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Increased groundwater dependence of riparian vegetation in response to drought

Published Article

California

Publication date: July 25, 2025

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Dryland riparian ecosystems face rising drought stress as climate change lowers water tables and reduces soil moisture. To understand how plants adjust water sourcing, a stable‑isotope study (2018–2020) along the Santa Clara River in southern California examined δ¹⁸O signatures in xylem water of four native woody species (Salix exigua, S. laevigata, Populus trichocarpa, P. fremontii) and the invasive Arundo donax. Mixing models revealed opportunistic water use: natives relied on shallow soil moisture in wet periods but shifted up to 60% toward groundwater during drought, demonstrating hydraulic flexibility and drought tolerance. Arundo, by contrast, consistently depended on shallow soil moisture (64–86%), making it more vulnerable to prolonged drought. These species‑specific patterns highlight the need to integrate ecological water‑use insights into groundwater sustainability planning to protect riparian vegetation under intensifying climate change.

Subject Tags

  • Climate resilience
  • Groundwater

Abstract

Riparian ecosystems in drylands face increasing risks from intensifying droughts, which lower water tables, reduce soil moisture and suppress streamflow—threatening vegetation and risking ecosystem collapse. Although riparian vegetation relies on subsurface water, the relative importance of groundwater versus rainfall-infiltrated soil moisture during drought remains unclear. As climate change prolongs drought severity, understanding how plants shift between water sources is key to predicting ecosystem resilience and guiding sustainable groundwater management. We conducted a stable isotope study along the Santa Clara River in southern California (2018–2020) during recovery from a severe (2012–2019) drought. We sampled δ18Op in plant xylem water from four native riparian woody species (Salix exigua, S. laevigata, Populus trichocarpa, P. fremontii) and the non-native grass Arundo donax. Shallow soil moisture and groundwater were sampled to characterize endmember δ18O signatures. Isotope mixing models were developed to track shifts in water source contributions for each species over three growing seasons. Riparian plants showed opportunistic water use, relying on shallow soil moisture during wet periods and shifting to groundwater during droughts. Native taxa including Populus and Salix species increased groundwater use by up to 60% during drought, reflecting hydraulic flexibility and drought tolerance. In contrast, the invasive A. donax depended on shallow soil moisture for 64–86% of its water under all conditions. These findings underscore the importance of quantifying species- and site-specific groundwater use. Incorporating such ecological insights into groundwater sustainability planning will be critical for protecting riparian vegetation and maintaining ecosystem function in a changing climate.

Citation

Kui, L., Williams, J., Singer, M. B., Stella, J. C., Kibler, C. L., Dawson, T. E., ... & Roberts, D. A. (2025). Increased groundwater dependence of riparian vegetation in response to drought. Ecohydrology, 18(5), e70060. https://doi.org/10.1002/eco.70060

TNC Authors

  • Melissa M. Rohde
    The Nature Conservancy