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 Δ¹⁸O 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 Δ¹⁸O. 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 Δ¹⁸O. 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.

中文翻译：

---

两种松树物种中沿针的逐步同位素富集的季节性和昼夜趋势

克雷格-戈登型（C-G）叶水同位素浓缩模型假设的富集水通过叶片表面的均匀分布，尽管观察结果Δ ¹⁸ O可从叶基部到尖端日趋富集。“渐进式同位素富集”的数据集是有限的，因此无法全面了解（a）渐进式同位素富集的大小和变异性，以及（b）渐进富集如何影响C–G叶片水模型预测的准确性。在这里，我们提出了在环境条件下捕获季节和昼夜变化的两个针叶树种中逐渐富集的观察结果。我们进一步研究其叶水同位素模型最好的拍摄逐渐富集对散装针水Δ的影响¹⁸O.观察到的渐进富集在两个物种中都很大并且大小相等。这种影响的幅度随蒸气压不足而随季节而波动，但在气象条件下面对昼夜周期时却是静止的。尽管大逐渐富集的C-G模型的三个变种相当成功预测散装针Δ ^18项从而O.我们的研究结果表明，逐渐富集的存在不会影响C-g型的预测成功，反而产生了关于新的见解引起逐步同位素富集的生理和解剖机制。