PREDICTING RADIATION ATTENUATION IN STANDS OF DOUGLAS-FIR

Forty-one fixed-area plots established in even-aged second-growth Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) stands were sampled for plot average attenuation of photosynthetically active radiation. Light transmission was modeled as a function of Curtis' relative density and a surrogate for the coefficient of variation of diameter at breast height (dbh). This surrogate, Dratio, was defined as: (Dmax-Dmin)/Dq where Dq is quadratic mean diameter, Dmax is the dbh of the maximum dbh tree, and Dmin is the dbh of the minimum dbh tree in a plot. Dratio helped account for variation in light attenuation associated with different stocking levels for a given relative density. Curtis' relative density and Dratio are easily computed and accurately measured during routine cruising of stands and can be extracted from most stand growth models. The results are applicable to flat or south-facing slopes within the Vancouver Island dry variant of the Coastal Western Hemlock (CWHa1) subzone including second-growth stand habitat of potential importance for the winter range and survival of black-tailed deer (Odocoileus hemionus columbianus Richardson) during severe winters. Results were validated from 27 independent plots.