One mechanism of invasive species success is the production of allelopathic chemicals that negatively affect native competitors. A highly invasive shrub, Cytisus scoparius, impedes Douglas-fir tree establishment in clearcuts, even years after its removal. This impediment may be from the allelopathic alkaloids of C. scoparius that could indirectly hinder Douglas-fir by inhibiting their mutualistic ectomycorrhizal fungi (EMF). I extracted and quantified alkaloids from C. scoparius tissue for use in a laboratory bioassay. I then tested if and how these alkaloids affected EMF growth. In a second assay, I tested the effects of three concentrations of pure sparteine, the primary alkaloid in C. scoparius , on fungal growth. Sparteine was the only alkaloid recovered from the extraction which yielded 0.32 mg sparteine/g fresh weight, a lower concentration than previously reported values. Both the crude extract and pure sparteine significantly affected fungal growth, but only sparteine produced a species-specific response. Growth was inhibited by increasing sparteine concentrations, and most species were inhibited at 1.4 mM, the concentration found in C. scoparius. One common EMF, Wilcoxina mikolae, was unaffected by sparteine while others, like Suillus caerulescens and Cenococcum geophilum, were more sensitive and stopped growing entirely at 5 to 10 mM. These results suggest that the alkaloids of C. scoparius may seriously hinder EMF, and indirectly Douglas-fir, contributing to the competitive dominance of the invasive shrub.
