Graduate Thesis Or Dissertation
 

Diversity and abundance of birds across a hardwood gradient in early seral Douglas-fir plantations

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  • Recent evidence suggests that population declines of some avian species may be driven primarily by reduced quantity and diversity of early-successional habitat on the breeding grounds. Increasing intensity of forest management on private lands and decreased harvest rates on federal lands has resulted in a loss of the diverse early-successional stage of forest succession, typically called early seral forest. Previous studies suggest that the amount of early seral broadleaf cover within conifer forests is important to foliage-gleaning bird communities. However, information regarding bird abundance, diversity, productivity and juvenile post-breeding habitat use in highly modified plantation habitat is scant. I investigated the association between broadleaved hardwood cover and avian abundance and diversity in intensively managed early seral Douglas-fir (Pseudotsuga menziesii) stands of the Pacific Northwest. I examined this relationship at the community-level, within the insectivorous foliage-gleaning guild, and for five leaf gleaning Neotropical migrant species: orange-crowned warbler (Oreothlypis celata), Wilson's warbler (Wilsonia pusilla), MacGillivray's warbler (Oporornis tolmiei), Swainson's thrush (Catharus ustulatus), and black-headed grosbeak (Pheucticus melanocephalus). Bird species richness decreased across an elevational gradient, but did not appear to vary as a function of either local vegetation composition or structure. In contrast, bird abundance was strongly associated with hardwood cover at local and landscape scales, especially for foliage-gleaning species. We found strong support for the existence of a threshold in relative bird abundance as a function of hardwood at the stand scale; abundance doubled with an increase from 1% to ~6% hardwood and then reached a plateau. Though abundance of leaf gleaners increased even more strongly across a gradient in hardwood, evidence for a distinct threshold was less clear. Though in some cases unexplained variation was quite large, all species except MacGillivray's warbler were strongly positively associated with early seral hardwood cover as fledglings, breeding adults, or both. Management practices aimed at retaining increased amounts of early seral hardwood cover at the stand level is thus likely to accommodate a greater number of breeding pairs and young. If adopted at landscape and regional scales, such practices may positively influence population trends of many declining bird species. We conclude that when early seral hardwood forest is scarce, even small increases in hardwood may provide substantial conservation benefits. However, for some species (i.e., foliage gleaners), there may be more direct trade-offs in abundance and juvenile recruitment with management intensity.
  • Keywords: plantation, bird diversity, early seral forest, Pacific Northwest, ecological thresholds, intensive forest management, hardwood, bird conservation, clearcut
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