Hydraulic Traits That Buffer Phreatophytes From the Effects of Groundwater Abstractions and Climate Change
Kevin Hultine1*, Ray Froend2, Davis Blasini3, Susan E Bush4, Melissa Karlinski5, Dan F Koepke6
1Department of Research, Conservation and Collections, Desert Botanical Garden, Phoenix Arizona, USA
2Centre for Ecosystem Management, Edith Cowan University, Joondalup, Western Australia, Australia
3Department of Research, Conservation and Collections, Desert Botanical Garden, Phoenix Arizona, USA
4Department of Research, Conservation and Collections, Desert Botanical Garden, Phoenix Arizona, USA
2Centre for Ecosystem Management, Edith Cowan University, Joondalup, Western Australia, Australia
6Department of Research, Conservation and Collections, Desert Botanical Garden, Phoenix Arizona, USA
Groundwater dependent ecosystems are largely defined by the presence of deeply-rooted phreatophytic plants. When connected to groundwater, phreatophytes in arid regions decouple ecosystem net primary productivity from precipitation inputs, underscoring a disproportionately high biodiversity and net ecosystem exchange of resources relative to surrounding areas. However, groundwater dependent ecosystems are widely threatened due to the effects of water diversions, groundwater pumping, and higher frequencies of episodic drought and heat waves. The resilience of these ecosystems to shifting ecohydrological-climatological conditions may depend on the capacity of dominant, phreatophytic vegetation to cope with dramatic reductions in water availability and increases in atmospheric water demand. We disentangle the broad range of hydraulic traits expressed by phreatophytic vegetation to better understand their capacity to survive, or even thrive under dramatically shifting ecohydrological conditions. We focus on three primary elements of plant water relations: 1) hydraulic architecture (including root area to leaf area ratios and rooting depth), 2) xylem structural and functional traits, and 3) stomatal regulation. We place the expression of these traits across a continuum of phreatophytic habits from obligate to semi-obligate to semi-facultative to facultative. We hypothesize that long-term reductions in available groundwater will result in either 1) stand replacement of obligate phreatophytic species with more facultative phreatophytic species as a function of wide-spread mortality in highly groundwater dependent populations, or 2) directional selection in semi-obligate and semi-facultative phreatophytes towards the expression of traits associated with highly facultative phreatophytes in the absence of replacement by other species. Shifts in the expression of hydraulic traits and phreatophytic habits may have profound impacts on water cycling processes, species assemblages and habitat structure of groundwater dependent woodlands and riparian forests.