Search

The successful utilization of stable carbon isotope approaches in investigating forest carbon dynamics has relied on the assumption that the carbon isotope compositions (δ13C) therein have detectable temporal variations. However, interpreting the δ13C signal transfer can be challenging, given the complexities involved in disentangling the effect of a single environmental factor, the isotopic dilution effect from background CO2 and the lack of high-resolution δ13C measurements. In this study, we conducted continuous in situ monitoring of atmospheric CO2 (δ13Ca) across a canopy profile in an old-growth temperate forest in northeast China during the normal year 2020 and the wet year 2021. Both years exhibited similar temperature conditions in terms of both seasonal variations and annual averages. We tracked the natural carbon isotope composition from δ13Ca to photosynthate (δ13Cp) and to ecosystem respiration (δ13CReco). We observed significant differences in δ13Ca between the two years. Contrary to in 2020, in 2021 there was a δ13Ca valley in the middle of the growing season, attributed to surges in soil CO2 efflux induced by precipitation, while in 2020 values peaked during that period. Despite substantial and similar seasonal variations in canopy photosynthetic discrimination (Δ13Ccanopy) in the two years, the variability of δ13Cp in 2021 was significantly lower than in 2020, due to corresponding differences in δ13Ca. Furthermore, unlike in 2020, we found almost no changes in δ13CReco in 2021, which we ascribed to the imprint of the δ13Cp signal on above-ground respiration and, more importantly, to the contribution of stable δ13C signals from soil heterotrophic respired CO2. Our findings suggest that extreme precipitation can impede the detectability of recent photosynthetic δ13C signals in ecosystem respiration in forests, thus complicating the interpretation of above- and below-ground carbon linkage using δ13CReco. This study provides new insights for unravelling precipitation-related variations in forest carbon dynamics using stable isotope techniques., (© The Author(s) 2024. Published by Oxford University Press.)

Atmospheric particulate matter (PM) pollution has become a major environmental risk, and green plants can mitigate air pollution by regulating their enzymatic activity, osmoregulatory substances, photosynthetic pigments, and other biochemical characteristics. The present investigation aims to evaluate the mitigation potential of five common evergreen tree species ( Photinia serrulata , Ligustrum lucidum , Eriobotrya japonica , Euonymus japonicus , Pittosporum tobira ) against air pollution and to assess the effect of dust retention on plant physiological functions exposed to three different pollution levels (road, campus, and park). The results found that the amount of dust retained per unit leaf area of the plants was proportional to the mass concentration of atmospheric particulate matter in the environment, and that dust accumulation was higher on the road and campus than in the park. There were significant differences in dust retention among the five tree species, with the highest leaf dust accumulation observed for E. japonica (5.45 g·m -2 ), and the lowest for P. tobira (1.53 g·m -2 ). In addition, the increase in PM adsorption by different plants was uneven with increasing pollution levels, with significant decreases in chlorophyll content, photosynthetic and transpiration rate. From a physiological perspective, P. tobira exhibited greater potential to respond to PM pollution. Biochemical indicators suggested that PM pollution caused changes in plant protective enzyme activities, with a decrease in superoxide dismutase (SOD) and peroxidase (POD) activities, as well as promoting membrane lipid peroxidation, and appropriate stress also enables plants to counteract oxidative damage. In particular, PM exposure also induced stomatal constriction. Overall, PM retention was significantly associated with physiological and photosynthetic traits. In conclusion, our study contributes to the understanding of the effects of PM on plant physiology. Furthermore, it also provides insights into the selection of plants that are tolerant to PM pollution., Competing Interests: The authors declare that they have no competing interests., (© 2024 He et al.)

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., (© 2024. The Author(s).)

Increasing the carbon sink capacity of terrestrial ecosystems is a primary strategy to mitigate climate change and achieve the "carbon neutrality" goal. Clarifying the status and future dynamics of carbon sink of terrestrial ecosystems in Northeast China is crucial for achieving "carbon neutrality" as this region is a core contributor to carbon sink in China's terrestrial ecosystems. Here, we systematically summarized current research on carbon sink of terrestrial ecosystems across Northeast China, including the measurements and spatial-temporal patterns of carbon sinks, driving mechanisms of carbon sinks, the assessments of carbon sink potential, and technologies for increasing carbon sequestration. There are substantial uncertainties in quantifying terrestrial ecosystem carbon sink in Northeast China due to differences in data sources and methods, especially for forest carbon sink measurements, ranging from 0.020 to 0.157 Pg C·a -1 . Carbon sink function depends on carbon exchange processes across plant-soil-atmosphere interfaces. The key pathways to enhance carbon sequestration in Northeast China under different temporal and spatial scales remains unclear. Improving terrestrial ecosystem quality is the key and core of carbon sequestration and sink enhancement. However, there is an urgent need to develop a multi-ecosystem collaborative carbon sequestration and sink enhancement technology system for the "dual carbon" goal. Future research needs to develop an accurate carbon sink measurement system that integrates multi-source data and multi-scale technologies to accurately assess the function and potential of carbon sink in Northeast China, focus on the multi-scale driving mechanism of carbon sink functions, develop new technical systems for coordinated enhancement of carbon sink for the Northeast terrestrial ecosystems, and carry out demonstrations of carbon sink enhancement technologies. These efforts will provide the scientific and technological supports for achieving the "carbon neutrality" goal.

The widespread utilization of fossil fuels has emitted large amounts of CO 2 into the atmosphere since the Industrial Revolution, leading to climate warming and frequent occurrence of extreme climate events. To effectively alleviate climate change, the international community has made various efforts to reduce carbon emissions and eliminate CO 2 from the atmosphere. In 2020, the Chinese government announced that carbon emission peaking and carbon neutrality will be achieved by 2030 and 2060, respectively. According to the current forecast, by the time carbon neutrality is achieved in 2060, even under the minimum conditions of fossil energy use, production, and living emissions, China will still have to emit about 1/4 of the current total emissions. These carbon must primarily be absorbed by ecosystems. Furthermore, approximately 140 ppm increase in CO 2 in the atmosphere since the Industrial Revolution still needs to be removed by ecosystems. Forests are the main component of terrestrial ecosystems, contributing more than 80% of the carbon sequestration capacity of all terrestrial ecosystems. However, due to the long periodicity, complexity and dynamic variability of forests, the basic concepts of ecosystem carbon sink and its time effect are still unclear, leading to problems, such as lacking technologies for improving carbon sink capacity and disorganized rules in the carbon sink trading market. In this review, we introduced carbon sink concept according to the processes of absorbing and fixing CO 2 by plant photosynthesis in forest ecosystems. Then, we analyzed the processes of time-scale-dependent carbon sinks of forest ecosystems, discussed the time effects of forest carbon sinks, and suggested using "t-year" as the unit of carbon sink (taking 3-6 months as the minimum measurement time, i.e ., the beginning of carbon sequestration). Third, we proposed the approaches to improve the carbon sink capacity of forest ecosystems. One way is to improve the carbon sink capacity (expanding forest area, improving forest quality, and increasing forest soil carbon storage) of forest ecosystems. Another approach is to maintain the carbon sink of forest ecosystems as long as possible, i.e ., to reduce temporary carbon sink (definition: carbon in the forest ecosystems emit into the atmosphere for a certain period) and to increase persistent carbon sink (definition: carbon in the forest ecosystems no longer emit into the atmosphere for a certain period; according to the relevant provisions of the Paris Agreement, the upper time limit for carbon sink measurement can be considered to be the year 2100. In order to maintain the persistent carbon sink, strateges such as efficient use of wood products (replace steel, cement, plastic with wood), control of forest fires or other disturbances-induced emissions, and turning forest biomass into biochar should be taken. Finally, we proposed to develop climate-smart forestry driven by artificial intelligence (AI), which would provide new theoretical and technical support for improving the carbon sink of forest ecosystems and facilitating sustainable forest management.

Background: Leaf nitrogen (N) and phosphorus (P) resorption is a fundamental adaptation strategy for plant nutrient conservation. However, the relative roles that environmental factors and plant functional traits play in regulating N and P resorption remain largely unclear, and little is known about the underlying mechanism of plant functional traits affecting nutrient resorption. Here, we measured leaf N and P resorption and 13 plant functional traits of leaf, petiole, and twig for 101 representative broad-leaved tree species in our target subtropical transitional forests. We integrated these multiple functional traits into the plant economics spectrum (PES). We further explored whether and how elevation-related environmental factors and these functional traits collectively control leaf N and P resorption., Results: We found that deciduous and evergreen trees exhibited highly diversified PES strategies, tending to be acquisitive and conservative, respectively. The effects of PES, rather than of environmental factors, dominated leaf N and P resorption patterns along the elevational gradient. Specifically, the photosynthesis and nutrient recourse utilization axis positively affected N and P resorption for both deciduous and evergreen trees, whereas the structural and functional investment axis positively affected leaf N and P resorption for evergreen species only. Specific leaf area and green leaf nutrient concentrations were the most influential traits driving leaf N and P resorption., Conclusions: Our study simultaneously elucidated the relative contributions of environmental factors and plant functional traits to leaf N and P resorption by including more representative tree species than previous studies, expanding our understanding beyond the relatively well-studied tropical and temperate forests. We highlight that prioritizing the fundamental role of traits related to leaf resource capture and defense contributes to the monitoring and modeling of leaf nutrient resorption. Therefore, we need to integrate PES effects on leaf nutrient resorption into the current nutrient cycling model framework to better advance our general understanding of the consequences of shifting tree species composition for nutrient cycles across diverse forests., (© 2024. The Author(s).)

