Local groundwater decline exacerbates response of dryland riparian woodlands to climatic drought

• Published in: Global change biology Vol. 28; no. 22; pp. 6771 - 6788
• Main Authors: Williams, Jared, Stella, John C., Voelker, Steven L., Lambert, Adam M., Pelletier, Lissa M., Drake, John E., Friedman, Jonathan M., Roberts, Dar A., Singer, Michael Bliss
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.11.2022; John Wiley and Sons Inc
• Subjects: Arid zones; Carbon isotopes; Carbon Isotopes - analysis; Climate change; climate gradient; Climatic conditions; Correlation; dendroecology; Drought; Droughts; Ecosystem; Forests; Groundwater; Groundwater levels; Groundwater recession; Groundwater supply; Growth; intermittent river; Populus spp; Riparian forests; riparian phreatophyte; Rivers; Santa Clara River (California); semi‐arid; Stomata; Stress; Stress (physiology); Trees; Trees - physiology; Vulnerability; Woodlands; climate gradient; Santa Clara River (California); intermittent river; dendroecology; semi-arid; riparian phreatophyte; Populus spp; climate change
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.16376

Dryland riparian woodlands are considered to be locally buffered from droughts by shallow and stable groundwater levels. However, climate change is causing more frequent and severe drought events, accompanied by warmer temperatures, collectively threatening the persistence of these groundwater dependent ecosystems through a combination of increasing evaporative demand and decreasing groundwater supply. We conducted a dendro‐isotopic analysis of radial growth and seasonal (semi‐annual) carbon isotope discrimination (Δ13C) to investigate the response of riparian cottonwood stands to the unprecedented California‐wide drought from 2012 to 2019, along the largest remaining free‐flowing river in Southern California. Our goals were to identify principal drivers and indicators of drought stress for dryland riparian woodlands, determine their thresholds of tolerance to hydroclimatic stressors, and ultimately assess their vulnerability to climate change. Riparian trees were highly responsive to drought conditions along the river, exhibiting suppressed growth and strong stomatal closure (inferred from reduced Δ13C) during peak drought years. However, patterns of radial growth and Δ13C were quite variable among sites that differed in climatic conditions and rate of groundwater decline. We show that the rate of groundwater decline, as opposed to climate factors, was the primary driver of site differences in drought stress, and trees showed greater sensitivity to temperature at sites subjected to faster groundwater decline. Across sites, higher correlation between radial growth and Δ13C for individual trees, and higher inter‐correlation of Δ13C among trees were indicative of greater drought stress. Trees showed a threshold of tolerance to groundwater decline at 0.5 m year−1 beyond which drought stress became increasingly evident and severe. For sites that exceeded this threshold, peak physiological stress occurred when total groundwater recession exceeded ~3 m. These findings indicate that drought‐induced groundwater decline associated with more extreme droughts is a primary threat to dryland riparian woodlands and increases their susceptibility to projected warmer temperatures. We conducted a dendro‐isotopic analysis of radial growth and carbon isotope discrimination at seven sites along the Santa Clara River to investigate the response of riparian cottonwood stands to the unprecedented California‐wide drought from 2012 to 2019. Groundwater decline was the principal driver of site differences in drought stress for these dryland riparian woodlands, and trees showed greater sensitivity to temperature at sites subjected to faster groundwater decline. Our results demonstrate that despite their location within the floodplain, these groundwater‐dependent ecosystems are highly vulnerable to projected climate change, the effects of which are likely to be exacerbated by groundwater decline.