Tree-ring δ13C Research Articles

Growth inhibition and recovery of Pinus massoniana in Chongqing since the 1980s

Over recent decades, the forest ecosystems of southern China have been substantially affected by acid rain, a principal biogeochemical driver. Despite recent efforts to mitigate acid rain, the long-term adaptive responses of trees of varying ages remain insufficiently understood. In this study, we employed the tree-ring basal area increment (BAI), δ13C, δ15N, and δ18O values to identify patterns in growth inhibition and recovery in young and mature Pinus massoniana within acid rain-afflicted zones in China. A marked growth inhibition phase occurred from 1983 to 2010, characterized by a significant decline in BAI, particularly in mature trees. A growth recovery phase occurred from 2011 to 2020, with considerable BAI increases. Throughout the growth-inhibition phase, the intrinsic water-use efficiency (iWUE) increased, whereas δ15N levels were stable, suggesting gradual stomatal closure influenced by both acid rain and elevated CO2 levels. During this phase, mature and young trees exhibited differing strategies in response to environmental stressors. The growth recovery phase was characterized by significant reductions in both iWUE and δ15N, leading to a more closed nitrogen cycle. Structural equation modeling and correlational analyses identified SO42−, NO3−, and precipitation as key determinants of growth variations in both young and mature trees. Our findings underscore the effect of prolonged acid rain exposure on growth suppression across P. massoniana age groups, highlighting iWUE as a reliable predictor of tree growth, although age should be considered. With the ongoing reduction in acidic gas emissions, subtropical P. massoniana forests have exhibited significant resilience, highlighting rapid ecological recovery. We advocate for continued large-scale monitoring and model-based assessments of acid rain effects, emphasizing the need for long-term evaluations to comprehend the ecological resurgence and carbon sequestration potential within these ecosystems.

Inter- and Intra-Annual Variations in Oak Tree Ring δ13C Values across Different Elevations and Their Climatic Responses in Qinling Mountains

The Qinling Mountains, serving as a natural geographical and climatic boundary in China, require comprehensive climatic records to elucidate the trends in climate changes across the country. While stable isotopes in tree rings are widely employed to indicate historical environmental changes, investigations into tree ring isotopes in the Qinling Mountains, particularly within the widespread broad-leaf oaks, remain limited. In this study, we investigated both intra- and inter-annual variations in the δ13C values of tree rings and their correlations with climatic signals over the past two decades for Quercus aliena var. acuteserrata, a dominant species among oak trees on the main peak of the Qinling Mountains. Our results reveal that responses to climate differ among altitudes and individual trees, with trees at higher altitudes exhibiting higher sensitivity to extreme climate, which is low temperatures and rainfall fluctuations during the growth period in intra-annual δ13C variations. Furthermore, the positive correlations are observed between temperature during growing season and both tree growth and the inter-annual δ13C variations. However, the climate signal appears to be hampered by oak-specific factors, such as intense competition among individuals and the age of trees. Therefore, we suggest a more rigorous selection of sampling and propose further investigations into isotopic fractionation processes in oaks for future studies.

Tree-Ring δ13C and Intrinsic Water-Use Efficiency Reveal Physiological Responses to Climate Change in Semi-Arid Areas of North China

Climate change has had a widespread and profound impact on global temperature and precipitation patterns, especially in semi-arid areas. Plant δ13C and iWUE indicate the trade-off between carbon uptake and water loss, which is pivotal for understanding plant responses to climate change. Information about the long-term responses of the physiological and ecological processes of different tree species to climate change is also required. To investigate the impact of different forest stand structures and site conditions on long-term growth and physiological processes of coniferous and broad-leaved trees in the mountainous area of Beijing, we analyzed the tree-ring δ13C variation of four tree species (Platycladus orientalis, Pinus tabuliformis, Quercus variabilis, Robinia pseudoacacia) sampled from 64 plots with varying site and stand conditions. We found that the tree-ring δ13C of the four tree species varied from each other and was mainly affected by density and slope aspect, followed by slope and age. Both tree-ring δ13C and iWUE of the four tree species showed increasing trends over time, mechanistically linked to long-term changes in global CO2 concentration. This indicates the four native tree species have adapted well to climate change, and the risk of decline is relatively low. The increased iWUE translated into different growth patterns which varied with tree species, site, and stand condition. Different tree species have varying sensitivities to environmental factors. The iWUE of coniferous tree species is more sensitive to climate change than that of broad-leaved tree species, especially to temperature (T), the Standardized Precipitation Evapotranspiration Index (SPEI), and vapor pressure deficit (VPD).

