The ecosystem wilting point defines drought response and recovery of a Quercus‐Carya forest

Abstract

Soil and atmospheric droughts increasingly threaten plant survival and productivity around the world. Yet, conceptual gaps constrain our ability to predict ecosystem‐scale drought impacts under climate change. Here, we introduce the ecosystem wilting point (ΨEWP), a property that integrates the drought response of an ecosystem's plant community across the soil–plant–atmosphere continuum. Specifically, ΨEWP defines a threshold below which the capacity of the root system to extract soil water and the ability of the leaves to maintain stomatal function are strongly diminished. We combined ecosystem flux and leaf water potential measurements to derive the ΨEWP of a Quercus‐Carya forest from an “ecosystem pressure–volume (PV) curve,” which is analogous to the tissue‐level technique. When community predawn leaf water potential (Ψpd) was above ΨEWP (=−2.0 MPa), the forest was highly responsive to environmental dynamics. When Ψpd fell below ΨEWP, the forest became insensitive to environmental variation and was a net source of carbon dioxide for nearly 2 months. Thus, ΨEWP is a threshold defining marked shifts in ecosystem functional state. Though there was rainfall‐induced recovery of ecosystem gas exchange following soaking rains, a legacy of structural and physiological damage inhibited canopy photosynthetic capacity. Although over 16 growing seasons, only 10% of Ψpd observations fell below ΨEWP, the forest is commonly only 2–4 weeks of intense drought away from reaching ΨEWP, and thus highly reliant on frequent rainfall to replenish the soil water supply. We propose, based on a bottom‐up analysis of root density profiles and soil moisture characteristic curves, that soil water acquisition capacity is the major determinant of ΨEWP, and species in an ecosystem require compatible leaf‐level traits such as turgor loss point so that leaf wilting is coordinated with the inability to extract further water from the soil.

Constraining Plant Hydraulics With Microwave Radiometry in a Land Surface Model: Impacts of Temporal Resolution

“…The prescribed “true” values of the retrieved parameters were chosen to represent a temperate deciduous forest, using the rich ecological and physiological data available from the oak and hickory dominated Missouri Ozarks Ameriflux (MOFLUX) site to prescribe parameter values when possible (Gu et al., 2016; Wood et al., 2023). Soil depth was chosen to be 2 m; observed soil depths at the MOFLUX site range from 0.38 to 2 m (Wood & Gu, 2022).…”

“…Soil hydraulic conductivity was chosen based on a typical value for the soil type found at the site, Weller silt loam soil (Young et al., 2001). The soil water retention curve was fitted to laboratory measurements on soil collected at the MOFLUX site (Wood et al., 2022).…”

Linking vegetation to climate using ecosystem pressure-volume relationships

“…Remote sensing applications hold promise for monitoring the water balance within ecosystems, and help us to identify tipping points where vegetation may transition from one type to another (Konings et al 2021). A key aspect of recognising thresholds is whether there is an acute transition point between the pre-and post-Ψthreshold parts of the EPV curve (Wood et al 2023). The transition point is often apparent in sapwood PV curves, and is mechanistically related to the cavitation threshold.…”

“…Typically measured in units of pressure, water potential is related to an absolute quantity of water using water retention curves in soil and pressure-volume curves in plants, both subsequently referred to as 'PV curves'. While PV curves tend to be performed on small samples (i.e., leaves, stems, or exsitu soil samples), there is mounting interest in understanding how these fundamental measures of water are linked at larger spatial scales (Konings et al 2021;Wood et al 2023).…”

“…Moreover, understanding large-scale relationships between water pressure and volume may shed novel insights into ecosystem function and response to climate variability (Wood et al 2023). The conceptual ecosystem-level PV curve (EPV) introduced here represents an effort to reconcile our detailed understanding of biophysical processes at sample-scale, with the scale at which land surface models operate, and at which tower-based, airborne and satellite remote sensing can provide information.…”