Matthias Saurer

Purpose of Review: Water Use Efficiency (WUE) is widely used to describe carbon-water coupling, but its meaning varies among disciplines, measurement approaches and spatial and temporal scales. We review commonly used WUE definitions and formulations and clarify their conceptual differences. Recent Findings: Ratio-based metrics, including WUE ( A /E) and intrinsic WUE ( A /g _s w), describe realized carbon–water exchange but remain sensitive to environmental conditions. Optimality-based approaches distinguish these ratios from the marginal water cost of carbon gain (∂E/∂ A ), providing a mechanistic perspective on water-use strategy. We also highlight differences among leaf, plant, ecosystem, isotope, and model-derived estimate. Summary: WUE is best understood as a context-dependent metric rather than a universal functional trait. We propose a framework linking each formulation to its carbon and water variables, spatial and temporal scale, environmental dependencies, and underlying assumptions. This approach supports transparent metric selection and improves comparisons across studies, scales, and methods.

Understanding how carbon–water coupling scales from leaves to ecosystems remains a major challenge because water-use efficiency (WUE) metrics integrate physiological, hydrological, and ecological processes. We integrated tree-ring width, triple stable isotopes (δ¹³C, δ ¹⁸ O and δ²H), eddy covariance (EC) observations, and a process-based model to investigate the drivers of intrinsic ( _i WUE) and ecosystem water-use efficiency (WUE _eco ) across biological scales in a mature Fagus sylvatica forest between 2004 and 2023. Modelled and isotope-derived _i WUE were strongly correlated (r = 0.61, p < 0.05) and both indicated an approximately 20% long-term increase, consistent with increasing gross primary productivity. In contrast, WUE _eco derived from EC was lower than model estimates and exhibited a positive trend not reproduced by the model. This divergence primarily reflected differences in biological representation and hydrological integration, as the single-species model underestimated ecosystem evapotranspiration by excluding co-occurring deciduous and evergreen species together with understory vegetation. Triple-isotope analyses showed that the long-term increase in _i WUE was driven by enhanced photosynthetic assimilation rather than sustained stomatal closure, whereas δ ¹⁸ O and δ²H variability was mainly controlled by source-water dynamics. These findings provide a mechanistic framework for reconciling leaf- and ecosystem-scale WUE and improving predictions of forest carbon–water coupling under climate change.

European white oaks grow from the Mediterranean coast to Southern Scandinavia, a huge environmental gradient that has likely fostered environmental adaptation. In the face of climate change, leveraging adaptations through assisted gene flow could help improve drought tolerance and maintain forest health, but requires an understanding of the species-specific patterns of adaptation to be successful at the target location. In this study, three common gardens were established in Switzerland, Tuerkiye, and Austria for two European white oak species (Quercus robur, and Q. pubescens) using provenances from Central and Southern Europe. Almost 900 oak seedlings were measured at key water-use efficiency and life history traits for their first two year of life and genotyped with low coverage whole-genome sequencing. Trait heritability and environmental adaptation were then explored through pedigree-free animal models, while the genomic architecture of traits was mapped using a genome wide association study ("GWAS"). Across the species, the heritability of measured traits was moderate to high, but common garden had a strong impact, signalling an environmental effect on the phenotype. Adaptation to precipitation seasonality was detected in key productivity and growth traits for both species, but had a small effect on absolute trait values. The GWAS identified a striking 150 kbp association in the Cyclic Nucleotide-Gated Ion Channel gene family with leaf d13C values. This gene family is involved in stomata opening and likely impacts the intrinsic water use efficiency under stress. Together, the strong signals of phenotypic plasticity and rather weak signals of climatic adaptation in seedlings suggest that assisted gene flow in these two white oaks is relevant only for highly drought-sensitive populations, if conducted managers should focus on seeds sources with high precipitation seasonality and smaller leaf sizes.

We present annually resolved millennial d ¹³ C tree-ring cellulose chronologies from 6 regions covering northeastern and eastern Russia, western Canada, Sweden, Norway, and Finland, and 3 regions from the Swiss Alps and Inner and Central Asia. We evaluated iWUE responses of conifer forests to changes in CO ₂ and climate during pre-industrial (CE 1000-1899) and anthropogenically influenced (CE 1900-1989) periods. Our results show that after moderate iWUE variations throughout most of the record, a major increase occurred in the 20 ^th century. During the industrial period, iWUE increased across the HL gradient from west (15.7 %) to east (33.0 %), while along high-elevated sites range 13.3 - 25.1 %. The most pronounced increase was observed at the most continental sites in Siberia and in the Altai mountains. We revealed that iWUE and related eco-physiological processes over the strongly human-influenced 20 ^th century were primarily modulated by changes in atmospheric CO ₂ concentration, although this effect has diminished in the last decades. In contrast, climate change dominated changes in iWUE in the pre-industrial period. The industrial iWUE increase is unprecedented for HL and HA over the past millennium. However, since the 2000s, Siberian forests demonstrate significant resilience to increased human-induced CO ₂ , associated with stable iWUE.