We examined the metabolic response of microbial respiration to glucose addition with the topsoil (0-10 cm) from five plantation types, including Quercus glauca , Castanopsis kawakamii , Pinus massoniana , Phoebe bournei , and Cinnamomum camphora plantations, in the Sanming Forest Ecosystem National Field Observation and Research Station in Fujian Province. The results showed that glucose addition significantly increased microbial respiration by 82.4%-349.5%, with significant difference among tree species. In the control, microbial respiration significantly correlated with microbial biomass carbon, soil organic carbon, and the fungi/bacteria ratio, indicating that microbial metabolism was regulated by soil organic carbon content and was associated with microbial biomass and community structure in the absence of labile carbon supply. In the glucose addition treatment, microbial respiration positively correlated with soil total nitrogen, dissolved organic nitrogen, and mineral nitrogen, indicating that microbial metabolism was mainly constrained by soil nitrogen content and its availability in the presence of adequate labile carbon supply. The metabolic response of microbial respiration, as indicated by the ratio of microbial respiration in the glucose addition treatment to that in the control, was primarily affected by soil carbon/nitrogen ratio, with a decrease in the ratio leading to an increase in the microbial metabolic response. Additionally, soil pH played an important role in mediating microbial metabolic response. The effect of the content and availability of soil carbon and nitrogen on microbial respiration depended on whether microbes were carbon-limited. Soil carbon content media-ted microbial respiration when microbes were carbon-limited, whereas soil nitrogen content and availability mediated microbial respiration after the alleviation of microbial carbon limitation.

Plantations actively participate in the global carbon cycle and play a significant role in mitigating global climate change. However, the influence of forest management strategies, especially planting density management, on the biomass carbon storage and production value of plantations for ensuring carbon sink benefits is still unclear. In this study, we estimated the carbon sequestration and economic value of Pinus massoniana plantations with various stand densities and rotation ages using a growth model method. The results revealed that with increasing stand age, low-density plantations at 2000 trees·ha -1 (358.80 m 3 ·ha -1 ), as well as high-density plantations at 4500 trees·ha -1 (359.10 m 3 ·ha -1 ), exhibited nearly identical standing volumes, which indicated that reduced inter-tree competition intensity favors the growth of larger trees during later stages of development. Furthermore, an increase in planting density led to a decrease in the average carbon sequestration rate, carbon sink, and number of trees during the rapid growth period, indicating that broader spacing between trees is favorable for biomass carbon accumulation. Further, extending the rotation period from 15 to 20 years or 25 years and reducing the optimal planting density from 3000 to 2000 trees·ha -1 increased the overall benefits of combined timber and carbon sink income by 2.14 and 3.13 times, respectively. The results highlighted that optimizing the planting density positively impacts the timber productivity and carbon sink storage of Pinus massoniana plantations and boosts the expected profits of forest managers. Thus, future afforestation initiatives must consider stand age and planting density management to shift from a scale-speed pattern to a quality-benefit design., (© 2024. The Author(s).)

Understanding the mechanisms controlling forest carbon accumulation is crucial for predicting and mitigating future climate change. Yet, it remains unclear whether the dominance of ectomycorrhizal (EcM) trees influences the carbon accumulation of entire forests. In this study, we analyzed forest inventory data from over 4000 forest plots across Northeast China. We find that EcM tree dominance consistently exerts a positive effect on tree, soil, and forest carbon stocks. Moreover, we observe that these positive effects are more pronounced during unfavorable climate conditions, at lower tree species richness, and during early successional stages. This underscores the potential of increasing the dominance of native EcM tree species not only to enhance carbon stocks but also to bolster resilience against climate change in high-latitude forests. Here we show that forest managers can make informed decisions to optimize carbon accumulation by considering various factors such as mycorrhizal types, climate, successional stages, and species richness., (© 2024. The Author(s).)

Improving the availability of soil phosphorus (P) and promoting tree growth through tree species selection and assembly are the critical issue. We conducted an afforestation experiment following randomized block experimental design with 1, 2, 4, and 6 tree species richness in south subtropics, including Pinus massoniana , Mytilaria laosensis , Erythrophleum fordii , Castanopsis hystrix , Michelia macclurei , Manglietia glauca , Aquilaria sinensis , and Dalbergia odorifera . We measured the bioavailable P components (CaCl 2 -P, citrate-P, enzyme-P and HCl-P) and examined the effects of different tree species assembly on bioavailable P components and tree growth. The results showed that, compared with non-nitrogen-fixing tree species, the mixing of nitrogen-fixing tree species ( E. fordii and D. odorifera ) effectively increased the contents of soil water, total nitrogen, total phosphorus, and microbial biomass P (MBP). The assembly of specific tree species improved the accumulation of bioavailable P. Mixing of nitrogen-fixing tree species significantly increased CaCl 2 -P content by 46.2% to 160.3%, the enzyme-P content produced by microbial mineralization by 69.3% to 688.2%, and HCl-P by 31.5% to 81.3%, increased MBP by 81.8% to 149.4%, and microbial biomass N (MBN) by 88.1% to 160.6%, respectively. Redundancy and correlation analysis results showed that MBP, available P, total phosphorus, L-leucine aminopeptidase, cellobiose, acid phosphatase, MBN and soil organic carbon were key factors driving the variation of rhizosphere soil bioavailable P. Mixing of nitrogen-fixing tree species increased enzyme-P and citrate-P, and the availability of which were positively correlated to tree basal area. In this study, mixing of nitrogen-fixing tree species increased the rhizosphere soil bioavailable P content, which facilitates tree growth.

The contribution of litterfall nutrient return to the maintenance of soil carbon pool and nutrient cycling is a crucial aspect of forest ecosystem functioning. Taking 21 tree species in subtropical young plantations as subjects, we investigated the correlation between litterfall nutrient return characteristics and functional traits of leaf and root and. The results showed notable variations in litterfall production, standing crop, and nutrient return across all the examined tree species. Mytilaria laosensis exhibited the highest litterfall production (689.2 g·m -2 ·a -1 ) and standing crop (605.1 g·m -2 ), while Cryptomeria fortunei demonstrated the lowest litterfall production (36.0 g·m -2 ·a -1 ) and standing crop (10.0 g·m -2 ). The nitrogen and phosphorus return amounts of 21 species ranged from 3.0 to 48.3 kg·hm -2 and from 0.1 to 2.0 kg·hm -2 , respectively. Castanopsis fissa demonstrated the highest nitrogen return, while Liquidambar formosana exhibited the highest phosphorus return. C. fortunei had the lowest nitrogen and phosphorus return. Results of the stepwise regression analysis indicated that litterfall production exhibited a significant negative correlation with leaf nitrogen content and leaf dry matter content, and a significant positive correlation with fine root tissue density. Additionally, leaf nitrogen content, leaf dry matter content, and specific root length had a significant negative impact on standing crop. The structural equation modelling results indicated that leaf dry matter content had a direct or indirect negative effect on nitrogen return amount through the reduction of litterfall production. Conversely, fine root tissue density had a significant positive impact on nitrogen return amount by increasing litter leaf nitrogen content. Both leaf nitrogen content and leaf dry matter content had direct or indirect negative effects on phosphorus return amount through the reduction of litterfall production. In conclusion, the tree species with low leaf nitrogen content and dry matter content, as well as high fine root tissue density, was recommended for the establishment of plantations in the subtropical zone in order to enhance nutrient cycling through litter decomposition and improve soil fertility and forest productivity.

Transpiration is a significant part of water cycle in forest ecosystems, influenced by meteorological factors and potentially constrained by soil moisture. We used Granier-type thermal dissipation probes to monitor xylem sap flow dynamics of three tree species ( Quercus liaotungensis , Platycladus orientalis , and Robinia pseudoacacia ) in a semi-arid loess hilly region, and to continuously monitor the key meteorological factors and soil water content (SWC). We established the SWC thresholds delineating soil moisture-limited and -unlimited sap flow responses to transpiration drivers. The results showed that mean sap flux density ( J s ) of Q. liaotungensis and R. pseudoacacia was significantly higher during period with higher soil moisture compared to lower soil moisture, while the difference in J s for P. orientalis between the two periods was not significant. We used an exponential saturation function to fit the relationship between the J s of each tree species and the integrated transpiration variable (VT) which reflected solar radiation and vapor pressure deficit. The difference in the fitting curve parameters indicated that there were distinct response patterns between J s and VT under different soil moisture conditions. There was a threshold in soil moisture limitation on sap flow for each species, which was identified as 0.129 m 3 ·m -3 for Q. liaotungensis , 0.116 m 3 ·m -3 for P. orientalis , and 0.108 m 3 ·m -3 for R. pseudoacacia . Below the thresholds, J s was limited by soil moisture. Above these points, the normalized sensitivity index (NSI) for Q. liaotungensis and P. orientalis reached saturation, while that of R. pseudoacacia did not reach saturation but exhibited a significant reduction in moisture limitation. Among the three species, P. orientalis was the most capable of overcoming soil moisture constraints.

