Local adaptation to habitat-specific herbivory and light levels in <i>Cardamine cordifolia</i>

Abstract

In this study I examined the effects of phenotypic plasticity and local adaptation to light environment in the crucifer Cardamine cordifolia, at the Rocky Mountain Biological Laboratory, in Gothic, Colorado. Previous work by Louda and colleagues showed that plants in sun environments were subject to heavy herbivore attack, whereas plants in shaded environments essentially escaped herivory. Louda et al.’s reciprocal transplant experiments in adjacent sun and willow-shade habitats showed that sun and shade plants exhibited phenotypic plasticity in response to low and high sun environments and that they were not locally adapted to these habitats. However, plant genotypes in deep shade habitats (under spruce trees) that have potentially been subjected to many generations of low light and low herbivory rates are likely to be locally adapted to this environment, as opposed to plants in high light, high herbivore environments. This dichotomy has not been studied previously and was the aim of my summer research project. I conducted a common garden, reciprocal transplant experiment using plants from sun and deep shade environments and added an herbivory component as well. I planted a total of 520 individual rhizomes from nine sun source sites and nine deep shade source sites in 3 shade and 3 sun gardens. Plants in the shade source habitats had significantly higher rates of herbivory, high canopy coverage, and received significantly lower photosynthetically available radiation than plants from sun sources. After planting in common gardens, I found that plants from sun and shade environments exhibited phenotypic plasticity when placed in the opposite habitat. I also found, however, that plants exhibit evidence of local adaptation. In general I found that plants in shade gardens tended to have bigger leaves, longer petioles, fewer leaves and lower chlorophyll concentration than plants in sun gardens. I also found that plants from sun habitats tended to grow more quickly than shade plants regardless of which garden they were planted, suggesting local adaptation. One interpretation of this pattern is that because plants living in sun habitats exhibit 10-40x higher rates of herbivory than plants in shade habitats, this could select for a more rapid growth rate in plants in the sun than in the shade because more rapid growth rate could increase plant fitness in the face of high herbivore pressure. We are currently finishing experiments in which we added herbivores to each rhizome in order to determine how plants in sun and shade gardens from each source habitat (sun or shade) respond to herbivores. I predict that plants in shade gardens will be less well-defended than sun plants and that plants from shade sources will be less well defended than sun plants.

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