Abstract
Vegetation distribution, composition and health in arid regions are largely dependent on water availability controlled by climate, local topography and geology. Despite a general understanding of climatic and geologic drivers in plant communities, trends in plant responses to water distribution and storage across areas under different local controls are poorly understood. Here we investigate the multi-decadal interactions between spatial heterogeneity of geologic controls and temporal variation of climate, and their impacts on water availability to vegetation and plant responses (via normalized difference vegetation index, NDVI) in a monsoon-driven arid region of southeastern Arizona. We find that grasslands display low NDVI and respond directly to monsoonal rainfall. In the uplands, vegetation on west-facing slopes and in canyons share similar NDVI averages and variability, suggesting that they both use water from surface-groundwater flow paths through fractured rocks. Along the San Pedro River, streamflow, groundwater, and NDVI in deciduous riparian woodlands are strongly responsive to monsoonal rainfall, but water availability stratifies between wet (perennial), intermediate, and dry reaches, underlain by different local geologic controls that affect water table elevation. These controls interact with the driving climate to affect water availability in the shallow alluvial aquifer of the riparian zone, a primary water source to the gallery phreatophytes. A recent shift toward a strengthened monsoon in the region has led to an increase in water availability for grasslands and for dry reaches of the San Pedro, while the benefit is more muted along wetter reaches, where the riparian forest shows signs of having reached its maturity, with diminished trends in NDVI. These results have implications for the future vulnerability of dryland vegetation to climate change, which may be either dampened or intensified by local controls such as geology.
Highlights
Plants in dryland ecosystems may experience differential seasonal access to water and distinct longterm trends in their responses to water availability changes, based on rooting depth, as well as the local expression of hydrology and water storage at their rooting location
In arid environments, where evapotranspiration exceeds precipitation, vegetation is typically concentrated at locations in the landscape where runoff accumulates and/or where the water table is close to the surface (Dawson and Ehleringer 1991, Patten 1998, Lite and Stromberg 2005, Rodriguez-Iturbe et al 2007), yielding potentially strong differences in vegetation types and density across a region with the same driving climate, depending on the local geologic controls and geomorphology (Caylor et al 2005, 2009, Franz et al 2010)
Our goal in this paper is to address how climate variation through time and/or geologic controls in space affect water availability to vegetation growing across a diverse landscape under the same climate regime by using various time series datasets including satellite-derived vegetation density, groundwater wells, as well as streamflow and rainfall gauges
Summary
Plants in dryland ecosystems may experience differential seasonal access to water and distinct longterm trends in their responses to water availability changes, based on rooting depth, as well as the local expression of hydrology and water storage at their rooting location. Precipitation brings water to the land surface where it may become available to vegetation as a function of local storage, yet the amount and distribution of water (in streamflow, soil moisture and groundwater) depends on the rainfall intensity, duration, location and seasonal distribution throughout the year, as well as the fluxes in the hydrological cycle including evapotranspiration, infiltration of rainfall into the soil, runoff generation, and percolation to aquifers. In arid environments, where evapotranspiration exceeds precipitation, vegetation is typically concentrated at locations in the landscape where runoff accumulates and/or where the water table is close to the surface (Dawson and Ehleringer 1991, Patten 1998, Lite and Stromberg 2005, Rodriguez-Iturbe et al 2007), yielding potentially strong differences in vegetation types and density across a region with the same driving climate, depending on the local geologic controls and geomorphology (Caylor et al 2005, 2009, Franz et al 2010). Dryland riparian forests are vulnerable to shifts in climate that affect root zone water availability to the key plant species because they cannot expand their range (Malagnoux et al 2007, Loarie et al 2009, Bertrand et al 2011, Reidmiller et al 2018), making them sensitive to climate change
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