Abstract:Summary
Atmospheric carbon dioxide concentration ([CO2]) is increasing, which increases leaf‐scale photosynthesis and intrinsic water‐use efficiency. These direct responses have the potential to increase plant growth, vegetation biomass, and soil organic matter; transferring carbon from the atmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sink would slow the rate of [CO2] increase and thus climate change. However, ecosystem CO2 responses are complex or confounded … Show more
“…For example, in several forests, elevated CO 2 concentrations, temperature, and drought increase seem to promote an increase in plant water use efficiency with an enhancement of photosynthesis and a generally modest reduction in stomatal conductance [66]. The influence of each factor is still under debate and difficult to disentangle [67]. Tree-ring δ 13 C samples from around the globe show an increase in i WUE since pre-industrial times [68,69].…”
Section: Using Carbon and Oxygen Stable Isotopes To Study Air Pollutionmentioning
Purpose of Review
Society is concerned about the long-term condition of the forests. Although a clear definition of forest health is still missing, to evaluate forest health, monitoring efforts in the past 40 years have concentrated on the assessment of tree vitality, trying to estimate tree photosynthesis rates and productivity. Used in monitoring forest decline in Central Europe since the 1980s, crown foliage transparency has been commonly believed to be the best indicator of tree condition in relation to air pollution, although annual variations appear more closely related to water stress. Although crown transparency is not a good indicator of tree photosynthesis rates, defoliation is still one of the most used indicators of tree vitality. Tree rings have been often used as indicators of past productivity. However, long-term tree growth trends are difficult to interpret because of sampling bias, and ring width patterns do not provide any information about tree physiological processes.
Recent Findings
In the past two decades, tree-ring stable isotopes have been used not only to reconstruct the impact of past climatic events, such as drought, but also in the study of forest decline induced by air pollution episodes, and other natural disturbances and environmental stress, such as pest outbreaks and wildfires. They have proven to be useful tools for understanding physiological processes and tree response to such stress factors.
Summary
Tree-ring stable isotopes integrate crown transpiration rates and photosynthesis rates and may enhance our understanding of tree vitality. They are promising indicators of tree vitality. We call for the use of tree-ring stable isotopes in future monitoring programmes.
“…For example, in several forests, elevated CO 2 concentrations, temperature, and drought increase seem to promote an increase in plant water use efficiency with an enhancement of photosynthesis and a generally modest reduction in stomatal conductance [66]. The influence of each factor is still under debate and difficult to disentangle [67]. Tree-ring δ 13 C samples from around the globe show an increase in i WUE since pre-industrial times [68,69].…”
Section: Using Carbon and Oxygen Stable Isotopes To Study Air Pollutionmentioning
Purpose of Review
Society is concerned about the long-term condition of the forests. Although a clear definition of forest health is still missing, to evaluate forest health, monitoring efforts in the past 40 years have concentrated on the assessment of tree vitality, trying to estimate tree photosynthesis rates and productivity. Used in monitoring forest decline in Central Europe since the 1980s, crown foliage transparency has been commonly believed to be the best indicator of tree condition in relation to air pollution, although annual variations appear more closely related to water stress. Although crown transparency is not a good indicator of tree photosynthesis rates, defoliation is still one of the most used indicators of tree vitality. Tree rings have been often used as indicators of past productivity. However, long-term tree growth trends are difficult to interpret because of sampling bias, and ring width patterns do not provide any information about tree physiological processes.
Recent Findings
In the past two decades, tree-ring stable isotopes have been used not only to reconstruct the impact of past climatic events, such as drought, but also in the study of forest decline induced by air pollution episodes, and other natural disturbances and environmental stress, such as pest outbreaks and wildfires. They have proven to be useful tools for understanding physiological processes and tree response to such stress factors.
Summary
Tree-ring stable isotopes integrate crown transpiration rates and photosynthesis rates and may enhance our understanding of tree vitality. They are promising indicators of tree vitality. We call for the use of tree-ring stable isotopes in future monitoring programmes.
“…Future terrestrial carbon sensitivity is mainly driven by two feedback mechanisms: the concentration-carbon and the climate-carbon feedbacks (M. Collins et al, 2013;Friedlingstein et al, 2006;Gregory et al, 2009). The first one is connected to the CO 2 fertilization effect (Walker et al, 2020) where elevated atmospheric CO 2 concentrations increase photosynthesis rates and generally lead to a higher terrestrial carbon uptake, thus constituting a negative feedback. The second one, the climate-carbon feedback, is driven by temperature and precipitation changes leading to a smaller land carbon uptake because of increased temperature and water stress on photosynthesis and higher ecosystem respiration costs, as well as increased fire frequency.…”
The terrestrial biosphere is currently slowing down global warming by absorbing about 30% of human emissions of carbon dioxide (CO 2). The largest flux of the terrestrial carbon uptake is gross primary production (GPP) defined as the production of carbohydrates by photosynthesis. Elevated atmospheric CO 2 concentration is expected to increase GPP ("CO 2 fertilization effect"). However, Earth system models (ESMs) exhibit a large range in simulated GPP projections. In this study, we combine an existing emergent constraint on CO 2 fertilization with a machine learning approach to constrain the spatial variations of multimodel GPP projections. In a first step, we use observed changes in the CO 2 seasonal cycle at Cape Kumukahi to constrain the global mean GPP at the end of the 21st century (2091-2100) in Representative Concentration Pathway 8.5 simulations with ESMs participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) to 171 ± 12 Gt C yr −1 , compared to the unconstrained model range of 156-247 Gt C yr −1. In a second step, we use a machine learning model to constrain gridded future absolute GPP and gridded fractional GPP change in two independent approaches. For this, observational data are fed into the machine learning algorithm that has been trained on CMIP5 data to learn relationships between present-day physically relevant diagnostics and the target variable. In a leave-one-model-out cross-validation approach, the machine learning model shows superior performance to the CMIP5 ensemble mean. Our approach predicts an increased GPP change in northern high latitudes compared to regions closer to the equator. Plain Language Summary About a quarter of human emissions of carbon dioxide (CO 2) is absorbed by vegetation and soil on the Earth's surface and hence does not contribute to global warming caused by CO 2 in the atmosphere. Thus, in order to better define the remaining carbon budgets left to meet particular warming targets like the 1.5°C of the Paris Agreement, it is important to accurately quantify the carbon uptake by plants in the future. Currently, this is modeled by Earth system models yet with great uncertainties. In this work, we present an alternative machine learning approach to reduce spatial uncertainties of vegetation carbon uptake in future climate projections using observations of today's conditions.
“…In this issue of New Phytologist , Walker et al . (2021; pp. 2413–2445) synthesize data from an incredibly broad range of sources, including herbaria, free air CO 2 enrichment (FACE) studies, ice cores, eddy covariance sites, and remote sensing, and examine an enormous diversity of measurements (such as soil respiration rates, glucose isotopomers from leaves, tree ring width data, stream‐gauges for runoff, and direct atmospheric CO 2 measurements) to address the question of how increasing CO 2 concentrations are affecting the carbon uptake capacity of our planet.‘… changes in plant carbon, nitrogen and water dynamics due to rising CO 2 concentrations cascade throughout ecosystems, with global impacts that are much less well understood’…”
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