Danthonia spicata (Poaceae) and Atkinsonella hypoxylon (Balansiae): Environmental dependence of a symbiosis

Epiphytic and endophytic fungal infections often enhance plant growth. Howe ver, supporting active fungal tissue may be costly to plants in low-nutrien t conditions and may affect the spatial distribution of host plants in hete rogeneous environments. We examined the field distribution of Danthonia spi cata infected and uninfected by the epiphytic fungus Atkinsonella hypoxylon relative to soil resource levels. We also conducted a greenhouse experimen t to determine how D. spicata growth and performance responded to soil fert ility and moisture. In two of three field populations, locations where A. h ypoxylon occurred had higher ammonia, but lower soil moisture, than locatio ns where D. spicata were uninfected. Infected and uninfected plants had sim ilar growth rates across greenhouse treatments, but infected plants had a p erformance (size x survival) disadvantage relative to uninfected plants in high-nutrient, high-moisture and low-nutrient, low-moisture conditions. Fie ld locations with D. spicata had low soil moisture, thus the performance di sadvantage of infected plants in low-nutrient, low-moisture conditions corr esponds to field observations that infected plants are rare in habitats wit h low ammonia. In a field common garden, infected plants had higher nitroge n concentrations than uninfected plants, suggesting that high nitrogen dema nd by A. hypoxylon may exclude infected plants from low-fertility field loc ations.