Growing‐Season Microclimatic Gradients from Clearcut Edges into Old‐Growth Douglas‐Fir Forests

Abstract

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 clearcut edges into the old‐growth Douglas‐fir forests as influenced by edge exposures and local weather conditions. Microclimatic gradients are described along transects extending from recently clearcut edges 240 m into stands of old‐growth Douglas‐fir (Pseudotsuga menziesii (Mirb.) Franco) forest west of the Cascade Range in the U.S. Pacific Northwest. Data for air temperature, soil temperature, relative humidity, short‐wave radiation, and wind speed were collected over the course of the day from 16 different edges representing a range of edge orientations and local weather conditions over two growing seasons (1989‐1990). Data for soil moisture were collected over three consecutive days in September 1990. Two indices, significance of edge influence (SEI) and depth of edge influence (DEI), were used to evaluate the effects of edges on microclimatic variables. Edge effects typically extended 30 to >240 m into the forest. From the edge into the forest, air temperatures decreased during the day and increased at night; the reversal produced mid‐morning and late‐afternoon periods when a gradient was absent. Changes in soil temperature from the edge into the forest were comparable to those for air temperature, except that edge effects did not extend as deeply into the forest. The gradient for relative humidity increased from the edge and was steepest in mid‐afternoon. Humidity effects sometimes extended >240 m into the forest. Short‐wave radiation decreased rapidly with distance from the edge, reaching interior forest levels by 30‐60 m. Wind speed decreased exponentially from the edge into the forest, depending on the relationship of edge orientation to wind direction; stronger winds influenced conditions deeper inside the forest, sometimes >240 m from the edge. Edge orientation played a critical role for all variables; for air and soil temperature and humidity, it affected the times of day at which maximum and minimum values peaked. Influence of local weather conditions on gradients was highly variable. 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 on microclimatic variables, which responded differently depending on time of day, edge orientation, and local weather. No single value could be calculated for DEI. Because many ecological features near edges, such as tree stocking and regeneration, dispersal of flying insects, and decomposition of woody debris, seem related to microclimatic gradients, 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 landscape levels.