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• Trees are sources, sinks, and conduits for gas exchange between the atmosphere and soil, and effectively link these terrestrial realms in a soil–plant–atmosphere continuum. • We demonstrated that naturally produced radon-222 (222Rn) gas has the potential to disentangle the biotic and physical processes that regulate gas transfer between soils or plants and the atmosphere in field settings where exogenous tracer applications are challenging. • Patterns in stem radon emissions across tree species, seasons, and diurnal periods suggest that plant transport of soil gases is controlled by plant hydraulics, whether by diffusion or mass flow via transpiration. • We establish for the first time that trees emit soil gases during the night when transpiration rates are negligible, suggesting that axial diffusion is an important and understudied mechanism of plant and soil gas transmission.

China’s large-scale tree planting programs are critical for achieving its carbon neutrality by 2060, but determining where and how to plant trees for maximum carbon sequestration has not been rigorously assessed. Here, we developed a comprehensive machine learning framework that integrates diverse environmental variables to quantify tree growth suitability and its relationship with tree numbers. Then, their correlations with biomass carbon stocks were robustly established. Carbon sink potentials were mapped in distinct tree-planting scenarios. Under one of them aligned with China’s ecosystem management policy, 44.7 billion trees could be planted, increasing forest stock by 9.6 ± 0.8 billion m³ and sequestering 5.9 ± 0.5 PgC equivalent to double China’s 2020 industrial CO 2 emissions. We found that tree densification within existing forests is an economically viable and effective strategy and so it should be a priority in future large-scale planting programs. China’s large-scale tree planting could sequester 5.9 ± 0.5 PgC by planting 44.7 billion trees. Tree densification in existing forests may be a more cost-effective strategy than afforestation.

Structurally intact tropical forests sequestered about half of the global terrestrial carbon uptake over the 1990s and early 2000s, removing about 15 per cent of anthropogenic carbon dioxide emissions 1 – 3 . Climate-driven vegetation models typically predict that this tropical forest ‘carbon sink’ will continue for decades 4 , 5 . Here we assess trends in the carbon sink using 244 structurally intact African tropical forests spanning 11 countries, compare them with 321 published plots from Amazonia and investigate the underlying drivers of the trends. The carbon sink in live aboveground biomass in intact African tropical forests has been stable for the three decades to 2015, at 0.66 tonnes of carbon per hectare per year (95 per cent confidence interval 0.53–0.79), in contrast to the long-term decline in Amazonian forests 6 . Therefore the carbon sink responses of Earth’s two largest expanses of tropical forest have diverged. The difference is largely driven by carbon losses from tree mortality, with no detectable multi-decadal trend in Africa and a long-term increase in Amazonia. Both continents show increasing tree growth, consistent with the expected net effect of rising atmospheric carbon dioxide and air temperature 7 – 9 . Despite the past stability of the African carbon sink, our most intensively monitored plots suggest a post-2010 increase in carbon losses, delayed compared to Amazonia, indicating asynchronous carbon sink saturation on the two continents. A statistical model including carbon dioxide, temperature, drought and forest dynamics accounts for the observed trends and indicates a long-term future decline in the African sink, whereas the Amazonian sink continues to weaken rapidly. Overall, the uptake of carbon into Earth’s intact tropical forests peaked in the 1990s. Given that the global terrestrial carbon sink is increasing in size, independent observations indicating greater recent carbon uptake into the Northern Hemisphere landmass 10 reinforce our conclusion that the intact tropical forest carbon sink has already peaked. This saturation and ongoing decline of the tropical forest carbon sink has consequences for policies intended to stabilize Earth’s climate. Unlike Amazonian forests, African forests have maintained their carbon sink until recently but by 2030 the African carbon sink will have shrunk by 14 per cent and the Amazonian sink will reach almost zero.

