Landscape Greening Following Unseasonal Precipitation Along a Desert–Alpine Gradient

Abstract

In seasonally arid systems, where water is a key limiting resource, unseasonal precipitation events may promote landscape greening. Because herbivores track spatial variation in fresh plant growth, dry-season precipitation could be an important catalyst of out-of-season foraging opportunities in seasonal arid environments. The eastern escarpment of California’s Sierra Nevada Mountains in the western United States features an intense ecoclimatic gradient from the Owens Valley’s cold desert to the High Sierra’s alpine tundra, in which patterns of temperature and precipitation vary seasonally and elevationally, and along which critically endangered Sierra Nevada bighorn sheep (“Sierra bighorn”) migrate seasonally. Immediately preceding the unusually warm 2021–2022 winter, the eastern Sierra experienced an historic autumn precipitation event, presenting an opportunity to investigate how unseasonal weather events shape patterns of plant green-up, and whether these conditions stand to impact herbivore space use. Here, we report on landscape greenness, indexed using the green chromatic coordinate, measured from 26,222 images collected by an in situ array of 20 time-lapse cameras deployed across the desert–alpine gradient of the eastern Sierra, over 5 years centered on the 2021–2022 fall-winter season. We compare greenness trends between time windows following rainfall and random timepoints to identify how unseasonal precipitation impacts plant greening using hierarchical linear models with a continuous first-order autocorrelation structure. Greenness trends were significantly more positive after rain than at random timepoints for low-elevation perennial grasses (p < 0.001), indicating that these plants readily uptake moisture regardless of the season. We additionally compare in situ greenness measurements with NDVI derived from satellite instrumentation to examine whether fine-scale patterns of green-up following unseasonal rains are detectable from space. Although in situ camera greenness and satellite NDVI were positively correlated, unseasonal species-specific greening was not detectable at the comparatively coarse scale of satellite analysis. Finally, we use GPS collar data from a population of Sierra bighorn to explore whether migratory behaviors could be associated with unseasonal landscape greening. Together, this work reveals that dry-season precipitation coupled with cold-season warm spells can lead to unseasonal landscape greenness and suggests that this process may facilitate herbivore access to high-quality forage during a normally barren time of year.