Nutrient heterogeneity is a ubiquitous feature of the natural world. There has been substantial research into how plants respond to changes in resource distributions. In pot and mesocosm studies, plants typically proliferate root biomass into high quality soil. This root foraging behaviour has been shown to cause increases in competitive intensity, community productivity, and shifts in species evenness in common gardens. However, little is known both about how plants forage, and the consequence of that behavior, in natural systems. We performed a field experiment where we manipulated heterogeneity without disturbing the intact roots of a grassland community, testing whether nutrient heterogeneity alters plant community structure and function. To simulate natural heterogeneity caused by ungulate defecation, we applied mesh bags filled with sterilized steer manure. Nutrients were applied to field plots in patches, uniformly, or not at all. We measured percent cover of each species within twenty-five 10cm square subplots distributed throughout each plot. After two years, we harvested aboveground and belowground biomass in 10 of our 25 subplots, and inside each patch. Soil nitrogen was also measured under patches, using resin bags.
Results/Conclusions
Nutrient heterogeneity elevated total plant cover, and species evenness compared to homogeneous and no-nutrient controls. However, spatial analysis revealed that these effects were highly localized, and limited to within 20cm of a patch. There was no effect on species richness. There was also no evidence of increased root foraging (measured as root length) inside patches, despite the fact that our patches elevated nitrogen. We suggest two possible explanations, which are not mutually exclusive, for the shifts in aboveground communities with no shifts in community level foraging behaviour. First, there may have been a reorganization of the belowground root community under our patches where species that became dominate/rare aboveground also became dominant/rare belowground with no overall shift in total biomass. Second, in natural field conditions where roots are extremely densely packed in the soil, small increases in root biomass may not be as easily detected as in root-free pots. It is possible that plants responded by shifting uptake physiology, or root demography; both of these responses have been demonstrated in pot experiments. Our results demonstrate that small scale nutrient heterogeneity can alter community structure and function, and that this effect is strongly dependant on the number of patches in the system.
