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Managing for ecological objectives

Managing for ecological objectives often involves maintaining or enhancing ecological complexity in a stand, as determined by structural and compositional characteristics. It is a fact that traditional silvicultural approaches, especially those that are aimed primarily at management for timber, simplify the structure and composition of forest stands relative to conditions occurring in unmanaged (and naturally disturbed) stands. If your objectives include management for native species diversity, non-game wildlife habitat, and more aesthetically diverse conditions, than you should consider managing stands to enhance ecological complexity.

Ecologically complex stands are typically composed of more than one tree species, often including species that have little or no commercial value. Complex stands often include more than one age class, or at least some older individuals within the stand. Complex stands tend to have a wider range of tree sizes, including some (or sometimes many) very large individuals. Ecologically complex stands may contain abundant and sometimes diverse understory and groundlayer plant communities. Complex stands tend to have abundant numbers of standing snags and dead trees on the ground, as well as trees of limited commercial but high ecological value, such as cavity trees and wolf trees (Figure 1).

[graphic] Mature and young forest

Figure 1. Stylized representation of structurally simple versus structurally complex stands for both mature (top) and young (bottom) stand conditions.

Ecological complexity also is expressed as spatial variation or heterogeneity in the above characteristics. In other words, complex stands vary in structure and composition from one spot to another, compared to simplified stands that are often uniform throughout in their structure and composition. The sustainability of native species populations (one aspect of biological diversity) is often dependent on the availability of structures and the heterogeneity that exists in complex forest stands.

This is not to say that all stands should be equally (or highly) complex, indeed a range of variation should exist in the broader landscape. The level of ecological complexity you might desire will depend on the dominant species in your forest and your objectives (for example, a mixture of objectives including timber management or primarily songbird watching). Not all forest types are equally complex naturally. For instance, jack pine forests are naturally less complex in structure and composition than are northern hardwood forests. However, virtually all forests, including jack pine, that are managed principally for timber are less complex when compared to unmanaged, naturally occurring stands of the same type. Another point to remember is that stand complexity is not simply a feature of mature and old-growth stands. Even young stands regenerating after stand-replacing natural disturbance display significant complexity in terms of residual live trees that occurred singly or in patches, as well as abundant snags and logs on the ground, and patches of undisturbed understory vegetation (Figure 1).

Emulation of Natural Disturbance and Stand Development

A general framework for adding ecological complexity to forest stands is to implement management activities, particularly regeneration harvesting and thinning, in ways that emulate the outcomes of natural canopy disturbances (harvesting) and natural stand development processes (thinning). You can accomplish regeneration harvests in ways that sustain or enhance the number of tree species in the stand, as well as the size and age range of trees, the number of snags and downed logs, and the abundance and types of understory plant species. Similarly, you can implement commercial thinning, and other intermediate stand treatments, in ways that increase spatial heterogeneity in structure and composition.

An essential first step in applying principles from natural disturbance regimes and stand development to ecological forestry is to understand that creating and perpetuating appropriate structural, functional, and compositional attributes in stands is often the primary management objective. Adopting a mind-set of forest continuity rather than forest termination is helpful in opening yourself to the possibilities inherent in ecological forestry. You must put aside the notion that all forest stands must be terminated and new ones regenerated at some point in the future! Ecologist have learned that at least some structural elements of the stand (e.g., large live trees, snags, downed trees) are continuously maintained even under stand-replacement disturbance regimes and that such stands can often be managed with sufficient levels of retention so as to maintain at least some level of continuous forest influence over the site. Of course, where tree- and gap-based disturbance regimes are characteristic of the forest type or site, natural stands are rarely, if ever, terminated. In a very real sense, natural situations in which forest stands are completely terminated and replaced by a new stand are rare rather than common , however common we have made even-aged stands in production-oriented landscapes.

Principles of Ecological Forestry

Forest management focused heavily on enhancing ecological complexity is an evolving area of interest. It can be a major objective or be incorporated into management for objectives such as income, wildlife habitat, or recreation. It involves consideration of three basic principles:

  • Incorporation of biological legacies (features of pre-disturbance forests) into harvesting prescriptions
  • Incorporation of natural stand development processes into intermediate treatments
  • Allowing appropriate recovery periods between regeneration harvests

Principle 1: Incorporation of biological legacies into harvesting prescriptions

Biological legacies are the organisms, structures, and biologically-created patterns that persist from the pre-disturbance forest and influence development in the post-disturbance stand. These legacies include those described below in Table 1.

