Water Stress in Riparian Woodlands from Groundwater Decline and Climate Change –Ecosystem Indicators at Multiple Scales
John C. Stella1*, Jared Williams1, Christopher Kibler2, Melissa M. Rohde1, Lissa Pelletier1, Michael Singer2,3, Dar Roberts2, Adam Lambert2, Kelly Caylor2
1State University of New York College of Environmental Science and Forestry
2University of California, Santa Barbara
3Cardiff University
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. Tree-ring analyses 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 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. Furthermore, we found greater seasonal coupling of canopy greenness to groundwater for vegetation along streams with natural flow regimes in comparison with anthropogenically altered streams, particularly in the most water-limited regions. Together these studies pave the way for developing complementary climate and groundwater sensitivity indicators to help manage vulnerable riparian woodlands experiencing global change.
Abstract type: Oral preferred (Powerpoint)
Session topic: Climate Change and Adaptation