Three-year survival and growth of Douglas-fir seedlings under various vegetation-free regimes

Responses of Douglas-fir seedlings were studied for 3 yr following eight ve getation-control treatments in three western Oregon clearcuts. The objectiv es were to determine seedling growth response to different areas of spot ve getation control and to determine the relative influence of early woody and herbaceous com petition on seed ling growth. Herbicide treatment areas Var ied in size from those receiving no control to full control (9.3 m(2)). Con trolled areas were maintained free of herbaceous vegetation for 2 yr and ai l woody vegetation was controlled for 3 yr. Two additional treatments, comp lete control of woody vegetation only and complete control of herbaceous ve getation only, were also examined. On two sites (Sum mit and Marcola), seed ling growth parameters were maximized at or near full vegetation control wi th a tree spacing of 3 m x 3 m. On the third site (Pedee), maximum growth r esponse occurred between 5 and 6 m(2) of control. Herbaceous vegetation con trol resulted in increased seedling growth at all sites while woody vegetat ion control yielded increased seedling growth only at the Pedee site. Cumul ative 3 yr herbaceous cover accounted for 68% and 41% of the variability in stem volume at Summit and Marcola, respectively. Adding cumulative 3 yr wo ody cover to the model accounted for an additional 18% and 49% of the varia bility in stem volume at Summit and Marcola, respectively. At Pedee, neithe r herbaceous nor woody cover significantly influenced 3 yr stem volume, sug gesting that factors other than vegetation cover were responsible for diffe rences measured.