Summary: The oxygen (δ ¹⁸ O) and hydrogen (δ ² H) isotope compositions of leaf and xylem water shape tree‐ring isotope baselines, while the fraction of sugars undergoing isotopic modification downstream of leaves ( f _O , f _H ) determines the dominant hydrologic signal. However, limited information on the seasonal dynamics of these isotope sources and on the drivers of f variation constrains tree‐ring isotope interpretation. We measured intra‐annual δ ¹⁸ O and δ ² H in stem water, sugar, starch, and tree‐ring α‐cellulose of beech and spruce over two growing seasons. Using modelled leaf water δ ¹⁸ O and δ ² H, we estimated seasonal f values and examined their relationships with nonstructural carbohydrate concentrations and climate. Tree rings primarily recorded δ ¹⁸ O and δ ² H signatures of leaf water, despite seasonal changes in f _O and f _H . We found no clear transfer of intra‐annual xylem water isotopic signals into sugars, starch, or cellulose. Seasonal f _O and f _H can be negatively correlated. Both were related to climate variables, but only f _O was correlated with nonstructural carbohydrate concentrations. Thus, isotopic fractionation downstream of leaves does not always override the seasonal imprint of leaf water in tree rings. These findings provide insight into the controls on the f _O – f _H covariation, supporting more robust interpretations of climate variability from tree‐ring isotope records.

Broadleaf afforestation increasingly requires selecting planting material that combines valuable wood properties with resilience to climate stress. High heartwood content is a desirable trait in pedunculate oak (Quercus robur), but it is unclear whether selecting for it could compromise resistance and resilience to drought performance. We addressed this question in a progeny trial started in 2002 in southern Denmark where trees were ranked by heartwood production and the 2018 drought was used as a natural stress event. Trees with contrasting heartwood content were assessed using multiple indicators of drought response, including annual ring width, vessel anatomy, wood density, and dual stable isotopes (δ¹³C and δ¹⁸O) measured in earlywood and latewood over five years. The 2018 summer drought (SPEI3 approximately -1.5) was clearly reflected in isotopic signals, indicating that trees perceived the water deficit. However, radial growth did not decline, and ring widths remained above average in 2018. Climate-growth relationships suggested that growth at this site is more strongly limited by cool springs or wet summers than by moderately dry early-summer conditions. Across all metrics, trees with lower and higher heartwood content showed broadly similar physiological and wood-structural responses. Within the environmental conditions and drought intensity observed here, these findings do not provide evidence for a strong trade-off between heartwood content and drought response. They therefore support, with appropriate caution, the continued consideration of heartwood content as a selection trait in breeding and afforestation programs in comparable environments.

Restoration of broadleaf forests increasingly requires selecting planting material that combines valuable wood properties with resilience to climate stress. High heartwood content is a desirable trait in pedunculate oak (Quercus robur), but it is unclear whether selecting for it could compromise resistance and resilience to drought performance. We addressed this question in a progeny trial started in 2002 in southern Dernark where trees were ranked by heartwood production, using the 2018 European drought as a natural stress event. Trees with differing heartwood amounts were assessed using multiple indicators of drought response: annual ring width, vessel anatomy, wood density, and dual stable isotopes (δ¹³C and δ¹⁸O) measured in earlywood and latewood over five years. The 2018 summer drought (SPEI3 about -1.5) was clearly detected in isotopic signals, indicating that trees perceived the water deficit. However, growth did not decline; ring widths were above average in 2018. Climate-growth correlations suggested that cold springs or wet summers limit growth more strongly than moderately dry early-summer conditions at this site. Across all metrics, trees with lower and higher high amount of heartwood showed similar physiological and wood-structural responses, with no evidence of reduced drought resistance or resilience in high-heartwood trees. These results indicate that selecting Q. robur for increased heartwood content does not impose detectable trade-offs in drought performance and can be pursued safely in breeding and reforestation programs in this region.

High-resolution SIMS imaging can reveal the function of temporary starch stores in the needles of conifers to transfer carbon from assimilating to exporting tissues, under conditions of limited water availability.

Stable isotopes of carbon, oxygen and hydrogen in tree rings provide a record of plant physiological processes and environmental variability. Although an increasing number of studies now apply triple-isotope approaches, no investigation has yet tested their temporal stability over millennial timescales or assessed the relative impacts of physiology versus climate on long-term isotopic signals. Here, we used 9000 years of multi-isotope records from co-occurring deciduous larch (Larix decidua) and evergreen cembra pine (Pinus cembra) at the Alpine treeline. We found a high interspecies coherence for δ18O throughout the Holocene with a robust summer hydroclimate sensitivity, confirming its dominance by environmental drivers. In contrast, δ13C and δ2H show weaker and less stable coherence, reflecting species-specific physiology. Larch exhibits tight δ2H–δ18O and δ2H–δ13C correlations and stronger climate sensitivity, consistent with its reliance on freshly assimilated carbon. Pine, by contrast, shows weaker δ2H–climate relationships and frequent decoupling from δ13C and δ18O, reflecting potential storage use and metabolic fractionations. Thus, inter-isotope relationships reveal that δ18O is a robust long-term climate proxy, while δ13C and δ2H encode contrasting carbon-use strategies and metabolic processes across species that may vary over time. Together, these findings demonstrate that multi-isotope, multi-species approaches not only strengthen climate reconstructions but also provide a physiological dimension to long-term isotope records.