In a groundbreaking study published in Nature Climate Change this year, researchers have cast doubt on a long-held assumption in ecology: that increasing the intrinsic water-use efficiency (iWUE) of trees will naturally boost their growth rates. This revelation challenges decades of belief surrounding the effects of rising atmospheric carbon dioxide on forest productivity and has profound implications for our understanding of plant physiology and climate feedback loops.
At the heart of this research is the complex interplay between atmospheric CO₂ concentration (cₐ), stomatal conductance (gₛ), photosynthesis rates (Aₙ), and tree productivity. Increasing CO₂ levels have been shown to enhance photosynthesis while simultaneously reducing stomatal conductance, the openings on leaves through which water vapor exits and CO₂ enters. This combination elevates iWUE, defined as the ratio of photosynthesis to stomatal conductance (Aₙ/gₛ), often interpreted as a sign of improved plant water use under stress. However, whether this physiological efficiency translates into real-world productivity gains for trees has remained contentious.
Utilizing a novel stomatal optimality theory, the team formulated a mathematical framework that delineates the maximal potential increase in tree growth attainable from observed increases in iWUE. This approach unifies physiological kinetics with environmental limitations, providing an envelope within which relative productivity increases can be realistically inferred. The findings emphasize a disconnect between water-use efficiency and biomass accumulation, highlighting the importance of considering multiple constraints beyond CO₂ fertilization.
One of the pivotal insights from the study is the recognition that rising atmospheric dryness—a factor exacerbated by climate change—can counterbalance the benefits of improved stomatal efficiency. As the air becomes drier, trees experience greater evaporative demand which challenges their hydraulic architecture and limits carbon assimilation, regardless of enhanced iWUE. This environmental flux blunts potential productivity gains, a nuance often overlooked in previous models that emphasized carbon inputs without factoring hydric stress.
In addition to abiotic constraints, the authors underscore the role of intrinsic anatomical and physiological factors, such as increased tree height and resultant hydraulic path-lengths. Taller trees must transport water over greater distances, increasing resistance and potentially offsetting gains in water-use efficiency. The research elegantly integrates these physiological dimensions, underscoring that tree growth responses to elevated CO₂ are inherently nonlinear and context-specific.
To validate theoretical predictions, the group examined a comprehensive dataset drawn from controlled manipulation experiments and isotopic analysis of tree rings, offering a multi-dimensional and temporal perspective on how trees have responded historically to rising CO₂. The isotopic evidence, reflecting intrinsic water-use efficiency trends over decades, aligned with the model’s forecast that increased efficiency does not straightforwardly translate into proportional growth increases.
This disconnect has profound implications for global carbon cycle models that commonly assume enhanced tree growth as a primary mitigation mechanism against anthropogenic CO₂ emissions. If forest productivity gains are overestimated, the carbon sink strength of terrestrial ecosystems may decline more rapidly than expected, accelerating climate feedback loops.
Moreover, the research surfaces critical questions about ecosystem resilience and forest management strategies under future climate scenarios. If trees cannot leverage improved water-use efficiency into sustained biomass accumulation, then forest productivity and their ecological services might be more vulnerable to intensified droughts and warming.
The study also highlights the complexity of optimizing water-use efficiency in a changing climate. While iWUE is widely regarded as an adaptation to drought stress, this research clarifies that it is not necessarily synonymous with increased carbon gain or improved growth. Instead, iWUE may serve as a conservative strategy that prioritizes survival over expansion under stress.
From a physiological standpoint, these findings call for a deeper examination of stomatal behavior and plant hydraulics beyond steady-state assumptions. Dynamic responses to environmental fluctuations, hydraulic limitations, and maintenance costs need to be factored into future models that seek to predict vegetation responses to global change reliably.
Furthermore, the research underscores the value of integrating theoretical frameworks with empirical data derived from long-term ecological records and isotope biogeochemistry, bridging the gap between mechanistic understanding and real-world ecosystem observations.
It also places emphasis on the need to revisit assumptions embedded in Earth system models. Incorporating constraints posed by atmospheric dryness and hydraulic architecture could improve projections of future carbon sequestration by forests, which are essential for guiding climate mitigation efforts.
By proposing an envelope of maximal relative increases in productivity associated with rises in iWUE, the authors offer a pragmatic tool for interpreting diverse experimental outcomes and resolving apparent contradictions in the literature regarding CO₂ fertilization effects.
The approach and findings also invite skepticism toward simplistic narratives that rising atmospheric CO₂ inherently benefits tree growth or forest carbon storage, emphasizing instead multifactorial influences and feedbacks.
While the research does not diminish the ecological importance of improved water-use efficiency, it reframes the discussion toward holistic assessments of how environmental stressors modulate the growth responses of trees under future atmospheric conditions.
In summary, this investigation shifts paradigms by demonstrating that although intrinsic water-use efficiency increases under elevated CO₂, proportional enhancements in tree productivity are unlikely or considerably constrained. This nuanced understanding stresses that tree growth and water relations are subject to complex trade-offs and environmental modulation, challenging optimistic assumptions about forest responses to climate change.
As forests continue to play a pivotal role in mitigating global warming, refining our grasp of plant physiological processes and their ecological outcomes remains imperative for robust predictions and effective climate policy efforts.
Subject of Research: Plant physiology, tree water-use efficiency, and growth responses under rising atmospheric CO₂ concentrations.
Article Title: Increased efficiency of water use does not stimulate tree productivity.
Article References:
Zhang, Q., Zhang, J., Adams, M.A. et al. Increased efficiency of water use does not stimulate tree productivity. Nat. Clim. Chang. (2025). https://doi.org/10.1038/s41558-025-02504-w
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