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Effects of Forest Management on Wildlife

 

Christopher Moorman
North Carolina State University

The central thesis of game management is this: game can be restored by the creative use of the same tools which have heretofore destroyed it – axe, plow, cow, fire, and gun.
                                                                                    Aldo Leopold, Game Management (1933)

Aldo Leopold, a forester and wildlife biologist, recognized that humans either over exploit forests for their resources or manage them for sustained use.  He also noted that land managers have a variety of tools with which to manipulate forests.  Timber harvests and prescribed fire, when appropriately implemented, often are the most practical means by which wildlife habitats can be managed.  Alternatively, where timber is not harvested and prescribed fires are not set, natural disturbances will shape forests and the wildlife habitats therein.  However, many natural disturbance regimes have been altered by humans (e.g., wildfire suppression, changes in flooding cycles below hydroelectric dams, invasion by non-native pests), so unmanaged forests generally will not yield habitat conditions similar to those common in the past, except perhaps in extremely large forests with undisturbed interiors. 

Forest management, especially timber harvest, affects wildlife habitats by altering succession and thereby plant communities.  For example, clearcutting a 100-yr-old mixed hardwood stand shifts the habitat from late successional to early successional plant communities.  Different wildlife species occur in different successional stages, so clearcutting would eliminate habitat for animals that prefer older forests but create habitat for species that use young, regenerating forests. 

Because habitat requirements vary so widely among species, it is impossible to provide habitat for all wildlife species at the same time in a small-scale area.  Generally, any management activity will improve habitat conditions for some animals and degrade conditions for others – there are always some winners and some losers.  For this reason, it is important for wildlife managers to identify focal wildlife species or communities and/or desired habitat conditions before initiating forest management practices.

In general, habitat quality, and hence wildlife species diversity, improves as plant species diversity and structural diversity of vegetation within and among forest stands increase.  Therefore, forest management practices that increase plant diversity and structure, especially ground and shrub cover, benefit wildlife.  However, the effects of management on wildlife often vary with region, forest type, forest area, soil type, climate, elevation, and many other environmental factors.  Managers must account for local variations in wildlife-habitat relationships when making prescriptions for forest management.

Final Harvest and Regeneration

 

Final Harvest – Two types of silvicultural systems are used to regenerate forests.  Even-aged systems regenerate an entire stand at once and contain trees of either one (e.g., clearcut) or two age classes.  Two-aged stands are created when some overstory trees are retained during the harvest (e.g., seedtree or shelterwood).  Uneven-aged systems use repeated, frequent harvests of scattered individual trees or small patches of trees, creating a stand with three or more age classes of trees.  Even-aged stands of timber contain shade-intolerant plant species that prefer full sun, whereas uneven-aged stands generally promote regeneration of shade-tolerant plants that grow in full or partial shade.  Group-selection harvest is a form of uneven-aged system that creates canopy gaps large enough (e.g., 0.5 ha) to allow sufficient sunlight to reach the forest floor so that more shade-intolerant species occur.

 
Habitat conditions created by the two silvicultural systems vary significantly.  Even-aged harvests yield large areas (usually >2.5 ha) of early-successional habitat for several years.  Such areas are favored by species attracted to dense undergrowth.  When some residual overstory trees are retained as in two-aged stands, both understory- and overstory-dwelling wildlife should be present.  When even-aged stands of timber mature, they offer large areas of homogeneous (i.e., vegetation structure within the stand is relatively consistent throughout), late-successional habitat. 
 

Horizontal habitat diversity may be high where even-aged harvests are used because a variety of different successional stages are present across the landscape.  Conversely, uneven-aged timber harvests, if applied broadly across a landscape, may limit horizontal diversity relative to even-aged harvests.  But, uneven-aged harvests yield heterogeneous conditions within a stand with many small canopy openings and high vertical vegetation structure (i.e., there are plants growing in all layers of the forest from the ground to the canopy).  Uneven-aged harvests can be used to mimic natural disturbance in stand types historically influenced by treefall dynamics, but care must be taken to avoid soil damage during the frequent entries for timber harvest and regeneration of desirable tree species can be difficult.
 

Regeneration – Stand regeneration can be accomplished either artificially with planting (i.e., plantations) or naturally using seed from residual trees (e.g., seed tree harvest), dispersal of seed from surrounding stands, or seed left in the soil.  Before plantations are established, sites often are prepared with heavy machinery, fire, or chemicals.  Site preparation helps control vegetation that might compete with crop trees.  Seedlings in plantations usually are spaced evenly and in rows. 
 
