Quantifying direct and indirect effects of fungicide on an old-field plant community: an experimental null-community approach

Environmental perturbations can alter the composition of plant communities, either directly, by altering growth of some species more than others, or indirectly, by altering the strength of interspecific interactions among species. The relative importance of direct and indirect effects is not at all well known. We used an experimental approach to quantifying direct and indirect effects of fungicide on the composition of a plant community. To separate the direct and indirect impacts of fungicide we grew plant species in monoculture and mixed communities, and with and without the systemic fungicide benomyl. We predicted that direct effects of fungicide would be important at low but not high nutrient availability, while indirect effects would be more important at high nutrient availability. After 3 years there was little impact of fungicide on arbuscular mycorrhizal fungal colonization, and on soil microbial community composition assessed as the relative abundance of different phospholipid fatty acids. Like fertilizer, fungicide increased plant biomass. However, in contrast to fertilizer, this did not result in a decline in species evenness. Although not significant, the direct effects of fungicide tended to oppose the indirect effects of both fungicide and interspecific interactions on plant community composition. Experiments relying on fungicide treatments must be interpreted extremely cautiously, because the impact of fungicide is potentially the integrated response of plants to multiple factors, including arbuscular mycorrhizal fungi, pathogenic and saprophytic fungi, and nutrient inputs.