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

Abstract

In a multilevel study to determine limits to underplanted conifer seedling growth, Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco), grand fir (Abies 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 thinned to basal areas ranging from 16 to 31 m 2 /ha. Understory vegetation was treated with a broadcast herbicide application prior to thinning, a directed release herbicide application 2 years later, or no treatment beyond harvest disturbance. Residual overstory density was negatively correlated with percent survival for all four species. Broadcast herbicide application improved survival of grand fir and western hemlock. Western redcedar, grand fir, and western hemlock stem volumes were inversely related to overstory tree density, and this effect increased over time. There was a strong indication that this was also the case for Douglas-fir. Reduction of competing understory vegetation resulted in larger fourth-year stem volumes in grand fir and western hemlock. Resume : Des semis de douglas de Menzies (Pseudotsuga menziesii (Mirb.) Franco), de sapin grandissime (Abies gran- dis (Dougl. ex D. Don) Lindl.), de thuya geant (Thuja plicata Donn ex D. Don) et de pruche de l'Ouest (Tsuga hetero- phylla (Raf.) Sarg.) ont ete plantes dans des peuplements de seconde venue de douglas de Menzies qui avaient ete