BIRD COMMUNITIES IN THINNED VERSUS UNTHINNED SIERRAN MIXED CONIFER STANDS

Abstract

We used point counts and nest monitoring to compare avian community composition and nesting success in thinned and unthinned stands of commercially managed Sierran mixed conifer forest. We conducted point counts and monitored 537 active nests of 37 species on 10 study plots during three consecutive breeding seasons in the northern Sierra Nevada. All 10 study plots had experienced a similar long term man- agement history that included fire suppression and single-tree selection logging, but five of the plots also un- derwent a protocol of combined commercial and biomass thinning 5-8 years prior to the beginning of the study. Pooling species by nest substrate, we found that detections of ground-nesting bird species were similar on thinned and unthinned plots, but we detected canopy-, cavity-, and especially shrub-nesting species much more frequently on the thinned plots. Nest success rates were not statistically different between thinned and unthinned plots for ground-, shrub-, canopy-, or cavity-nesting species. Thinned stands were characterized by significantly less canopy cover, significantly lower density of small and medium conifers, and significantly greater understory cover and deer brush (Ceanothus integerrimus) cover than the unthinned stands. We surmise that the thinning protocol stimulated vigorous shrub growth, and conclude that forest conditions associated with a relatively open canopy and a well-developed shrub understory are highly beneficial to numerous breeding bird species in the Sierran mixed conifer community, including many species that may not nest or forage in the understory. Forest thinning that promotes vigorous shrub growth may correlate with an increased abundance of nesting birds, at least within stands affected by historical fire suppression and single-tree selection logging. Received 1 October 2002, accepted 5 March 2003. Logging practices and human-altered fire regimes have changed forest structure and composition across much of the Sierra Nevada since the mid-Nineteenth Century (Franklin and Fites-Kaufmann 1996, Gruell 2001). Tim- ber harvest practices and fire suppression throughout much of the region generally have reduced the frequency of low intensity fires, reduced the number of large trees, increased the density of smaller trees, and possibly re- duced the extent of shrub cover (Weaver 1974,