Invasive Plants in PNW Ecosystems
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Factors Influencing Regeneration of Scotch Broom (Cytisus scoparius)Timothy B. Harrington, USDA Forest Service, Pacific Northwest Research Station, 3625 93rd Ave SW, Olympia, WA 98512, tharrington@fs.fed.us Scotch broom (Cytisus scoparius) is a large, leguminous shrub found extensively throughout 16 eastern and 6 western U.S. states. Vigorous seedling regeneration enables dense stands to form quickly, increasing hazardous fuels, excluding native plants, and altering community structure of meadows, prairies, and young forests. Laboratory research was initiated in 2003 to determine how seedling emergence varied with temperature, light and stratification period, or following application of two soil-active herbicides on soils of differing texture. Seedling emergence also was studied under several densities of 40- to 70-year old Douglas-fir (Pseudotsuga menziesii) forest near Olympia, WA. In the laboratory, maximum germination rates occurred between 15○ and 25○ C, and effects of heavy shading switched from detrimental to beneficial at temperatures above 20○ C. Initial germination rates increased dramatically with duration of cold stratification, but cumulative germination was relatively unaffected. Seedling germination and biomass were moderately inhibited by single or combined applications of sulfometuron and metsulfuron herbicides, but mortality averaged only 7% for treated soils versus 1% for non-treated soils. In forested areas, seedling emergence was greater in low or moderate density stands of Douglas-fir versus in clearcuts, and it was greater in organic versus mineral seedbeds. However, cumulative emergence of seedlings remained low (<6%) two years after sowing seed, suggested that relatively intact forest communities have low susceptibility to invasion.
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