The natural abundance of stable carbon and nitrogen isotopes (δ 13 C and δ 15 N) in leaves can provide comprehensive information on the physiological and ecological processes of plants and has been widely used in ecological research. However, recent studies on leaf δ 13 C and δ 15 N have focused mainly on woody species, few studies have been conducted on herbs in different vegetation types, and their differences and driving factors are still unclear. In this study, we focused on the herbs in subalpine coniferous forests, alpine shrublands, and alpine mea-dows on the eastern Qinghai-Tibet Plateau, and investigated the differences in leaf δ 13 C and δ 15 N of herbs and the driving factors. The results showed that there were significant differences in leaf δ 13 C and δ 15 N values of herbs among different vegetation types, with the highest δ 13 C and δ 15 N values in alpine meadows, followed by alpine shrublands, and the lowest in subalpine coniferous forests. Using variation partitioning analysis, we revealed that differences in leaf δ 13 C and δ 15 N of herbs among various vegetation types were driven by both leaf functional traits and climate factors, with the contribution of leaf functional traits being relatively higher than that of climate factors. Hierarchical partitioning results indicated that mean annual temperature (MAT), chlorophyll content index, leaf nitrogen content per unit area (N area ), and leaf mass per area were the main drivers of leaf δ 13 C variations of herbs across different vegetation types, while the relative importance of N area and MAT for variation in leaf δ 15 N of herbs was much higher than those other variables. There was a strong coupling relationship between leaf δ 13 C and δ 15 N as indicated by the result of the ordinary least squares regression. Our findings could provide new insights into understanding the key drivers of leaf δ 13 C and δ 15 N variations in herbs across different vegetation types.

Phyllostachys pubescens (moso bamboo) has extensively expanded to subtropical broadleaf forests. However, how moso bamboo expansion influences litter-leached dissolved organic matter (DOM) biodegradation is unclear. In this study, we collected fresh leaf litter of moso bamboo and 10 broadleaf tree species from a subtropical forest in southern China and extracted litter-leached dissolved organic carbon (DOC), dissolved total nitrogen (DTN), and dissolved total phosphorus (DTP). Then, using a 42-day incubation experiment, we measured litter-leached DOM biodegradation of the selected 11 species and assessed the relative mixing effects on biodegradation of bamboo litter- and broadleaf tree litter-leached DOM mixtures with volume mixing ratios of 1:3, 1:1, and 3:1. In the litter leachates, bamboo had lower DOC:DTN ratio, DOC:DTP ratio, and DOM aromaticity (i.e., lower SUVA 254 and SUVA 350 values) than most broadleaf tree species. Litter-leached DOM biodegradation did not differ among bamboo, Liquidambar formosana, Vernicia fordii, and Cyclobalanopsis glauca, but was greater for bamboo than for the other seven broadleaf tree species. Leaf litter-leached DOM biodegradation correlated negatively with DOC:DTN and DOC:DTP ratios, but exhibited no significant relationship with DOM aromaticity. Regardless of volume mixing ratios, antagonistic effects were observed when bamboo litter-leached DOM was mixed with broadleaf tree litter-leached DOM with comparable biodegradation, whereas synergistic effects occurred when bamboo litter-leached DOM was mixed with broadleaf tree litter-leached DOM with lower biodegradation. The relative mixing effects on DOM biodegradation increased linearly with elevated interspecific difference in litter-leached DOM biodegradation between bamboo and broadleaf tree species across the incubation periods. These findings indicate that moso bamboo expansion will substantially alter litter-leached DOM biodegradation by improving substrate quality and changing species interactions, and the magnitudes of such changing trends are dependent on the native tree litter-leached DOM biodegradation in subtropical broadleaf forests., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

In this paper, the effects of species diversity, tree growth, and spatial clustering on mycorrhizal carbon and nitrogen sequestration and the interaction of soil physicochemical properties in Northeast China were investigated. Based on 720 10 m ∗ 10 m plots in Harbin Experimental Forest Farm of Northeast Forestry University, we determined mycorrhizal biomarkers of easily extractable Glomalin-related soil protein (EEG) and total Glomalin-related soil protein (TG). Four plant diversity indices, seven structural metrics, and five soil properties were also measured. We found that: 1) The low tree diversity plots had 1.08-1.23 times higher TG, EEG, TG-N/TN (proportion of N in TG to TN), and TG-C/SOC (proportion of C in TG to SOC) than the high plots. 2) Tree diameter was negatively correlated with EEG and TG, but positively correlated with the EEG and TG contribution to soil TN and SOC. Soil EEG and TG were positively correlated with under-branch height and tree density. W (Uniform Angle Index, higher W indicates more clustering of tree distribution in the plot) was negatively correlated with the above four ratios and positively correlated with EEG/TG. 3) pH was the most powerful explainer for the GRSP variations (6.8 %, strongest negative association with GRSP/TN, R 2  > 0.13), followed by soil electrical conductance (6.5 %, positive relation with TG, p < 0.05), AP (3.2 %). 4) Plant diversity mainly affected GRSP traits through the interaction with soils (0.07), tree growth and density directly increased TG, TG-N/TN, and TG-C/SOC, while tree spatial distribution directly reduced TG-N/TN. Our finding highlighted the important effects of tree diversity and forest structural traits on GRSP amount, carbon sequestration, and nutrient retentions, and could support glomalin-related forest soil management of temperate forests in the high-latitude northern hemisphere., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

The effects of volatile organic compounds on urban air quality and the ozone have been widely acknowledged, and the contributions of relevant biogenic sources are currently receiving rising attentions. However, inventories of biogenic volatile organic compounds (BVOCs) are in fact limited for the environmental management of megacities. In this study, we provided an estimation of BVOC emissions and their spatial characteristics in a typical urbanized area, Shenzhen megacity, China, based on an in-depth vegetation investigation and using remote sensing data. The total BVOC emission in Shenzhen in 2019 was estimated to be 3.84 × 10 9  g C, of which isoprene contributed to about 24.4%, monoterpenes about 44.4%, sesquiterpenes about 1.9%, and other VOCs (OVOCs) about 29.3%. Metropolitan BVOC emissions exhibited a seasonal pattern with a peak in July and a decline in January. They were mainly derived from the less built-up areas (88.9% of BVOC emissions). Estimated BVOCs comprised around 5.2% of the total municipal VOC emissions in 2019. This percentage may increase as more green spaces emerge and anthropogenic emissions decrease in built-up areas. Furthermore, synergistic effects existed between BVOC emissions and relevant vegetation-based ecosystem services (e.g., air purification, carbon fixation). Greening during urban sprawl should be based on a trade-off between BVOC emissions and ecosystem benefits of urban green spaces. The results suggested that urban greening in Shenzhen, and like other cities as well, need to account for BVOC contributions to ozone. Meanwhile, greening cites should adopt proactive environmental management by using plant species with low BVOC emissions to maintain urban ecosystem services while avoid further degradation to ozone pollution., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Afforestation and reforestation are pivotal in mitigating land degradation and bolstering the carbon sink capacity of terrestrial ecosystems. However, the potential economic ramifications of afforestation and reforestation in the context of climate change remain largely unexplored. In this study, we employed an interdisciplinary methodology to establish a framework for assessing future forest potential and carbon sequestration in the Eastern Loess Plateau region of China. Our findings indicate that an estimated 17,392.99 km 2 of land suitable for afforestation still existed within the region, exhibiting a propensity to aggregate around existing forests rather than being dispersed randomly. Notably, 4385.36 km 2 was prioritized for afforestation initiatives. Projections suggest a significant enhancement of the forest carbon sink within the study area by 2050, ranging from 36.93 Mt to 105.38 Mt. The corresponding economic value for this enhancement is estimated to vary between US$3.25 billion and US$17.68 billion. Of significance is the observed polarization of the region's carbon sink capacity over time, with half of the total carbon sinks concentrated within 10% of the districts. Additionally, approximately 26% of the counties are expected to transition from carbon sinks to carbon sources. These findings underscore the substantial impact of climate change on forest distribution and suggest a targeted approach to combat forest degradation by circumventing ineffective afforestation activities., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

We have clarified the study area has a history of 65 years and has been restored for 6 years. This study investigated the carbon storage characteristics of undisturbed natural forests and restored mining vegetation in Yunnan Province, China. The goal was to quantify carbon reserves and increments to inform ecological restoration strategies. Four vegetation components (tree, shrub, herb, litter) and five soil layers (0-10, 10-20, 20-30, 30-40, 40-60 cm) were analyzed. In natural forest, the tree layer stored 60% of carbon (273 Mg ha -1 ), overwhelmingly dominating vegetation carbon stocks. Shrub, herb, and litter layers each comprised < 1%. Surface soil layers (0-30 cm) stored 64% of soil carbon. In the restored mining area, the tree layer contributed 75% of vegetation carbon increment (16 Mg ha -1 ), though stocks were lower than natural forest. Soil layers showed the highest carbon increment (69%) despite lower biomass than natural conditions. Unexploited forests thus exhibit robust carbon storage, while restored mining areas have weaker carbon gains, indicating recovery potential. Strategic interventions targeting soil quality, stimulating vegetation growth, and increasing carbon sequestration could significantly augment reserves and ecological functionality. Prioritizing vegetation succession and soil revitalization are paramount to ensuring ecological integrity and sustainable development. Fostering a positive regional ecological feedback loop will be pivotal. This research quantifies carbon storage differences between undisturbed and restored mining areas, highlighting soil and vegetation as critical targets for optimizing carbon sequestration and ecosystem recovery in degraded environments., (© 2023. The Author(s).)

