Diversity of Shrub-Dwelling Arthropods to Thinning of Young Douglas-Fir Forests in the Pacific Northwest, USA

Abstract

Abstract We examined an unthinned control stand and three different thinning intensities in 40- to 60-year-old Douglas-fir forests in western Oregon to assess the effects of thinning on the abundance, species richness, and diversity of shrub-dwelling arthropods. Shrub-dwelling arthropods, collected with a bagging technique, decreased significantly as thinning intensity increased in deciduous foliage (vine maple, Acer circinatum Pursh), but arthropods showed no response to thinning on the two types of conifer foliage (Douglas-fir, Pseudotsuga menziesii [Mirb.] Franco and western hemlock, Tsuga heterophylla [Raf.] Sarg.). The species richness and diversity of shrub-dwelling arthropods showed significantly higher values on the coniferous branches than on the deciduous foliage. Functional group composition for the two foliage types revealed consistently different proportions. Deciduous branch supported a higher proportion of sapsuckers and leaf chewers, and conifer branch hosted a higher proportion of predators and detritivores. Nonmetric multidimensional scaling ordination showed a distinct difference between the species assemblages inhabiting the deciduous and conifer branches. The effects of thinning on the arthropods were complex. Seasonal effects on arthropods exceed treatment effects because the species composition in the dry season was very different from that in the wet season. The shrub-dwelling arthropod community reveals little seasonal difference on coniferous branch, but significant differences occur on deciduous branch. In this study, treatment effects were very limited, but our results indicate that as thinning increased, the response of arthropod community on deciduous branch increased more. This large-scale experiment provided an opportunity to assess responses of shrub-dwelling arthropods to levels and patterns of forest management.