POTENTIAL VOLUME YIELD FOR MIXED-SPECIES DOUGLAS-FIR STANDS IN THE NORTHERN ROCKY-MOUNTAINS

A procedure for estimating potential volume yield is developed for stands containing dominant Douglas-fir in the Northern Rocky Mountains. Earlier work on site index and height growth and maximum stand density was the basis for deriving a stand form factor equation that provides potential volume estimates for a continuum of even- to uneven-aged mixed-species stands. The coefficients of this equation (which explained 84% of the variation) differ with respect to habitat type. The combination of asymptotic dominant height, potential basal area, and stand form factor allows for estimating potential total volumes as a function of site index and age of dominant Douglas-fir, habitat type, and the skewness of the dbh(1.5)-distribution (which characterizes stand structure). Potential volume yield levels vary with respect to habitat type and stand structure (skewness), indicating that much information on potential total volume is lost if only site index is used to measure productivity. These discrepancies between productivity estimates based on site index and potential volume are the result of different yield levels sensu Assmann existing within a given site index class, For the majority of habitat types, volume growth potential would have been underestimated using site index alone.