Negative mycorrhizal feedback and plant species coexistence in a serpentine grasslan

A relatively unexplored mechanism that may promote species coexistence in plant communities is plant-soil feedback, an important component of which are mycorrhizal associations between plant roots and arbuscular mycorrhizal fungi (AMF). Positive or negative plant-soil feedback occurs when the soil microbial community associated with a plant species either increases or decreases, respectively, the performance of conspecific plants compared to co-occurring heterospecific plants. This research has focused on examining the nature, mechanistic basis, and diversity implications of mycorrhizally mediated plant-soil feedback in an eastern U.S. serpentine grassland system. Chapter one describes a study that found several plants in this system were most highly colonized by the AMF associated with conspecifics. However, these plants achieved the largest biomass when inoculated with the AMF associated with heterospecifics, indicating that negative mycorrhizal feedback was occurring, possibly as a result of over-optimum AMF colonization. Chapter two describes a study that detected species-level specificity between plant species and their associated AMF in this system. Particular AMF species made up a higher proportion of the total AMF spores found in the root zone of particular plant species based on spore counts from underneath plants in the field and from cultures started with field collected roots. Chapter three describes a study that found evidence suggesting the observed negative mycorrhizal feedback among these grasses may be due to plant species-specific root zone compositional changes in both AMF species and AMF clones. Over time these plants appear to accumulate AMF species and clones in their root zones which are relatively less beneficial towards conspecifics than heterospecifics. Chapter four describes a study demonstrating that negative plant-soil feedback generally has a larger effect on plant biomass than does competition in this system. This study found that negative plant-soil feedback was most important in the absence of competition, suggesting that it may be most important during seedling or ramet recruitment into gaps formed by the removal of existing vegetation. By differentially favoring the establishment of seedlings or ramets of a different species than the one previously occupying a given soil space, negative plant-soil feedback may act as a mechanism promoting species coexistence.