Effects of soil nutrient heterogeneity and parasitic plant infection on an experimental grassland community

Abstract

Soil nutrient heterogeneity and parasitic plants have been well-documented to affect plant growth. However, their interactive effects on plant communities have not been tested yet. We assembled plant communities consisting of eight grassland species of three functional groups in one homogeneous soil and four heterogeneous soils consisting of high and low nutrient patches. The heterogeneous soils were manipulated by combining factorially two patch size treatments (large or small patch) with two patch contrast treatments (high or low contrast). All communities were either infected with the holoparasitic plant Cuscuta australis or not. Seven weeks after infection with C. australis, aboveground biomass of the communities was significantly reduced through suppressing the growth of forbs, but not that of grasses or legumes within the communities. Compared to the homogeneous soil, soil heterogeneity overall did not influence the growth of the communities or that of the component functional groups. However, aboveground biomass of the communities was greater in the heterogeneous soil with high patch contrast than in the heterogeneous soil with low patch contrast. A similar pattern was found for the legumes, but aboveground biomass of the forbs or grasses did not show any difference. Patch size did not influence aboveground biomass of the communities, forbs or legumes within the communities. Aboveground biomass of the grasses was greater in the heterogeneous soil with small patches than in the heterogeneous soil with large patches. The effects of C. australis infection on the relative abundance of the forbs, legumes, and grasses varied from negative, neutral to positive. However, the interaction of C. australis infection with soil nutrient heterogeneity, its patch size or patch contrast had little impact on the growth and functional composition of the communities. Our results suggest that parasitic plant infection can influence plant communities by altering the performance of component functional groups, independent of soil nutrient heterogeneity.