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1. Precipitation alters the CO 2 effect on water‐use efficiency of temperate forests

   https://doi.org/10.1111/gcb.15491  

   Belmecheri, Soumaya ; Maxwell, R. Stockton ; Taylor, Alan H. ; Davis, Kenneth J. ; Guerrieri, Rossella ; Moore, David J. P. ; Rayback, Shelly A. ( January 2021 , Global Change Biology)

   Increasing water‐use efficiency (WUE), the ratio of carbon gain to water loss, is a key mechanism that enhances carbon uptake by terrestrial vegetation under rising atmospheric CO₂(c_a). Existing theory and empirical evidence suggest a proportional WUE increase in response to risingc_aas plants maintain a relatively constant ratio between the leaf intercellular (c_i) and ambient (c_a) partial CO₂pressure (c_i/c_a). This has been hypothesized as the main driver of the strengthening of the terrestrial carbon sink over the recent decades. However, proportionality may not characterize CO₂effects on WUE on longer time‐scales and the role of climate in modulating these effects is uncertain. Here, we evaluate long‐term WUE responses toc_aand climate from 1901 to 2012 CE by reconstructing intrinsic WUE (iWUE, the ratio of photosynthesis to stomatal conductance) using carbon isotopes in tree rings across temperate forests in the northeastern USA. We show that iWUE increased steadily from 1901 to 1975 CE but remained constant thereafter despite continuously risingc_a. This finding is consistent with a passive physiological response toc_aand coincides with a shift to significantly wetter conditions across the region. Tree physiology was driven by summer moisture at multi‐decadal time‐scales and did not maintain a constantc_i/c_ain response to risingc_aindicating that a point was reached where rising CO₂had a diminishing effect on tree iWUE. Our results challenge the mechanism, magnitude, and persistence of CO₂'s effect on iWUE with significant implications for projections of terrestrial productivity under a changing climate.

    

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2. The North American Monsoon buffers forests against the ongoing megadrought in the Southwestern United States

   https://doi.org/10.1111/gcb.16762  

   Strange, Brandon M. ; Monson, Russell K. ; Szejner, Paul ; Ehleringer, Jim ; Hu, Jia ( June 2023 , Global Change Biology)

   The US Southwest has been entrenched in a two‐decade‐long megadrought (MD), the most severe since 800 CE, which threatens the long‐term vitality and persistence of regional montane forests. Here, we report that in the face of record low winter precipitation and increasing atmospheric aridity, seasonal activity of the North American Monsoon (NAM) climate system brings sufficient precipitation during the height of the summer to alleviate extreme tree water stress. We studied seasonally resolved, tree‐ring stable carbon isotope ratios across a 57‐year time series (1960–2017) in 17 Ponderosa pine forests distributed across the NAM geographic domain. Our study focused on the isotope dynamics of latewood (LW), which is produced in association with NAM rains. During the MD, populations growing within the core region of the NAM operated at lower intrinsic and higher evaporative water‐use efficiencies (WUE_iand WUE_E, respectively), compared to populations growing in the periphery of the NAM domain, indicating less physiological water stress in those populations with access to NAM moisture. The disparities in water‐use efficiencies in periphery populations are due to a higher atmospheric vapor pressure deficit (VPD) and reduced access to summer soil moisture. The buffering advantage of the NAM, however, is weakening. We observed that since the MD, the relationship between WUE_iand WUE_Ein forests within the core NAM domain is shifting toward a drought response similar to forests on the periphery of the NAM. After correcting for past increases in the atmospheric CO₂concentration, we were able to isolate the LW time‐series responses to climate alone. This showed that the shift in the relation between WUE_iand WUE_Ewas driven by the extreme increases in MD‐associated VPD, with little advantageous influence on stomatal conductance from increases in atmospheric CO₂concentration.

    

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3. Global tree intrinsic water use efficiency is enhanced by increased atmospheric CO 2 and modulated by climate and plant functional types

   https://doi.org/10.1073/pnas.2014286118  

   Mathias, Justin M. ; Thomas, Richard B. ( February 2021 , Proceedings of the National Academy of Sciences)

   We conducted a meta-analysis of carbon and oxygen isotopes from tree ring chronologies representing 34 species across 10 biomes to better understand the environmental drivers and physiological mechanisms leading to historical changes in tree intrinsic water use efficiency (iWUE), or the ratio of net photosynthesis (A_net) to stomatal conductance (g_s), over the last century. We show a ∼40% increase in tree iWUE globally since 1901, coinciding with a ∼34% increase in atmospheric CO₂(C_a), although mean iWUE, and the rates of increase, varied across biomes and leaf and wood functional types. While C_awas a dominant environmental driver of iWUE, the effects of increasing C_awere modulated either positively or negatively by climate, including vapor pressure deficit (VPD), temperature, and precipitation, and by leaf and wood functional types. A dual carbon–oxygen isotope approach revealed that increases inA_netdominated the observed increased iWUE in ∼83% of examined cases, supporting recent reports of global increases inA_net, whereas reductions ing_soccurred in the remaining ∼17%. This meta-analysis provides a strong process-based framework for predicting changes in tree carbon gain and water loss across biomes and across wood and leaf functional types, and the interactions between C_aand other environmental factors have important implications for the coupled carbon–hydrologic cycles under future climate. Our results furthermore challenge the idea of widespread reductions ing_sas the major driver of increasing tree iWUE and will better inform Earth system models regarding the role of trees in the global carbon and water cycles.

    

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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. Altered cell wall hydroxycinnamate composition impacts leaf- and canopy-level CO2 uptake and water use in rice

   https://doi.org/10.1093/plphys/kiad428  

   Pathare, Varsha_S ; Panahabadi, Rahele ; Sonawane, Balasaheb_V ; Apalla, Anthony_Jude ; Koteyeva, Nuria ; Bartley, Laura_E ; Cousins, Asaph_B ( July 2023 , Plant Physiology)

   Cell wall properties play a major role in determining photosynthetic carbon uptake and water use through their impact on mesophyll conductance (CO2 diffusion from substomatal cavities into photosynthetic mesophyll cells) and leaf hydraulic conductance (water movement from xylem, through leaf tissue, to stomata). Consequently, modification of cell wall (CW) properties might help improve photosynthesis and crop water use efficiency (WUE). We tested this using 2 independent transgenic rice (Oryza sativa) lines overexpressing the rice OsAT10 gene (encoding a “BAHD” CoA acyltransferase), which alters CW hydroxycinnamic acid content (more para-coumaric acid and less ferulic acid). Plants were grown under high and low water levels, and traits related to leaf anatomy, CW composition, gas exchange, hydraulics, plant biomass, and canopy-level water use were measured. Alteration of hydroxycinnamic acid content led to statistically significant decreases in mesophyll CW thickness (−14%) and increased mesophyll conductance (+120%) and photosynthesis (+22%). However, concomitant increases in stomatal conductance negated the increased photosynthesis, resulting in no change in intrinsic WUE (ratio of photosynthesis to stomatal conductance). Leaf hydraulic conductance was also unchanged; however, transgenic plants showed small but statistically significant increases in aboveground biomass (AGB) (+12.5%) and canopy-level WUE (+8.8%; ratio of AGB to water used) and performed better under low water levels than wild-type plants. Our results demonstrate that changes in CW composition, specifically hydroxycinnamic acid content, can increase mesophyll conductance and photosynthesis in C3 cereal crops such as rice. However, attempts to improve photosynthetic WUE will need to enhance mesophyll conductance and photosynthesis while maintaining or decreasing stomatal conductance.

    

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