HARVESTING EFFECTS ON MICROCLIMATIC GRADIENTS FROM SMALL STREAMS TO UPLANDS IN WESTERN WASHINGTON

Riparian zones are vital components of the landscape. Much attention h as been focused on the question of how wide a buffer is needed to prot ect the original riparian environment. We sampled five streams 2-4 m w ide and associated riparian ecosystems before and after clearcutting i n western Washington. Buffers ranging from 17 to 72 m wide were left i ntact at all sites when harvesting. Our objectives were: (1) to charac terize preharvest microclimatic gradients across riparian ecosystems, from the stream to the upland; (2) to identify effects of harvesting o n these gradients; and (3) to describe effects of buffer width and nea r-stream microclimate on stream microclimate. Six weather stations mea suring air temperature, soil temperature, surface air temperature, rel ative humidity, short-wave solar radiation, and wind speed were instal led along transects running across the stream and into the upland, and two reference stations were established, one in an upland clearcut an d one in an upland interior forest. Pairwise comparison tests were use d to evaluate statistical differences between stations along transects for determination of gradient extent. Pre-harvest riparian gradients existed for all variables except solar radiation and wind speed, and v alues generally approached forest interior values within 31-62 m from the stream. After harvesting, microclimate values at the buffer edge a nd each subsequent location toward the upland began to approximate cle arcut values instead of forest interior values, indicating an interrup tion or elimination of the stream-upland gradient. In addition, regres sion analyses showed that stream microclimate was affected to some deg ree by buffer width and microclimate in the surrounding area. We concl ude that a buffer at least 45 m on each side of the stream is necessar y to maintain a natural riparian microclimatic environment along the s treams in our study, which were characterized by moderate to steep slo pes, 70-80% overstory coverage (predominantly Douglas-fir and western hemlock), and a regional climate typified by hot, dry summers and mild , wet winters. This buffer width estimate is probably low, however, si nce it assumes that gradients stabilize within 30 m from the stream an d that upslope edge effects extend no more than 15 m into the buffer ( a low estimate based on other studies). Depending on the variable, req uired widths may extend up to 300 m, which is significantly greater th an standard widths currently in use in the region (i.e., similar to 10 -90 m). Our results indicate that even some of the more conservative s tandard buffer widths may not be adequate for preserving an unaltered microclimate near some streams. Additional site-specific data are need ed for different site conditions in order to determine whether general izations can be made regarding near-stream microclimate.