Forest Residuals and Bird Communities

Variation in bird communities among harvest areas depended on the type and amount of residual “skips” retained in those areas. Plots in harvested areas that had abundant high shrubs and few large trees (or snags) hosted bird species that are commonly associated with open forest, parkland, and shrubby habitats in boreal and sub-boreal regions of western North America.

This section is adapted from Schieck, J., K. Stuart-Smith, and M. Norton. 2000. Bird communities are affected by amount and dispersion of vegetation retained in mixedwood boreal forest harvest areas. Forest Ecology and Management 126:239–254.

This study found that as the amount of live residuals and snags increased, bird species associated with mature and old boreal forest became more common. Conversely, bird species associated with open country and shrubby areas became less common with more residuals. That pattern was consistent for all of the three studies analysed here. Although the relationship between birds and residuals was difficult to separate, large deciduous trees were more closely related to bird communities than large coniferous trees or large snags. 

Some bird species (e.g., Warbling Vireo, Western Tanager) appeared to respond more, large deciduous trees, other bird species (e.g., Brown Creeper, Black-throated Green Warbler) appeared to prefer more large coniferous trees, and other species (e.g., Hairy Woodpecker) probably responded to increases in large snags. 

This study found that bird communities in harvest areas were more similar to those in old-growth forest when >80% of the large residual trees were retained in clumps. This pattern was consistent for all three studies and was present regardless of whether few or many large residual trees were retained in harvest areas. Clumps appear to retain key features of forest structure and microclimates. The effect of clumps on bird communities may have been underestimated in this study because tree size and composition varied greatly among clumps, diluting the strong relationship many boreal birds have with large trees.

Even though many bird species were common between harvested and old-growth plots the authors caution managers from assuming that residual clumps will be effective habitat for old growth forest birds. Many bird species had higher densities in old forest than they had in structured cut-blocks, even when >100 large trees per ha had been retained. This study did not evaluate the reproductive success of the birds in harvest areas, but these areas may be low quality habitat for some old-growth bird species because in other studies bird density was positively related to habitat quality (Donovan et al., 1995; Villard et al., 1993). Thus, only after more extensive studies will it be possible to evaluate whether retaining large trees in harvest areas can be used to replace some of the requirements for old-growth boreal forest in a harvest landscape. In the interim, it would be prudent to retain adequate amounts of old forest in the landscape. 

Although managers may wish to retain standing trees in cut-blocks so that these blocks retain some characteristics found in old-growth forest, that management strategy probably is not desirable for all cutblocks. Historically wildfire was the dominant mechanism resetting succession in boreal forests, but created areas with no live residual trees and other areas with many live residual trees. To match that variation, managers need to create some cut-blocks that have few or no residual live trees, other cut-blocks with scattered residual trees and still other cut-blocks with loose or dense clumps of residuals. By better understanding the impacts of tree retention on forest biodiversity, managers can develop plans that improve the integrity of native biotic communities in harvested landscapes.