The uptake of carbon dioxide (CO 2 ) by terrestrial ecosystems is critical for moderating climate change 1 . To provide a ground-based long-term assessment of the contribution of forests to terrestrial CO 2 uptake, we synthesized in situ forest data from boreal, temperate and tropical biomes spanning three decades. We found that the carbon sink in global forests was steady, at 3.6 ± 0.4 Pg C yr −1 in the 1990s and 2000s, and 3.5 ± 0.4 Pg C yr −1 in the 2010s. Despite this global stability, our analysis revealed some major biome-level changes. Carbon sinks have increased in temperate (+30 ± 5%) and tropical regrowth (+29 ± 8%) forests owing to increases in forest area, but they decreased in boreal (−36 ± 6%) and tropical intact (−31 ± 7%) forests, as a result of intensified disturbances and losses in intact forest area, respectively. Mass-balance studies indicate that the global land carbon sink has increased 2 , implying an increase in the non-forest-land carbon sink. The global forest sink is equivalent to almost half of fossil-fuel emissions (7.8 ± 0.4 Pg C yr −1 in 1990–2019). However, two-thirds of the benefit from the sink has been negated by tropical deforestation (2.2 ± 0.5 Pg C yr −1 in 1990–2019). Although the global forest sink has endured undiminished for three decades, despite regional variations, it could be weakened by ageing forests, continuing deforestation and further intensification of disturbance regimes 1 . To protect the carbon sink, land management policies are needed to limit deforestation, promote forest restoration and improve timber-harvesting practices 1 , 3 . Data from boreal, temperate and tropical forests over the past three decades reveal that the global forest carbon sink has remained steady during that time, despite considerable regional variation.

A revised estimate of Chinese carbon emissions from fossil fuel burning and cement production is presented, based on updated energy consumption and clinker production data and two new sets of measured emission factors for Chinese coal. Downward revision of China's carbon emissions China emits large amounts of anthropogenic carbon, but its carbon emission estimates are highly uncertain. This paper presents a revised estimate of Chinese carbon emissions from fossil fuel burning and cement production based on updated energy consumption and clinker production data, and two new sets of measured emission factors for Chinese coal. The authors estimate of China's cumulative carbon emissions for the period 2000 to 2013 is 13% lower than previous estimates. Nearly three-quarters of the growth in global carbon emissions from the burning of fossil fuels and cement production between 2010 and 2012 occurred in China 1 , 2 . Yet estimates of Chinese emissions remain subject to large uncertainty; inventories of China’s total fossil fuel carbon emissions in 2008 differ by 0.3 gigatonnes of carbon, or 15 per cent 1 , 3 , 4 , 5 . The primary sources of this uncertainty are conflicting estimates of energy consumption and emission factors, the latter being uncertain because of very few actual measurements representative of the mix of Chinese fuels. Here we re-evaluate China’s carbon emissions using updated and harmonized energy consumption and clinker production data and two new and comprehensive sets of measured emission factors for Chinese coal. We find that total energy consumption in China was 10 per cent higher in 2000–2012 than the value reported by China’s national statistics 6 , that emission factors for Chinese coal are on average 40 per cent lower than the default values recommended by the Intergovernmental Panel on Climate Change 7 , and that emissions from China’s cement production are 45 per cent less than recent estimates 1 , 4 . Altogether, our revised estimate of China’s CO 2 emissions from fossil fuel combustion and cement production is 2.49 gigatonnes of carbon (2 standard deviations = ±7.3 per cent) in 2013, which is 14 per cent lower than the emissions reported by other prominent inventories 1 , 4 , 8 . Over the full period 2000 to 2013, our revised estimates are 2.9 gigatonnes of carbon less than previous estimates of China’s cumulative carbon emissions 1 , 4 . Our findings suggest that overestimation of China’s emissions in 2000–2013 may be larger than China’s estimated total forest sink in 1990–2007 (2.66 gigatonnes of carbon) 9 or China’s land carbon sink in 2000–2009 (2.6 gigatonnes of carbon) 10 .

