COS 109-5 - Critical drought stress thresholds for riparian woodlands converge with multi-scalar evidence from isotopes to satellites

Tuesday, August 16, 2022

4:30 PM – 4:45 PM EDT

Location: 514B

Presenting Author(s)

John C. Stella, PhD

Professor and Vice President of Research
SUNY College of Environmental Science and Forestry
Syracuse, New York, United States

Slides (PDF)

Coauthor(s)

JW

Jared Williams

SUNY College of Environmental Science and Forestry, United States
CK

Christopher Kibler

University of California, Santa Barbara, United States
MR

Melissa M. Rohde

SUNY College of Environmental Science and Forestry, United States
LP

Lissa Pelletier

SUNY College of Environmental Science and Forestry, United States
MS

Michael Singer

Station CNRS d'Écologie Théorique et Expérimentale, France
DR

Dar Roberts

University of California, Santa Barbara, United States
AL

Adam Lambert

University of California, Santa Barbara, United States
KC

Kelly Caylor

University of California, Santa Barbara, United States

Background/Question/Methods

Though riparian woodlands are thought to be buffered against water stress by their landscape position and favorable hydrology, climate change and groundwater extraction increasingly threaten their long-term sustainability, particularly in drylands globally. Here we synthesize findings on the water stress response of riparian woodlands during and after the exceptional California (USA) drought of 2012–2019 from concurrent studies at different spatial and temporal scales. We coupled tree-ring studies from riparian stands along the Santa Clara River in Southern California with a basin-scale remote sensing investigation and a state-wide satellite imagery analysis to compare the timing and severity among indicators, and as well as ecosystem resilience.

Results/Conclusions

Tree-ring analyses of riparian cottonwoods revealed strong reductions in radial growth and carbon isotope discrimination as well as enrichment in δ¹⁸O during the driest years, indicating severe drought stress which was determined more by the rate of groundwater decline than by climate drivers. This pattern was reinforced at the landscape scale, where we observed decreased canopy greenness and increased dead biomass in riparian woodland stands progressing downstream as a “brown wave” from 2012 to 2016. Immediately after the drought, individual trees showed strong recovery of canopy-integrated leaf gas exchange, as indicated by tree-ring Δ¹³C and δ¹⁸O, as well as radial growth, except at sites subjected to the greatest water stress. Overall there were consistent relationships between groundwater depth, healthy vegetation cover, and tree growth and function, indicating that woodland health deteriorated in a predictable fashion as the water table declined at different sites and different times. The statewide analysis of Sentinel satellite imagery reinforced these results, showing woodland stress responses to deeper groundwater across all riparian ecotypes, as evidenced by concurrent declines in NDVI. Together these studies pave the way for developing complementary climate and groundwater sensitivity indicators to help manage vulnerable riparian woodlands experiencing global change.