Short‐term variation in leaf‐level water use efficiency in a tropical forest

Abstract

Summary The representation of stomatal regulation of transpiration and CO2 assimilation is key to forecasting terrestrial ecosystem responses to global change. Given its importance in determining the relationship between forest productivity and climate, accurate and mechanistic model representation of the relationship between stomatal conductance (gs) and assimilation is crucial. We assess possible physiological and mechanistic controls on the estimation of the g1 (stomatal slope, inversely proportional to water use efficiency) and g0 (stomatal intercept) parameters, using diurnal gas exchange surveys and leaf‐level response curves of six tropical broadleaf evergreen tree species. g1 estimated from ex situ response curves averaged 50% less than g1 estimated from survey data. While g0 and g1 varied between leaves of different phenological stages, the trend was not consistent among species. We identified a diurnal trend associated with g1 and g0 that significantly improved model projections of diurnal trends in transpiration. The accuracy of modeled gs can be improved by accounting for variation in stomatal behavior across diurnal periods, and between measurement approaches, rather than focusing on phenological variation in stomatal behavior. Additional investigation into the primary mechanisms responsible for diurnal variation in g1 will be required to account for this phenomenon in land‐surface models.

Wood‐density has no effect on stomatal control of leaf‐level water use efficiency in an Amazonian forest

“…Measurement of the stomatal slope parameter is fraught with trade‐offs and compromises and a number of approaches have been used to estimate this important physiological parameter (Ball et al, 1987; Bernacchi et al, 2006; Davidson et al, 2023; Medlyn et al, 2011; Miner et al, 2017; Wu et al, 2020). Here, using an in situ steady‐state‐survey approach—enabled by canopy access and typically low wind speeds at the field site—we showed that the time since inserting a leaf into the leaf chamber impacted A and g sw and that the leaf‐WUE was higher (lower m ) when measured at steady‐state compared with measurements made within 90 s of inserting the leaf (Figure 2).…”

“…Here, using an in situ steady‐state‐survey approach—enabled by canopy access and typically low wind speeds at the field site—we showed that the time since inserting a leaf into the leaf chamber impacted A and g sw and that the leaf‐WUE was higher (lower m ) when measured at steady‐state compared with measurements made within 90 s of inserting the leaf (Figure 2). Most of the survey‐type gas exchange measurements are made after a short acclimation time, typically less than 90 s (Bernacchi et al, 2006; Davidson et al, 2023; Leakey et al, 2006; Lin et al, 2015; Wu et al, 2020). However, it is known that stomata react more slowly to changes at the leaf surface than photosynthesis (Lawson & Vialet‐Chabrand, 2018).…”

“…We considered that R dark corresponded to the mean value of R dark25 scaled to the measured leaf temperature using an Arrhenius function as described previously. Finally, we compared g 1 and c g1 estimated at 90 s, a typical time at which survey measurements are taken (Davidson et al, 2023; Wu et al, 2020) and at steady‐state using a paired t‐test. All the other analyses presented here use the gas exchange values taken at steady state.…”

“…Most of the survey-type gas exchange measurements are made after a short acclimation time, typically less than 90 s (Bernacchi et al, 2006;Davidson et al, 2023;Leakey et al, 2006;Lin et al, 2015;Wu et al, 2020). However, it is known that stomata react more slowly to changes at the leaf surface than photosynthesis (Lawson & Vialet-Chabrand, 2018).…”

“…Furthermore, measurements were made on intact branches where care was taken to ensure that stomatal conductance fully acclimated to the measurement conditions. This approach was taken to avoid any possible bias associated with the slow response of stomata or with the excision of branches (Davidson et al, 2023; Miner et al, 2017; Wolz et al, 2017).…”