Is there a risk to living large? Large size correlates with reduced growth when stressed for knapweed populations

Abstract

A central hypothesis in ecology is that plant life history evolution is constrained by funda- mental ''compromises between the conflicting selec- tion pressures resulting from particular combinations of competition, stress, and disturbance'', with stress being defined as abiotic conditions that restrict pro- duction. Biogeographic differences among native and non-native ranges of invasive plants may provide unique opportunities for tests of this theory. We conducted a greenhouse experiment with Centaurea stoebe plants from North American and European populations. We compared the total biomass and phenotypic plasticity indices for plants from the native and non-native ranges under stressed and non-stressed conditions. The average size of Centaurea stoebe plants from 13 North American populations was greater than that of plants from 18 European popula- tions regardless of stress treatment. However, when plants from the same populations were exposed to lower resources the differences in biomass between plants from North American and European populations were significantly less, suggesting that large plants were poorer stress tolerators. For all 31 populations the regression slope for the relationship between mean mass for populations in non-stressful conditions and mean mass in stressful conditions was less than 1.0, indicating that populations that produced large plants in good conditions also produced plants that grew disproportionately less in stressful conditions. These findings suggest that Centaurea stoebe may be evolv- ing towards being a good ''competitor'' (sensu Grime, 1977) in its invasive range, but at the cost of being a good ''stress tolerator''.