Complex interactions in a streamside plant community

Ecologists are increasingly finding that complex combinations of competitiv e 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 Cali fornia. Specifically, I tested the hypothesis that the sedge Carex nudata p rovides critical stable substrate for other plants during winter floods and protection from herbivores over the growing season. In addition to these f acilitative effects, Carer is also hypothesized to compete with the associa ted species, and thus limit their size and reproduction. To evaluate these hypotheses, I followed the performance of transplanted in dividuals of Mimulus guttatus, M. cardinalis, Juncus covillei, Conocephalum conicum, and Brachythecium frigidum and naturally occurring individuals of Epipactis gigantea on Carer tussocks with dense, thinned, pinned back, or completely clipped Carer stems. The five transplanted species were also pla nted directly onto the emergent streambed. Though streambed transplants gre w as well as those on tussocks over the summer, they experienced significan tly greater winter mortality, up to 100%, supporting the hypothesis that tu ssocks provide a critical stable substrate. In contrast, growing season com petition by Carer reduced biomass by over 50% for five of the six species a nd reduced reproductive performance by over 60%. Also, over the growing sea son, Carex protected M. guttatus and Epipactis from insect larvae and deer, respectively, reducing herbivory by >75%. Additional results from a deer e xclosure treatment showed that the positive effects of this "associational defense" were equal in magnitude to the negative effects of Carer competiti on on Epipactis biomass. The mechanisms underlying these associational defe nses and the implications of my results for the relationship between distur bance and facilitation are discussed. I suggest that regarding plant intera ctions as combinations of facilitative and competitive components may enhan ce our understanding of natural communities.