Effect of thinning on flying insect communities using window traps in young douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests in the Pacific Northwestern America

Abstract

The response of insects to four thinning intensities (control, light-thin, heavy-thin, or light-thin with gaps) was examined in 40-to 60-year-old Douglas-fir stands on the Willamette National Forest in Oregon, USA. In both 2000 and 2001, flying insect assemblages were collected from window traps placed over the forest floor in the center of each treatment block during two annual intervals: 1 ) June through August (early-season) and 2) August through October (late-season). The seasonal responses by these insects showed statistically significant differences in species richness (F= 22.21,P 0.05) and L/G (IV=29.2, P ≤0.05). For all thinning treatments, the numbers of species and individuals were higher in the early season than the late season. Non-metric multidimensional scaling showed that Axis 1 (65%) and Axis 2 (27%) explained 92% of the variance. The former was strongly associated with thinning intensity, having higher coefficients of species richness (r=0.645) and species diversity (r=0.583). The multi-response permutation procedures showed a statistically significant difference for thinning intensity (T-statistics=-4.6322,A-statistics=0.0479,P <0.0001 ). These results suggest that heavier thinning can result in more diverse populations of flying insects, including herbivores and predators.