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

Physical properties of old-growth Douglas-fir [Pseudotsuga rnenzjesll (Mirb.)Frankco] forests in the west coast regions of North America have received very httle 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 intenor area of an old-growth Douglas-fir forest of the Wind River Experimental 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, and Ts), relative humidity (h), wind speed (v), and solar radiation (R,) The maximum differences in Ta, T,, h, R,, and v recorded over 40 sampling days were 2.7"C, 5.g°C, 7 6 % , 0 8 kW m-2, and 0.5 m S -' , respectively; clear diurnal patterns were detected for all 5 variables with greater varlabll~ty during the day than at night, changes in temperatures and relative humidity over time appeared to be more predictable than those in wlnd speed and solar radiation.Local weather condition was linearly related to the above variability a n d , therefore, can be used as a reliable mdependent vanable to predict a forest's ~I C I -o c l m a t e and variability.Results concluded in this study are essential in providing microclimatic perspective In evaluating silv~cultural alternatives for the region in creation of old-growth-like forest structure.