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Scotch broom (cytisus scoparius) and soil nitrogen : ecological implications Shaben, Jacqueline

Abstract

Scotch broom (Cytisus scoparius), a leguminous shrub with nitrogen-fixing rhizobial root associations, is an invasive plant in the endangered Garry oak ecosystem (GOE) of Southern British Columbia. Broom frequently spreads from disturbed areas such as power line right-of-ways, and thus is a threat to native ecosystems. To unravel the ecological relationship between Cytisus scoparius and soil nitrogen, I conducted two independent studies in 2004-2005. The first study assessed broom’s effect on nutrient availability and plant community composition in two Garry oak sites. The second evaluated the impact on seedling recruitment of broom following fertilization with biosolids obtained from sewage treatment. To determine if broom increased soil nitrogen availability, adjacent broom-invaded and non-invaded plots at two GOE sites (Rocky Point and Bamberton) were investigated. Here, soil-nutrient availability was compared using PRS[superscript TM] ion-exchange membrane probes and concurrent plant surveys. No differences were observed in nitrogen availability between broom-present and -absent plots except for a weak trend of higher Nl₄⁺ at Rocky Point. By contrast, phosphorus availability at Rocky Point was significantly lower in the broom-present plots. Plant richness was independent of broom presence, however multivariate analysis showed that species identity and abundance differed, with native species declining and introduced species increasing in the broominvaded plots. I conducted a complementary bioassay in which a native grass and an exotic grass were grown individually either with Cytisus or the native Holodiscus discolor, to test if grasses benefited from the presence of Cytisus. No difference in grass growth between treatment combinations was observed. The second study tested a novel method of broom control. With fertilization trials, I compared the efficacy of sewage biosolids and ammonium nitrate on the suppression of broom seedlings following broom removal. In addition, I conducted a greenhouse germination experiment to explore the mechanism of broom suppression. Broom seedlings were fewest and vegetation biomass tended to be highest in the biosolids plots. The germination experiment indicated no difference in broom germination rate when grown without plant competition. This suggests that the negative effects of biosolids on broom are due to increased competition from other plants for light and water. The management implications of these two studies are that Cytisus scoparius, though not correlated with dramatic increases in nitrogen availability, may alter phosphorus availability in GOE and thus influence plant composition. Addition of sewage biosolids to disturbed sites however, causes broom seedlings to lose their competitive advantage in dense vegetation that is facilitated by complete nutrient addition from biosolids.

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