EXPLAINING PATTERNS OF DOMINANCE IN OLD-FIELD COMMUNITIES: TRADE-OFFS, FEEDBACKS, MUTUALISMS AND ENEMIES

Although one of the oldest observations within ecology is that within communities there are few common species with many uncommon and rare species, the mechanisms that shape this relationship remain elusive. The primary explanations for repeated dominance in plant communities lie in competition theory, which predicts specific allocation to resource acquisition, low resource tolerance, colonization, and herbivore tolerance and resistance. Alternatively, soil community feedback theory predicts positive plant-soil feedbacks to alter competitive dynamics and lead to dominance. Finally, specific mutualistic clades of the soil community, such as mycorrhizal fungi, may increase resource acquisition or herbivore tolerance, thus promoting positive feedback. To address these mechanisms that contribute to relative abundance and dominance, I used a light limited, old-field model system. Although these systems are relatively diverse, there is a striking pattern of repeated dominance by Solidago canadensis. By using a series of greenhouse and manipulative, long-term in situ experiments, I found no “smoking gun” mechanism to explain the dominance of So. canadensis, but rather an entire suite of processes that likely contribute to relative abundance and the maintenance of diversity. I found no evidence of life history trade-offs across old-field species, with S. canadensis consistently violating long-standing theory by being the best light competitor, most shade tolerant, most herbivore resistant, most herbivore tolerant, and among the fastest growing species. Looking belowground, I found that old-field plant species, even coexisting congeneric species, culture significantly different soil microbial communities, which altered plant performance, changed the intensity of interspecific competition and reversed whether plant species were limited by conspecifics or heterospecifics. Although this mosaic of shifting competitive abilities due to soil feedbacks is predicted to maintain diversity, the ability of S. canadensis to grow well in its own and competitor soil communities may foster S. canadensis invasion and subsequent defense of territory. Finally, mycorrhizal fungi increased herbivore tolerance across old-field species, while having little or negative effects on plants in the absence of herbivory. This process may promote diversity within old-fields, but offers insight into how So. canadensis maintain dominance in the face of dozens of specialist herbivores