Resistance to Centaurea solstitialis invasion from annual and perennial grasses in California and Argentina

A common explanation for Centaurea solstitialis invasion in California is that it occupies an "empty niche" created by the replacement of native perennial grasses by exotic annual grasses and concomitant increases in soil water availability. This hypothesis, however, cannot explain C. solstitialis invasion into perennial-dominated grasslands of central Argentina. We assessed invasibility of annual versus perennial grass communities in these regions through parallel field experiments where we created grass plots and, after one year of establishment, measured effects on water and light, and added C. solstitialis seeds in two successive trials. Additionally, we removed vegetation around naturally occurring C. solstitialis in both regions, and examined the performance of Californian and Argentinean C. solstitialis individuals when growing under common conditions simulating climate in California and Argentina. In California, both grass types offered high resistance to C. solstitialis invasion, water was generally greater under perennials than annuals, and light was similarly low beneath both types. In Argentina, invasibility was generally greater in annual than perennial plots, water was similar between groups, and light was much greater beneath annuals. Removal experiments showed that competition from annual grasses in California and perennial grasses in Argentina greatly reduce C. solstitialis performance. Additionally, Californian and Argentinean individuals did not exhibit genetic differentiation in studied traits. Our results suggest that dominant plant functional groups in both California and Argentina offer substantial resistance to C. solstitialis invasion. The success of this species might be tightly linked to a remarkable ability to take advantage of disturbance in both regions.