DOUGLAS-FIR AND WHITE-FIR ADVANCE REGENERATION FOR RENEWAL OF MIXED-CONIFER FORESTS

Stem analysis of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and white fir (Abies concolor (Gord. & Glend.) Lindl.) advance regeneration 20 years after overstory removal showed that both species can respond in height growth to become dominant components of future stands on medium- to low-quality sites with a Mediterranean climate. Average annual height growth of both species nearly doubled on site classes III and IV during the first 5 years after release and continued to increase over the 20-year period of observation. Trees on the poorer site class V responded more slowly, but achieved similar 5-year periodic annual height growth after 20 years. Regression analyses indicated that prerelease height growth and site class were generally the most important variables in describing height growth after overstory removal, but regression equations left much variability in growth unexplained. Discriminant analysis was used to develop functions that identified trees meeting or exceeding specified levels of average annual height growth 5, 1 0, or 20 years after release; the classifications were generally successful at least 70% of the time.