Variation in water-use strategies of Prosopis velutina in Southern Arizona

Authors: GILLESPIE, C. - University Of Arizona; BODNER, G. - The Nature Conservancy; Scott, Russell; HU, J. - University Of Arizona

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2026
Publication Date: 2/7/2026
Citation: Gillespie, C., Bodner, G., Scott, R.L., Hu, J. 2026. Variation in water-use strategies of Prosopis velutina in Southern Arizona. Ecohydrology. 2026:19:e70183. https://doi.org/10.1002/eco.70183.
DOI: https://doi.org/10.1002/eco.70183

Interpretive Summary: Expansive, dense stands of large mesquite trees were once the most abundant type of riparian woodlands in the southwestern U.S., but now they are increasingly rare and face many threats such as groundwater pumping, land clearing, and competition from invasive species. However, younger mesquite stands, often growing in dense thickets because of farmland left fallow on the same near-stream terraces frequently receive scrutiny over concern about the impact these trees may have on groundwater levels. With few studies done in these thickets, there is a gap in our understanding of how water use in these younger thickets compare with mature mesquite trees on similar settings. This study investigated the water use strategies of mesquite trees located in the flood plain of the San Pedro River in southern Arizona 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) on source water use, with a focus on understanding how these trees access both deep (groundwater) and shallow soil moisture (recent precipitation). We found that despite differences in stand characteristics, the trees opportunistically used both deep and shallow water sources. 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 in both years. 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.

Technical 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.