Soil disturbance and compaction during site preparation may kill fossorial wildlife or destroy their habitats.  Additionally, non-selective chemical control of competing vegetation may reduce plant species diversity and eliminate some sources of food and cover for wildlife (see Herbicides section).  Dense plantings, with limited space between seedlings, facilitate rapid canopy closure, which also reduces habitat quality as the understory is shaded out.  However, plantations managed to allow sunlight to the forest floor can provide excellent habitat for a variety of wildlife species.

Intermediate Stand Treatments

As a timber stand matures and the canopy closes, shaded understory vegetation dies and vertical structure declines.  However, wildlife diversity, especially bird diversity, generally increases with vertical height diversity of the vegetation (i.e., vertical height diversity, and the presence of vegetation in multiple layers of the forest, provides a high number of habitat niches).  An exception would be pine savannas that harbor high plant and animal diversity and are characterized by low vertical height diversity but lush understory cover.  Once the forest canopy closes, management activities that increase sunlight in the forest understory benefit wildlife by increasing vertical height diversity.  Intermediate stand treatments that open the forest canopy also improve growing conditions for crop trees. 

Thinning – Thinning is the cutting or removal of certain trees from a stand to regulate the number, quality, and distribution of the remaining crop trees.  Thinning opens the canopy and allows sunlight to reach the forest floor, which stimulates the understory and thereby can improve wildlife habitat quality.  Early thinning before timber is commercially valuable encourages growth of grasses and other herbaceous plants in the understory.  Heavy thinnings that significantly reduce canopy cover often are recommended for wildlife.

Prescribed Burning – Fire is a natural force that shapes plant and animal communities around the World.  Because many animal species evolved with fire, wildlife are well adapted to its presence on the landscape.  Many species benefit from fire and some even require it to produce habitat conditions necessary for survival.  Most species have behavioral adaptations to avoid being killed during fires.  Small mammals, reptiles, and amphibians remain underground or under rocks and downed logs, and larger animals and birds generally escape a fire by running or flying, respectively. 

Prescribed fire is set in a controlled manner to forest fuels under selected weather conditions to achieve a pre-determined management goal.  Prescribed burning commonly is conducted to reduce forest fuels and thereby reduce the potential for devastating wildfire, control competing vegetation, encourage regeneration of specific plant species, revert successional growth, control forest pests, and improve wildlife habitat.  Prescribed fire can improve wildlife habitat by: (1) encouraging lush groundcover of grasses and forbs that provide food and cover; (2) increasing palatability and nutrition of new growth for at least one year after a burn; and (3) increasing fruit, seed, and beneficial insect production.  When used in wildlife habitat management, prescribed burning generally is conducted with frequencies and intensities that mimic historical regimes.  
 

Dead Wood

Standing dead trees, called snags, and downed logs provide valuable microhabitats for a multitude of wildlife.  Animals use snags for nesting, roosting, and feeding.  Ground-dwelling species use downed logs for cover from predators and as travel corridors, and many animals feed on the arthropods attracted to the decaying wood.  Forest management can increase dead wood in a stand by killing live trees or leaving downed logging slash following harvests.  Conversely, removal of snags or logging debris during harvests adversely impacts habitat quality.  Specifically, piling and burning woody debris during site preparation lessens downed wood accumulations. 

 

During forestry operations, residual slash and several larger diameter snags should be left across the stand.  Forest plantations managed intensively to produce timber on short rotations tend to lack snags and downed logs of sufficient diameter to benefit wildlife; therefore, mature trees retained during final harvest increase large diameter dead wood in the subsequent rotation.  Also, prescribed fire in these plantations should be used with caution so large snags are not lost.

 

Herbicides

 

Technological advancements in chemical development and decreased use of prescribed fire have led to wide-scale use of herbicides in forest management, especially in conifer plantations.  Herbicides typically are used in young plantations to release crop trees from competitive vegetation, but they also may be used later in the rotation to limit understory or midstory competitors. 

 

 

Although herbicides pose relatively little risk of acute toxicity to wildlife when applied appropriately, their application can lead to dramatic changes in wildlife habitat conditions.  More frequent applications of non-selective herbicides can yield monocultures that offer poor habitat conditions.  Conversely, targeted application (e.g., spot or banded rather than broadcast) of single herbicides has relatively minor impacts on habitat quality.  Most research studies have determined that the impacts of herbicides on plant diversity are short-lived (e.g., <3 years), but less is known about repeated application of herbicides throughout a rotation or the use of tank mixes that contain multiple herbicides. 

