In the face of accelerating climate change, understanding how plants respond to rising temperatures is paramount. A groundbreaking study published in Nature Plants by Xia et al. (2026) reveals that vegetation transpiration—the process by which plants release water vapor—reaches its optimal operating temperature at a significantly higher threshold than photosynthesis, the fundamental mechanism plants use to assimilate carbon. This discovery reshapes our comprehension of plant physiological responses under heat stress and carries profound implications for predicting ecosystem resilience amid global warming.
Photosynthesis and transpiration are intimately linked processes within plant physiology. Photosynthesis involves carbon dioxide uptake to produce energy-rich compounds essential for growth, while transpiration refers to the loss of water vapor through stomata, facilitating nutrient transport and leaf cooling. Traditionally, the temperature optimum for gross primary productivity (GPP)—the total carbon fixed by photosynthesis—has been extensively characterized, reflecting a nonlinear response curve with a distinct peak at an intermediate temperature. However, the thermal optima for transpiration (Trans) have remained elusive until now, despite transpiration’s critical role in plant temperature regulation and water cycling.
Xia and colleagues approached this gap by integrating a diverse array of data sources: global eddy covariance flux tower measurements, sap flow records, and simulations from a sophisticated Earth system model. Eddy covariance allows continuous monitoring of ecosystem-scale carbon and water fluxes, while sap flow provides direct measures of water movement within plant stems. This multi-faceted dataset enabled a comparative analysis of temperature responses in both photosynthetic carbon assimilation and transpiration across numerous biomes, spanning a wide climatic gradient.
The results compellingly demonstrated that the temperature optimum for transpiration ((T{mathrm{opt}}^{mathrm{Trans}})) consistently exceeded the optimal temperature for photosynthesis ((T{mathrm{opt}}^{mathrm{GPP}})) across ecosystems. This pattern held true not only at localized sites but also when scaled up to global simulations. The divergence between these optima suggests a decoupling under heat stress: transpiration mechanisms can sustain functionality even when photosynthetic carbon uptake begins to decline. This delayed thermal peak hints at a higher heat tolerance for water loss processes, reflecting the plant’s priority to use transpiration as a cooling strategy to prevent thermal damage during hot periods.
Such a decoupling challenges prevailing assumptions that photosynthesis and transpiration are tightly coordinated. While it is well-established that stomatal conductance regulates both CO₂ intake and water vapor loss, the asynchronous thermal optima underscore a nuanced physiological trade-off. Plants may maintain elevated transpiration rates beyond the photosynthetic optimum to dissipate excess heat, thus safeguarding the structural and enzymatic components of photosynthesis. However, this strategy is not without risks; prolonged water loss under extreme heat can lead to hydraulic stress and eventual stomatal closure to conserve water, ultimately suppressing transpiration itself.
Beyond establishing the existence of this thermal decoupling, the study leveraged machine learning techniques to identify the factors most influential in determining these temperature optima. Maximum air temperature emerged as the principal driver for both (T{mathrm{opt}}^{mathrm{Trans}}) and (T{mathrm{opt}}^{mathrm{GPP}}), aligning with expectations given the direct impact of ambient thermal conditions on plant physiology. Intriguingly, the difference between the two optima ((Delta T_{mathrm{opt}})) correlated closely with vegetation water content. High water content appeared to enable a larger temperature gap, suggesting that well-hydrated plants can sustain transpiration at higher temperatures relative to photosynthesis, thereby enhancing leaf cooling capacity.
Comparing observations with model simulations revealed additional insights and shortcomings. While the Earth system model captured the broad spatial patterns of temperature optima, it substantially underestimated the magnitudes of both (T{mathrm{opt}}^{mathrm{Trans}}) and (T{mathrm{opt}}^{mathrm{GPP}}), as well as their difference. This discrepancy points to limitations in how current models simulate plant water-carbon coupling, particularly under heat stress conditions. Models may insufficiently represent hydraulic traits, leaf cooling processes, or stomatal dynamics, potentially biasing projections of ecosystem responses to warming.