Forestry has long been considered an effective means of restoring degraded drylands worldwide. Often, afforestation in such lands relies on the establishment of runoff harvesting systems that are formed as contour bench terraces on hillslopes, increasing water availability for the planted trees and shrubs. The construction of terraces requires intensive earthworks by heavy machinery. This study assessed the long-term (>10 yrs) effects of forestry-related land-use change on soil properties and herbaceous vegetation in 16-year-old and 12-year-old afforestation sites (established in 2005 and 2009), and in nearby control ("natural") areas in the semi-arid northern Negev, Israel. Mean herbaceous vegetation height in the 2005 afforestation sites (12.1 cm) was significantly (P = 0.0009) and 23% greater than in the control areas (9.8 cm), whereas in the 2009 afforestation sites (6.2 cm) it was 37% lesser than in the control areas. Mean herbaceous vegetation aboveground biomass was similar in the 2005 afforestation (0.39 Mg ha -1 ) and control areas (0.38 Mg ha -1 ), and almost significantly (P = 0.0510) and twofold greater than in the 2009 afforestation sites (0.19 Mg ha -1 ). The effect of hillslope aspect on these variables was substantial; their mean values were higher in the northern (mesic) hillslopes than in the southern (xeric) hillslopes. Soil samples were obtained from depths of 0-5 and 5-10 cm and physio-chemo-biological properties were assessed in the laboratory. The overall soil quality - as calculated by two soil quality indices (SQIs), including the generalized SQI (SQI gen ) and the minimum dataset SQI (SQI MDS ) - was significantly (P < 0.0001 for both indices) and 13-22% greater in the control areas (0.52 and 0.61, respectively) than that in the afforestation treatments (0.44-0.46 and 0.50-0.51, respectively). These results are generally attributed to the removal of soil's A-horizon during earthworks, and the exposure of the underlying B-horizon. The similar SQI values of both hillslope aspects, as well as of both soil depths, indicate the generally degraded state of the entire region. In conclusion, while contour bench terracing may facilitate the recovery of herbacaeous vegetation to some extent, the effectiveness of this practice for soil restoration is questionable. Overall, insights of this study demonstrate a caveat that converting natural drylands to forestry systems may not yield sufficient ecological benefits, and therefore should be implemented with caution., Competing Interests: Declaration of competing interest The authors declare no competing interests., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The global atmospheric nitrogen (N) deposition has intensified in recent years, resulting in a complex impact on forest ecosystems. This study investigated the effects of canopy (CAN) and understory additions of N (UAN) on leaf carbon (C) and N assimilations, as well as growth parameters of representative woody plant species in an evergreen broad-leaved forest, i.e. Castanea henryi, Schefflera heptaphylla, Blastus cochinchinensis, and Lasianthus chinensis. The results showed that leaf N assimilation key enzyme nitrate reductase (NR) activities of B. cochinchinensis and S. heptaphylla were significantly decreased by UAN, and were significantly decreased by CAN for C. henryi. CAN significantly decreased the nitrite reductase activity of C. henryi, while significantly increased that of L. chinensis. However, the A max values of each woody species were not significantly different among control (CK), CAN, and UAN. Community surveys demonstrated that CAN and UAN inhibited the growth (diameter at breast height, height, or crown width) of the representative large tree, C. henryi, while promoting the growths of understory woody species (B. cochinchinensis and L. chinensis). Overall, N addition was found to change the physiological processes of N and C metabolisms of the dominant woody species in an evergreen broad-leaved forest. The community of subtropical evergreen broad-leaved forests may further decline and its C fixation capacity may be detrimentally changed under N deposition in the future., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

Although effects of atmospheric nitrogen (N) deposition on forest plants have been widely investigated, N interception and absorption effects by forest canopy should not be neglected. Moreover, how N deposition change the molecular biological process of understory dominant plants, which was easily influenced by canopy interception so as to further change physiological performance, remains poorly understood. To assess the effects of N deposition on forest plants, we investigated the effects of understory (UAN) and canopy N addition (CAN) on the transcriptome and physiological properties of Ardisia quinquegona, a dominant subtropical understory plant species in an evergreen broad-leaved forest in China. We identified a total of 7394 differentially expressed genes (DEGs). Three of these genes were found to be co-upregulated in CAN as compared to control (CK) after 3 and 6 h of N addition treatment, while 133 and 3 genes were respectively found to be co-upregulated and co-downregulated in UAN as compared to CK. In addition, highly expressed genes including GP1 (a gene involved in cell wall biosynthesis) and STP9 (sugar transport protein 9) were detected in CAN, which led to elevated photosynthetic capacity and accumulation of protein and amino acid as well as decrease in glucose, sucrose, and starch contents. On the other hand, genes associated with transport, carbon and N metabolism, redox response, protein phosphorylation, cell integrity, and epigenetic regulation mechanism were affected by UAN, resulting in enhanced photosynthetic capacity and carbohydrates and accumulation of protein and amino acid. In conclusion, our results showed that the CAN compared to UAN treatment had less effects on gene regulation and carbon and N metabolism. Canopy interception of N should be considered through CAN treatment to simulate N deposition in nature., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

The high northern latitudes (>50°) experienced a pronounced surface stilling (i.e., decline in winds) with climate change. As a drying factor, the influences of changes in winds on the date of autumn foliar senescence (DFS) remain largely unknown and are potentially important as a mechanism explaining the interannual variability of autumn phenology. Using 183,448 phenological observations at 2,405 sites, long-term site-scale water vapor and carbon dioxide flux measurements, and 34 y of satellite greenness data, here we show that the decline in winds is significantly associated with extended DFS and could have a relative importance comparable with temperature and precipitation effects in contributing to the DFS trends. We further demonstrate that decline in winds reduces evapotranspiration, which results in less soil water losses and consequently more favorable growth conditions in late autumn. In addition, declining winds also lead to less leaf abscission damage which could delay leaf senescence and to a decreased cooling effect and therefore less frost damage. Our results are potentially useful for carbon flux modeling because an improved algorithm based on these findings projected overall widespread earlier DFS than currently expected by the end of this century, contributing potentially to a positive feedback to climate., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

DNA methylation is an important epigenetic regulator of gene expression. Application of 5-azacytidine (a methylation inhibitor) significantly promoted bud sprouting rate and the elongation of branches and leaves in "Luhehong" and "Fengdanbai." In total, 11,166 and 11,443 fragments were obtained by methylation-sensitive amplified polymorphism (MSAP) analysis during chilling-induced dormancy release in the two varieties, respectively. Total methylation levels were high in dormant buds, mainly for hemimethylation, which were slowly increased by short-term chilling (7 days) and decreased by long-term chilling. Compared with 0 day, the ratio of the methylation downregulated group increased during dormancy release, whereas that of the upregulated group declined gradually. These variations were consistent with the dynamic expressions of DNA methyltransferase/demethylase genes and their enzyme activity changes. In total, 13 polymorphic MSAP fragments were similar to known proteins ( E -value <1e-5), and their methylation statuses were consistent with their expression patterns. The expression change of PsCWH , encoding cell wall hydrolase, might be due to DNA methylation ratios of CpG sites identified by bisulfite sequencing. These results indicated that chilling accumulation promoted bud dormancy release and sprouting through DNA methylation modification of specific genes. This study would provide new insights into the molecular mechanism underlying dormancy release in tree peony.

