Competitive abilities of rare, common, and invasive Cirsium species in different nutrient environments

The relative competitive ability of species may differ systematically across abiotic gradients.  Stressful habitats, mainly those with nutrient poor soils, have been shown to suppress the growth and competitive abilities of common and invasive plant species, allowing competitively inferior endemic and rare plant species to persist.  Serpentine habitats, which have continuously low resource levels, are thought for this reason to be invasion resistant and to serve as refuges for narrowly distributed, rare, and native species.  We test the hypothesis that rare, serpentine-restricted plants are inferior competitors but can thrive in nutrient-poor habitats by comparing closely related plants (congeners) that differ in their abundance and distribution.  Specifically, we consider six species of Cirsium (thistles) that occur in northern California and range from federally endangered to noxious invader.  Our rarest species, the endangered Cirsium fontinale var. fontinale, is endemic to California and is only found in serpentine habitats.  Our invasive species, Cirsium vulgare, is native to Europe and has successfully established on most continents.  We conducted a factorial greenhouse experiment where we manipulated competition (none, intraspecific, interspecific) and five levels of nutrients.  We found that rare species have similar growth and competitive abilities in each nutrient environment as the common and invasive species.  Our results suggest that the trade-off between competitive ability and growth in resource-poor habitats may not explain the disparate distributions of closely related plant species.