On the use of leaf water to determine plant water source: A proof of concept

Abstract

Source water apportionment studies using the dual isotopes of oxygen and hydrogen have revolutionized our understanding of ecohydrology. But despite these developments-mostly over the past decade-many technical problems still exist in terms of linking xylem water to its soil water and groundwater sources. This is mainly

Leaf water δ

“…The experimental component of our study validated the notion that under high humidity, the gross diffusive exchanges of water vapour can dominate leaf water isotopic signatures (Farquhar and Cernusak 2005;Seibt et al 2006;Helliker and Griffiths 2007;Seibt et al 2007;Reyes-García et al 2008;Helliker 2011Helliker , 2014Lehmann et al 2020). Understanding NSS controls on the isotopic signature of plant water and tissue organic material is important during gas exchange by certain species (Farquhar and Cernusak 2005;Cernusak et al 2008;Lai et al 2008;Cernusak et al 2016;Benettin et al 2021). Our simulations with a NSS leaf water model illustrate how the contrasting diurnal patterns of gas exchange affect d 18 O lw for the two species during the humidity chamber experiments.…”

“…The distribution of CAM and C 3 epiphytic bromeliads across the continental divide in western Panama was used as the basis for a comparison of the coupling in water budgets for the two photosynthetic pathways. The study set out to define whether leaf water oxygen stable isotopes (d 18 O lw ) reflect the seasonal variability in precipitation, changes in d 18 O of tank water reserves, and contrasting daily gas exchange patterns, along an altitudinal transect running north from the Pacific to montane cloud forest at Fortuna (Cavelier et al 1996;Zotz et al 2005;Gómez González et al 2017;Benettin et al 2021). The overall aim was to investigate whether water vapour exchange across leaf surfaces, when atmospheric water vapour concentrations are close to saturation, leads to significant inward fluxes of water which 'reset' the d 18 O lw under natural conditions in the field (Farquhar and Cernusak 2005;Seibt et al 2006;Helliker and Griffiths 2007;Helliker 2011;Lehmann et al 2020) in addition to potential liquid water uptake (Pierce et al 2001;Dawson and Goldsmith 2018;Berry et al 2019).…”

“…2), and we explore the integration of bromeliad hydraulic properties and d 18 O signals in ecological terms in the final section below. We also set out to test the hypothesis that the gas exchange of C 3 and CAM bromeliads would result in contrasting temporal and spatial drivers for d 18 O lw , when compared with source water d 18 O inputs measured in terms of precipitation, tank water and water vapour (Dubbert et al 2017;Benettin et al 2021). Depending on seasonal conditions (dry or wet season), we thought that daytime gas exchange associated with C 3 species should be subject to evaporative enrichment in d 18 O lw at lower altitudes in the dry season.…”

“…The d 18 O signal in leaf water and organic material in bromeliads reflects the relative inputs from contrasting water sources such as precipitation, tank water (which may become evaporatively enriched between rain events) and atmospheric water vapour (Farquhar and Cernusak 2005;Seibt et al 2008;Cernusak et al 2016;Lehmann et al 2020;Benettin et al 2021).…”

On the Spatio-Temporal Under-Representation of Isotopic Data in Ecohydrological Studies

“…Indeed, the overall uncertainty in isotope-based estimated proportions of sources for plant water use is rarely quantified. Methods that include uncertainty propagation exist (e.g., Moore and Semmens, 2008;Parnell et al, 2010Parnell et al, , 2013Rothfuss and Javaux, 2017;Bowen et al, 2018;Benettin et al, 2021) but they often rely on assumptions that are difficult to verify and meet (e.g., no fractionation during root water uptake, homogeneity of soil water isotope profiles at depth, knowledge of all water sources). Attempts to quantify uncertainty have been carried out but addressing only a limited part of the effects presented in Figure 1 (e.g., Evaristo et al, 2015;Zhang et al, 2017;Beyer et al, 2018).…”