The leaves of riparian plants are the main source of energy and nutrients in riparian ecosystems. In order to evaluate the nutrient release of reforested trees in a riparian zone, a field litterbag experiment involving three foliar types (the leaves of either coniferous and broadleaf trees as single-leaf treatment, or a mixture of coniferous and broadleaf leaves as a heterogenous-leaf treatment) and different submergence depths [no submergence (CK), shallow submergence (SS), and deep submergence (DS)] was conducted in situ in the Three Gorges Reservoir (TGR) for one year. The results showed that, when compared to the single-leaf treatment, the heterogenous-leaf treatment exhibited greater mass loss at both SS and DS, in contrast to a greater nitrogen release rate only at DS and a greater phosphorous release rate only at SS. Overall, submergence facilitated decomposition and nutrient release, although the decomposition rate was higher in SS than in DS. The results suggested that the decomposition and nutrient release of the three foliar types may increase the potential pollution risk to the TGR water environment. Thus, we propose that the leaves of the reforested riparian stands be harvested prior to submergence to preserve the water quality of the TGR.

Drought and nitrogen (N) deposition are important components of global climate and environmental change. In this greenhouse study, we investigated the ecophysiological responses of the seedlings of three subtropical forest plant species (Schima superba, Castanopsis fissa, and Michelia macclurei) to short-term experimental drought stress, N addition, and their interaction. The results showed that drought stress reduced the activities of antioxidant enzymes [superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)] and total antioxidant capacity (T-AOC), but increased the malondialdehyde (MDA), abscisic acid (ABA), and proline (PRO) contents in plants. The PRO content, T-AOC, and antioxidant enzyme activities were increased, and ABA and MDA contents were decreased by N addition alone. Furthermore, N addition under drought stress increased antioxidant enzymes activities, PRO content, and T-AOC. The treatments, however, did not significantly affect the chlorophyll fluorescence parameters of the species. T-AOC was positively correlated with antioxidant enzyme activities in each species, indicating that antioxidant enzymes were important for plant resistance to oxidative stress. MDA content increased with the increase of ABA content, indicating that ABA may help regulate stomatal movement and drought-induced oxidative injury in plants. T-AOC was positively correlated with PRO content, probably because PRO participated in osmotic regulation of cells and increased osmotic stress resistance. These results indicate that N addition can reduce drought stress of subtropical forest plants and will help researchers predict how evergreen broad-leaved forests will respond to global change in the future., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Urbanization, the migration of populations from rural to urban areas, has been causing great stress on natural environments, leading to air pollution and nitrogen (N) deposition, negatively affecting forest health. Although there is evidence that urbanization has changed forest N cycling, little is known about whether urbanization also changes the availability of phosphorus (P), which is important for plant growth and forest productivity. To address this question, we carried out a survey in the Pearl River Delta region, the world's largest urban area in southern China, using two types of representative forests, the evergreen broadleaf forests (BFs) and pine plantations (PPs). The leaf N:P ratios in the two forest types were high (20-50) with a significant increasing pattern along the rural-to-urban gradient. The ratios of leaf P:K and P:Na declined along the rural-to-urban gradient, whereas leaf P content did not change in BF but decreased in PP along the rural-to-urban gradient, suggesting that leaf P became limiting along urbanization. The abundance of actinomycetes and gram-negative bacteria decreased along the rural-to-urban gradient, indicating the negative effects of urbanization on soil microorganisms. Principal component analysis indicated that divergent key factors respond to the urbanization and affect plant P limitation in the two forest types. In BF, broadleaf trees showed a greater response to N deposition from urbanization indicating direct leaf N uptake from N deposition is a key factor for plant P limitation. Alternatively, in PP, our findings suggest soil acidification is an important factor accelerating plant P limitation. Our study revealed that urbanization intensifies plant P limitation in subtropical forests, and the effects vary depending on forest types. Our findings provide empirical information to support the management of forest ecosystems and evaluation of urbanization effects on forest health., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Land use and land cover change induced by large scale ecological restoration programs has a significant impact on the terrestrial ecosystem carbon cycle, especially on the net primary productivity (NPP) in arid and semi-arid regions. This study investigated the change in NPP caused by the large-scale ecological restoration in the Chinese Loess Plateau (LPR) region from 1986 to 2015 based on land cover datasets and NPP calculated using the Carnegie-Ames-Stanford Approach model. The results indicated that the annual total NPP exhibited a significant uptrend (P < 0.01) throughout the whole vegetation restoration region during the last 30 years, with an annual increase of 0.137 Tg C. A significant abrupt change was detected in 2006 for the annual total NPP series. Over half of the restoration region showed an increase in NPP in the past three decades, however, about 30~40% of the vegetation restoration region exhibited NPP loss before 2006, but subsequently NPP loss was found in only approximately 20% of the study region. Overall, the increase in NPP attributed to the vegetation restoration reached 51.14 Tg C in the past three decades, indicating that these large-scale vegetation restoration programs increased the carbon sequestration capacity of terrestrial ecosystems in the Loess Plateau. The findings of this study improve our understanding of the effects of the green campaign on terrestrial ecosystems., Competing Interests: The authors have declared that no competing interests exist.

Soil nitrogen (N) deficiencies can affect the photosynthetic N-use efficiency (PNUE), mesophyll conductance (g m ), and leaf N allocation. However, lack of information about how these physiological characteristics in N-fixing trees could be affected by soil N deficiency and the difference between N-fixing and non-N-fixing trees. In this study, we chose seedlings of two N-fixing (Dalbergia odorifera and Erythrophleum fordii) and two non-N-fixing trees (Castanopsis hystrix and Betula alnoides) as study objects, and we conducted a pot experiment with three levels of soil N treatments (high nitrogen, set as Control; medium nitrogen, MN; and low nitrogen, LN). Our results showed that soil N deficiency significantly decreased the leaf N concentration and photosynthesis ability of the two non-N-fixing trees, but it had less influence on two N-fixing trees. The LN treatment had lower g m in D. odorifera and lower leaf N allocated to Rubisco (P R ), leaf N allocated to bioenergetics (P B ), and g m in B. alnoides, eventually resulting in low PNUE values. Our findings suggested that the D. odorifera and E. fordii seedlings could grow well in N-deficient soil, and adding N may increase the growth rates of B. alnoides and C. hystrix seedlings and promote the growth of artificial forests.

Tropical and subtropical forest ecosystems play an important role in the global carbon regulation. Despite increasing evidence for effects of biodiversity (species diversity, functional diversity and functional dominance), stand structural attributes, stand age and environmental conditions (climate and topography) on tree carbon storage, the relative importance of these drivers at large scale is poorly understood. It is also still unclear whether biodiversity effects on tree carbon storage work through niche complementarity (i.e. increased tree carbon storage due to interspecific resource partitioning) or through the mass-ratio effect (tree carbon storage regulated by dominant traits within communities). Here we analyze tree carbon storage and its drivers using data of 480 plots sampled across subtropical forests in China. We use multiple regression models to test the relative effects of biodiversity, stand structural attributes, stand age and environmental conditions on tree carbon storage, and use a partial least squares path model to test how these variables directly and/or indirectly affect tree carbon storage. Our results show that tree carbon storage is most strongly affected by stand age, followed by climate, biodiversity and stand structural attributes. Stand age and climate had both direct and indirect (through species diversity, functional dominance and stand structural attributes) effects. We find that tree carbon storage correlates with both species diversity and functional dominance after stand age and environmental drivers are accounted for. Our results suggest that niche complementarity and the mass-ratio effect, not necessarily mutually exclusive, both play a role in maintaining ecosystem functioning. Our results further indicate that biodiversity conservation might be an effective way for enhancing tree carbon storage in natural, species-rich forest ecosystems., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Polycyclic aromatic hydrocarbons (PAHs) have toxic, teratogenic, mutagenic and carcinogenic effects on living organisms. Plants can function as pollutant bioindicators and bioaccumulators due to their wide surface distribution and specific responses to atmospheric pollutants. However, various plants exhibit significant differences in their capacities to accumulate PAHs. At present, research has mainly focused on the effects of leaf morphology and physiological characteristics, and few studies have evaluated the effects of the leaf surface on PAH accumulation. We aimed to assess the factors impacting the uptake and accumulation of PAHs by leaves. We selected 8 common tree species in Shanghai, China, and used supercritical fluid extraction technology to determine the content of PAHs in their leaves. Specific measurements of leaf area, width/length, wax content, and stomatal density were applied to index the morphological and physiological characteristics; surface roughness, surface free energy, polar components, and dispersion components were compiled into an adsorption performance index. Principal component analysis (PCA) and canonical correlation analysis (CCA) were used to assess the effects of different leaf characteristics on PAH accumulation. We found that the mean concentrations of ΣPAHs ranged from 300 to 2000 ng·g -1 and that the proportions of different benzene rings were significantly different among the different tree species. Leaf morphology and physiological characteristics had more significant effects compared to surface adsorption. CCA showed a significant negative correlation between leaf morphological characteristics and wax content, but had no significant correlation with surface adsorption. Low-molecular-weight PAHs were found to be mainly affected by the morphological characteristics, while medium- and high-molecular-weight PAHs were influenced by wax content and adsorption. Our conclusions provide a theoretical basis for the establishment of a reliable plant atmosphere-monitoring system and a method for screening tree species with strong PAH adsorption capacity., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Large-scale planted forests (PF) have been given a higher priority in China for improving the environment and mitigating climate change relative to natural forests (NF). However, the ecological consequences of these PF on water resource security have been less considered in the national scale. Moreover, a critically needed comparison on key ecological effects between PF and NF under climate change has rarely been conducted. Here, we compare carbon sequestration and water consumption in PF and NF across China using combination of remote sensing and field inventory. We found that, on average, NF consumed 6.8% (37.5 mm per growing season) less water but sequestered 1.1% (12.5 g C m -2  growing season -1 ) more carbon than PF in the period of 2000-2012. While there was no significant difference in water consumption (p = 0.6) between PF and NF in energy-limited areas (dryness index [DI] < 1), water consumption was significantly (p < 0.001) higher in PF than that in NF in water-limited regions (DI > 1). Moreover, a distinct and larger shift of water yield was identified in PF than in NF from the 1980s to the 2000s, indicating that PF were more sensitive to climate change, leading to a higher water consumption when compared with NF. Our results suggest NF should be properly valued in terms of maximizing the benefits of carbon sequestration and water yield. Future forest plantation projects should be planned with caution, particularly in water-limited regions where they might have less positive effect on carbon sequestration but lead to significant water yield reduction., (© 2018 John Wiley & Sons Ltd.)

