Underplanted conifer seedling survival and growth in thinned Douglas-fir stands

In a multilevel study to determine limits to underplanted conifer seedling growth, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), grand fir (Abie s grandis (Dougl. ex D. Don) Lindl.), western redcedar (Thuja plicata Donn ex D. Don), and western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings were planted beneath second-growth Douglas-fir stands that had been thinne d to basal areas ranging from 16 to 31 m(2)/ha. Understory vegetation was t reated with a broadcast herbicide application prior to thinning, a directed release herbicide application 2 years later, or no treatment beyond harves t disturbance. Residual overstory density was negatively correlated with pe rcent survival for all four species. Broadcast herbicide application improv ed survival of grand fir and western hemlock. Western redcedar, grand fir, and western hemlock stem volumes were inversely related to overstory tree d ensity, and this effect increased over time. There was a strong indication that this was also the case for Douglas-fir. Reduction of competing underst ory vegetation resulted in larger fourth-year stem volumes in grand fir and western hemlock.