The effect of defoliation intensity and season on Douglas-fir growth were studied in three replications in the Siuslaw National Forest during 1988-1990. Four intensities of defoliation were applied once in either spring or summer 1988. Stem diameter and canopy area were linearly inversely proportional to the level of defoliation. Defoliation intensity did not affect height growth (P > 0.05). Greater losses in height occurred when seedlings were defoliated in spring than in summer. No differences in predawn and mid-day xylem water potential were observed between trees defoliated in summer or spring 1988. However, in 1989, 25% defoliation reduced mid-day moisture stress whereas 50 and 75% defoliation increased mid-day moisture stress (P < 0.05). Effects of defoliation on length, width, and area of dominant and subdominant twigs as well as number of twigs were separated into different whorls. As indicated by the sharp slopes of the response surfaces, spring defoliation affected seedlings more (P < 0.5) than summer defoliation. No season or intensity effects (except second whorls) on area of dominant twigs were carried over to 1990. No effects (P > 0.5) of season or intensity on twig width were observed in 1990. Subdominant twigs were generally more sensitive to defoliation than dominant twigs. Greater losses (P < 0.5) in area of subdominant twigs occurred when seedlings were defoliated in spring than in summer. Similar to area, length of twigs continued to respond to defoliation intensity in 1990. Length and width of dominant and subdominant twigs were smaller for spring compared to summer defoliated trees. Number of twigs were linearly inversely proportional to the level of defoliation intensity in 1989. Intensity did not affect number of twigs in 1990 except second whorls. No effect of defoliation intensity and season on new whorls was detected. / Graduation date: 1992