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1. Comparing optimal and empirical stomatal conductance models for application in Earth system models

   https://doi.org/10.1111/gcb.14445  

   Franks, Peter J. ; Bonan, Gordon B. ; Berry, Joseph A. ; Lombardozzi, Danica L. ; Holbrook, N. Michele ; Herold, Nicholas ; Oleson, Keith W. ( October 2018 , Global Change Biology)

   Earth system models (ESMs) rely on the calculation of canopy conductance in land surface models (LSMs) to quantify the partitioning of land surface energy, water, andCO₂fluxes. This is achieved by scaling stomatal conductance,g_w, determined from physiological models developed for leaves. Traditionally, models forg_whave been semi‐empirical, combining physiological functions with empirically determined calibration constants. More recently, optimization theory has been applied to modelg_winLSMs under the premise that it has a stronger grounding in physiological theory and might ultimately lead to improved predictive accuracy. However, this premise has not been thoroughly tested. Using original field data from contrasting forest systems, we compare a widely used empirical type and a more recently developed optimization‐typeg_wmodel, termedBBandMED, respectively. Overall, we find no difference between the two models when used to simulateg_wfrom photosynthesis data, or leaf gas exchange from a coupled photosynthesis‐conductance model, or gross primary productivity and evapotranspiration for aFLUXNETtower site with theCLM5 communityLSM. Field measurements reveal that the key fitted parameters forBBandMED,g_1Bandg_1M,exhibit strong species specificity in magnitude and sensitivity toCO₂, andCLM5 simulations reveal that failure to include this sensitivity can result in significant overestimates of evapotranspiration for high‐CO₂scenarios. Further, we show thatg_1Bandg_1Mcan be determined from meanc_i/c_a(ratio of leaf intercellular to ambientCO₂concentration). Applying this relationship withc_i/c_avalues derived from a leaf δ¹³C database, we obtain a global distribution ofg_1Bandg_1M, and these values correlate significantly with mean annual precipitation. This provides a new methodology for global parameterization of theBBandMEDmodels inLSMs, tied directly to leaf physiology but unconstrained by spatial boundaries separating designated biomes or plant functional types.

    

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2. Vascular epiphytes show low physiological resistance and high recovery capacity to episodic, short‐term drought in Monteverde, Costa Rica

   https://doi.org/10.1111/1365-2435.13613  

   Williams, Cameron B. ; Murray, Jessica G. ; Glunk, Andrew ; Dawson, Todd E. ; Nadkarni, Nalini M. ; Gotsch, Sybil G. ; Oliveira, ed., Rafael ( July 2020 , Functional Ecology)

   Tropical montane cloud forests support abundant epiphytic vascular plant communities that serve important ecosystem functions, but their reliance on atmospheric inputs of water may make them susceptible to the drying effects of rising cloud bases and more frequent droughts.

   We conducted a common garden experiment to explore the combined effects of decreasing cloud influence—lower humidity, warmer temperature, brighter light—and meteorological drought (i.e. absence of rain) on the physiology and morphology of vascular epiphytes native to primary forests of Monteverde, Costa Rica. The epiphytes, which exhibited C₃photosynthesis, were sourced from a lower montane cloud forest (CF) or a rainforest (RF) below the current cloud base and transplanted into nearby shadehouses (CF or RF shadehouse respectively). Vapour pressure deficit (VPD) and light availability, measured as photosynthetically active radiation, were 2.5 and 3.1 times higher in the RF than the CF shadehouse. Half of the plants were subjected to a severe 4‐week drought followed by a recovery period, and the other half were watered controls.

   Plants subjected to low VPD/light conditions of the CF shadehouse were physiologically and morphologically resistant to the drought treatment. However, compared to control plants, both sources of plants subjected to high VPD/light conditions of the RF shadehouse experienced declines in maximum net photosynthesis (A_max), stomatal conductance (g_s) and the proportion of healthy leaves (those not exhibiting chlorosis, desiccation or necrosis). At peak drought, leaves from the RF were 19% thinner than controls. Within 7–14 days after rewatering,A_max,g_sand leaf health recovered to nearly the levels of controls. Growth rate, mortality and phenology were unaffected by the treatments.

   The divergent responses to drought in the CF versus RF shadehouses, combined with the recovery in the RF shadehouse, indicate that these epiphytes possess adaptive properties that confer low resistance, but high recovery capacity, to episodes of short‐term drought over a range of cloud influence. In addition, the reduction inA_maxsuggests stomatal regulation that favours water conservation over carbon acquisition, a strategy that may inform epiphyte responses to rising clouds and increasing drought frequency expected in the long term.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

    

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3. Vein network redundancy and mechanical resistance mitigate gas exchange losses under simulated herbivory in desert plants

   https://doi.org/10.1093/aobpla/plad002  

   Duarte, Miguel A ; Woo, Sabrina ; Hultine, Kevin ; Blonder, Benjamin ; Aparecido, Luiza Maria ( February 2023 , AoB PLANTS)

   Crous, Kristine (Ed.)

