Global tree intrinsic water use efficiency is enhanced by increased atmospheric CO ₂ and modulated by climate and plant functional types

Abstract

We conducted a meta-analysis of carbon and oxygen isotopes from tree ring chronologies representing 34 species across 10 biomes to better understand the environmental drivers and physiological mechanisms leading to historical changes in tree intrinsic water use efficiency (iWUE), or the ratio of net photosynthesis (Anet) to stomatal conductance (gs), over the last century. We show a ∼40% increase in tree iWUE globally since 1901, coinciding with a ∼34% increase in atmospheric CO2 (Ca), although mean iWUE, and the rates of increase, varied across biomes and leaf and wood functional types. While Ca was a dominant environmental driver of iWUE, the effects of increasing Ca were modulated either positively or negatively by climate, including vapor pressure deficit (VPD), temperature, and precipitation, and by leaf and wood functional types. A dual carbon–oxygen isotope approach revealed that increases in Anet dominated the observed increased iWUE in ∼83% of examined cases, supporting recent reports of global increases in Anet, whereas reductions in gs occurred in the remaining ∼17%. This meta-analysis provides a strong process-based framework for predicting changes in tree carbon gain and water loss across biomes and across wood and leaf functional types, and the interactions between Ca and other environmental factors have important implications for the coupled carbon–hydrologic cycles under future climate. Our results furthermore challenge the idea of widespread reductions in gs as the major driver of increasing tree iWUE and will better inform Earth system models regarding the role of trees in the global carbon and water cycles.

The three‐dimensional construction of leaves is coordinated with water use efficiency in conifers

“…Global increases in temperature coupled with rising vapour pressure deficit (VPD) place increased strain on plant hydraulic and photosynthetic systems (Choat et al ., 2018; Grossiord et al ., 2020). There is strong evidence that tree water use efficiency (WUE) has increased in recent decades, most probably the result of rising atmospheric CO 2 , allowing plants to open their stomata less frequently, thereby conserving water (Keenan et al ., 2013; Mathias & Thomas, 2021). However, the underlying physiological mechanisms behind this trend need to be further elucidated (Guerrieri et al ., 2019).…”

The physiological acclimation and growth response of Populus trichocarpa to warming

“…Although evidence for changes in tree growth rates and biomass production is mixed (Canham et al 2018; McMahon et al 2010; Peñuelas et al 2011), some effects of increasing temperatures on tree physiology are already being observed. These include geographic range shifts (Monleon & Lintz 2015; Walther 2003), an increase in tree water use efficiency (Adams et al 2020; Mathias & Thomas 2021), and changes in rates of photosynthesis and respiration (Dusenge et al 2019; Kamarathunge et al 2019). The degree of physiological plasticity to climate change within dominant tree genera, such as Populus is not well known, yet will affect carbon (C) and water fluxes at the ecosystem and global levels (King et al 2006; Lombardozzi et al 2015; Smith & Dukes 2013) and the provisioning of essential ecosystem services (e.g., net ecosystem exchange, tree plantation biomass production) in the future.…”

Responses of Sap Flux Densities of Different Plant Functional Types to Environmental Variables Are Similar in Both Dry and Wet Seasons in a Subtropical Mixed Forest

“…Thus, the short-term changes can reveal subtle plant responses to their environment [22]. Ecophysiologists usually study plant water use at narrow temporal scales (e.g., diurnal or hourly), as plant physiological processes generally respond to environmental conditions in real-time (e.g., water use efficiency, stomatal conductance, and photosynthesis) [25][26][27][28]. A better understanding of the patterns of hourly-or diurnal-scales water use among plant functional types can be highly valuable in revealing internal variations among species as well as proportionate contributions to the overall forest water use.…”