Variation in microclimate associated with dispersed-retention harvests in coniferous forests of western Washington. Forest Ecology and Management 226

Green-tree or structural retention is becoming increasingly common as a method of regeneration harvest in the Pacific Northwest. Amelioration of microclimatic stress is assumed to be one mechanism by which overstory retention enhances the survival of forest organisms and the potential for ecosystem recovery following timber harvest. We examined patterns of transmitted light (photosynthetic photon flux density, PPFD), air and soil temperature, and soil moisture across a broad gradient of dispersed retention in mature, coniferous forests at three locations in western Washington. Treatment means and within-treatment variation (coefficients of variation among sample points within treatments) were compared for warm, sunny days in 6- to 7-year-old experimental harvest units representing 0, 15, 40, and 100 % retention of original basal area. Multiple linear regression was used to explore relationships between microclimate and plot-scale measures of forest structure (including overstory attributes, understory vegetation, and logging slash). PPFD and mean and maximum air and soil temperatures decreased with level of retention. PPFD showed the strongest response, but did not differ between 40 and 100 % retention. Mean and maximum air temperatures were significantly greater at 0 and 15 % retention than at 100%. Among harvest treatments (0, 15, and 40%), mean air temperature was significantly lower at 40 than at 0%, but maximum air temperature did not differ among treatments. Mean and maximum soil temperatures differed only between 0 an