Riparian understory response to upland harvest along small streams in the Washington Cascades

Background/Question/Methods

Riparian areas are ecologically important: they add to biological and structural diversity, provide habitat for a variety of species, and maintain thermal regulation and bank stabilization of streams.  Riparian buffers intended to protect aquatic resources are an integral part of forest management in the Pacific Northwest.  Although there has been considerable research evaluating the effect of buffers on aquatic systems, relatively little is known about how upslope logging impacts the ecological characteristics of the buffers themselves.  The objective of this study is to characterize and compare forest understory community composition and species diversity between riparian buffers and adjacent harvest areas over time.  We conducted a chronosequence study at 70 sites along small forested streams in western Washington.  Each site consisted of a riparian buffer 50-70 years old and an upslope managed stand, which were categorized into 7 age classes (1-2, 3-4, 5-6, 7-10, 11-30, 31-50 and 51-70 years since harvest).  At each site, plant cover and species richness were recorded on understory vegetation plots (1 m²) established along transects located in the buffer and harvest area.  Data were analyzed using various multivariate techniques, including ANOVA, PERMANOVA and NMDS.

Results/Conclusions

Species compositions differed between riparian and upslope communities within each age class.  In riparian buffers adjacent forestry activities did not effect species richness over time but plant cover varied with the highest cover (82%) occurring 7-10 years post harvest.  Upslope managed forests sites increased simultaneously in species richness and plant cover three years post harvest, peaked by year eleven, and declined in following years as tree canopy cover increased.  Our data suggest that riparian areas along small forested streams in this area have a unique understory plant community that can temporarily be altered by adjacent forest harvesting activities, but the effect of management activities is short-lived.