The effects of spring burning and grass seeding in forest clearcuts on native plants and conifer seedlings in coastal Washington
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Seeding clearcuts with grasses and legumes is used to increase ungulate forage, reduce browsing on seedlings, reduce competition with seedlings, or control erosion, but the effects on native plants and conifer seedlings are poorly known. Also, a shift to spring or no burning of clearcuts may alter the effectiveness of seeding and interactions with native species and conifer seedlings. The effects of burning (none, spring) and forage grass seeding (0, 22, or 44 kg/ha seed, plus fertilizer) on seeded forage production, native and exotic plant cover and richness, and the survival and growth of planted western hemlock (Tsuga heterophylla) seedlings on two clearcuts in coastal forest of western Washington, USA, were quantified over 5 years. Forest vegetation in the study area is in the westerm hemlock series, with adjacent upslope areas in the Pacific silver fir (Abies amabilis) series. Burning doubled production of seeded grass, but seeding rate (22 kg/ha vs. 44 kg/ha) had no effect. Grass biomass peaked in 3 years, then declined to negligible levels. Detrimental effects of both burning and seeding on native species appeared small and short-lived over a 5-year period. Native plant species richness decreased after burning, but after 5 years nearly equalled that of unburned areas. Native plant species richness was unaffected by seeding rate. Native plant cover increased with time after clear felling regardless of treatment. In burned areas, seeding reduced native cover slightly. Exotic species richness and cover were highest in burned areas. About 25% of the planted western hemlock seedlings died, mostly during the first year. Mortality was 64% higher in seeded, burned areas than in unseeded, burned areas. Final seedling height was 4% higher in the burned than unburned areas, and seeding rate had no effect on height. Results of this study likely would apply to similarly treated western hemlock plant associations in coastal forests.