Long-term effects of tanoak competition on Douglas-fir stand development

In 1- to 2-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) plantations near Cave Junction and Glendale, Oregon, sprout clumps of tanoak (Lahocarpus densiflorus (Hook. & Arn.) Rehd.) and other hardwoods were removed with herbicides in April 1983 to leave relative covers of 0, 25, 50, or 100 percent of the nontreated cover, which averaged 15 percent. In 1996 (Cave Junction) and 1998 (Glendale), precommercial thinning (PCT) of Douglas-fir and cutting of nonconifer woody species were operationally applied across the four densities of tanoak. In 2005, Douglas-fir in 0 percent relative cover of tanoak averaged 5 to 8 cm larger at breast height and 3 to 6 m taller, and had two to four times the net stand volume of those growing in 100 percent relative cover. From 1999 to 2005, Douglas-fir stand growth accelerated more rapidly in tanoak relative covers of 0 and 25 percent than in covers of 50 and 100 percent. Differential development of Douglas-fir and hardwoods in relative covers of 0, 25, and 100 percent, followed by selection of crop trees via PCT, resulted in three distinct stand structures: pure stands of Douglas-fir with a single canopy layer 12 to 16 m tall, mixed stands with overstory Douglas-fir (12 m) and midstory hardwoods (7 m), and mixed stands with a single canopy layer (8 to 9 m).