Defining Groundwater-Dependent Ecosystems and Assessing Critical Water Needs for Their Foundational Plant Communities
John C. Stella1
1 State University of New York College of Environmental Science and Forestry (SUNY-ESF), Syracuse, NY, USA; stella@esf.edu
In many water-limited regions, human water use in conjunction with increased climate variability threaten the sustainability of groundwater-dependent plant communities and the ecosystems that depend on them (GDEs). Identifying and delineating vulnerable GDEs and determining critical functional thresholds for their foundational species has proved challenging, but recent research across several disciplines shows great promise for reducing scientific uncertainty and increasing applicability to ecosystem and groundwater management. Combining interdisciplinary approaches provides insights into indicators that may serve as early indicators of ecosystem decline, or alternatively demonstrate lags in responses depending on scale or sensitivity, or that even may decouple over time. At the plant scale, miniaturization of plant sap flow sensors and tensiometers allow for non-destructive, continual measurements of plant water status in response to environmental stressors. Novel applications of proven tree-ring and stable isotope methods provide multi-decadal chronologies of radial growth, physiological function (using d13C ratios) and source water use (using d18O ratios) in response to annual variation in climate and subsurface water availability to plant roots. At a landscape scale, integration of disparate geospatial data such as hyperspectral imagery and LiDAR, as well as novel spectral mixing analysis promote the development of novel water stress indices such as vegetation greenness and non-photosynthetic (i.e., dead) vegetation, as well as change detection using time series. Furthermore, increases in resolution across numerous data types can increasingly differentiate individual plant species, including sensitive taxa that serve as early warning indicators of ecosystem impairment. Combining and cross-calibrating these approaches promises to provide insight into the full range of GDE response to environmental change, including increased climate drought and variability, human groundwater extraction and flow regulation. We review the range of emerging water stress indicators at multiple scales, discuss tradeoffs in data resolution and efficiency of data acquisition, and illustrate their application in current projects in several arid-land ecosystems.