EXTERNAL HYPHAL PRODUCTION OF VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI IN PASTURE AND TALLGRASS PRAIRIE COMMUNITIES

External hyphae of vesicular-arbuscular mycorrhizal (VAM) fungi were q uantified over a growing season in a reconstructed tallgrass prairie a nd an ungrazed cool-season pasture. In both sites, hyphal lengths incr eased throughout the growing season. Peak external hyphal lengths were 111 m cm(-3) of soil in the prairie and 81 m cm(-3) of soil in the pa sture. These hyphal lengths calculate to external hyphal dry weights o f 457 mu g cm(-3) and 339 mu g cm(-3) of soil for prairie and pasture communities, respectively. The relationships among external hyphal len gth, root characteristics, soil P and soil moisture were also determin ed. Measures of gross root morphology [e.g., specific root length (SRL ) and root mass] have a strong association with external hyphal length . Over the course of the study, both grassland communities experienced a major drought event in late spring. During this period a reduction in SRL occurred in both the pasture and prairie without a measured red uction in external hyphal length. Recovery for both the pasture and pr airie occurred not by increasing SRL, but rather by increasing externa l hyphal length. This study suggests that growth is coordinated betwee n VAM hyphae and root morphology, which in turn, are constrained by pl ant community composition and soil nutrient and moisture conditions.