Nonstructural carbohydrates (NSC) have been proposed to play an important role in maintaining the hydraulic integrity of trees, particularly in environments with high risks of embolism formation, but knowledge about the interaction between NSC reserves and xylem hydraulics is still very limited. We studied the variation of NSC reserves and hydraulic traits in Pinus koraiensis Sieb. et Zucc. (Korean pine) in March and June across a relatively large altitudinal gradient in Changbai Mountain of Northeast China. One of the major aims was to investigate the potential role NSC plays in maintaining hydraulic integrity of overwintering stems in facing freezing-induced embolism. Consistent with our hypotheses, substantial variations in both NSC contents and hydraulic traits were observed across altitudes and between the two seasons. In March, when relatively high degrees of winter embolism exist, the percentage loss of conductivity (PLC) showed an exponential increase with altitude. Most notably, positive correlations between branch and trunk soluble sugar content and PLC (P = 0.053 and 0.006) were observed across altitudes during this period. These correlations could indicate that more soluble sugars are required for maintaining stem hydraulic integrity over the winter by resisting or refilling freezing-induced embolism in harsher environments, although more work is needed to establish a direct causal relationship between NSC dynamics and xylem hydraulics. If the correlation is indeed directly associated with varying demands for maintaining hydraulic integrity across environmental gradients, greater carbon demands may compromise tree growth under conditions of higher risk of winter embolism leading to a trade-off between competitiveness and stress resistance, which may be at least partially responsible for the lower dominance of Korean pine trees at higher altitudes.

Theoretical and eddy covariance studies demonstrate that aerosol-loading stimulates canopy photosynthesis, but field evidence for the aerosol effect on tree growth is limited. Here, we measured in situ daily stem growth rates of aspen trees under a wide range of aerosol-loading in China. The results showed that daily stem growth rates were positively correlated with aerosol-loading, even at exceptionally high aerosol levels. Using structural equation modeling analysis, we showed that variations in stem growth rates can be largely attributed to two environmental variables covarying with aerosol loading: diffuse fraction of radiation and vapor pressure deficit (VPD). Furthermore, we found that these two factors influence stem growth by influencing photosynthesis from different parts of canopy. Using field observations and a mechanistic photosynthesis model, we demonstrate that photosynthetic rates of both sun and shade leaves increased under high aerosol-loading conditions but for different reasons. For sun leaves, the photosynthetic increase was primarily attributed to the concurrent lower VPD; for shade leaves, the positive aerosol effect was tightly connected with increased diffuse light. Overall, our study provides the first field evidence of increased tree growth under high aerosol loading. We highlight the importance of understanding biophysical mechanisms of aerosol-meteorology interactions, and incorporating the different pathways of aerosol effects into earth system models to improve the prediction of large-scale aerosol impacts, and the associated vegetation-mediated climate feedbacks., (© 2018 John Wiley & Sons Ltd.)

Assessing the uptake of trace gases by forests contributes to understanding the mechanisms of gas exchange between vegetation and the atmosphere and to evaluating the potential risk of these pollutant gases to forests. In this study, the multi-timescale characteristics of the stomatal uptake of NO, NO 2 , SO 2 and O 3 by Schima superba, Eucalyptus citriodora and Acacia auriculiformis were investigated by continuous sap flow measurements for a 3-year period. The peak canopy stomatal conductance (G C ) for these three species appeared between 9:00 and 12:00, which was jointly regulated by the vapour pressure deficit (VPD) and photosynthetically active radiation (PAR). Additionally, annual and seasonal variations in the stomatal uptake of trace gases for these three tree species suggested that there was a combination effect between canopy stomatal conductance and ambient concentration on the uptake of trace gases. Furthermore, the result demonstrated that the trace gas absorption capacities among these three forest types followed the order of S. superba > E. citriodora > A. auriculiformis. The findings of this study have theoretical significance and application value in assessing air purification and the risk of harm to forests in Southern China.

The low-temperature limited alpine treeline is one of the most obvious boundaries in mountain landscapes. The question of whether resource limitation is the physiological mechanism for the formation of the alpine treeline is still waiting for conclusive evidence and answers. We therefore examined non-structural carbohydrates (NSC) and nitrogen (N) in treeline trees (TATs) and low-elevation trees (LETs) in both summer and winter in 11 alpine treeline cases ranging from subtropical monsoon to temperate continental climates across Eurasia. We found that tissue N concentration did not decrease with increasing elevation at the individual treeline level, but the mean root N concentration was lower in TATs than in LETs across treelines in summer. The TATs did not have lower tissue NSC concentrations than LETs in summer. However, the present study with multiple tree species across a large geographical scale, for the first time, revealed a common phenomenon that TATs had significantly lower NSC concentration in roots but not in the aboveground tissues than LETs in winter. Compared with LETs, TATs exhibited both a passive NSC storage in aboveground tissues in excess of carbon demand and an active starch storage in roots at the expense of growth reduction during the growing season. This starch accumulation disappeared in winter. Our results highlight some important aspects of the N and carbon physiology in relation to season in trees at their upper limits. Whether or to what extent the disadvantages of winter root NSC and summer root N level of TATs affect the growth of treeline trees and the alpine treeline formation needs to be further studied.

Premise of the Study: The growth limitation hypothesis (GLH) and carbon limitation hypothesis (CLH) are two dominant explanations for treeline formation. The GLH proposes that low temperature drives the treeline through constraining C sinks more than C sources, and it predicts that non-structural carbohydrate (NSC) levels are static or increase with elevation. Although the GLH has received strong support globally for evergreen treelines, there is still no consensus for deciduous treelines, which experience great asynchrony between supply and demand throughout the year., Methods: We investigated growth and the growing-season C dynamics in a common deciduous species, Erman's birch (Betula ermanii), along an elevational gradient from the closed forest to the treeline on Changbai Mountain, Northeast China. Samples were collected from developing organs (leaves and twigs) and main storage organs (stems and roots) for NSC analysis., Key Results: Tree growth decreased with increasing elevation, and NSC concentrations differed significantly among elevations, organs, and sampling times. In particular, NSC levels varied slightly during the growing season in leaves, peaked in the middle of the growing season in twigs and stems, and increased continuously throughout the growing season in roots. NSCs also tended to increase or vary slightly in developing organs but decreased significantly in mature organs with increasing elevation., Conclusions: The decrease in NSCs with elevation in main storage organs indicates support for the CLH, while the increasing or static trends in new developing organs indicate support for the GLH. Our results suggest that the growth limitation theory may be less applicable to deciduous species' growth than to that of evergreen species., (© 2018 Botanical Society of America.)

Cladoxylopsida included the earliest large trees that formed critical components of globally transformative pioneering forest ecosystems in the Mid- and early Late Devonian (ca. 393-372 Ma). Well-known cladoxylopsid fossils include the up to ∼1-m-diameter sandstone casts known as Eospermatopteris from Middle Devonian strata of New York State. Cladoxylopsid trunk structure comprised a more-or-less distinct cylinder of numerous separate cauline xylem strands connected internally with a network of medullary xylem strands and, near the base, externally with downward-growing roots, all embedded within parenchyma. However, the means by which this complex vascular system was able to grow to a large diameter is unknown. We demonstrate-based on exceptional, up to ∼70-cm-diameter silicified fossil trunks with extensive preservation of cellular anatomy from the early Late Devonian (Frasnian, ca. 374 Ma) of Xinjiang, China-that trunk expansion is associated with a cylindrical zone of diffuse secondary growth within ground and cortical parenchyma and with production of a large amount of wood containing both rays and growth increments concentrically around individual xylem strands by normal cambia. The xylem system accommodates expansion by tearing of individual strand interconnections during secondary development. This mode of growth seems indeterminate, capable of producing trees of large size and, despite some unique features, invites comparison with secondary development in some living monocots. Understanding the structure and growth of cladoxylopsids informs analysis of canopy competition within early forests with the potential to drive global processes., Competing Interests: The authors declare no conflict of interest., (Published under the PNAS license.)

