Distribution of arbuscular mycorrhizal fungi in stands of the wetland grass Panicum hemitomon along a wide hydrologic gradient.

Although wetland plant species usually aggregate into zones that correspond with their water depth/dryness tolerances, it is not known whether associated arbuscular mycorrhizal (AM) fungi show a similar zonation. We assessed the distribution of AM fungi in two similar depressional wetlands dominated by the semi-aquatic grass Panicum hemitomon by sampling soil in plots along dry-to-wet gradients that spanned 80 cm in relative elevation, and identifying/counting viable AM fungal spores. We found that eight of nine AM fungal species were common to both of the wetlands. Within each wetland, there were significant differences in species composition related to relative water depth. The zonation patterns were not identical between wetlands but revealed that certain species were relegated to the drier portions of the gradient in both. No species were relegated to only the wet portions of the wetlands; those that dominated there were also present in the drier areas. Our data show that water depth is an important factor determining the distribution of the AM fungi, even when, as in our study wetlands, the host plant remains constant along a dry-to-wet gradient. This suggests that the fungi are not physiologically equivalent in their tolerance to wetland conditions.