Effects of current forest practices on organic matter dynamics in headwater streams at the Trask river watershed, Oregon

Abstract

Forest management in riparian ecosystems can significantly alter biotic and abiotic processes in streams. Forest harvest without the retention of buffers along small streams can affect organic matter dynamics, and drive instream characteristics like trophic food webs. To investigate the extent to which differing levels of tree retention adjacent to the channel mitigated changes in organic matter dynamics, we examined coarse particulate organic matter delivery, transport, and retention, as well as canopy cover, along small streams with four harvest treatments, both before and after harvest. Our research was part of a larger effort of the Trask River Watershed Study (TRWS) in the northern Oregon Coast Range, which examined the long-term physical, chemical, and biological effects of forest management on aquatic ecosystems at multiple spatial scales. Canopy cover at reference and treatment sites prior to harvest was approximately 91%, but following harvest, mean canopy cover at treatment sites decreased with increasing harvest intensity: 3% decrease after clearcut with buffer, 14% after clearcut with leave trees, and 57% after complete clearcut. Organic matter delivery (i.e., the amount of leaf, needle, wood, reproductive, or miscellaneous litter material) to streams was dominated by leaves and varied seasonally but decreased overall after harvest with complete clearcut. Organic matter transport (i.e., the amount of material netted during a sampling period at the end of each stream reach) values fell within the observed range of reference values at all harvested sites. Organic matter retention (i.e., the amount of material sampled in plots systematically placed along the stream reach) post-harvest was dominated by woody material and was consistent with pre-harvest measurements and reference sites for all the harvested streams. The number of log jams, and the total weight of log jams, increased for all treatments after harvest, although the most dramatic increase in log jam weight (140% increase) was observed for the complete clearcut and clearcut with leave tree sites. On these five sites, our results indicated that while complete clearcutting reduced organic matter delivery, retention of even a small number of trees along these streams appeared to support litter delivery rates after harvesting comparable to before harvest. We acknowledge the limitations of results from single treatment sites; however, our results were generally consistent with studies that had full replication. As part of the Trask River Watershed study, our research adds to the comprehensive effort to understand the ecological significance of riparian buffers and forest harvest in headwater streams.