Light reduction predicts widespread patterns of dominance between asters and goldenrods

Here we investigate the long-cited pattern that throughout the eastern United States, Solidago species (goldenrods), and in particular S. canadensis displace Aster species and dominate old-field communities. Theory predicts that such a ubiquitous pattern of repeated dominance should be linked to competitive ability for a limiting resource. However, no one has investigated this possibility in old-fields, representing a potentially significant gap in our understanding of a common human-altered environment. We tested the hypothesis that S. canadensis is the superior competitor for light compared to other common co-occurring goldenrod species, and that the goldenrods in general are the superior competitors for light compared to coexisting aster species, which are typically less abundant. We tested this hypothesis by comparing the light attenuation abilities of four goldenrod species, S. canadensis, S. rugosa, S. gigantea, and Euthamia graminifolia, and three aster species, Aster novae-angliae, A. pilosus, and A. prenanthoides. Consistent with our hypothesis, S. canadensis had a greater ability to attenuate light than any of the other goldenrods at higher densities, and the goldenrods overall had a greater ability to attenuate light than the asters. By conducting a census in our study area, we verified that S. canadensis is locally the most abundant goldenrod and that goldenrods are more locally abundant than asters. Furthermore, by conducting a literature survey we found evidence that S. canadensis replaces A. pilosus through time. Thus we found a close correspondence between relative abundance in the field and light attenuation ability in field experiments. These results are consistent with theory predicting that competition for limiting resources, in this case light, explains patterns of dominance and relative abundance in old-field plant communities.