We tried to establish compatible carbon content models of individual trees for a Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) plantation from Fujian province in southeast China. In general, compatibility requires that the sum of components equal the whole tree, meaning that the sum of percentages calculated from component equations should equal 100%. Thus, we used multiple approaches to simulate carbon content in boles, branches, foliage leaves, roots and the whole individual trees. The approaches included (i) single optimal fitting (SOF), (ii) nonlinear adjustment in proportion (NAP) and (iii) nonlinear seemingly unrelated regression (NSUR). These approaches were used in combination with variables relating diameter at breast height (D) and tree height (H), such as D, D2H, DH and D&H (where D&H means two separate variables in bivariate model). Power, exponential and polynomial functions were tested as well as a new general function model was proposed by this study. Weighted least squares regression models were employed to eliminate heteroscedasticity. Model performances were evaluated by using mean residuals, residual variance, mean square error and the determination coefficient. The results indicated that models with two dimensional variables (DH, D2H and D&H) were always superior to those with a single variable (D). The D&H variable combination was found to be the most useful predictor. Of all the approaches, SOF could establish a single optimal model separately, but there were deviations in estimating results due to existing incompatibilities, while NAP and NSUR could ensure predictions compatibility. Simultaneously, we found that the new general model had better accuracy than others. In conclusion, we recommend that the new general model be used to estimate carbon content for Chinese fir and considered for other vegetation types as well.

The Grain for Green Program (GGP) is the largest afforestation and reforestation project in China in the early part of this century. To assess carbon sequestration in stands under the GGP in Southwest China, the carbon stocks and their annual changes in the GGP stands in the region were estimated based on the following information: (1) collected data on the annually planted area of each tree species under the GGP in Southwest China from 1999 to 2010; (2) development of empirical growth curves and corresponding carbon estimation models for each species growing in the GPP stands; and (3) parameters associated with the stands such as wood density, biomass expansion factor, carbon fraction and the change rate of soil organic carbon content. Two forest management scenarios were examined: scenario A, with no harvesting, and scenario B, with logging at the customary rotation followed by replanting. The results showed that by the years 2020, 2030, 2040, 2050 and 2060, the expected carbon storage of the GGP stands in Southwest China is 139.58 TgC, 177.50-207.55 TgC, 196.86-259.65 TgC, 240.45-290.62 TgC and 203.22-310.03 TgC (T = 1012), respectively. For the same years, the expected annual change in carbon stocks is 7.96 TgCyr-1, -7.95-5.95 TgCyr-1, -0.10-4.67 TgCyr-1, 4.31-2.24 TgCyr-1 and -0.02-1.75 TgCyr-1, respectively. This indicates that the stands significantly contribute to forest carbon sinks in this region. In 2060, the estimated carbon stocks in the seven major species of GGP stands in Southwest China are 4.16-13.01 TgC for Pinus armandii, 6.30-15.01 TgC for Pinus massoniana, 11.51-13.44 TgC for Cryptomeria fortunei, 15.94-24.13 TgC for Cunninghamia lanceolata, 28.05 TgC for Cupressus spp., 5.32-15.63 TgC for Populus deltoides and 5.87-14.09 TgC for Eucalyptus spp. The carbon stocks in these seven species account for 36.8%-41.4% of the total carbon stocks in all GGP stands over the next 50 years.

Soil respiration is a major pathway in the global carbon cycle and its response to environmental changes is an increasing concern. Here we explored how total soil respiration (RT) and its components respond to elevated acid rain in a mixed conifer and broadleaf forest, one of the major forest types in southern China. RT was measured twice a month in the first year under four treatment levels of simulated acid rain (SAR: CK, the local lake water, pH 4.7; T1, water pH 4.0; T2, water pH 3.25; and T3, water pH 2.5), and in the second year, RT, litter-free soil respiration (RS), and litter respiration (RL) were measured simultaneously. The results indicated that the mean rate of RT was 2.84 ± 0.20 μmol CO2 m(-2) s(-1) in the CK plots, and RS and RL contributed 60.7% and 39.3% to RT, respectively. SAR marginally reduced (P = 0.08) RT in the first year, but significantly reduced RT and its two components in the second year (P < 0.05). The negative effects were correlated with the decrease in soil microbial biomass and fine root biomass due to soil acidification under the SAR. The temperature coefficients (Q10) of RT and its two components generally decreased with increasing levels of the SAR, but only the decrease of RT and RL was significant (P < 0.05). In addition, the contribution of RL to RT decreased significantly under the SAR, indicating that RL was more sensitive to the SAR than RS. In the context of elevated acid rain, the decline trend of RT in the forests in southern China appears to be attributable to the decline of soil respiration in the litter layer.

Increased anthropogenic nitrogen emissions and more severe environmental issues (e.g. air pollution, soil acidification, and plant nutrient imbalances) are striking forest ecosystems. Data on NH4+ and NO3- concentrations in throughfall and stemflow were collected to estimate variation characteristics of nitrogen concentrations through forest hydrological processes across China. A typical study was carried out in the three forest types in the Jinyun Mountain region of Chongqing, from May to October 2012. Nitrogen concentrations in throughfall and stemflow are higher than those in atmospheric precipitation. DIN concentrations in atmospheric precipitation, throughfall, and stemflow, across China and in the Jinyun Mountain region, were 2.18 and 1.51, 3.19 and 3.88, and 5.14 and 3.92 mg N L(-1), respectively. NH4+ concentration was higher than NO3- concentration, suggesting NH4+ is the dominant nitrogen component in China. Additionally, across China, a linear relationship existed between DIN and NH4+, and between DIN and NO3- in atmospheric precipitation. DIN concentrations in throughfall and stemflow changed with the observed changes in precipitation, and DIN concentrations in precipitation positively correlated with those in throughfall and in stemflow were also observed. Moreover, average DIN concentrations in throughfall and stemflow varied in different forest types, resulting from differences in forest canopy structures and tree species characteristics. In the Jinyun Mountain region, both throughfall and stemflow DIN concentrations were the highest in the mixed broadleaved/coniferous forest, followed by evergreen broadleaved forest, and the lowest in moso bamboo forest. Monthly variations of NH4+ and NO3- concentrations, in throughfall and stemflow, were observed in the Jinyun Mountain region.

Urban plants are capable of reducing environmental pollutions through bioaccumulation contaminants in their tissues. The accumulation of heavy metals (Pb, Cu, Cd, Cr, and Zn) in leaves of nine tree species and five shrub species from Yan׳an city of China were investigated, and total metal accumulation capacities of different plants were evaluated using the metal accumulation index (MAI). The results indicated that plants in polluted environments are enriched in heavy metals relative to those in pristine environments, this is mainly caused by traffic emissions and coal combustion. Species with the highest accumulation of a single metal did not have the highest total metal accumulation capacity, the MAI should be an important indicator for tree species selection in phytoextraction and urban greening. Considering total accumulation capacities, Sabina chinensis, Juniperus formosana, Ailanthus altissima and Salix matsudana var. matsudana could be widely used in the Loess Plateau., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Previous studies show that Masson pine (Pinus massoniana L.) stands grown at the industrially-polluted site have experienced unprecedented growth decline, but the causal mechanisms are poorly understood. In this study, to understand the mechanisms of growth decline of Mason pine strands under pollution stresses, we determined the reactive oxygen species levels and chemical composition of the current-year (C) and one-year-old (C + 1) needles, and calculated the needle construction costs (CCmass) of Masson pine trees grown at an industrially-polluted site and an unpolluted remote site. Pine trees grown at the polluted site had significantly higher levels of hydroxyl radical and superoxide anion in their needles than those grown at the unpolluted site, and the former trees eventually exhibited needle early senescence. The contents of lipids, soluble phenolics and lignins in C and C + 1 needles were significantly higher at the polluted site than at the unpolluted site, but the total amounts of non-construction carbohydrates were lower in non-polluted needles than in polluted needles. Elevated levels of the reactive oxygen species and early senescence in polluted needles together led to significant increases in CCmass and a longer payback time. We infer that the lengthened payback time and needle early senescence under pollution stress may reduce the Masson pine tree growth and consequently accelerate tree decline.

