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A biogeographical test of plant community invasibility: Annuals versus perennials in California and central Argentina.

Hierro, José*,1, Villarreal, Diego2, Callaway, Ragan1, 1 The University of Montana, Missoula2 Universidad Nacional de La Pampa, Santa Rosa, Argentina

ABSTRACT- It has been hypothesized that the widespread replacement of perennial by annual grasses in California grasslands has facilitated the later invasion of Centaurea solstitialis. The rationale for this proposition is that the combination of shallow root systems and early senescence of annuals could result in unused deep soil water that can be exploited by the extensive taproot of this exotic annual herb. We tested this idea in two regions of C. solstitialis' introduced range that differ remarkably in dominant plant functional groups, California, dominated by annual grasses, and central Argentina, dominated by perennial grasses. In each region, we built communities composed of either annual or perennial grasses in field plots. After one year of establishment, we invaded field plots with 200 C. solstitialis seeds. Additionally, in each plot we measured soil moisture at different depths of the soil profile (from 0.10 m to 1.20 m) and photosynthetic active radiation (PAR). We found no support for greater invasibility in annual versus perennial grasslands. In California, no plant establishment was observed. In Argentina, plant establishment was also low (less than 1% of added seeds), and although it was higher in annual than perennial experimental plots, the differences were not significant. Surprisingly, soil moisture was higher in perennial than annual plots down to 0.75 m below the surface and no differences were detected at deeper distances of the soil profile in California. No differences in soil moisture were detected at any depth in Argentina between plant community types. PAR was higher in annual plots in California only in the summer; whereas in Argentina PAR was consistently higher in annual than perennial plots throughout the year. These findings are consistent with other work assessing the invasion success of C. solstitialis in these two regions that points towards disturbance as a critical factor in this success.

Key words: biogeographic approach to invasions, centaurea solstitialis, synthetic plant communities, seed addition experiment

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