The ecological implications of these findings are profound. As global temperatures continue to climb, carbon uptake by vegetation may become increasingly vulnerable, peaking at lower temperatures than those tolerated by transpiration. This asymmetry suggests ecosystems might endure higher heat stress periods by prioritizing leaf cooling through water loss rather than carbon assimilation. While this temporary adaptation might stave off heat damage, it raises concerns over long-term water availability and carbon sequestration capacity, both critical factors for climate mitigation and ecosystem services.
From a broader perspective, this research also emphasizes the importance of considering thermal thresholds beyond photosynthesis alone when evaluating vegetation resilience. Transpiration represents more than a mere side effect of photosynthetic gas exchange; it is an active thermoregulatory process with its own distinct temperature sensitivity. Such knowledge could refine Earth system models and improve predictions of plant community dynamics, drought vulnerability, and feedback loops in the climate system.
Importantly, the study’s integration of empirical measurements with modeling underscores the value of interdisciplinary approaches in plant ecology. It sets a new standard for addressing complex physiological traits at scales ranging from leaf to biome, thereby bridging the knowledge gap between molecular mechanisms and ecosystem functioning. The deployment of machine learning further highlights how data-driven insights can unravel multifactorial controls on plant responses to environmental change.
Looking forward, these findings provoke a host of new research questions. How do different plant functional types or species modify their transpiration optimum relative to photosynthesis? What role does soil moisture, atmospheric vapor pressure deficit, or nutrient availability play in modulating these temperature thresholds? Investigating these aspects will be critical to understanding the resilience and adaptation strategies of diverse vegetation communities in a warming world.
Further refining Earth system models to better capture the distinct thermal responses of carbon and water cycles will be essential for predicting future climate-vegetation feedbacks. Incorporating explicit hydraulics, stomatal regulation, and leaf thermal buffering mechanisms could enhance model realism and help anticipate tipping points in ecosystem functioning. Such improvements could also inform forest management and agricultural practices aimed at optimizing water use efficiency and protecting carbon stocks under extreme heat events.
In sum, the demonstration of a higher temperature optimum for vegetation transpiration than for photosynthesis reshapes our understanding of plant-environment interactions. This decoupling underscores the sophisticated balancing act plants perform to maintain physiological processes under heat stress. As global climate change intensifies, accounting for the differential temperature sensitivities of these two fundamental processes will be crucial for better forecasting ecological trajectories and managing natural resources sustainably.
The work of Xia et al. thus provides a pivotal advance in the plant sciences, revealing a critical dimension of plant thermal biology that had hitherto been overlooked. By integrating observations across continents and biomes with machine learning and modeling, the study offers a robust framework for probing the complex interplay between temperature, carbon, and water in terrestrial ecosystems. These insights chart a path toward more nuanced and effective climate adaptation strategies for global vegetation.
Subject of Research: Plant physiological responses to temperature, specifically the thermal optima of photosynthesis and transpiration.
Article Title: Higher optimal temperature for vegetation transpiration than for photosynthesis.
Article References:
Xia, H., Zhang, F., Ciais, P. et al. Higher optimal temperature for vegetation transpiration than for photosynthesis. Nat. Plants (2026). https://doi.org/10.1038/s41477-026-02263-2
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41477-026-02263-2
Keywords: plant physiology, photosynthesis, transpiration, thermal optimum, gross primary productivity, climate change, heat stress, ecosystem modeling, water-carbon coupling
Tags: carbon assimilation temperature peakclimate change plant physiologyeddy covariance flux tower dataglobal warming ecosystem resiliencegross primary productivity temperature curveheat stress effects on plantsphotosynthesis temperature responseplant temperature regulation mechanismsplant transpiration temperature optimumplant water cycling under heatstomatal water vapor releasetranspiration vs photosynthesis temperature
