Do aggregated harvests with structural retention conserve cavity users in old forest in the boreal plains?

摘要

Many cavity-using vertebrates are dependent on large-diameter trees in old forest and thus are at high risk from clearcutting. Old forest species may be better conserved if harvesting is more similar to natural disturbances: trees and forested patches left in cutblocks (structural retention) and cutblocks spatially aggregated into large (1000s ha) harvests. I evaluated the short- and long-term effects of spatially-aggregated harvests with structural retention on cavity users in old upland forest in the boreal plains of Alberta and Saskatchewan. First, I described the interactions between cavity excavators and non-excavators (i.e. cavity web) by tracking reuse of 317 cavities from 2004 to 2007 in two unharvested and two aggregated-harvest landscapes. Three species produced the majority of cavities reused by six avian and mammalian non-excavators in unharvested forest: the dominant excavator (yellow-bellied sapsucker ( Sphyrapicus varius)) and two keystone excavators (northern flicker (Colaptes auratus), pileated woodpecker (Dryocopus pileatus)). These woodpeckers retained their roles as keystone and dominant excavators in aggregated harvests but numbers of sapsuckers declined and flickers increased. Next, I examined multi-scale nest site selection by excavators. Large (>35 cm) aspen with numerous false tinder conks ( Phellinus tremulae) were important nest trees for most woodpeckers. In aggregated harvests, sapsuckers preferred forest patches >5 ha and composed of mature and old upland stands whereas flickers selected open cutblock areas for nesting. Finally, I evaluated structural retention as a long-term, regional conservation strategy by simulating 150 years of aggregated harvest with different retention levels (0-20%) and patch size distributions. While 20% retention performed better at conserving old upland forest than low (5%) and no retention, fewer large patches and less mature forest were conserved. No retention strategy consistently conserved more habitat than aggregated harvest alone for four focal species (sapsucker, pileated woodpecker, black-capped chickadee, northern flying squirrel (Glaucomys sabrinus)), which all experienced significant habitat losses. Overall, I conclude that structural retention that targets key habitat requirements is a valuable short-term strategy for conserving old forest cavity users in spatially-aggregated harvests. However, long-term, regional conservation of the old forest cavity web requires additional strategies, such as reduced harvest volumes and permanent and floating reserves.