Amount or pattern? Grassland responses to the heterogeneity and availability of two key resources

Patterns of resource availability and heterogeneity shape the composition, productivity, and dynamics of plant assemblages in a wide variety of ecosystems. Despite this, plant responses to simultaneous changes in the availability and heterogeneity of more than a single resource are virtually unknown. To fill this gap, microcosms consisting on assemblages formed by Lolium perenne, Plantago lanceolata, Anthoxantum odoratum, Holcus lanatus and Trifolium repens were grown in a factorial experiment with the following treatments: nutrient availability (NA), water availability (WA), spatial nutrient heterogeneity (NH), and temporal water heterogeneity (WH). Assemblages exhibited precise root foraging patterns in response to nutrient heterogeneity, which were modified by NA and WA. A series of two- and three-way interactions involving the four factors evaluated determined biomass production, the below: aboveground biomass ratio, the patterns of root biomass allocation with depth, and the relative contribution to aboveground biomass of Lolium and Anthoxanthum. In all cases, these interactions explained significant amounts of the variation found in the data at the assemblage level. At the individual level, the magnitude of plastic responses was higher when nutrient heterogeneity was one of the factors being compared for most species and/or traits. Increases in plasticity in Holcus and Trifolium were related to increases in the proportion of aboveground biomass accounted by these species. Our study demonstrates that considering the interactions between resource availability and heterogeneity allows for a refinement of predictions at the individual and assemblage level that can detectably reduce the error associated with extrapolating from single factor analyses.