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 (
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(
- NSF-PAR ID:
- 10453877
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Global Change Biology
- Volume:
- 27
- Issue:
- 8
- ISSN:
- 1354-1013
- Page Range / eLocation ID:
- p. 1560-1571
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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(Ca), although mean iWUE, and the rates of increase, varied across biomes and leaf and wood functional types. While Cawas a dominant environmental driver of iWUE, the effects of increasing Cawere 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 Caand 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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Abstract 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 of
Quercus 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 (NOx ) and sulfur (SO2) air pollution overwhelming climate. We find evidence forQuru leaf gas exchange being less tightly regulated thanLitu through an analysis of isotope‐derived leaf internal CO2(Ci), 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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Abstract 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 (WUEiand WUEE, 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 WUEiand WUEEin 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 WUEiand WUEEwas 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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c a ), and decline in sea ice extent over the past century. The tree‐ring records were assessed during three periods with contrasting climatic conditions: (a) the early 20th century warming, (b) the 1940–1970 cooling period, and (c) the anthropogenic late 20th century warming period. We found opposing growth trends between the two sites, but similar carbon isotope discrimination (Δ13C) and intrinsic water‐use efficiency (iWUE) trajectories. While tree growth (defined as basal area increment) increased at the site nearer to the Arctic Ocean during the 20th century following the rise in temperature and sea ice loss, growth declined after 1950 at the more interior site. At both sites, Δ13C slightly increased over these periods. However, trees showed a nonlinear response to increasedc a , shifting after 1970 from a passive stomatal response (i.e., no changes iniWUE) to an active response (i.e., a moderate ∼12% increase iniWUE). Further, our isotope‐based findings do not support the idea that temperature‐induced drought stress caused the divergent growth trends at our treeline sites. This study thus highlights nonlinear and complex physiological and growth adjustments to concomitant changes in temperature, sea ice extent, andc a over the last century at the northern treeline. -
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Abstract Nitrogen (N)‐fixing trees are thought to break a basic rule of leaf economics: higher leaf N concentrations do not translate into higher rates of carbon assimilation. Understanding how leaf N affects photosynthesis and water use efficiency (WUE) in this ecologically important group is critical.
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A sat), stomatal conductance (g sw) and WUE (WUEiand δ13C).A sat, WUEiand δ13C, but notg sw, increased with higher leaf N. Surprisingly, N‐fixing and non‐fixing trees displayed similar scaling between leaf N and these physiological variables, and this finding was supported by reanalysis of a global dataset. N fixers generally had higher leaf N than non‐fixers, even when non‐fixers were not N‐limited at the leaf level. Leaf‐level N limitation did not alter the relationship ofA sat,g sw, WUEiand δ13C with leaf N, although it did affect the photosynthetic N use efficiency. Higher WUE was associated with higher productivity, whereas higherA satwas not.Synthesis : The ecological success of N‐fixing trees depends on the effect of leaf N on carbon gain and water loss. Using a field fertilization experiment and reanalysis of a global dataset, we show that high leaf‐level photosynthesis and WUE in N fixers stems from their higher average leaf N, rather than a difference between N fixers and non‐fixers in the scaling of photosynthesis and WUE with leaf N. By clarifying the mechanism by which N fixers achieve and benefit from high WUE, our results further the understanding of global N fixer distributions.