The Chinese government has made a strategic decision to reach ‘carbon neutrality’ before 2060. China’s terrestrial ecosystem carbon sink is currently offsetting 7–15% of national anthropogenic emissions and has received widespread attention regarding its role in the ‘carbon neutrality’ strategy. We provide perspectives on this question by inferring from the fundamental principles of terrestrial ecosystem carbon cycles. We first elucidate the basic ecological theory that, over the long-term succession of ecosystem without regenerative disturbances, the carbon sink of a given ecosystem will inevitably approach zero as the ecosystem reaches its equilibrium state or climax. In this sense, we argue that the currently observed global terrestrial carbon sink largely emerges from the processes of carbon uptake and release of ecosystem responding to environmental changes and, as such, the carbon sink is never an intrinsic ecosystem function. We further elaborate on the long-term effects of atmospheric CO 2 changes and afforestation on China’s terrestrial carbon sink: the enhancement of the terrestrial carbon sink by the CO 2 fertilization effect will diminish as the growth of the atmospheric CO 2 slows down, or completely stops, depending on international efforts to combat climate change, and carbon sinks induced by ecological engineering, such as afforestation, will also decline as forest ecosystems become mature and reach their late-successional stage. We conclude that terrestrial ecosystems have nonetheless an important role to play to gain time for industrial emission reduction during the implementation of the ‘carbon neutrality’ strategy. In addition, science-based ecological engineering measures including afforestation and forest management could be used to elongate the time of ecosystem carbon sink service. We propose that the terrestrial carbon sink pathway should be optimized, by addressing the questions of ‘when’ and ‘where’ to plan afforestation projects, in order to effectively strengthen the terrestrial ecosystem carbon sink and maximize its contribution to the realization of the ‘carbon neutrality’ strategy.

Several initiatives have been proposed to mitigate forest loss and climate change through tree planting as well as maintaining and restoring forest ecosystems. These initiatives have both inspired and been inspired by global assessments of tree and forest attributes and their contributions to offset carbon dioxide (CO₂) emissions. Here we use data from more than 130,000 national forest inventory plots to describe the contribution of nearly 1.4 trillion trees on forestland in the conterminous United States to mitigate CO₂ emissions and the potential to enhance carbon sequestration capacity on productive forestland. Forests and harvested wood products uptake the equivalent of more than 14% of economy-wide CO₂ emissions in the United States annually, and there is potential to increase carbon sequestration capacity by ∼20% (−187.7 million metric tons [MMT] CO₂ ±9.1 MMT CO₂) per year by fully stocking all understocked productive forestland. However, there are challenges and opportunities to be considered with tree planting. We provide context and estimates from the United States to inform assessments of the potential contributions of forests in climate change mitigation associated with tree planting.

China's carbon balance The publication of a comprehensive assessment of China's terrestrial carbon budget fills a major gap in the geographical spread of carbon balance data, and helps to further reduce uncertainties in the Northern Hemisphere carbon balance. Three different indicators were used to monitor China's carbon balance and its driving mechanisms during the 1980s and 1990s: biomass and soil carbon inventories extrapolated from satellite greenness measurements, ecosystem models and atmospheric inversions. The three methods produce similar estimates for the net carbon sink at 0.19 to 0.26 petagrams per year. Global terrestrial ecosystems, in comparison, have absorbed carbon at a rate of 1 to 4 Pg carbon per year during the 1980s and 1990s, which offsets 10–60% of fossil fuel emissions. Northeast China is a net source of CO 2 to the atmosphere as a result over-harvesting and degradation of forests. In contrast, southern China accounts for over 65% of the carbon sink, attributable to regional climate change, tree planting and shrub recovery. This paper analyses the terrestrial carbon balance of China during the 1980s and 1990s using biomass and soil carbon inventories extrapolated by satellite greenness measurements, ecosystem models and atmospheric inversions. These three methods produce similar estimates of a net sink of 0.19–0.26 billion tonnes of carbon per year, indicating that China absorbed 28–37 per cent of its fossil carbon emissions over these two decades, mainly attributable to regional climate change, large-scale plantation programmes and shrub recovery. Global terrestrial ecosystems absorbed carbon at a rate of 1–4 Pg yr -1 during the 1980s and 1990s, offsetting 10–60 per cent of the fossil-fuel emissions 1 , 2 . The regional patterns and causes of terrestrial carbon sources and sinks, however, remain uncertain 1 , 2 , 3 . With increasing scientific and political interest in regional aspects of the global carbon cycle, there is a strong impetus to better understand the carbon balance of China 1 , 2 , 3 . This is not only because China is the world’s most populous country and the largest emitter of fossil-fuel CO 2 into the atmosphere 4 , but also because it has experienced regionally distinct land-use histories and climate trends 1 , which together control the carbon budget of its ecosystems. Here we analyse the current terrestrial carbon balance of China and its driving mechanisms during the 1980s and 1990s using three different methods: biomass and soil carbon inventories extrapolated by satellite greenness measurements, ecosystem models and atmospheric inversions. The three methods produce similar estimates of a net carbon sink in the range of 0.19–0.26 Pg carbon (PgC) per year, which is smaller than that in the conterminous United States 5 but comparable to that in geographic Europe 6 . We find that northeast China is a net source of CO 2 to the atmosphere owing to overharvesting and degradation of forests. By contrast, southern China accounts for more than 65 per cent of the carbon sink, which can be attributed to regional climate change, large-scale plantation programmes active since the 1980s and shrub recovery. Shrub recovery is identified as the most uncertain factor contributing to the carbon sink. Our data and model results together indicate that China’s terrestrial ecosystems absorbed 28–37 per cent of its cumulated fossil carbon emissions during the 1980s and 1990s.