Afforestation is a mitigation option to reduce the increased atmospheric carbon dioxide levels as well as the predicted high possibility of climate change. In this paper, vegetation survey data, statistical database, National Forest Resource Inventory database, and allometric equations were used to estimate carbon density (carbon mass per hectare) and carbon storage, and identify the size and spatial distribution of forest carbon sinks in plantation ecosystems in sand source areas of north Beijing, China. From 2001 to the end of 2010, the forest areas increased more than 2.3 million ha, and total carbon storage in forest ecosystems was 173.02 Tg C, of which 82.80 percent was contained in soil in the top 0-100 cm layer. Younger forests have a large potential for enhancing carbon sequestration in terrestrial ecosystems than older ones. Regarding future afforestation efforts, it will be more effective to increase forest area and vegetation carbon density through selection of appropriate tree species and stand structure according to local climate and soil conditions, and application of proper forest management including land-shaping, artificial tending and fencing plantations. It would be also important to protect the organic carbon in surface soils during forest management.

By using eddy covariance technique, a year-round (November, 2011-October, 2012) continuous measurement of net ecosystem carbon dioxide exchange (NEE) was conducted in a 4-year old mixed forest plantation in Badaling of Beijing. The forest plantation ecosystem was a net carbon sink in July and August, but a carbon source in the rest months. The monthly net carbon loss and uptake were the largest in April and July, respectively. The annual net ecosystem productivity was (-256 +/- 21) g C x m(-2) x a(-1), in which, the ecosystem respiration was (950 +/- 36) g C x m(-2) x a(-1), and the gross ecosystem productivity was (694 +/- 17) g C x m(-2) x a(-1). The nighttime NEE increased exponentially with the soil temperature at 10 cm depth, with the estimated temperature sensitivity of ecosystem respiration (Q10 ) being 2.2. During the growth season (May-September), the daytime NEE increased with photosynthetically active radiation (PAR) as described by the Michaelis-Menten rectangular hyperbola. The ecosystem quantum yield varied seasonally, ranging from 0.0219 micromol CO2 x micromol(-1) in May to 0.0506 micromol CO2 x micromol(-1) in July. The maximum carbon assimilation rate and the average daytime respiration followed the seasonal trends of PAR and air temperature. In July and August, vapor pressure deficit and soil moisture played a significant role in determining daytime NEE.

Understory vegetation plays a crucial role in carbon and nutrient cycling in forest ecosystems; however, it is not clear how understory species affect tree litter decomposition and nutrient dynamics. In this study, we examined the impacts of understory litter on the decomposition and nutrient release of tree litter both in a pine (Pinus sylvestris var. mongolica) and a poplar (Populus × xiaozhuanica) plantation in Northeast China. Leaf litter of tree species, and senesced aboveground materials from two dominant understory species, Artemisia scoparia and Setaria viridis in the pine stand and Elymus villifer and A. sieversiana in the poplar stand, were collected. Mass loss and N and P fluxes of single-species litter and three-species mixtures in each of the two forests were quantified. Data from single-species litterbags were used to generate predicted mass loss and N and P fluxes for the mixed-species litterbags. In the mixture from the pine stand, the observed mass loss and N release did not differ from the predicted value, whereas the observed P release was greater than the predicted value. However, the presence of understory litter decelerated the mass loss and did not affect N and P releases from the pine litter. In the poplar stand, litter mixture presented a positive non-additive effect on litter mass loss and P release, but an addition effect on N release. The presence of understory species accelerated only N release of poplar litter. Moreover, the responses of mass loss and N and P releases of understory litter in the mixtures varied with species in both pine and poplar plantations. Our results suggest that the effects of understory species on tree litter decomposition vary with tree species, and also highlight the importance of understory species in litter decomposition and nutrient cycles in forest ecosystems.

Growing evidence has revealed high heterogeneity of fine root networks in both structure and function, with different root orders corporately maintaining trees' physiological activities. However, little information is available on how fine root heterogeneity of trees responds to environmental stresses. We examined concentrations of seven potentially toxic metals (Cr, Ni, Cu, Zn, As, Cd, and Pb) within fine root networks and their correlations with root morphological and macro-elemental traits in six Chinese subtropical trees. The contributions of different orders of roots to fine-root metal storage and return were also estimated. Results showed no consistent pattern for the correlation among different metal concentration against root traits. Unlike root metal concentration that generally decreased with root order, root metal storage was commonly lowest in middle root orders. Root senescence was at least comparable to leaf senescence contributing to metal removal. Although the first-order roots constituted 7.2-22.3% of total fine root biomass, they disproportionately contributed to most of metal return fluxes via root senescence. The two distinct root functional modules contributed differentially to metal uptake, allocation, and return, with defensive (lower-order) roots effectively stabilizing and removing toxic metals and bulk buffering (higher-order) roots possessing a persistent but diluted metal pool. Our results suggest a strong association of physiological functions of metal detoxification and metal homeostasis with the structural heterogeneity in fine root architecture.

Model simulation and in situ observation are the two most important means in studying the water and carbon cycles of terrestrial ecosystems, but have their own advantages and shortcomings. To combine these two means would help to reflect the dynamic changes of ecosystem water and carbon fluxes more accurately. Data assimilation provides an effective way to integrate the model simulation and in situ observation. Based on the observation data from the Harvard Forest Environmental Monitoring Site (EMS), and by using ensemble Kalman Filter algorithm, this paper assimilated the field measured LAI and remote sensing LAI into the Biome-BGC model to simulate the water and carbon fluxes in Harvard forest area. As compared with the original model simulated without data assimilation, the improved Biome-BGC model with the assimilation of the field measured LAI in 1998, 1999, and 2006 increased the coefficient of determination R2 between model simulation and flux observation for the net ecosystem exchange (NEE) and evapotranspiration by 8.4% and 10.6%, decreased the sum of absolute error (SAE) and root mean square error (RMSE) of NEE by 17.7% and 21.2%, and decreased the SAE and RMSE of the evapotranspiration by 26. 8% and 28.3%, respectively. After assimilated the MODIS LAI products of 2000-2004 into the improved Biome-BGC model, the R2 between simulated and observed results of NEE and evapotranspiration was increased by 7.8% and 4.7%, the SAE and RMSE of NEE were decreased by 21.9% and 26.3%, and the SAE and RMSE of evapotranspiration were decreased by 24.5% and 25.5%, respectively. It was suggested that the simulation accuracy of ecosystem water and carbon fluxes could be effectively improved if the field measured LAI or remote sensing LAI was integrated into the model.

The purpose of this study was to assess the relationship between tree leaf micro-morphology and features in adsorbing air suspended particulate matter and accumulating heavy metals. Seven tree species, including Ginkgo biloba, at heavy traffic density site in Huainan were selected to analyze the frequency of air particulate matter retained by leaves, the particle amount of different sizes per unit leaf area retained by leaves and its related micro-morphology structure, and the relationship between particle amount of different sizes per unit leaf area retained by leaves and its related accumulation of heavy metals. We found that the species characterized by small leaf area, special epidemis with abundant fax, and highly uneven cell wall, as well as big and dense stomata and without trichomes mainly absorbed fine particulate matter; while those species with many trichomes mainly retained coarse particulate matter. Accumulation of heavy metals in leaves of the seven species was significantly different except for Ph. Tree species with high capacities in heavy metal accumulation were Ginkgo biloba, Ligustrum lucidum, and Cinnamomum camphora. Accumulation of Cd, Cr, Ni, Zn, Cu and total heavy metal concentration for seven tree species was positively related to the amount of particulate matter absorbed. Correlation coefficients of d10 vs d2.5, d10 vs d1.0, d2.5 vs d1.0 were 0.987, 0.971, 0.996, respective, and the correlate level was significant. The ratios of d2.5/d10, d1.0/d10, d1.0/d2.5 were 0.844, 0.763, 0.822, indicating that the particulate matter from traffic was mainly fine particulates.

Using the measurement data from an 8-level vertical profile of CO2/H2 0 in a temperate deciduous broadleaved forest at the Maoershan Forest Ecosystem Research Station, Northeast China, this paper quantified the errors of CO2 storage flux (Fs ) calculated with three scalar variables, i. e. , CO2 density (rho c), molar fraction (cc), and molar mixing ratio relative to dry air (Xc). The dry air storage in the control volume of flux measurement was not a constant, and thus, the fluctuation of the dry air storage could cause the CO2 molecules transporting out of or into the control volume, i. e. , the variation of the dry air storage adjustment term (Fsd). During nighttime and day-night transition periods, the relative magnitude of Fsd to eddy flux was larger, and ignoring the Fsd could introduce errors in calculating the net CO2 exchange between the forest ecosystem and the atmosphere. Three error sources in the Fs calculation could be introduced from the atmospheric hydrothermal processes, i. e. , 1) air temperature fluctuation, which could cause the largest error, with one order of magnitude larger than that caused by atmospheric pressure (P) , 2) water vapor, its effect being larger than that of P in warm and moist summer but smaller in cold and dry winter, and 3) P, whose effect was generally smaller throughout the year. In estimating the effective CO2 storage (Fs&#95;E) , the Fs value calculated with rho c, cc, and Xc was overestimated averagely by 8. 5%, suggested that in the calculation of Fs, adopting the Xc conservation to atmospheric hydrothermal processes could be more appropriate to minimize the potential errors.