Effects of soil resource and herbivory manipulation on temperate-mesic grassland vegetation under a changing climate
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Water availability is obviously a primary regulator of plant productivity and species interactions in arid ecosystems, where many previous studies have documented these effects by manipulating soil moisture availability. The impact of altered precipitation patterns — predicted as a consequence of climate change — however, is less clear for more temperate-mesic habitats. How much does variation in soil moisture availability matter here relative to other key environmental factors? To address this, we used a long-term field experiment to explore how the interaction of below-ground water and nutrient availability and above-ground herbivory, interact as regulators of neighbourhood biomass production and species composition in an old-field meadow in Eastern Ontario. After five years of soil water and nutrient level manipulation, with and without herbivore exclosures, we recorded above-ground dry biomass for each resident species within replicate plots. Analysis of treatment effects showed that community above-ground biomass increased with the addition of soil nutrients, decreased under reduced water levels, and was unaffected by increasing water levels or the presence of herbivore exclosures. By Contrast, species richness was altered by all applied treatments with the soil water level manipulations having the greatest effects. The soil nutrient manipulation induced changes in diversity consistent with the traditional ‘humped-back’ model, which predicts a unimodal relationship between productivity and diversity, but the soil moisture manipulation did not. Body size metrics (i.e. maximum potential body size or minimum reproductive threshold size) did not predict species biomass representation under any of the experimental manipulations. However, the biomass of the three most dominant plant species — each of agricultural and ecological significance — was altered by the soil resource manipulations. These results suggest that future precipitation regime changes are likely to negatively impact eastern Ontario’s native grassland plant species and its economically valuable hay and pasture lands. Furthermore, they challenge conventional theory regarding plant body size and competition and the relationship between productivity and diversity along gradients of soil moisture availability versus soil nutrient availability.