Table 1. Categories of biological legacies with some examples of types.

Legacy Category

Examples

Organisms

Sexually mature and intact live trees

Tree reproduction (seedling and sapling banks)

Vegetatively reproducing parts (e.g., roots)

Seed banks

Shrub, herb, bryophyte species

Mature and immature animals and microbes

Organic matter

Fine litter

Particulate material

Organically-derived structures

Standing dead trees

Downed trees and other coarse woody debris

Root wads and pits from uprooted trees

Organically-derived patterns

Soil chemical, physical, microbial properties

Forest understory composition and distribution

Biological legacies persist even through the most intense stand-replacement disturbances; they play critical roles as habitat and modifiers of the physical environment; and they are difficult or impossible to re-create in managed stands, hence the interest in carrying them over from the pre-disturbance stand.

  • Even-aged Management: One can easily modify even-aged regeneration harvests to incorporate biological legacies by retaining some large (healthy) trees of the dominant species as well as other species, by not cutting decadent trees and snags, and by protecting or creating some dead and downed trees. Consider including a range of sizes of retained trees, snags, and down trees (coarse wood debris), including large and very large trees. Also, you might give special consideration to retaining actual or potential habitat trees, including cavity trees, mast trees, nest trees, etc. Finally, try to retain and protect natural regeneration of desired species when it occurs.

The number or amount of structures (live trees, snags, logs, etc.) you might retain is dependent on management objectives and desired future conditions. For instance, retention of a low stand density or basal area of residual trees (e.g., 10-20 ft 2 /ac), followed by regeneration, will result in a largely single age stand containing scattered older trees. In contrast, retention of 30-50 ft 2 /ac of residual basal area will result in stands better described as two-aged. Keep in mind that there may be growth reductions when regenerating intolerant species under even modest levels of a residual overstory. However, maximizing regeneration growth throughout the stand is not of primary concern when managing for ecological complexity. Moreover, the continued growth of the residual trees may help to compensate for growth losses of regeneration.

You also should consider the spatial pattern of biological legacies within your harvest unit. Some ecological objectives are best sustained by dispersing retained structures (live trees, snags, coarse woody debris, etc.) over the harvest unit while other objectives are best served by aggregating structures (Figure 2). Within a single harvest unit, the retention pattern may vary from dispersed to aggregate by alternating between patch and dispersed cutting across the stand. In this way, spatial variation in stand structure is assured. Moreover, retaining some large patches of live trees is a straightforward way to insure protection of undisturbed understory plant communities and forest floor environments.

[graphic} Uncut stand, dispersed retention and aggregate retention

Figure 2. Stylized representation of overstory retention harvesting. (top) unharvested stand; (middle) dispersed retention; (bottom) aggregate retention.

  • Two-Aged Management: With minor adaptations, two-aged systems provide excellent opportunities to incorporate biological legacies in management. Many of the same consideration outlined for even-aged management are also applicable to two-aged systems. As with even-aged systems, you might retain overstory trees (and other legacies) in spatial patterns that range from dispersed to aggregated within the same harvest unit.

    Two-aged stands, that leave significant numbers of overstory trees in aggregates, or clumps, provide good opportunities to protect understory plant communities and forest floor environments since there can be places in the stand where harvesting and traffic is easily excluded.
  • Uneven-aged Management: Retention of biological legacies is also relevant to silvicultural prescriptions for uneven-aged management, i.e., single-tree and small group selection. You can easily modify selection prescriptions to incorporate biological legacies. Marking guidelines can explicitly incorporate objectives of maintaining old and large trees and their derivatives (large snags and downed boles) as part of the stand. If group selection is used, you can retain snags and downed boles, and occasionally live trees, in the gaps. The size and shape of openings used with group selection can also be chosen to match the sizes and shapes of gaps created by natural disturbances, which typically include a greater number of small openings than large ones .

Principle 2: Incorporation of natural stand development processes into intermediate treatments

This principle involves activities that are comparable to intermediate stand-level treatments in traditional silviculture, such as thinning and pruning, and may include such practices. However, the objective is to create structural and compositional diversity and heterogeneity throughout the stand, rather than to concentrate growth on selected trees and create spatially uniform stands, which is the case with usual stand tending treatments.

Foresters generally model intermediate stand-level treatments such as thinning and pruning on natural stand development processes, including competitive tree mortality and tree decline with age. In application, traditional thinning and pruning regimes create spatially uniform stands. To increase ecological complexity in established stands, consider modifying intermediate treatments to include variable density thinning and ecological under burning (in appropriate forest types), as well as non-traditional activities such as decadence creation and introduction of compositional diversity.

