Basal Area and Diameter Distribution Prediction Equations for Young Douglas-Fir Plantations with Hardwood Competition: Coast Ranges

A stand-level modeling system for young Douglas-fir plantations applicable to the Coast Ranges of the Pacific Northwest was developed by using data from 84 growth-monitoring plots. Specific components of the system for predicting current size distributions of planted Douglas-fir include: dominant height and survival of planted Douglas-fir; a basal area projection equation for hardwoods; and diameter distribution percentile prediction functions for planted Douglas-fir. Prediction equations for height, crown width, and height to crown base of individual trees describe stand structure of the planted Douglas-fir. The stand- and tree-level equations include both direct and indirect effects of hardwood competition and site preparation, competition release, and precommercial thinning treatments. With the exception of minimum dbh, stand-level equations for planted Douglas-fir explained at least 92% of the observed variation. Basal area dynamics were simulated through plantation age 20 years for site preparation and competition release treatments of different effectiveness by varying hardwood basal area at plantation age 5 years. The simulations indicate that site preparation must reduce hardwood basal area at plantation age 5 years by 75% or more to realize an increase in Douglas-fir basal area and shift of the diameter distribution toward larger trees. This is due to increased growth of hardwoods that survive site preparation. In contrast, competition release treatments exhibited positive Douglas-fir response with 50% efficacy or more. Therefore, ineffective site preparation treatments need to be followed with competition release treatments having at least 50% efficacy to produce similar Douglas-fir basal area and diameter distributions at age 20 as in stands with no site preparation. West. J. Appl. For. 20(2):77–93.