Tenth-year survival and size of underplanted seedlings in the Oregon Coast Range

Abstract

This study initiated a two-aged forest stand structure by underplanting 50-year-old stands, primarily of Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco var. menziesii) and Douglas-fir – western hemlock ( Tsuga heterophylla (Raf.) Sarg.), thinned to 19–33 m2/ha on interior and coastal sites in the Oregon Coast Range. Douglas-fir, grand fir ( Abies grandis (Dougl. ex D. Don) Lindl.) (interior site only), western hemlock, and western redcedar ( Thuja plicata Donn ex D. Don) were planted following thinning either uniformly or in gaps of 0.06 or 0.1 ha. Understory vegetation treatments included (i) a preharvest site preparation herbicide application and an untreated control at both sites and (ii) a postharvest herbicide release treatment at the interior site. Planting conditions and stock at the interior site were not ideal, so survival was adjusted for first-year mortality. Adjusted 10 year survival ranged from 27% to 56% for Douglas-fir, 47% to 65% for western hemlock, 61% to 80% for grand fir, and 78% to 96% for western redcedar. Tenth-year survival at the coastal site ranged from 79% to 92% for Douglas-fir, 61% to 75% for western hemlock, and 67% to 86% for western redcedar. All species grew moderately well beneath the lowest-density overstories, and size was better within gaps than matrices for most species. Understory site preparation improved size for most species. Browsing on Douglas-fir and western redcedar impacted size on both sites.