Response of bluebunch wheatgrass to invasion: differences in competitive ability among invader-experienced and invader-naïve populations.
Invasive species may alter selective pressures on native plant populations. Although there is some evidence that competition with invasive plants may lead to differences in competitive ability between populations that have experienced invasion and those that have not, previous results have varied among species but also among populations of the same species. We conducted a glasshouse experiment to determine whether there was variation in traits or in ability to tolerate or suppress an invasive species among populations of a common native grass that had different histories of exposure to competition from an invasive species. Specifically, we grew seeds of a native grass (Pseudoroegneria spicata) collected from 14 wild populations (six from invaded populations and eight from uninvaded populations) and a cultivar (Anatone) alone or in competition with the invasive aster (Centaurea stoebe) and measured traits of both species during and at the end of a 100-day growing period. Pseudoroegneria spicata seedlings from invader-experienced populations had more leaves than invader-naïve populations, and juvenile plants from experienced populations were less affected by competition with C. stoebe than were plants from naïve populations. There were significant differences in traits among populations at the seedling and juvenile life stages, and at both life stages variation among populations was greater than variation among experience types. The most predictive traits of P. spicata tolerance to competition were number of leaves (seedling and juvenile stage) and total and root biomass (juvenile only). No traits significantly predicted suppression of C. stoebe. There was not a significant relationship between a population's suppression of C. stoebe and its tolerance of competition. Our results suggest that, in P. spicata, invasion selects for larger plants and traits that can influence tolerance of competition. If land managers are interested in identifying highly competitive seed sources for revegetation in invaded areas, both population and invader experience type should be considered. As tolerance and suppression do not appear to be related in P. spicata, seed source selection should be driven by the element of competitive ability (either tolerance or suppression) that is most important to project goals.