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1. Resolving the contrasting leaf hydraulic adaptation of C₃ and C₄ grasses

   https://doi.org/10.1111/nph.20341  

   Baird, Alec_S; Taylor, Samuel_H; Pasquet‐Kok, Jessica; Vuong, Christine; Zhang, Yu; Watcharamongkol, Teera; Cochard, Hervé; Scoffoni, Christine; Edwards, Erika_J; Osborne, Colin_P; et al (January 2025, New Phytologist)

   Summary Grasses are exceptionally productive, yet their hydraulic adaptation is paradoxical. Among C₃grasses, a high photosynthetic rate (A_area) may depend on higher vein density (D_v) and hydraulic conductance (K_leaf). However, the higherD_vof C₄grasses suggests a hydraulic surplus, given their reduced need for highK_leafresulting from lower stomatal conductance (g_s).Combining hydraulic and photosynthetic physiological data for diverse common garden C₃and C₄species with data for 332 species from the published literature, and mechanistic modeling, we validated a framework for linkages of photosynthesis with hydraulic transport, anatomy, and adaptation to aridity.C₃and C₄grasses had similarK_leafin our common garden, but C₄grasses had higherK_leafthan C₃species in our meta‐analysis. Variation inK_leafdepended on outside‐xylem pathways. C₄grasses have highK_leaf : g_s, which modeling shows is essential to achieve their photosynthetic advantage.Across C₃grasses, higherA_areawas associated with higherK_leaf, and adaptation to aridity, whereas for C₄species, adaptation to aridity was associated with higherK_leaf : g_s. These associations are consistent with adaptation for stress avoidance.Hydraulic traits are a critical element of evolutionary and ecological success in C₃and C₄grasses and are crucial avenues for crop design and ecological forecasting. 

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2. Quantitative critique of leaf‐based paleo‐CO ₂ proxies: Consequences for their reliability and applicability

   https://doi.org/10.1002/gj.3807  

   Konrad, Wilfried; Royer, Dana L.; Franks, Peter J.; Roth‐Nebelsick, Anita (March 2020, Geological Journal)

   A variety of proxies have been developed to reconstruct paleo‐CO₂from fossil leaves. These proxies rely on some combination of stomatal morphology, leafδ¹³C, and leaf gas exchange. A common conceptual framework for evaluating these proxies is lacking, which has hampered efforts for inter‐comparison. Here we develop such a framework, based on the underlying physics and biochemistry. From this conceptual framework, we find that the more extensively parameterised proxies, such as the optimisation model, are likely to be the most robust. The simpler proxies, such as the stomatal ratio model, tend to under‐predict CO₂, especially in warm (>15^°C) and moist (>50%humidity) environments. This identification of a structural under‐prediction may help to explain the common observation that the simpler proxies often produce estimates of paleo‐CO₂that are lower than those from the more complex proxies and other, non‐leaf‐based CO₂proxies. The use of extensively parameterised models is not always possible, depending on the preservation state of the fossils and the state of knowledge about the fossil's nearest living relative. With this caveat in mind, our analysis highlights the value of using the most complex leaf‐based model as possible. 

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3. Climate‐change‐driven shifts in C ₃ and C ₄ grass distributions and leaf traits could lead to changes in community‐level flammability

   https://doi.org/10.1002/ajb2.70081  

   Raubenheimer, Sarah L; Zheng, Liting; Stefanski, Artur; Reich, Peter B (October 2025, American Journal of Botany)

   Abstract PremiseClimate change poses challenges to grasslands, including those of the North American Great Plains Region, where shifts in species distributions and fire dynamics are expected. Our present analysis focuses on remaining grasslands within this largely developed and agricultural region. The differential responses of C₄and C₃grass species to future climate conditions, particularly in habitat suitability and flammability, are critical for understanding ecosystem changes. MethodsWe used species distribution models to predict shifts in habitat suitability for 37 grass species under future climate scenarios and assessed flammability traits in a free‐air CO₂‐enrichment study, focusing on species' physiological responses to elevated CO₂, warming, and drought. ResultsOur models predicted that C₄species will retain higher habitat suitability, while C₃species will decline. Leaf‐level flammability analysis showed that species with higher water‐use efficiency under elevated CO will have lower flammability than under non‐elevated, potentially decreasing the predicted rate of fire spread when such species dominate. In contrast, species with higher growth rates but lower water‐use efficiency may be more flammable. Species‐specific responses varied within functional types. Anticipated shifts in species distributions suggest C₄species will become more dominant, potentially altering competitive dynamics and reducing C₃diversity. Changes in flammability under future conditions are expected to influence fire regimes, with a predicted decrease in mean community rate of spread due to the dominance of less‐flammable C₄species. ConclusionsThese findings highlight the need for adaptive fire management and conservation strategies to maintain biodiversity and ecosystem function in North American grasslands under climate change. 

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4. Acclimation of Photosynthesis to CO ₂ Increases Ecosystem Carbon Storage due to Leaf Nitrogen Savings

   https://doi.org/10.1111/gcb.17558  

   Smith, Nicholas G; Zhu, Qing; Keenan, Trevor F; Riley, William J (November 2024, Global Change Biology)

   ABSTRACT Photosynthesis is the largest flux of carbon between the atmosphere and Earth's surface and is driven by enzymes that require nitrogen, namely, ribulose‐1,5‐bisphosphate (RuBisCO). Thus, photosynthesis is a key link between the terrestrial carbon and nitrogen cycle, and the representation of this link is critical for coupled carbon‐nitrogen land surface models. Models and observations suggest that soil nitrogen availability can limit plant productivity increases under elevated CO₂. Plants acclimate to elevated CO₂by downregulating RuBisCO and thus nitrogen in leaves, but this acclimation response is not currently included in land surface models. Acclimation of photosynthesis to CO₂can be simulated by the photosynthetic optimality theory in a way that matches observations. Here, we incorporated this theory into the land surface component of the Energy Exascale Earth System Model (ELM). We simulated land surface carbon and nitrogen processes under future elevated CO₂conditions to 2100 using the RCP8.5 high emission scenario. Our simulations showed that when photosynthetic acclimation is considered, photosynthesis increases under future conditions, but maximum RuBisCO carboxylation and thus photosynthetic nitrogen demand decline. We analyzed two simulations that differed as to whether the saved nitrogen could be used in other parts of the plant. The allocation of saved leaf nitrogen to other parts of the plant led to (1) a direct alleviation of plant nitrogen limitation through reduced leaf nitrogen requirements and (2) an indirect reduction in plant nitrogen limitation through an enhancement of root growth that led to increased plant nitrogen uptake. As a result, reallocation of saved leaf nitrogen increased ecosystem carbon stocks by 50.3% in 2100 as compared to a simulation without reallocation of saved leaf nitrogen. These results suggest that land surface models may overestimate future ecosystem nitrogen limitation if they do not incorporate leaf nitrogen savings resulting from photosynthetic acclimation to elevated CO₂. 

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5. Importance of Timing of Dark Acclimation for Estimating Light Inhibition of Leaf Respiratory CO ₂ Efflux

   https://doi.org/10.1111/pce.15335  

   Bruhn, Dan; Griffin, Kevin L; Møller, Ian M (December 2024, Plant, Cell & Environment)

   SUMMARY STATEMENT The degree of inhibition of leaf respiration by light is often studied, but the methods used and the results obtained are variable. We suggest that in the future daytime leaf respiration is measured 3 min after dark acclimation to avoid under‐estimating the degree of light inhibition of leaf respiration. This will most likely speed up future surveys and perhaps also result in less inter‐study variation in the calculated degree of light inhibition of leaf respiration. 

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