Arbuscular mycorrhizal fungi (order Glomales) form mycorrhizae with roughly 95% of vascular plant families, and are among the most abundant and widespread fungi on the planet. They have been shown to affect the composition, structure, and diversity of their host-plant communities, and they are obligate biotrophs, gaining all of their carbohydrates from their hosts' photosynthesis. Because of this, AMF effects on plant community composition and structure should vary along light gradients, but this has not been studied. Based on the putative nature of interactions between AMF and their hosts, we predict that AMF should enhance host-plant diversity and growth in sunny microsites, but have little effect in dense shade. We have initiated a series of comparative and experimental studies to explore this idea, focusing on Wisconsin oak savannas, which show strong internal gradients in light availability and herb diversity over short distances. Here we present preliminary results from the first of these studies, an analysis of AMF and understory composition along these internal gradients. Over one hundred 1-m2 plots were established in three remnant savannas. Plant species coverages were estimated several times per year, and soil cores were analyzed to determine soil nutrient concentrations and the identity and abundance of AMF. Light levels were estimated from computer analysis of hemispheric canopy photographs. Preliminary results show a positive correlation between AMF richness and plant species richness, and apparent interactions among abiotic factors, AMF composition, and understory composition. These results have implications for the conservation and restoration of oak savannas, which are among the most species-rich and endangered plant communities in the upper Midwest.

Key words: arbuscular mycorrhizae, diversity, Glomales, gradients, oak savannas, plant community structure