The process of silvicultural thinning has become very controversial recently with regards to fire protection and management for old-growth conditions and biodiversity. Therefore, an unthinned control stand and 3 different thinning intensities were examined for their effects on the abundance, species richness, and diversity of arthropods in thinning treatments of silvicultural practices. Study sites were 40–50 year-old young stands of typical plantation Douglas-fir forests in the Willamette National Forest, Oregon. Shrub-, ground-, and litter-dwelling arthropods were collected with a bagging technique, pitfall traps, and Berlese extraction during 2000 and 2001.; Abundance of shrub-dwelling arthropods decreased with the thinning intensity for deciduous foliage, but did not show any response for conifer foliage. Species richness and diversity of shrub-dwelling arthropods showed higher values in the conifer foliage types. Functional group composition for the two foliage types revealed consistently different proportions; the deciduous foliage type had a higher proportion of plant suckers and the conifer foliage type had higher proportion of predators and detritivores. NMS ordination (Non-metric Multidimensional Scaling) showed a very distinct difference between the species inhabiting the two contrasting foliage types.; Abundance and diversity of ground-dwelling arthropods were higher in Heavy Thin and Light Thin with Gap treatments than the Control and Light Thinning treatments. Five groups of arthropods with relatively high abundance (such as Formicidae (ants), Araneae (spiders), Carabidae (ground-beetles), Gryllacrididae (camel-crickets), and Polydesmida (millipedes)) permitted in depth analysis. Four groups (i.e., ants, spiders, camel-crickets, and millipedes) were more abundant in the more intense thinning treatment areas. However, the abundance of Carabidae (ground-beetles), the third most abundant group, was higher at the unthinned control than in any thinning treatments; densities were much higher during the wet season than dry season. NMS ordination showed that seasonal effects outweighed the thinning effects. Though the disturbance associated with thinning would be expected to decrease populations and density of fauna, I hypothesize that the principal effect of the thinning disturbance was to increase habitat heterogeneity and subsequently species richness.; Abundance of litter-dwelling arthropods decreased in proportion to the thinning treatments. The litter-dwelling fauna was primarily correlated with seasonal moisture and secondarily positively correlated with thinning intensity. The proportion of predators decreased with the advancing seasons.
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