Climatic and management determinants of large herbivore production in semiarid grassland

摘要

Knowledge of climatic and management influences on large herbivore production (LHP, kg ha(-1)) is needed for low productivity, semiarid grasslands to address potential consequences of both increasing climate variability and the need to increase animal protein for human consumption. Here, we evaluate the influence of climatic variability and herbivore density on LHP in semiarid grassland using a unique long-term (80 years: 1939-2018) grazing study with three grazing intensities based on forage utilization (light, moderate and heavy). Seasonal variation in precipitation, but not temperature, was the primary influence on LHP. Winter (October-March) and spring (April-June), but not summer (July-September), precipitation during the current year positively influenced LHP across the 3 grazing intensities, whereas prior growing season (prior April-September) precipitation was consistently a negative influence. Although spring precipitation was the most influential seasonal weather variable for LHP, the effect of winter precipitation closely followed under all three grazing intensities, suggesting that non-growing season precipitation is essential for soil water storage to initiate production of sufficient high-quality forage in the subsequent grazing season, resulting in a positive feedback on LHP. A key finding from our analysis was that the effect of summer precipitation is smaller than the combined effects of winter and spring precipitation. As such, much of the variation in LHP can be predicted by seasonal weather parameters that are known early in the growing season. The magnitude of seasonal precipitation effects on LHP was greatest for heavy grazing; consequently LHP with heavy grazing is more reliant on primary production produced in the current year to increase LHP as forage quantity is more limiting than forage quality. Moreover, stability of LHP across years (range: 7.5 to 34.6 kg ha(-1)) was less with heavy grazing, which results in "boom-bust" economics that threaten sustainability of operations. Management adaptations to mitigate climatic variability, therefore, will be most necessary and advantageous when land managers employ heavy grazing intensities. Despite the substantial interannual variability in precipitation that characterizes semiarid grasslands, our results show that proactive flexibility by land managers in adjusting grazing management decisions to seasonal precipitation amounts forecasted for the winter and spring seasons would reduce enterprise risk and improve confidence in decision-making, profitability, production efficiency and environmental sustainability from semiarid grasslands.