GROWING-SEASON MICROCLIMATIC GRADIENTS FROM CLEAR-CUT EDGES INTO OLD-GROWTH DOUGLAS-FIR FORESTS

Edge is an important landscape feature of fragmented forest landscapes in the Pacific Northwest, USA. Our primary objective of this study is to characterize the changes in microclimatic variables from recent cl earcut edges into the old-growth Douglas-fir forests as influenced by edge exposures and local weather conditions. Microclimatic gradients a re described along transects extending from recently clearcut edges 24 0 m into stands of old-growth Douglas-fir (Pseudotsuga menziesii (Mirb .) France) forest west of the Cascade Range in the U.S. Pacific Northw est. Data for air temperature, soil temperature, relative humidity, sh ort-wave radiation, and wind speed were collected over the course of t he day from 16 different edges representing a range of edge orientatio ns and local weather conditions over two growing seasons (1989-1990). Data for soil moisture were collected over three consecutive days in S eptember 1990. Two indices, significance of edge influence (SEI) and d epth of edge influence (DEI), were used to evaluate the effects of edg es on microclimatic variables. Edge effects typically extended 30 to > 240 m into the forest. From the edge into the forest, air temperature s decreased during the day and increased at night; the reversal produc ed mid-morning and late-afternoon periods when a gradient was absent. Changes in soil temperature from the edge into the forest were compara ble to those for air temperature, except that edge effects did not ext end as deeply into the forest. The gradient for relative humidity incr eased from the edge and was steepest in mid-afternoon. Humidity effect s sometimes extended > 240 m into the forest. Short-wave radiation dec reased rapidly with distance from the edge, reaching interior forest l evels by 30-60 m. Wind speed decreased exponentially from the edge int o the forest, depending on the relationship of edge orientation to win d direction; stronger winds influenced conditions deeper inside the fo rest, sometimes > 240 m from the edge. Edge orientation played a criti cal role for all variables; for air and soil temperature and humidity, it affected the times of day at which maximum and minimum values peak ed. Influence of local weather conditions on gradients was highly vari able. Overall, however, gradients generally were longest and steepest on partially clear, warm, dry days, at southwest-facing edges, and for air temperature, soil temperature, and relative humidity. SEI and DEI were found to be necessary measurements for evaluating edge effects o n microclimatic variables, which responded differently depending on ti me of day, edge orientation, and local weather. No single value could be calculated for DEI. Because many ecological features near edges, su ch as tree stocking and regeneration, dispersal of flying insects, and decomposition of woody debris, seem related to microclimatic gradient s, forest management to protect interior conditions should shift from the traditional charge (''create as much edge as possible'') to a new charge in which the amount of edge is reduced at both the stand and la ndscape levels.