|GUIDELINES for||. . .|
|Developing Stand Density Management Regimes|
Stand density affects a range of forest resource values apart from timber, including wildlife habitat quality, wildlife use, species diversity and water yield. These values are all influenced by stand characteristics such as structure (canopy coverage, tree size, species mixture), pattern of tree mortality (large and fine woody debris, standing snags), and understorey species composition (abundance and vigour), as well as rate and direction of change (dynamics). Stand density management practices may have a positive, negative or neutral effect on other resource values by modifying these stand characteristics.
The report Stand Tending Impacts on Environmental Indicators (Greenough and Kurz 1996) identifies some of the effects that density management practices (pre-commercial and commercial thinning) may have on wildlife habitat, as measured by the following habitat indicators:
Thinning practices decrease the overstorey crown coverage of a stand, and delay crown closure in proportion to the intensity of thinning. The production of understorey shrubs and herbs responds inversely to overstorey cover, and is proportional to the intensity of sunlight reaching the forest floor. The timing of thinning (stage of stand development) also influences the quality of the understorey forage production response. For example, properly timed thinning in coastal ecosystems may enhance berry production, whereas a delay of 10 or more years following stand crown closure often yields a poor response of berry-producing shrubs due to exhaustion of the understorey bud bank.
Coarse woody debris production within a stand is a prerequisite for many species of fungi, plants, insects and small animals. Stand density management practices affect the volume and size of coarse woody debris produced in a stand. Relative to unthinned stands, thinning tends to reduce the total volume, but increases the average size of coarse woody debris created over a managed rotation. The effect of thinning on debris-dependent habitats is further complicated by stand dynamics; for instance, reduced competition and mortality following thinning delays the production of large diameter, coarse woody debris.
A similar relationship exists between thinning and standing snags. Habitat values, which depend on the functional maturity of stands, are affected by changes in the size and frequency of standing snags, total volume and production rate of coarse woody debris, and the size distribution of large living trees. The effects of thinning on the quality of these habitats may be predicted by considering each factor and its response over time. However, the likelihood of co-ordinating factors to create favourable conditions presents a difficult prescriptive challenge. Some species of plants and animals are more critically dependent on stand age rather than functional structural characteristics.
Stands usually support a number of different species. Stand density management practices can be designed to immediately enhance habitat quality for one or several species. However, these changes may result in decreased habitat quality for other co-dependent species. Long-term thinned stand dynamics increase, reduce or reverse the initial relationship. No stand or density management treatment can possibly meet the needs of all wildlife species through all stages of stand development.
In addition to the Biodiversity Guidebook, the following reference guides discuss the relationship between density management regimes and critical habitat needs for various wildlife species:
Managing Identified Wildlife: Procedures and Measures5
Guidelines for Integrating Grizzly Bear Habitat and Silviculture in Coastal British Columbia.6
Deer and Elk Habitats in Coastal Forests of Southern British Columbia (Nyberg and Janz 1990)
Handbook for Timber and Mule Deer Management Co-ordination on Winter Ranges in the Cariboo Region (Armleder et al. 1986)
Guidelines for Maintaining Biodiversity during Juvenile Spacing (Park and McCulloch 1993)
In order to balance a range of environmental, habitat, forage and timber requirements, a single, optimum density regime is neither feasible, nor desirable. Consequently, it is unwise to apply an economically optimum density management regime for timber production uniformly to all stands. Non-standard density regimes may be necessary to improve or maintain critical habitats under certain management scenarios.
A clear relationship does not exist between stand habitat quality, species utilization and population size. Factors other than local habitat quality affect the presence of a species in space and time. Wildlife habitat quality must be evaluated and planned at the landscape level, since the current and future distribution of different stand conditions, or the uniformity of stand conditions are important factors in assessing the habitat impacts of stand-level actions. Thinned and unthinned stand dynamics result in changes in the proportion, spatial distribution, linkage and fragmentation of habitats, all of which play a critical role in species distribution and biodiversity. Readers are encouraged to review Conservation biology principles for forested landscapes (Voller and Harrison 1998) for a better understanding of the effects of forest management disturbances in the landscape.
The effects of manipulating stand density on water quantity and quality are poorly understood. The current Watershed Assessment Procedures guidebooks compare unlogged forest conditions with newly logged areas to estimate the rate of recovery as the new stand regenerates and grows. These estimates are based on the assumption that, as the regenerated stand height increases, there is a proportionate decrease in snow accumulation and snowmelt rates. The effects of stand espacement and thinning intensity on the hydrological recovery rate is not known.
In a recent study in the Kamloops Forest Region, the snow accumulation and melt of a thinned and pruned, 6 to 8 m tall lodgepole pine stand was compared to a similar, but unthinned stand, and a recent clearcut. The study indicated similar values of snow water equivalent (SWE) for the three stand types at the onset of springtime snowmelt. All three stand types had considerably more snow water equivalent (SWE +40%) than an adjacent stand of mature Engelmann spruce, subalpine fir and lodgepole pine (SeBl[Pli]). The rate of snowmelt in the two young lodgepole pine stands and the clearcut was faster than in the mature stand. This study suggests that the hydrological recovery to old forest characteristics may be slower than indicated in the guidebook recovery curves. It does not, however, show any hydrological effect of thinning.
Many site and stand factors may be affected by stand density management practices; some may enhance the productive capacity of the stand, while others may introduce elements of uncertainty and risk. For instance:
The characteristics of the site and stand may indicate the potential significance of these biotic and abiotic responses to density management decision making.
Copyright 1999 Province of British Columbia