Background/Question/Methods
Environmental heterogeneity is a ubiquitous and important factor determining the distribution of plant species. Environmental heterogeneity occurring at large spatial scales, relative to the average dispersal distance of a species, should select for genetic differentiation among populations occurring in distinct patches. In contrast selection should favor adaptive phenotypic plasticity in response to heterogeneity at small spatial scales within the average dispersal distance of a species. Erodium cicutarium is an invasive annual plant which has recently invaded serpentine and non-serpentine soils at the UC McLaughlin reserve in Northern California. Previous work has demonstrated that serpentine soils have greater small scale heterogeneity in above ground interspecific competition, soil water availability, and soil chemistry than adjacent non-serpentine soils. In fall 2005 180 full sibling families were planted into six sites (4 serpentine and 2 non-serpentine) at the UC McLaughlin reserve in a full reciprocal transplant design. Phenotypic traits spanning early and late developmental stages were measured on every surviving individual, as well as lifetime fitness (number of viable seeds produced). Soil chemistry, soil water content, and above ground grass and forb biomass were quantified at every experimental block. Multiple linear regression was used to assess patterns of selection on characteristics of the reaction norms of all families in response to continuous environmental variation. Multivariate analysis of variance was used to assess genetic differentiation in characteristics of reaction norms among genotypes from serpentine and non-serpentine sites.

Results/Conclusions

In several cases selection on linear and quadratic components of reaction norms was apparent. If serpentine and non-serpentine genotypes exhibited genetic differentiation in these reaction norm components serpentine genotypes always produced the adaptive plastic response. Adaptive phenotypic plasticity would be expected in lineages exposed to greater environmental heterogeneity on small spatial scales, such as those collected from serpentine soils in this study. Adaptation to extreme edaphic environments, such as serpentine soils, is traditionally viewed in the context of differences in mean trait expression. This study demonstrates the importance of considering the implications of differences in the spatial scale of environmental heterogeneity among distinct environments.