Variation in Water-Use Strategies of Velvet Mesquite, Prosopis velutina, in Southern Arizona

Abstract

In semi-arid regions of the United States, mesquite trees are widely distributed across the landscape and play a pivotal ecological role, influencing hydrological processes and contributing to biodiversity. This is especially true in riparian areas, where understanding the adaptive water-use strategies of facultative phreatophytes is essential to understanding ecohydrology in this region and in similar dryland ecosystems worldwide. This study investigated the water-use strategies of mesquite trees (Prosopis velutina) located in the floodplain of the San Pedro River in southern Arizona, USA across two contrasting water years, a dry winter/wet summer (2022) versus a wet winter/dry summer (2023). We explored the impact of age, size and density of mesquite stands (characterized as mature trees, dense young thicket and thinned young thicket) to understand how these trees access both deep (groundwater) and shallow soil moisture (recent precipitation). Across stand types, trees opportunistically used both deep and shallow source water, a strategy that is consistent in woody plants that grow with a bimodal precipitation regime. That relatively young thickets show similar strategies to mature bosques demonstrates the potential for rapid restoration to bosque form and function. We also examined leaf water potential to assess seasonal water stress between the two water years and found that despite the summer precipitation in 2023 being 111% lower than in 2022, summer leaf water potential had similar dynamics. This unexpected result leads us to posit that the higher winter precipitation in 2023 compared to 2022 (91% higher) helped to buffer the trees in 2023 from water stress, even during a drier summer. This suggests that winter precipitation (deeper in the soil profile) remains an important water source for trees, even in dryland regions where surface water along riparian areas is readily available.