  • Variable density thinning: Thinning is typically distributed equally across a stand specifically to create a uniform distribution of equally sized crop trees, all having equal access to light, water, and soil nutrients. In contrast, most stands where natural thinning is occurring display greater spatial variation in tree densities, growth rates, and tree sizes. Moreover, competitive thinning is augmented by small-scale canopy disturbances from wind, lightning, insects, or fire that can occur at virtually anytime during stand development.

Variable density thinning is an approach that emulates the natural variation that results from both competitive mortality and small-scale canopy disturbance. With this approach, your thinning pattern will include unthinned areas and heavily thinned patches (i.e. gaps), along with variable levels of thinning and residual density between these endpoints (see Figure 3 below ). The result is greater spatial variability in stand densities, thus providing for greater complexity and heterogeneity in structural conditions across the stand. View some practical guidelines for implementing variable density thinning (PDF, 44K) .

[graphic] Variable thinning

Figure 3. Stylized representation of variable density thinning. (top) unthinned stand; (bottom) variable thinned stand.

  • Ecological under burning: Periodic surface fires were a natural occurrence in several regional forest types, including red pine, mixed-pine, and oak. Periodic use of prescribed surface fire can help maintain (or restore) understory conditions that are reflective of conditions occurring prior to regional fire suppression. To promote ecological complexity, consider prescribed burning yet allowing the fires to burn heterogeneously across the stand, that is, make no special effort to insure that the entire stand burns evenly. Moreover, periodic surface fires can be an effective means of inducing decadence creation (see below), if scattered trees are killed by the fire. Take care to avoid inducing excessive injury or mortality, as might occur if thick duff layers surround most trees or if the fire occurs during excessively dry.
  •   [photo] Red pin snag created using blasting cord to remove the live crown. Chippewa National Forest, Itasca County, MN
      Red pine snag created using blasting cord to remove the live crown. Chippewa National Forest, Itasca County, MN (H. Tjader)
       
    Decadence creation: Consider deliberate felling of live trees to increase the abundance and types of dead trees (coarse wood debris) on the ground. Also, consider girdling (or killing in some other way) living trees to create snags. A range of tree sizes should be considered, including large diameter stems. Use of blasting cord to damage the crowns of live trees is another approach for creating snags. This method creates conditions and rates of decline that are similar to that occurring after a fatal lightning strike.
  • Introduction and conservation of compositional diversity: When applying variable density thinning with large gaps, you may promote the establishment of additional mid- and intolerant tree, shrub, and herbaceous species in the stand. Moreover, during the course of stand development, shade tolerant species may become established in the canopy. Encourage the establishment of these species for their contributions to ecological complexity and native plant diversity. Moreover, when thinning, consider leaving non-commercial tree species in the stand, i.e., retaining these species for their contributions to ecological complexity, biological diversity, and wildlife habitat and food. Finally, consider underplanting tolerant species, e.g., eastern white pine under red pine, where seed sources or advance regeneration is lacking.

Principle 3: Allowing appropriate recovery periods between regeneration harvests

Recovery periods between regeneration harvests are needed for development of significant structural and compositional complexity in forest stands after disturbance. Stands managed primarily for timber are typically harvested before significant levels and types of complexity have developed (Figure 4). Most commercially managed stands lack trees of very large diameter, significant amounts of large dead wood, and trees with unique structures (cavities, large limbs, heartwood). Moreover, they may be compositionally limited, particularly in the canopy, because not enough time has passed for tolerant species to be recruited from the understory into gaps opened by natural thinning and disturbance. This problem is compounded if the stand was deliberately or inadvertently simplified during establishment, e.g., no legacies retained.

In general, stands (or individual trees within them) are harvested based on size and economic considerations. In general, economic rotation ages, typically 50-90 years, are shorter than those required to develop complex stand structures. If the primary management objective is development of ecological complexity, then management based on economic rotation ages may be inappropriate. Rather, the primary determinant of intermediate harvest or rotation age in such cases would be the development of desired or acceptable levels of structural complexity, compositional diversity, and within-stand heterogeneity.

[graphic] Harvest complexity stage

Figure 4. Stylized representation of the development of structural complexity during stand development. In traditional forest managment, stands are harvested prior to significant levels of complexity developing.

 

 
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North Central Region Forest Management Guide: A cooperative project of the USDA Forest Service and University of Minnesota.
USDA Forest Service - Northern Research Station
Last Modified:  05/25/2006