White spruce presence increases leaf miner effects on aspen growth in interior Alaska

Alaska’s boreal forests are experiencing rapid changes in climate that may favor deciduous-dominated systems, with important implications for global biogeochemical and energy cycles. However, aspen (Populus tremuloides Michx.) has experienced substantial defoliation from the aspen leaf miner (Phyllocnistis populiella Cham., hereafter ALM) in Alaska, resulting in significant growth reductions. We conducted a tree-ring and Δ13C study to test the hypothesis that moisture limitation may have predisposed aspen to leaf miner damage. Contrary to our hypothesis, differences in climate-growth correlations between relatively severely and lightly affected trees were negligible during the pre-outbreak decades. Stands with greater summer precipitation had more limited ALM impact, however differences among models were small and multiple climate variables were suitable predictors of ALM impact. The strong negative relationship we detected between tree-ring Δ13C and basal area increment (BAI) suggested that interannual variation in Δ13C was driven primarily by variation in photosynthesis, limiting the utility of Δ13C as a tool to detect stomatal responses to moisture-limitation. Instead, we found that larger, faster-growing individuals on gentler slopes showed a stronger absolute reduction in BAI (pre-ALM BAI−post-ALM BAI), but were similar in relative BAI reduction (pre-ALM BAI/post-ALM BAI), with smaller, slower growing trees. Older trees and stands with greater relative abundance of white spruce [Picea glauca (Moench) Voss] had greater relative ALM impact whereas slower growing trees on steeper slopes were less affected. The significant effect of white spruce abundance on ALM impact was likely due to favorable leaf miner overwintering habitat provided beneath white spruce trees, which can lead to increased leaf miner survival and thus greater reductions in aspen growth. Our results illustrate the subtle but complex biotic interaction between microclimate and pest physiology in determining ALM-induced aspen growth reductions, adding important nuance to a hypothesized increase in deciduous tree cover in Alaska’s boreal forest.

Tree-ring based forest model calibrations with a deep learning algorithm

Process-based forest growth models are important tools for forest management and studies for forest growth, carbon sequestration, and forest dynamics. However, their reliable simulations rely not only on the quality of model construction but also on accurate parameters to appropriately depict various physiological and biophysical processes in the models. While explicit physiological measurements are excellent sources for model parameterization, they are not always readily available. It would be ideal to use easy-to-measure tree-ring data as a benchmark for parametrization, but such applications have been rare. Here we present a new approach to reasonably parameterize a stand growth model based on tree-ring data using deep learning algorithms. We integrated a stable carbon isotope (δ13C) version of a simple process-based stand growth model, 3-PG, into a recurrent neural network (RNN). The new 3PG-RNN model trained the RNN network and calibrated 3-PG parameters to minimize the mean squared error between the 3-PG outputs and targeted values. We then tested 3PG-RNN based on two Abies grandis stands located in Northern Idaho, USA. The results showed that the 3PG-RNN model was efficient in calibrating key parameters related to gas-exchange processes, including specifically quantum yield, maximum canopy conductance, and the slope function for stomatal responses to vapor pressure deficit. These calibrated parameters provided reasonable simulations for gas-exchanges similar to those constrained by explicit physiological observations. This was particularly true when both long-term diameter growth (estimated from tree-ring width) and tree-ring δ13C were used for model training. The RNN tool made it possible to use tree ring data as the key benchmark to calibrate the forest model and provide unbiased simulations for gas exchanges and forest growth with the RNN approach, which would greatly facilitate forest studies and management.

Contrasting water-use strategies to climate warming in white birch and larch in a boreal permafrost region.

The effects of rising atmospheric CO2 concentrations (Ca) with climate warming on intrinsic water-use efficiency and radial growth in boreal forests are still poorly understood. We measured tree-ring cellulose δ13C, δ18O, and tree-ring width in Larix dahurica (larch) and Betula platyphylla (white birch), and analyzed their relationships with climate variables in a boreal permafrost region of northeast China over past 68years covering a pre-warming period (1951-1984; base period) and a warm period (1985-2018; warm period). We found that white birch but not larch significantly increased their radial growth over the warm period. The increased intrinsic water-use efficiency in both species was mainly driven by elevated Ca but not climate warming. White birch but not larch showed significantly positive correlations between tree-ring δ13C, δ18O and summer maximum temperature as well as vapor pressure deficit in the warm period, suggesting a strong stomatal response in the broad-leaved birch to temperature changes. The climate warming-induced radial growth enhancement in white birch is primarily associated with a conservative water-use strategy. In contrast, larch exhibits a profligate water-use strategy. It implies an advantage for white birch over larch in the warming permafrost regions.

Winter–spring minimum temperature variations inferred from tree-ring δ13C in southeastern China

Winter–spring minimum temperature variations inferred from tree-ring δ13C in southeastern China

Stress-tolerant trees for resilient cities: Tree-ring analysis reveals species suitable for a future climate

Cities are vulnerable to droughts and heatwaves due to the decline in permeable green spaces and the emergence of heat islands. To tackle these environmental challenges, many cities adopted ambitious tree-planting initiatives. However, tree tolerance to climate extremes is overlooked even though physiological and growth limitations jeopardize ecosystem services provision. We assessed the responses of Tipuana tipu to the 2013/2014 summer drought in São Paulo, Brazil. We sampled trees in a park and streets and assessed how tree-ring δ13C and growth responses to drought differ. The Regression Tree shows that microhabitats exert the strongest control on tree-ring δ13C seconded by drought. δ13C is on average 1.0‰ higher in streets where growth rate is 60% higher than in the park. Similarly, δ13C increased by up to 0.9‰ during the drought when growth increased by up to 45% in both sites. This positive association between δ13C and growth rate (R2 = 0.78) indicates that growth is limited by assimilation rate. Assimilation is up to 15% higher in streets and increased by up to 10% during the drought because of higher incident sunlight and air temperature. Thus, T. tipu is a drought-tolerant species with potential to contribute to (sub)tropical cities, thriving in a warming climate.

