GROWTH-RESPONSES OF DOUGLAS-FIR (PSEUDOTSUGA-MENZIESII [MIRB] FRANCO)TO DEFOLIATION

The effects of defoliation on Douglas-fir (Pseudotsuga menziesii [Mirb .] Franco) growth in 3-year-old plantations were studied at two sites near Alsea, Oregon during 1988-1990. Four intensities of defoliation ( 0, 25, 50, and 75% of current year's foliage removed) were applied onc e in either spring or summer 1988. Seedling diameter and canopy area h ad inverse linear relationships to the level of defoliation. Defoliati on intensity had no effect on seedling height growth (P > 0.05) throug hout the study period. Greater losses (P < 0.05) in height occurred wh en seedlings were defoliated in spring than in summer. The relatively shallow slopes of our response surfaces relating defoliation intensity to subsequent tree growth suggest that young Douglas-fir are very tol erant of defoliation provided that the terminal leader remains intact. Defoliation had no effect on tree mortality. Pre-dawn and midday xyle m water potentials were monitored using a pressure chamber during summ er 1988 and 1989 as an index of tree moisture stress. No differences i n pre-dawn and midday xylem water potential were observed (P > 0.05) b etween trees defoliated in summer or spring compared with undefoliated trees in 1988. However, in 1989, 25% defoliation increased midday xyl em water potential, whereas 50 and 75% defoliation decreased water pot ential (P < 0.05).