Abstract
The increasing frequency and magnitude of climatic extremes are altering
water availability in dryland ecosystems globally. However, riparian
vulnerability to hydroclimate whiplash remains poorly understood. Here, we
examined how riparian willow, cottonwood, and valley oak trees respond to
groundwater fluctuations and drought through their water use patterns and
phenology. To this end, we combined time-series analysis of in situ,
high-frequency groundwater monitoring with high-resolution PlanetScope
satellite imagery of a drought-prone and relatively pristine watershed in
California (Chalone Creek, Pinnacles National Park). We found that flow
regime dictates the potential for trees to access groundwater, while the
identity of tree species determines the timing and magnitude of their use.
Machine-learning models revealed that at intermittent sites, groundwater
depth predominantly controlled vegetation greenness, represented by
Normalized Difference Vegetation Index (NDVI). In contrast, variation in
photoperiod length dominated at the perennial site where water was more
reliably available. During the severe 2020-2022 drought, all species
experienced reduced greenness, but phenological responses differed by flow
regime. While the start of season was delayed across all sites, trees at
intermittent reaches exhibited a substantially earlier end of season
during drought, resulting in growing seasons shortened by as much as 37
days. These phenological shifts vastly exceed those documented across
aridity classifications in global datasets from satellite observations,
ground-based monitoring networks, and experimental precipitation
manipulations. Although riparian trees in drylands have been shaped by
exposure to drought over evolutionary timescales, our findings show that
trees in these intermittent systems may be operating close to critical
groundwater thresholds, rendering them particularly vulnerable to
increasingly long and severe droughts.