The effects of environment and competitive ability on invasiveness

Scotch broom (Cytisus scoparius) is a well-known invasive species in Washington, Oregon, and California but a common nursery and short-lived garden perennial in New England. Despite its presence on Cape Cod, MA since 1908, many horticulturalists insist that this plant cannot survive the more rigorous inland winters and will not become invasive in New England. On the other hand, Genista tinctoria, another non-native legume that is a close relative of C. scoparius, is common in low quality habitats in New England. We conducted growth chamber germination trials and greenhouse competition trials focusing on responses to temperature and soil conditions in competition among seedlings of these two species. We found lower germination for G. tinctoria than for C. scoparius at all temperatures and across all soil and competition treatments. While C. scoparius showed rapid initial growth and no significant changes in response to varied soil composition, G. tinctoria growth was delayed and increased with soil nutrients and soil drainage. These results suggest that large differences in competitive ability may explain invasiveness, but that differences in winter survival could be the key to naturalization. Both nursery imports and local adaptation could assist the arrival or creation of a low temperature ecotype of C. scoparius with invasive consequences for New England.