Danthonia Spicata (Poaceae) and Atkinsonella Hypoxylon (Balansiae): environmental dependence of a symbiosis.

Epiphytic and endophytic fungal infections often enhance plant growth. However, supporting active fungal tissue may be costly to plants in low-nutrient conditions and may affect the spatial distribution of host plants in heterogeneous environments. We examined the field distribution of Danthonia spicata infected and uninfected by the epiphytic fungus Atkinsonella hypoxylon relative to soil resource levels. We also conducted a greenhouse experiment to determine how D. spicata growth and performance responded to soil fertility and moisture. In two of three field populations, locations where A. hypoxylon occurred had higher ammonia, but lower soil moisture, than locations where D. spicata were uninfected. Infected and uninfected plants had similar growth rates across greenhouse treatments, but infected plants had a performance (size × survival) disadvantage relative to uninfected plants in high-nutrient, high-moisture and low-nutrient, low-moisture conditions. Field locations with D. spicata had low soil moisture, thus the performance disadvantage of infected plants in low-nutrient, low-moisture conditions corresponds to field observations that infected plants are rare in habitats with low ammonia. In a field common garden, infected plants had higher nitrogen concentrations than uninfected plants, suggesting that high nitrogen demand by A. hypoxylon may exclude infected plants from low-fertility field locations.