Environmental heterogeneity, fungal parasitism and the demography of the grass Stipa leucotricha.

This study investigated the demographic consequences of fungal infection of a perennial grass, Stipa leucotricha. The rate of parasitism of this grass by the host-specific, systemic fungus Atkinsonella texensis varies over short distances. Infection was frequent (57% of plants) in "mottes" (clusters of woody plants) but rarer in adjacent open grasslands (9%). To test the hypothesis that the relative performance of infected and uninfected plants differed in the two habitats, infected and uninfected genotypes were collected from mottes in a central Texas population, propagated in the green-house and then transplanted into the same site in replicate plots within mottes, at the edges of mottes, and in open grassy areas. Demographic data were recorded for 30 months over three growing seasons. Plants were observed to lose and gain infection. Infection had no significant effect on plant survival, tiller number or dry mass although infected plants tended to be larger. Uninfected plants had a significantly higher probability of setting seed but there were no differences in seed production by reproductive plants. There were significant effects of planting environment on all of these measures. Motte edges were most favorable for S. leucotricha transplants while motte interiors were least favorable and open areas were intermediate. There was no evidence of habitat x infection interactions; therefore the fungal infection had similar effects in different habitats. The high frequency of infection in motte habitats is best explained by more efficient contagious spread there. The favorability of motte edges for plant growth is substantially offset by higher infection rates at the edges of mottes.