Effects of past and present competition on evolutionary potential and adaptation in the colonizing plant, Erodium cicutarium

Background/Question/Methods

Despite the large amount of information on plant adaptation to physical stressors in the environment, there is remarkably little information on the role of competition as an agent of selection.  Spatial variation in the composition of plant communities likely represents a complex selective mosaic of interactions that may influence the genetic architecture of a plant species at two fundamentally different hierarchical levels. First, competition may affect phenotypic expression of genetic variation within populations and thus the evolutionary potential of the population to respond to future selective challenges. Second, local competitive regimes could represent historic selective forces that have resulted in adaptive variation among populations.

Using the annual plant Erodium cicutarium, a species invasive in North America and native to Europe, we investigated the following two research questions. First, how does competition influence the phenotypic expression of within-population variation in ecologically important plant traits related to morphology, physiology, and phenology? Second, does the past or “historic” competitive regime of a plant’s source population influence its phenotypic response to experimental manipulations of competition? Within its native range in Bavaria, Germany, we collected multiple seed families of E. cicutarium from 10 populations in low and high competitive environments. In the glasshouse, we planted seeds from every family in both competitive (using the annual grass Bromus hordeaceus as competitor) and non-competitive environments.

Results/Conclusions

Analyses indicate that, regardless of population seed source, there was a significant effect of competitive environment on phenotypic expression of within-population variability. Plants growing under competition exhibited less among-family variation in physiological traits (e.g., stomatal conductance) and morphology (e.g., leaf length).  The competitive regime of seed source populations also influenced plant response to the competition treatments. Plants originating from populations within highly competitive environments exhibited significantly more variability in chlorophyll content than plants originating from low competition populations.  Differences in chlorophyll content between high and low competition source populations were greater under the no competition glasshouse treatment. This reduction in the expression of population differences in chlorophyll content under higher competition in the glasshouse indicates a significant interaction between past and current competitive regimes on the expression of plant physiological response to competition. There were also indications of phenotypically plastic responses to competition treatments. Plants growing under competition exhibited clear signs of etiolation. Compared to plants growing without competition, they produced fewer and longer leaves that grew at a steeper angle and also contained lower levels of chlorophyll.