Hydraulic traits that buffer deep‐rooted plants from changes in hydrology and climate

Abstract

Groundwater‐dependent ecosystems are often defined by the presence of deeply rooted phreatophytic plants. When connected to groundwater, phreatophytes in arid regions decouple ecosystem net primary productivity from precipitation, underscoring a disproportionately high biodiversity and exchange of resources relative to surrounding areas. However, groundwater‐dependent ecosystems are widely threatened due to the effects of water diversions, groundwater abstraction, and higher frequencies of episodic drought and heat waves. The resilience of these ecosystems to shifting ecohydrological–climatological conditions will depend largely on the capacity of dominant, phreatophytic plants to cope with dramatic reductions in water availability and increases in atmospheric water demand. This paper disentangles the broad range of hydraulic traits expressed by phreatophytic vegetation to better understand their capacity to survive or even thrive under shifting ecohydrological conditions. We focus on three elements of plant water relations: (a) hydraulic architecture (including root area to leaf area ratios and rooting depth), (b) xylem structure and function, and (c) 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. Although many species occupy multiple phreatophytic niches depending on access to groundwater, we anticipate that populations are largely locally adapted to a narrow range of ecohydrological conditions regardless of gene flow across ecohydrological gradients. Consequently, we hypothesize that reductions in available groundwater and increases in atmospheric water demand will result in either (a) stand replacement of obligate phreatophytic species with more facultative species as a function of widespread mortality in highly groundwater‐dependent populations or (b) directional selection in semi‐obligate and semi‐facultative phreatophytes towards the expression of traits associated with highly facultative phreatophytes in the absence of species replacement. Anticipated shifts in the expression of hydraulic traits may have profound impacts on water cycling processes, species assemblages, and habitat structure of groundwater‐dependent woodlands and riparian forests.

UAV thermal image detects genetic trait differences among populations and genotypes of Fremont cottonwood (Populus fremontii, Salicaceae)

“…Significant differences were observed in most pairs of comparisons within both measured populations (Table 3). cooler temperatures illustrated greater transpiration rate (Hultine et al, 2020a) and presumably higher water content. Linking ground-based leaf and canopy temperature measurements with UAV-based thermal estimates need further testing and development in various climates and environments.…”

“…We completed the ground-based leaf temperature measurements in the common garden on May 30, 2019 at midday between 12:00 and 14:00 h. We measured leaf temperatures of 60 individual trees that represented two populations and 10 genotypes: five genotypes of the hot SD population from the local Agua Fria National Monument (Population #8, Table 1) and five genotypes of the cool HP population from Keams Canyon (Population #16, Table 1). These populations and genotypes were selected to represent Fremont cottonwood trees adapted to the very hot and cool origins of the broad environmental gradient sampled for the common garden and based on our previous ground-based results (Hultine et al, 2020a). Each population was sampled from two different blocks and two different plots.…”

Adaptive capacity in the foundation tree species Populus fremontii: implications for resilience to climate change and non-native species invasion in the American Southwest

“…A recent common garden experiment conducted at the mid-elevation common garden ( Fig. 3 ) revealed that chronic exposure to intense heat waves could impose strong selection pressures on P. fremontii to maximize canopy thermal regulation via a suite of hydraulic strategies ( Hultine et al ., 2020 ). Genotypes sourced from the extremely warm SD ecoregion had midday leaf temperatures in midsummer that were on average 2.0°C (SE ± 0.58) below ambient temperature; while genotypes sourced from the higher elevation, cooler UHP ecoregion had leaf temperatures that were on average 2.1°C (SE ± 0.67) above ambient temperature ( Fig.…”

“…4a ). The cooler leaf temperatures corresponded with the warm-adapted, SD ecotypes having a 35% higher midday leaf transpiration rate ( E ) relative to the UHP ecotypes, resulting in substantially greater leaf evaporative cooling ( Hultine et al ., 2020 ). Although predawn leaf water potentials (Ψ pd ) were similar between the two ecotypes, minimum leaf water potentials (Ψ min ) during midday were on average 0.2 MPa lower in the SD ecotypes ( Fig.…”

Landscape Dynamics in an Iconic Watershed of Northwestern Mexico: Vegetation Condition Insights Using Landsat and PlanetScope Data

“…In previous reports, obligate riparian species, such as cottonwoods, show they are restricted to places with specific groundwater levels [30,65]. On the other hand, mesquite and other facultative riparian species are distributed near streams with varying levels, but are usually found where groundwater is at deeper levels than those required for the establishment or sustenance of obligate riparian species [66,67]. Thus, the spatial differentiation between obligate riparian and facultative riparian species is crucial, since the absence of obligate riparian species could serve as an indicator of a decline in water availability [68].…”