   Herbivory can impact gas exchange, but the causes of interspecific variation in response remain poorly understood. We aimed to determine (1) what effects does experimental herbivory damage to leaf midveins have on leaf gas exchange and, (2) whether changes in leaf gas exchange after damage was predicted by leaf mechanical or venation traits. We hypothesized that herbivory-driven impacts on leaf gas exchange would be mediated by (1a/1b) venation networks, either by more vein resistance, or possibly trading off with other structural defenses; (2a/2b) or more reticulation (resilience, providing more alternate flow pathways after damage) or less reticulation (sectoriality, preventing spread of reduced functionality after damage). We simulated herbivory by damaging the midveins of four leaves from each of nine Sonoran Desert species. We then measured the percent change in photosynthesis (ΔAn%), transpiration (ΔEt%) and stomatal conductance (Δgsw%) between treated and control leaves. We assessed the relationship of each with leaf venation traits and other mechanical traits. ΔAn% varied between +10 % and −55%, similar to ΔEt% (+27%, −54%) and Δgsw% (+36%, −53%). There was no tradeoff between venation and other structural defenses. Increased damage resilience (reduced ΔAn%, ΔEt%, Δgsw%) was marginally associated with lower force-to-tear (P < 0.05), and higher minor vein density (P < 0.10) but not major vein density or reticulation. Leaf venation networks may thus partially mitigate the response of gas exchange to herbivory and other types of vein damage through either resistance or resilience.

    

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4. Differences in leaf gas exchange strategies explain Quercus rubra and Liriodendron tulipifera intrinsic water use efficiency responses to air pollution and climate change

   https://doi.org/10.1111/gcb.16673  

   Mathias, Justin M. ; Smith, Kenneth R. ; Lantz, Kristin E. ; Allen, Keanan T. ; Wright, Marvin J. ; Sabet, Afsoon ; Anderson‐Teixeira, Kristina J. ; Thomas, Richard B. ( March 2023 , Global Change Biology)

   Trees continuously regulate leaf physiology to acquire CO₂while simultaneously avoiding excessive water loss. The balance between these two processes, or water use efficiency (WUE), is fundamentally important to understanding changes in carbon uptake and transpiration from the leaf to the globe under environmental change. While increasing atmospheric CO₂(iCO₂) is known to increase tree intrinsic water use efficiency (iWUE), less clear are the additional impacts of climate and acidic air pollution and how they vary by tree species. Here, we couple annually resolved long‐term records of tree‐ring carbon isotope signatures with leaf physiological measurements ofQuercus rubra(Quru) andLiriodendron tulipifera(Litu) at four study locations spanning nearly 100 km in the eastern United States to reconstruct historical iWUE, net photosynthesis (A_net), and stomatal conductance to water (g_s) since 1940. We first show 16%–25% increases in tree iWUE since the mid‐20th century, primarily driven by iCO₂, but also document the individual and interactive effects of nitrogen (NO_x) and sulfur (SO₂) air pollution overwhelming climate. We find evidence forQuruleaf gas exchange being less tightly regulated thanLituthrough an analysis of isotope‐derived leaf internal CO₂(C_i), particularly in wetter, recent years. Modeled estimates of seasonally integratedA_netandg_srevealed a 43%–50% stimulation ofA_netwas responsible for increasing iWUE in both tree species throughout 79%–86% of the chronologies with reductions ing_sattributable to the remaining 14%–21%, building upon a growing body of literature documenting stimulatedA_netoverwhelming reductions ing_sas a primary mechanism of increasing iWUE of trees. Finally, our results underscore the importance of considering air pollution, which remains a major environmental issue in many areas of the world, alongside climate in the interpretation of leaf physiology derived from tree rings.

    

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5. Idioblasts and peltate hairs as distribution networks for water absorbed by xerophilous leaves

   https://doi.org/10.1111/pce.13985  

   Losada, Juan M. ; Díaz, Miriam ; Holbrook, N. Michele ( January 2021 , Plant, Cell & Environment)

   Capparis odoratissimais a tree species native to semi‐arid environments of South America where low soil water availability coexists with frequent night‐time fog. A previous study showed that water applied to leaf surfaces enhanced leaf hydration, photosynthesis and growth, but the mechanisms of foliar water uptake are unknown. Here, we combine detailed anatomical evaluations with water and dye uptake experiments in the laboratory, and use immunolocalization of pectin and arabinogalactan protein epitopes to characterize water uptake pathways in leaves. Abaxially, the leaves ofC. odoratissimaare covered with peltate hairs, while the adaxial surfaces are glabrous. Both surfaces are able to absorb condensed water, but the abaxial surface has higher rates of water uptake. Thousands of idioblasts per cm², a higher density than stomata, connect the adaxial leaf surface and the abaxial peltate hairs, both of which contain hygroscopic substances such as arabinogalactan proteins and pectins. The highly specialized anatomy of the leaves ofC odoratissimafulfils the dual function of minimizing water loss when stomata are closed, while maintaining the ability to absorb liquid water. Cell‐wall related hygroscopic compounds in the peltate hairs and idioblasts create a network of microchannels that maintain leaf hydration and promote water uptake.

    

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