The effects of riparian tree harvest intensity and woody debris addition on biotic and abiotic stream characteristics.

摘要

Forested headwater streams rely on riparian zones for many critical products and services. Riparian areas shade streams, stabilize stream banks, filter sediment, and provide allochthonous materials like leaves, large woody debris (LWD), and terrestrial invertebrates (TI's). Logging can affect the products and services of riparian zones. Best management practices (BMP's) have been developed to minimize the impacts of riparian timber harvest on streams. The main component of these guidelines is the establishment of a road-free riparian buffer called a streamside management zone (SMZ). The West Virginia Division of Forestry's BMP's require a SMZ that is at least 30 m wide for perennial and intermittent streams. There is no harvest limit within the SMZ, although heavy equipment cannot be operated within the area. This study quantified the effect of LWD additions and riparian tree harvest intensity at 50% and 90% basal area harvest (BAH) on various biotic and abiotic characteristics of eight Appalachian headwater streams.;Stream temperatures and pool formation rates were affected by the treatments. Stream temperature increased as canopy cover decreased. During the warmest days in summer, water temperature within 90% BAH sections reached levels that could be stressful or lethal to salmonids. The combination of 50% BAH sections and unharvested sections appeared to maintain natural thermal regimes. Neither the BAH nor the LWD treatments had an effect on stream turbidity or sedimentation rates within the experimental sections. The haphazard LWD additions resulted in no net increase in pool area. However, a greater number of pools were created and destroyed in the LWD addition sections. LWD additions also increased habitat complexity and most likely decreased the long-term LWD deficit that often occurs following logging operations.;Periphyton biomass increased with decreasing canopy coverage. Temperature and alkalinity did not significantly affect periphyton biomass, although the temperature gradient was small. Trends of increasing periphyton biomass were observed with canopy reductions as small as 6.4%. TI inputs, a critical food source for trout, were affected by the BAH treatments. TI input biomass was significantly higher in the 90% BAH sections.;TI input composition varied significantly between years (natural variation) and between the reference and 90% BAH sections. At greater harvest intensities, increases of Acrididae and Tettigoniidae (Orthoptera), wood-associated coleopterans, Formicidae (ants) and pollinating hymenopterans were observed.;Aquatic invertebrates (AI's) were, for the most part, unaffected by the treatments. AI abundance, richness, and biomass were not affected by the BAH or LWD treatments. AI abundance and biomass was significantly affected by natural annual variation. AI communities were not affected by the treatments, although a significant and more-direct relationship was observed between decreasing canopy coverage and increasing grazing taxa biomass.;The results of this study suggest that riparian tree harvesting affects stream temperature, periphyton biomass, TI inputs, and to a lesser degree, AI's. Following riparian harvesting, AI and fish production may increase via increases in periphyton biomass and TI input biomass. However, observed increases in stream temperature created conditions that would have been metabolically and physiologically stressful for salmonids. Riparian zones with 50% BAH and alternating unharvested sections appears to minimize the thermal impacts of riparian tree harvesting.