Twelfth-year response of Douglas-fir to area of weed control and herbaceous versus woody weed control treatments

Abstract

Coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) response to eight weed control treatments was measured 12 years after planting at two Oregon sites. Treatments included four areas of weed control around individual trees (0.375, 1.49, 3.35, and 5.95 m2), no weed control (check), total vegetation control, control of herbaceous competition only, or control of woody competition only. Douglas-fir growth and woody-species invasion differed between the Coast Range site (Summit) and the Cascade Range foothills site (Marcola). Woody species reinvasion was more intense at Summit, with Douglas-fir cumulative mortality in the check treatment reaching 23% in year 12. Woody-only control improved Douglas-fir growth at Summit but had no significant effect on growth at Marcola. Total vegetation control had a profound effect on stem volume growth 12 years after planting. At Summit, total vegetation control resulted in a 355% increase in volume per hectare relative to the check. At Marcola the increase was only 63%. At Summit, growth increased with each increase in area of weed control, whereas at Marcola growth increased with increasing area of weed control up to 3.35 m2 of control. Results suggest that much of the gain in volume growth attributable to weed control may be lost if weed-control treatments are not highly efficacious. The differential response to woody control indicates that its benefit at a given site is strongly related to the abundance of competitive hardwood species, which may be predicted from the preharvest stand structure and vegetation community.