Complex interactions in a streamside plant community

Ecologists are increasingly finding that complex combinations of competitive and facilitative interactions influence the distribution and abundance of plants. I conducted a two-year field experiment to explore these processes in a streamside community lining the South Fork Eel River in northern California. Specifically, I tested the hypothesis that the sedge Carex nudata provides critical stable substrate for other plants during winter floods and protection from herbivores over the growing season. In addition to these facilitative effects, Carex is also hypothesized to compete with the associated species, and thus limit their size and reproduction. To evaluate these hypotheses, I followed the performance of transplanted individuals of Mimulus guttatus, M. cardinalis, Juncus covillei, Conocephalum conicum, and Brachythecium frigidum and naturally occurring individuals of Epipactis gigantea on Carex tussocks with dense, thinned, pinned back, or completely clipped Carex stems. The five transplanted species were also planted directly onto the emergent streambed. Though streambed transplants grew as well as those on tussocks over the summer, they experienced significantly greater winter mortality, up to 100%, supporting the hypothesis that tussocks provide a critical stable substrate. In contrast, growing season competition by Carex reduced biomass by over 50% for five of the six species and reduced reproductive performance by over 60%. Also, over the growing season, Carex protected M. guttatus and Epipactis from insect larvae and deer, respectively, reducing herbivory by >75%. Additional results from a deer exclosure treatment showed that the positive effects of this "associational defense" were equal in magnitude to the negative effects of Carex competition on Epipactis biomass. The mechanisms underlying these associational defenses and the implications of my results for the relationship between disturbance and facilitation are discussed. I suggest that regarding plant interactions as combinations of facilitative and competitive components may enhance our understanding of natural communities.