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

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 <0.001) and mean abundance (F=34.87, P <0.001). The greatest numbers of taxa. generated via indicator species analysis were in the early season and within the treatment of light-thin with gap (L/ G). in particular, the woodborer beetle, Buprestis (Burprestidae), was strongly associated with L/G (indicator Value = 52.4, P <= 0.0011). Two beetle taxa, Ampedus (Elateridae, IV=84.7, P <= 0.001) and members of Cermabycidae (IV=58.4, P <= 0.001), were correlated significantly with early-season, whereas Melanoplus (Acrididae) was associated with late-season (IV=21.3, 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.