Ecosystem consequences of forest fragmentation in the Pacific Northwest: Biogeochemical edge effects within old-growth forest remnants.

摘要

Possible mechanisms are presented for biogeochemical edge effects observed within oldgrowth Douglas-fir/western hemlock forests fragmented by timber harvests in the U.S. Pacific Northwest. In a systematic manner with distance into remnant forest, I tested the general hypothesis that edge-altered microclimate and structure initiate positive feedbacks linking abiotic and biotic responses. Mixed-effects statistical models and Spearman correlations were used to describe spatiotemporal patterns and interactions among these variables.; By modifying material and energy flows, structural changes near edges impact forest microclimate. I found reductions in cover and live-crown volume of plant canopies to be the most significant structural changes near edges. As evidenced by distance effects on basal area and relative importance, altered competition among dominant tree species may affect succession in edge environments. Extreme variability in temperature and reductions in both floor moisture and snow retention were the most notable microclimatic effects near forest edges. The results include significant negative correlations between tree live-crown volume and microclimatic variability. Relative to other forest types, the longer persistence of abiotic edge effects in north temperate coniferous forest may result from the tall stature and slow side-canopy closure of the long-lived tree species.; As identified in the conceptual model, the dynamics of soil organic matter at the soil-plant interface provide key information for understanding how abiotic effects influence basic biogeochemical processes near forest edges. I investigated mechanisms for edge effects on soil organic matter by focusing on the major above-ground fluxes of litter fall and decomposition. Reduced litter fall near edges appears largely due to depleted forest canopies. The interplay of microclimatic variability and the quantity and quality of the litter fall may be responsible for the more complex patterning of litter decomposition rates with distance into remnant forest. The decoupling of basic ecosystem processes near edges, including litter fall and decomposition, predicate the polymodal distance effects observed for biologically available nitrogen, uptake of nitrogen by plants, and forest productivity. These results indicate an accurate quantification of edge effects is required within fragmented forest landscapes, in order to assess the cumulative impact of land use on the processing of organic matter and nutrients important to ecosystem sustainability and conservation planning.