CHANGES IN BIRD COMMUNITIES IN BOREAL MIXEDWOOD FOREST: HARVEST AND WILDFIRE EFFECTS OVER 30 YEARS

Abstract

A current paradigm in conservation biology is that forest harvest practices that better approximate natural disturbance processes are more likely to conserve biodi- versity. We contrasted bird communities in three replicate stands in each of 1, 13-15, and 22-28 yr old forests following wildfire and harvest in north-central Alberta, Canada. Stands were chosen from old (>120 yr) boreal mixedwood forests having -95% of the canopy trees killed during fire, and harvested sites retaining an average of 6% of the pre-harvest canopy trees. For all age classes, postharvest sites tended to have greater bird abundance. Species composition also differed between these treatment types. Two-Way Indicator Spe- cies Analysis (TWINSPAN) identified five major ecological groupings of species that dif- fered between wildfire and harvest, and among stand ages. Correspondence analysis (CA) identified similar bird communities. Greatest differences between bird communities oc- curred immediately following disturbance, and gradual convergence of communities oc- curred throughout the first 28 yr after disturbance. Species associated with open shrub and grassland or riparian habitats were associated primarily with 1-yr postharvest stands. Three- toed Woodpeckers (Picoides tridactyla) and Black-backed Woodpeckers (P. arcticus), to- gether with other species that use snags for foraging or nesting, occurred primarily in 1- yr postwildfire stands. Convergence in avian communities was correlated with the loss of standing snags on postwildfire sites. However, differences in bird communities were ap- parent up to 28 yr following disturbance, and this lack of complete convergence has im- portant consequences for sustainable forestry practices designed to maintain biodiversity in the boreal mixedwood forest. Notably, Connecticut Warbler (Oporornis agilis), Brown Creeper (Certhia americana), Winter Wren (Troglodytes troglodytes), and American Robin (Turdus migratorius) had higher densities on postwildfire than on postharvest stands. Lin- coln's Sparrow (Melospiza georgiana), Alder Flycatcher (Empidonax alnorum), Tennessee Warbler (Vermivora peregrina), Black-and-white Warbler (Mniotilta varia), American Red- start (Setophaga ruticilla), Mourning Warbler (Oporornis philadelphia), Rose-breasted Grosbeak (Pheucticus ludoviciana), Canada Warbler (Wilsonia canadensis), and Pine Siskin (Carduelis pinus) had higher densities on postharvest stands, possibly due to the greater abundance, after harvest, of larger live residual trees and a taller and more dense shrub layer. Harvest designed to approximate stand-replacing fires may require the retention of more snags than is currently practiced. New approaches to fire salvage logging are also required to ensure adequate retention of standing dead trees on the landscape.