The ever-increasing vehicle counts have resulted in a significant increase in air pollution impacting human and natural ecosystems including trees, and physical properties. Roadside plantations often act as a first defense line against the vehicular emissions to mitigate the impacts of pollutants. However, they are themselves vulnerable to these pollutants with varying levels of tolerance capacity. This demands a scientific investigation to assess the role of roadside plantation for better management and planning for urban sprawl where selected trees could be grown to mitigate the impacts of harmful pollutants. The present study assesses the impacts of vehicular emissions on the adaptation and mitigation potential of two important roadside tree species i.e. Grevillea robusta and Mangifera indica planted along roadsides in the capital city of Uttarakhand. Uttarakhand is one of the Indian Western Himalayan State and its capital city is situated on the foothills of Himalaya. The adaptation and mitigation potential were evaluated by studying the response of pollutants on the functional traits which drive the physiology of the trees. The CO2 assimilation rate, transpiration rate, stomatal conductance, water use efficiency (WUE), air pollution tolerance index (APTI), copper and proline accumulation, dust removal efficiency (DRE), leaf thickness and cooling created by plantation were studied to evaluate the response of trees exposed to roadside traffics. To compare the influence of pollutants, traits of trees grown in a control site with few or absence of vehicular movement were compared with the roadside trees. The control site represented part of a reserve forest where human interference is controlled and human-induced activities are prohibited. The vehicular frequency was found to modulate tree characteristics. The tree characteristics representing WUE, APTI, proline and copper accumulation, leaf thickness, cooling impact, and DRE were enhanced significantly, while the decreased CO2 assimilation rate was observed near roadside trees compared to the control site. We found both of the species to perform well to be used as one of the potential species for roadside and urban greening. However, there is a need to assess the potential of other species in reference to the present study.

Conversion of tropical peat swamp forest to drainage-based agriculture alters greenhouse gas (GHG) production, but the magnitude of these changes remains highly uncertain. Current emissions factors for oil palm grown on drained peat do not account for temporal variation over the plantation cycle and only consider CO 2 emissions. Here, we present direct measurements of GHGs emitted during the conversion from peat swamp forest to oil palm plantation, accounting for CH 4 and N 2 O as well as CO 2 . Our results demonstrate that emissions factors for converted peat swamp forest is in the range 70–117 t CO 2 eq ha −1 yr −1 (95% confidence interval, CI), with CO 2 and N 2 O responsible for ca. 60 and ca. 40% of this value, respectively. These GHG emissions suggest that conversion of Southeast Asian peat swamp forest is contributing between 16.6 and 27.9% (95% CI) of combined total national GHG emissions from Malaysia and Indonesia or 0.44 and 0.74% (95% CI) of annual global emissions. The magnitude of greenhouse gas emissions from land use change on tropical peatlands is unclear. Here, the authors measure greenhouse gas fluxes throughout the conversion from peat swamp forest to oil palm plantation, and estimate the contribution to regional and global emissions.