Eastern US deciduous tree species respond dissimilarly to declining soil moisture but similarly to rising evaporative demand

Abstract

Hydraulic stress in plants occurs under conditions of low water availability (soil moisture; θ) and/or high atmospheric-demand for water (vapor pressure deficit; D). Different species are adapted to respond to hydraulic stress by functioning along a continuum where at one end they close stomata to maintain a constant leaf water potential (ΨL) (isohydric species), and at the other they allow ΨL to decline (anisohydric species). Differences in water use along this continuum are most notable during hydrologic stress, often characterized by low θ and high D; however, θ and D are often, but not necessarily coupled at timescales of weeks or longer, and uncertainty remains about the sensitivity of different water use strategies to these variables. We quantified the effects of both θ and D on canopy conductance (Gc) among widely-distributed canopy-dominant species along the iso-anisohydric spectrum growing along a hydroclimatological gradient. Tree-level Gc was estimated using hourly sap flow observations from three sites in the eastern United States: a mesic forest in western North Carolina, and two xeric forests in southern Indiana and Missouri. Each site experienced at least one year of substantial drought conditions. Our results suggest that sensitivity of Gc to θ varies across sites and species, with Gc sensitivity being greater in dry than in wet sites, and greater for isohydric compared to anisohydric species. However, once θ limitations are accounted for, sensitivity of Gc to D remains relatively constant across sites and species. While D limitations to Gc were similar across sites and species, ranging from 16–34% reductions, θ limitations to Gc ranged from 0–40%. The similarity in species sensitivity to D is encouraging from a modeling perspective, though it implies substantial reduction to Gc will be experienced by all species in a future characterized by higher D.

Exploring complex water stress–gross primary production relationships: Impact of climatic drivers, main effects, and interactive effects

“…Dryness stress on ecosystem production has often been characterized by low soil water content (SWC) and high atmospheric water demand (i.e., vapor pressure deficit, VPD; Liu et al, 2020). The impacts of VPD and SWC on plants have been studied in recent years against the background of global warming (Chen et al, 2021; Denham et al, 2021). However, the dominance of VPD versus SWC on plant water stress is still under debate (Fu et al, 2022; Liu et al, 2020; Sulman et al, 2016; Yuan et al, 2019).…”

The xylem of anisohydric Quercus alba L. is more vulnerable to embolism than isohydric codominants

“…We found that Q. alba had particularly high P50 (consistent with previous work: Kannenberg et al, 2019; Maherali et al, 2006) but were also more anisohydric. We used the variation in Ψ L to quantify the degree of isohydricity to incorporate stomatal responses to both declining soil water and increasing D , noting that the latter is the predominant factor limiting conductance for these sites and species (Denham et al, 2021; Novick et al, 2016; Yi et al, 2019). However, prior work using other approaches for quantifying isohydricity in these study sites and elsewhere also concludes that Quercus species are more anisohydric than many of their codominant counterparts (Abrams, 1990; Cavender‐Bares & Bazzaz, 2000; Ewers et al, 2007; Kannenberg et al, 2019; Meinzer et al, 2013; Roman et al, 2015).…”

“…Altogether, it appears that Q. alba sustain high rates of gas exchange at the cost of operating with damaging water potential gradients and low Ψ safety . Moreover, much of the variability in stomatal conductance and water potential for eastern US trees, and especially Quercus species, are determined by the dynamics of D (Denham et al, 2021; Novick et al, 2019; Yi et al, 2019). Thus, these species may be particularly vulnerable to hydraulic dysfunction linked to future droughts that will be characterized by increasingly high D (Ficklin & Novick, 2017).…”

“…Negative excursions in Ψ L driven by D may be especially important in eastern US forests, where limitations to stomatal conductance from D have been shown to dominate over soil water limitations, at both the stand (Novick et al, 2016) and tree‐scale (Denham et al, 2021; Yi et al, 2019). While substantial soil water deficits occurred in some of our sites (e.g., MO, NC_E, IN), the more mesic NC_W stands rarely experience soil water limitations, and soil water deficits were not observed during the study period (Figure ); however, Ψ L reductions during periods of elevated D occurred routinely (Figure ).…”

“…Q. alba , A. saccharum , and L. tulipifera are the 5th, 6th, and 17th most abundant species (out of 134) in eastern US forests (Iverson et al, 2008). These species differ widely in terms of xylem anatomy ( Q. alba are ring‐porous whereas A. saccharum and L. tulipifera and are diffuse‐porous) and in terms of stomatal regulation strategy ( Q. alba are more anisohydric than the other species, Denham et al, 2021; Matheny et al, 2017; Meinzer et al, 2013; Roman et al, 2015). We seek to understand: (1) to what extent is regulation of Ψ L coordinated with embolism resistant tissues across these three species?…”