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

Jared Williams; John C Stella; Steven L Voelker; Adam M Lambert; Lissa M Pelletier; John E Drake; Jonathan M Friedman; Dar A Roberts; Michael Bliss Singer

doi:10.1111/gcb.16376

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

Glob Chang Biol. 2022 Nov;28(22):6771-6788. doi: 10.1111/gcb.16376. Epub 2022 Aug 31.

Authors

Jared Williams^{1

2}, John C Stella^{1

3}, Steven L Voelker⁴, Adam M Lambert^{2

5}, Lissa M Pelletier¹, John E Drake³, Jonathan M Friedman⁶, Dar A Roberts^{7

8}, Michael Bliss Singer^{8

9

10}

Affiliations

¹ Graduate Program in Environmental Science, College of Environmental Science and Forestry, State University of New York, Syracuse, New York, USA.
² Marine Science Institute, University of California, Santa Barbara, California, USA.
³ Department of Sustainable Resources Management, State University of New York College of Environmental Science and Forestry, Syracuse, New York, USA.
⁴ College of Forest Resources and Environmental Science, Michigan Technological University, Houghton, Michigan, USA.
⁵ Cheadle Center for Biodiversity and Ecological Restoration, University of California, Santa Barbara, California, USA.
⁶ U.S. Geological Survey, Fort Collins Science Center, Fort Collins, Colorado, USA.
⁷ Department of Geography, University of California, Santa Barbara, California, USA.
⁸ Earth Research Institute, University of California, Santa Barbara, California, USA.
⁹ School of Earth and Environmental Sciences, Cardiff University, Cardiff, UK.
¹⁰ Water Research Institute, Cardiff University, Cardiff, UK.

Abstract

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 (Δ¹³ C) 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 Δ¹³ C) during peak drought years. However, patterns of radial growth and Δ¹³ C 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 Δ¹³ C for individual trees, and higher inter-correlation of Δ¹³ C 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.

Keywords: Populus spp.; Santa Clara River (California); climate change; climate gradient; dendroecology; intermittent river; riparian phreatophyte; semi-arid.

MeSH terms

Carbon Isotopes / analysis
Droughts*
Ecosystem
Forests
Groundwater*
Trees / physiology

Substances

Carbon Isotopes