CENTAUREA SOLSTITIALIS FROM A NON-NATIVE RANGE ARE BETTER COMPETITORS THAN CONSPECIFICS IN THE NATIVE RANGE

Centaurea solstitialis and Centaurea calcitrapa (yellow starthistle and purple starthistle) are two closely related species with highly overlapping native and non-native ranges in Spain and California, respectively. C. calcitrapa is naturalized but has not become invasive in California, whereas C. solstitialis has formed virtual monocultures in many parts of the state, decreasing both biodiversity and grazing potential. We sampled seeds from eight populations per species and region and grew plants alone and in competition with seven species of grasses from both ranges in a common garden greenhouse experiment. After 92 days plants were harvested, dried, and weighed. For both species, Californian populations were larger than Spanish conspecifics when grown with no competition. When plants were grown in competition with grasses, Californian populations of C. solstitialis out-performed their Spanish conspecifics, and in some cases competitors had no effect on North American C. solstitialis biomass. C. calcitrapa plants from California were consistently suppressed by competitors more than C. solstitialis, although plants from California tended to out-perform their Spanish conspecifics. Other studies indicate that some traits of C. solstitialis and C. calcitrapa have been selected for in their new, non-native ranges over the last two centuries. We cannot exclude potential founder effects, but our results suggest that such selection may extend to greater competitive abilities for both species in at least one non-native range. However, the invasive C. solstitialis demonstrated stronger competitive response to other species that the non-invasive C. calcitrapa, suggesting that inherent competitive abilities may contribute in part to invasiveness