Effects of Coarse Woody Debris and Soil Depth on the Density and Diversity of Soil Invertebrates on Clearcut and Forested Sites on the Olympic Peninsula, Washington (original) (raw)

1998, Environmental Entomology

Arthropod density and species richness were determined using high-gradient extraction for soil on clearcut and late successional forests on the Olympic Peninsula, WA. Study objectives were to compare the effects of harvesting at different soil depths and dishUlces from coarse woody debris. Canonical correspondence analysis (CCA) was used to determine the effects of soil moisture, CO2 evolution, and 0 and A horizon depth on species composition. Soil depth had a greater effect on changes in density than did distance to woody debris. Densities of Acari, as well as total species richness, were significantly higher at 0-5 em depths than at 5-10 em despite significantly lower moisture content at the soil surface. Density of Acari and Coleoptera and species richness were significantly higher on forested sites than on clearcuts. There was a trend toward higher Collembolan density on clearcuts, but the difference was not significant because of the very different responses on dry and wet clearcuts sites. The results of CCA suggests moisture and 0 and A horizon depths were the most important quantitative environmental factors we measured in determining differences in species composition. Clearcutting appeared to have site specific-effects on wet and dry site locations. Low moisture and temperature extremes did not appear to limit micro arthropod density. High rainfall accentuated by clearcutting was associated with lower 0 and A horizon depths and appeared to decrease Collembola and Coleoptera density on the wet clearcut. KEY WORDS coarse woody debris, arthropods, clearcutting, soils BECAUSE OF HlCH rates of input and relatively slow decomposition rates, forest floors in the Pacific Northwest are characterized by some of the highest biomass of coarse woody debris of any forest ecosystem measured (Spies and Cline 1988). Forests of the western Olympic Peninsula, WA, have the highest biomass measured with reports of up to +500 Mg. ha-J (Agee and Huff 1987). An increasing body of literature has identified woody debris as a critical habitat component for a number of vertebrate, invertebrate, and microbial organisms (Harmon et al. 1986, Speight 1989, Caza 1993, Samuelsson et al. 1994). Coarse woody debris is defined by logs >10 cm in diameter and one meter in length (Harmon and Hua 1991). Although, in the Pacific Northwest, a complex soil environment and a relatively mild climate is thought to contribute to an especially diverse and abundant microinvertebrate community (Moldenke 1990), the direct contribution of woody debris to microarthropod biodiversity is not fully known. The presence of woody debris is thought to enhance the diversity of soil organisms by increasing the physical, structural, and chemical heterogeneity of the forest floor (Warren and Key 1991). In addition woody debris also may be critical to the maintenance of the biological properties of the forest floor by contributing to soil organic matter, maintaining soil stability and increasing soil moisture levels (Amaranthus et al. 1989, Harvey et al. 1989, Edmonds 1991). Although annual precipitation is higher in the Pacific Northwest compared with most other parts of the United States, summers can be relatively dry with 90% of the average rainfall occurring from October to May (Franklin and Waring 1980). On dryer sites, summer soil moisture levels can approach levels that begin to inhibit ecosystem processes like primary production and litter decomposition (Edmonds 1979, Franklin and Waring 1980). The biological properties of the forest floor also can be affected by summer drought, particularly if these conditions are further accentuated by clearcut harvesting (Harvey et al. 1976, 1979; Perry et al. 1987). Clearcutting can reduce the abundance and diversity of soil arthropods primarily by reducing organic matter and by increasing temperature and moisture extremes beyond the tolerance range for most arthropods (Vlug and Borden 1973;