Thinning of Mature Lodgepole Pine Stands Increases Scolytid Bark Beetle Abundance and Diversity

Thinning of forests has been used as a management tool for bark beetles; however, its effects have only been studied in a limited number of bark beetle species, and the causes of its effectiveness remain unclear. We sampled the abundance and diversity of secondary bark beetles in mature thinned and unthinned lodgepole pine, Pinus contorta Dougl. ex Loud., stands (ca. 840 and 2500 trees/ha, respectively) near Whitecourt, Alberta. We examined the factors that might influence any differences in abundance and diversity between thinned and unthinned stands. Breeding habitat for secondary bark beetles (fresh coarse woody debris) was much more abundant in thinned stands than in unthinned stands in the first year after thinning but then returned to background levels. Temperature and wind speeds were higher in thinned stands in all 3 years after thinning. The abundance of striped ambrosia beetles, Trypodendron lineatum Olivier, and pine engravers, Ips pini (Say), captured in baited funnel traps and window traps remained significantly higher in thinned stands than in unthinned stands in all 3 years after thinning, while the diversity of bark beetles remained constant or increased over this period. Our data suggest that the persistent changes in microclimate following thinning, especially increased wind, were partly responsible for thinned stands having more secondary bark beetles than unthinned stands