance between stand stocking and cutting-cycle length is established, it can be maintained only by harvesting at fairly regular intervals. Excessive delays in cutting-cycle harvests will alter stand structure because the overstory stocking will increase and understory development will slow dramatically under overstocked conditions. In essence, the stand reverts to a more even-aged character, and the process of building good uneven-aged stand structure must be restarted.

*Uneven-Aged Silviculture for the Loblolly and Shortleaf Pine Forest Cover Types. 1996. Baker, James B., Cain, Michael D., Guldin, James M., Murphy, Paul A. and Shelton, Michael G. USDA Forest Service Southern Research Station General Technical Report SO-118. (Chapter I)