Long-term mean climate and seasonal variability drive spatial patterns of forage production fluctuation trend across California annual grasslands

Forage production is integral to the ecological and economic functions of rangeland ecosystems worldwide and is characteristically highly variable. Understanding temporal variability and its trends in forage production are essential for sustainable resource management. This study evaluates recent long-term (2001 to 2018) trends in year-to-year fluctuations in forage production and identifies key factors driving spatial variability of these trends across California annual grasslands. Using annual forage production estimations at 30 meters from fused remote sensing observations, we calculated coefficients of variation (CV) for 8-year moving windows to quantify interannual variability in forage production, vegetation phenology, climate, as well as other environmental factors. The trends in CVs were then assessed with Mann-Kendall tests and Sen’s slope, and three random forest models were built. Results showed that more than 36% of grasslands experienced significantly increasing trends in interannual forage production fluctuations. The random forest model based solely on climate variables revealed that spatial patterns of trends in temporal fluctuations of forage production were mostly driven by long-term climatic means; specifically, drier areas with a long-term mean growing season precipitation below ~500 mm, or warmer areas with long-term mean minimum temperatures above ~6 °C, were more likely to exhibit significant increasing trends in forage production fluctuations. Spatially, trends in the temporal variability of seasonal precipitation and late season maximum temperatures, as well as other environmental factors such as soil organic matter content and elevation, also significantly contributed to trends in forage production fluctuations, although to a lesser degree. Further analysis using a random forest model with remote sensing-based phenological metrics indicated statewide trends in forage production variability were linked to trends in peak growth variability, as well as trends in the end and length of the growing season. These results highlight the value of understanding year-to-year trends to inform the local, adaptive decisions that land managers must make in order to sustainably balance forage supply with livestock grazing demand.