Seasonal and diurnal trends in progressive isotope enrichment along needles in two pine species

Abstract

The Craig-Gordon type (C-G) leaf water isotope enrichment models assume a homogeneous distribution of enriched water across the leaf surface, despite observations that Δ 18 O can become increasingly enriched from leaf base to tip. Datasets of this 'progressive isotope enrichment' are limited, precluding a comprehensive understanding of (a) the magnitude and variability of progressive isotope enrichment, and (b) how progressive enrichment impacts the accuracy of C-G leaf water model predictions. Here, we present observations of progressive enrichment in two conifer species that capture seasonal and diurnal variability in environmental conditions. We further examine which leaf water isotope models best capture the influence of progressive enrichment on bulk needle water Δ 18 O. Observed progressive enrichment was large and equal in magnitude across both species. The magnitude of this effect fluctuated seasonally in concert with vapour pressure deficit, but was static in the face of diurnal cycles in meteorological conditions. Despite large progressive enrichment, three variants of the C-G model reasonably successfully predicted bulk needle Δ 18 O. Our results thus suggest that the presence of progressive enrichment does not impact the predictive success of C-G models, and instead yields new insight regarding the physiological and anatomical mechanisms that cause progressive isotope enrichment.

Insight into Canary Island pine physiology provided by stable isotope patterns of water and plant tissues along an altitudinal gradient

“…Observed values might then be related to the progressive 18 O enrichment (Fig. 3) that the exceptional long Canary Island pine needles (up to 30 cm length) showed, which is potentially driven by the diffusion of the heavy isotopologue from evaporative sites back to xylem water (through aquaporins), further enriching the longitudinal flow towards the tip (Cernusak et al 2016;Helliker and Ehleringer 2002;Kannenberg et al 2021). Differentiated transpiration rates along the needle surface could also trigger the longitudinal enrichment (Shu et al 2008).…”

Enhancing Understanding of the Hydrological Cycle via Pairing of Process‐Oriented and Isotope Ratio Tracers

“…Deviations from this relationship in the full-physics δ' 18 O may arise from variations in δ' 18 Ο 0 or the sensitivity of α to temperature and/or supersaturation. In the following analysis, we remove the deviations arising from variations in δ' 18 Ο 0 by calculating δ' 18 O as an isotopic discrimination from δ' 18 Ο 0, as is commonly done for plant waters (e.g., Barbour et al, 2004;Cernusak et al, 2016;Farquhar et al, 2007;Kannenberg et al, 2021):…”