Physiological Responses of a Desert Phreatophyte to Spatial and Temporal Variation in Groundwater Depth and Vadose Zone Water Availability

Desert plant responses to changing water resources and atmospheric conditions strongly influence ecosystem carbon and water fluxes. Yet, resolving desert plant responses to spatial and temporal variability in the environment remains challenging. Here, we determined responses of a deep-rooted phreatophyte shrub (Sarcobatus vermiculatus) growing in a dune ecosystem to spatial and temporal variation in groundwater (GW) depth and vadose zone plant available water (PAW), as well as temporal variation in atmospheric conditions. For over a year, we monitored GW depth, PAW, shrub predawn and midday stem water potential (Ψ_pd, Ψ_md), leaf gas exchange, leaf chlorophyll fluorescence, leaf nutrition and shoot growth across eight sites and across seasons spanning extremes in precipitation and temperature. At all sites, plant water status (Ψ_pd, Ψ_md), leaf N and shoot growth increased with increasing PAW. Plant responses to increasing GW depth were not consistent across sites, and site differences in GW depth did not explain variation in plant water status, nutrition, or growth responses to PAW. Leaf gas exchange and chlorophyll fluorescence were more directly influenced by temperature and vapour pressure deficit. The slope of the Ψ_md–Ψ_pd relationship (σ, metric of iso/anisohydry) varied among sites and was correlated with percent cover rather than site hydrology. Shrubs at higher cover sites were more isohydric, while shrubs at lower cover sites were more anisohydric. These results highlight the consistent positive influence of vadose zone PAW on growth and function of a desert phreatophyte shrub, the strong influence of atmospheric conditions on leaf function, and the potential influence of competition (higher shrub density) on Ψ regulation.