Variation in microclimate associated with dispersed-retention harvests in coniferous forests of western Washington

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 6to 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 and 100% retention. Minimum air and soil temperatures and late-summer soil moisture did not differ among treatments. Within-treatment variability (CV) did not differ significantly with level of retention for any of the variables sampled, but CVs for soil temperature showed a consistent increase with decreasing retention. In combination, topography, residual forest structure, and understory variables were good predictors of PPFD and mean and maximum temperatures (R of 0.55-0.85 in multiple regression models), but were poorer predictors of minimum temperatures and soil moisture (R of 0.10-0.51). Canopy cover appeared most frequently in the models and cover of understory vegetation was a significant predictor in models of soil temperature. Trends in microclimate among experimental treatments were consistent, in large part, with the early responses of bryophyte, herbaceous, and fungal communities at these sites. Our results suggest that 15% retention, the current minimum standard on federal forests within the range of the northern spotted owl, does little to ameliorate microclimatic conditions relative to traditional clearcut logging.