LITTER INVERTEBRATE RESPONSES TO VARIABLE DENSITY THINNING IN WESTERN WASHINGTON FOREST

Abstract

We evaluated the response of forest floor invertebrates to variable density thinning (VDT) of the Douglas‐fir (Pseudotsuga menziesii (Mirb.) Franco) forest overstory at Ft. Lewis, Washington, during 2000 and 2001 (7–8 years post‐thinning). We placed pitfall traps at 8–12 random grid points in each of four thinned units and four control (unthinned) units in each of two sites (blocks) representing different management histories. Most taxa showed significant seasonal trends, with peak abundances during summer. ANOVA indicated strongly significant effects of site for 9 of 39 species and combined taxa, probably reflecting factors associated with management history; only two taxa showed significant responses to the thinning treatment or to block × treatment interactions. Indicator analysis revealed three spider species as potential indicators of thinning treatment. Detrended correspondence analysis (DCA) and cluster analysis for 85 species indicated that the thinning treatments altered the invertebrate assemblage in different ways, depending on initial structure. Multiresponse permutation procedure (MRPP) confirmed that species assemblages, but not combined taxa or functional groups, in treated units differed significantly from those in control units and from each other. Therefore, VDT affected forest floor invertebrates in the short term, but the effect was strongly modified by site (block) factors, especially management history. These results indicated that previous management history is an important determinant of treatment effect and that compensatory shifts in relative importance among species within functional groups may maintain ecological function during environmental changes.