 

Herbicides can be used to improve wildlife habitat.  Because of the increasing selectively of chemicals, applications of specific herbicides can favor plants beneficial to wildlife by eliminating exotic and undesirable plant species.  Herbicides also can be used to maintain open habitats in a permanent state of early succession or to improve the effects of other management practices such as prescribed burning or thinning.  For example, where a forest midstory has grown too dense to effectively use prescribed fire as a management tool, herbicides can be used to kill midstory stems so that fire can be used.  In forests where snags and downed logs are lacking, live trees can be injected and killed using herbicides.  

 

Wetlands

 

River bottoms, depressional ponds, and other wetlands are high quality wildlife habitats.  Wetland habitats typically are characterized by high plant diversity and complex vegetation structure; they also are rich in insects and other arthropods and fruit- and nut-bearing plants.  Wetlands provide important habitat for reptile and amphibian species that require water during some part of their life cycle.  In dry regions, wetlands provide an important source of drinking water.  Most waterfowl species, such as ducks and geese, require wetlands year round.  Ephemeral pools, which hold water only during a portion of the year, often are free of fish and provide critical breeding sites for salamanders and other amphibians. 

 

 

Because of their extreme value to wildlife, wetlands should be protected during forest management activities.  Before initiating timber harvests, no-harvest buffers can be established parallel to rivers and streams or around small, isolated wetlands.  Forested buffers lessen water temperatures via shading and prevent sediment from logging activities from moving into wetlands.  Recommended buffer widths and management prescriptions within buffers vary based on the target wildlife species, stream width, intensity of adjacent harvest, and slope.
 

Landscape Issues

Spatial Arrangement – The distribution of certain wildlife species is affected by the arrangement (interspersion and juxtaposition) of stand types and ages on the surrounding land, and some species require multiple habitat types to survive. On the other hand, some species require large patches of a single habitat.  Instead of making management recommendations one stand at a time, it is important that decisions consider landscape-level conditions.  It also is critical that managers understand the relationship between target wildlife species and the surrounding landscape.  Dispersal of many successional stages, stand types, and stand sizes across a forested landscape can help to maximize wildlife diversity at large spatial scales.

 

 

Fragmentation and edge effects – Although timber harvests can fragment or divide wildlife habitats into smaller units, they do not fragment the forest.  True forest fragmentation occurs only when forest patches are isolated from one another by non-forest land uses such as agriculture or urbanization.  Fragmentation by non-forest land use divides remaining forest patches into smaller and more isolated blocks and decreases their suitability to wildlife species that prefer interior habitat away from the edge of a forest patch. 

 

 

Additionally, edges between forest and non-forest land uses may attract avian nest predators and brood parasites (e.g., brown-headed cowbirds), thereby leading to lower nest success of birds nesting in the forest patch.  These detrimental edge effects are especially pronounced in landscapes with extensive coverage of non-forest land uses.  Conversely, an edge between forest patches of different ages may harbor a high diversity of wildlife species, especially in landscapes characterized by extensive forest cover.  Edges between forest stands allow wildlife easy access to adjacent habitats and often are characterized by more complex vegetation structure.  Conversely some species require large contiguous forests, which are scarcer throughout the world.  Management for these wildlife species is particularly difficult and often less compatible with development, timber management, and people.

 

 

References

 

 

Allen, A. W., Y. K. Bernal, and R. J. Moulton. 1996. Pine plantations and wildlife in the southeastern United States: an assessment of impacts and opportunities. Information and Technology Report 3, USDI National Biological Service, Washington, DC.

 

Howell, D. L., K. V. Miller, P. B. Bush, and J. W. Taylor. 1996. Herbicides and wildlife (1954-1996): an annotated bibliography on the effects of herbicides on wildlife habitat and their uses in habitat management. USDA Forest Service Technical Publication R8-TP13.

 

Hunter, M. L. 1990. Wildlife, Forests, and Forestry: Principles of Managing Forests for Biological Diversity. Prentice Hall Career and Technology, Englewood Cliffs, New Jersey.

 

Russell, K. R., T. B. Wigley, W. M. Baughman, H. G. Hanlin, and W. M. Ford. 2004. Responses of southeastern amphibians and reptiles to forest management: a review. Pp. 319-334 in H. M. Rausher and K. Johnsen, (eds.). Southern Forest Science: Past, Present, and Future. USDA Forest Service General Technical Report SRS-75.

 

Smith, J. K., ed. 2000. Wildland fire in ecosystems: effects of fire on fauna. USDA Forest Service General Technical Report RMRS-GTR-42-volume 1.

 

Thompson, F. R., J. D. Brawn, S. Robinson, J. Faaborg, and R. L. Clawson. 2002. Approaches to investigate effects of forest management on birds in eastern deciduous forests: how reliable is our knowledge? Wildlife Society Bulletin 28:1111-1122.

 

 

 

Posted 8 September 2007

 

 

 

Updated 30 September 2007