A Comparison of the Breeding Bird Communities in Seral Stages of the Englemann Spruce Sub-alpine fir zone in East Central British Columbia
The bird communities of ESSFwk1 forests in the Cariboo Forest Region are rich in species when compared to other local biogeoclimatic subzones. Forest management activities in this subzone will result in changes in the availability of seral stages of forest, and bird species that require late seral forest will face the largest reduction in habitat. Knowledge of the bird species and required habitat attributes in the different seral stage communities can be used to evaluate and refine biodiversity guidelines.
This study found that bird communities in early seral (<40 years), mid-seral (40-120 years), and late seral (>120 years) forests were not significantly different in richness or diversity. Although bird community composition was very similar between mid- and late seral forests, there were a few species, such as Winter Wren and Boreal Chickadee, that were significantly more abundant in late seral forests. The similar bird communities in mid- and late seral forests may be due to structural similarities in the forests, such as greater basal area and live crown volume (CDI). In contrast to mid- and late seral forests, early seral forest had very low basal and CDI, but greater amounts of grass and forbs. This resulted in a bird community distinct from that in older forests. Alder Flycatcher and Olive-sided Flycatcher were exclusive to early seral forests. The abundance of four other species was far greater in early seral stands than in older forest (Warbling Vireo, Orange-crowned Warbler, MacGillivrayís Warbler, and Lincolnís Sparrow)>
Two habitat attributes, basal area and CDI, have the strongest correlation with bird abundance at both the community and individual species level. These features are negatively correlated with both the total number of observations and the number of territorial observations. At the individual level, the sign of the correlation is negative for species that prefer early seral forests and positive for those preferring older forests. For species such as Winter Wren, Varied Thrush, Alder Flycatcher, MacGFillivrayís Warbler, and Lincolnís Sparrow, the level of basal area where crown closure occurs results in a strong shift in relative abundance. The abundance of species associated with older forest, such as Winter Wren, increased dramatically in stands with basal area values greater than this level, while the abundance of species preferring early seral forests (like Alder Flycatcher), was very high in stands with basal area values below this level, and very low in stands with basal area values greater than this level.
The volume of coarse woody debris (CWD) per hectare was positively correlated with the abundance of Winter Wrens and Wilsonís Warblers. Species such as Boreal Chickadee, Red-breasted Nuthatch, and Mountain Chickadee were not significantly correlate with the density of wildlife trees, and wildlife trees in late seral forests may not currently be in limited supply for these secondary cavity nesters. The abundance of other species that use wildlife trees was too low for analysis.
The herb-shrub stage of forest development is important to many species. Several bird species that prefer early seral forests had positive correlations with the cover of forbs and grasses (Wilsonís Warbler, Lincolnís Sparrow and Chipping Sparrow). For these species, vegetation management prescriptions that prolong the duration of the herb-shrub stage will have the greatest benefit.
Forest management will result in
a reduction in the availability of late seral habitat. The results
of this study were used to model changes in species abundant through time,
given the various seral stage options in the Forest Practices Code Biodiversity
Guidebook (1995). In all cases, the area of mature and late seral
forest will decrease, resulting in declines in the abundance of species
such as Winter Wren and Boreal Chickadee. Conversely there will be
more habitat available for early seral stage species such as Orange-crowned
Warbler and Warbling Vireo, and their abundance will increase accordingly.
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