LB 24-212 - Drought and insect herbivory affect soil nutrients and primary production in a northern mixed grassland

Climate change is leading to modifications in precipitation patterns in grassland ecosystems, which can have strong impacts on plant production, ecosystem function and nutrient cycling, particularly in more semi-arid ecosystems. These impacts can also affect invertebrate herbivores given their strong plant linkages. Micronutrient cycling remains poorly understood in most grassland ecosystems, particularly in the context of drought and herbivory. We conducted a manipulative cage experiment combining drought manipulation with four grasshopper herbivory levels and control cages without herbivory to examine the impacts of herbivory intensity and drought on annual net primary production and nutrient cycling. The study was conducted at a northern mixed grass prairie site dominated by C3 grasses in eastern Montana to examine predictions from an earlier study in short grass prairie. We found nutrient uptake rates responded to both herbivory and precipitation treatments, but no interactions were evident. Zinc responded to both herbivory and precipitation treatments, while drought positively affected supply rates of nitrogen. The effects of herbivores on uptake rates were generally weaker than the effects of drought. Although grass damage increased with grasshopper density, particularly in the average precipitation treatment, plants were able to compensate for herbivory except with high levels of grasshopper herbivory considered a severe economic infestation. High densities of grasshoppers often lead to chemical control efforts focused on reducing densities of grasshoppers. Our results indicate that grasshopper chemical control efforts will not always result in increased forage for livestock in northern mixed grass rangeland, due to plant regrowth and compensation for herbivory.