LONG-TERM GROWTH RESPONSE OF DOUGLAS FIR TO WEED CONTROL

Survival and growth of young Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) transplants in the warm, dry-summer climate of Oregon are heavily influenced by competition from grasses and other herbaceous weeds. The ephemeral increase in available soil moisture resulting from weed control reduced tree moisture stress in the summer. This not only resulted in immediately increased growth, but also had significant or highly significant positive effects on tree growth for several years following, hastening the onset of exponential growth and thus shortening crop rotation.