CMY01 Mycorrhizal Colonization and Plant Community Responses to Long-term Suppression of Mycorrhizal Fungi

Twenty replicate permanent 2x2 m plots were established in early 1991 along a randomly located transect, with a 2m space between each plot, on the following watersheds: 1B, 1D, annually burned HQB, 10B, 20D and infrequently burned HQB. Ten of the plots were randomly assigned as long-term mycorrhizal suppression plots. In each of these plots, AM fungi were suppressed by the application of the fungicide benomyl as a soil drench (7.5 liters per plot) at the rate of 1.25 g/m2 (active ingredient). The mycorrhizal suppression plots were treated biweekly throughout each growing season (April through October) beginning in 1991. The control plots each received no fungicide, but an equivalent volume of water (7.5 liters) was applied biweekly. To evaluate the effectiveness of the fungicide, three soil cores (2.5 cm diameter x 14 cm deep) were removed from both fungicide-treated and control plots each October throughout the study. Roots were extracted from the soil, washed free of soil, stained in trypan blue (Phillips and Hayman, 1970), and examined microscopically to assess percentage root colonization by mycorrhizal fungi using a Petri dish scored in 1-cm squares (Daniels et al. 1981). The vegetation within all plots was sampled in May and September of 1991, 1993, and 1995. In each plot, the cover and frequency of each plant species was estimated using a modified point-frame method (Cook and Stubbendieck, 1986). A frame containing ten 1m long vertical pins arranged in parallel at 10 cm apart was placed systematically at 4 locations (each 25 cm apart) within the central 1 m2 of the plot (4 frames=40 pins per plot). Every contact of the aboveground structures of each plant species with each pin was recorded. From the pin-contact data, the relative cover was calculated for each plant species (total number of pin-contacts mad by individual of species x/total number of pin-contacts of all species) for each of the two sample dates each year and for each species the maximum value attained between the two sample dates was retained for analysis. The frequency ( percentage of the 10-pin frames in which species x was encountered) also was estimated for each plant species. The total number of pin contacts of all species was used as an index of total canopy density in each plot. Previous use of this pincontact method on these tallgrass prairie sites showed that the total number of pin contacts of all species is also strongly correlated with total aboveground plant biomass (Hickman, 1996). Plant species richness (mean number of species per plot), species diversity (Shannon’s H’), and evenness were calculated using both types of abundance data (frequency and cover).