Growing season microclimate variability within an old-growth Douglas-fir forest

Physical properties of old-growth Douglas-fir [Pseudosuga menziesii (Mirb.) Frankco] forests in the west coast regions of North America have received very Little attention aver 30 yr of extensive research. This study was aimed at characterizing the forest microclimate and its variability over the growing season (June to October). Three 200 m line transects in the interior area of an old-growth Douglas-fir forest of the Wind River Experim ental Forest, Gifford Pichot National Forest, Washington, USA, were used to install randomly 8 automatic recording systems to quantify growing-season microclimate and variability in 1990 and 1992, including air and surface soil temperature (T-a and T-s), relative humidity (h), wind speed (v), and solar radiation (R-t). The maximum differences in T-a, T-s, h, R-t, and v recorded over 40 sampling days were 2.7 degrees C, 5.9 degrees C, 7.6%, 0.8 kW m(-2), and 0.5 m s(-1), respectively; clear diurnal patterns were detected for all 5 variables with greater variability during the day than at night; changes in temperatures and relative humidity over time appeared to be more predictable than those in wind speed and solar radiation. Local weather condition was linearly related to the above variability and, therefore, can be used as a reliable independent variable to predict a forest's microclimate and variability. Results concluded in this study are essential in providing microclimatic perspective in evaluating silvicultural alternatives for the region in creation of old-growth-like forest structure.