Global environmental change and foliar fungal plant disease: Testing the potential for interactive effects in a grassland ecosystem

A combination of four experiments testing both the effects of foliar fungal disease on a grassland ecosystem and the effects of multiple environmental changes on foliar fungal disease severity suggest that increased foliar fungal disease severity may be an important mechanism by which global change impacts grassland ecosystems. Although plant disease is a ubiquitous ecological process, the effects of plant disease at the ecosystem level have rarely been examined. To test the influence of ambient levels of foliar fungal disease on a grassland ecosystem, I employed a classical exclusion experiment using foliar fungicide. The most striking effect of excluding foliar fungal disease was that root production increased by ⅓, leading to almost 50% greater root biomass within three years. In contrast, aboveground production was increased only marginally, by perhaps 5%. However, disease exclusion increased leaf longevity across the plant community by ¼, and increased late-season photosynthetic capacity of the dominant species, Andropogon gerardii , by ⅓, providing mechanisms linking foliar disease and root production. Will global environmental change alter these effects of disease on the ecosystem? To address this question, I quantified the effects of experimentally controlled environmental changes on the percent leaf area infected by over thirty foliar fungal pathogens (disease severity). In two experiments, decreased plant species diversity increased disease severity across the plant community by increasing the abundance of remaining plant species, facilitating transmission of their specialist foliar fungal pathogens. In a factorial experiment, elevated carbon dioxide concentration increased disease severity of C3 grasses, and elevated CO2 and decreased diversity interacted such that each amplified the effect of the other on C3 grass disease severity. In the same experiment, nitrogen addition increased C4 grass disease severity by increasing foliar nitrogen concentration; this effect was magnified by elevated CO2, and it magnified the effect of decreased diversity on C4 grass disease severity. In a final experiment examining nine foliar fungal diseases, fire suppression increased severity of seven diseases in the short-term (generally a single growing season) by allowing inoculum to accumulate, but decreased severity of four diseases in the long-term by either reducing host abundance or altering microclimate.