Intra-annual stable isotopes in the tree rings of Hymenaea courbaril as a proxy for hydroclimate variations in southern Amazonia

The hydrology of Amazonia is changing due to climate and land-use changes, especially in the southern region, which has warmed and dried faster than other tropical regions. Yet there are no long-term hydrological records to put these changes in a historical perspective. Here we investigate the use of tree-ring carbon (δ13C) and oxygen isotopes (δ18O) to assess the seasonal variation in climate for the southern Amazonia basin. We analysed the intra-annual variation of δ13C and δ18O in 10 segments of each tree ring from 2013 to 2017 from individuals of Hymenaea courbaril, a long-lived and widespread neotropical tree species. We find strong seasonal patterns of tree-ring δ13C supporting previous observations of annual growth rhythms for this species. The intra-annual variation in δ18O shows that the lowest values generally occur just after the middle point of ring formation, corresponding to the peak rainy season. We find strong correlations between the δ18O in the middle of the growth ring and vapour pressure deficit (r = 0.92, P = 0.02) and precipitation (r = −0.93, P = 0.02). We further find associations between the oxygen isotopic series and the discharge of the Araguaia basin’s main rivers during the rainy period. Our results show that these δ18O records are sensitive to fluctuations in rainfall and humidity, and thus to river discharge in the region. Longer reconstructions of based on tree-ring δ18O of Hymenaea courbaril could provide a novel proxy to assess past hydrological changes.

Growth responses to climate warming and their physiological mechanisms differ between mature and young larch trees in a boreal permafrost region

The effects of tree age-related intrinsic water-use efficiency (iWUE) under climate change such as warming and rising atmospheric CO2 concentrations (Ca) on tree growth are still poorly understood. We measured tree-ring δ13C, δ18O, tree-ring width, and their relationships with climate variables, calculated iWUE for relatively young (90 ± 5 yr) and mature (230 ± 40 yr) Larix dahurica trees in a boreal permafrost region in northeastern China. The period of tree-ring measurements covering 70 years includes a pre-warming period (1951–1984; base period) and a warm period (1985–2018; warm period). The growth rate of both mature and young trees did not change with time over the base period, while the mature trees but not the young ones significantly increased the growth rate with time over the warm period. The iWUE of mature trees consistently increased from 1951 to 2018, whereas that of the young trees increased only with time during the warm period from 1985 to 2018. The tree-ring δ13C and δ18O were not correlated for both young and mature trees over both base and warm periods, except for a significantly positive correlation between tree-ring δ13C and δ18O for mature trees during the warm period. We found that mature trees were more sensitive to Ca than young trees. Our results suggest that mature trees have a more ‘conservative’ (low stomatal conductance (gs), constant assimilation rate (A)) water use strategy than young trees (constant gs, high A) in the permafrost regions in a warming world.

Quercus ilex L. dieback is genetically determined: Evidence provided by dendrochronology, δ13C and SSR genotyping

Quercus ilex L. dieback has been reported in several Mediterranean forests, revealing different degree of crown damages even in close sites, as observed in two Q. ilex forest stands in southern Tuscany (IT). In this work, we applied a novel approach combining dendrochronological, tree-ring δ13C and genetic analysis to test the hypothesis that different damage levels observed in a declining (D) and non-declining (ND) Q. ilex stands are connected to population features linked to distinct response to drought. Furthermore, we investigated the impact of two major drought events (2012 and 2017), that occurred in the last fifteen years in central Italy, on Q. ilex growth and intrinsic water use efficiency (WUEi). Overall, Q. ilex showed slightly different ring-width patterns between the two stands, suggesting a lower responsiveness to seasonal climatic variations for trees at D stand, while Q. ilex at ND stand showed changes in the relationship between climatic parameters and growth across time. The strong divergence in δ13C signals between the two stands suggested a more conservative use of water for Q. ilex at ND compared to D stand that may be genetically driven. Q. ilex at ND resulted more resilient to drought compared to trees at D, probably thanks to its safer water strategy. Genotyping analysis based on simple-sequence repeat (SSR) markers revealed the presence of different Q. ilex populations at D and ND stands. Our study shows intraspecific variations in drought response among trees grown in close. In addition, it highlights the potential of combining tree-ring δ13C data with SSR genotyping for the selection of seed-bearing genotypes aimed to preserve Mediterranean holm oak ecosystem and improve its forest management.

Disentangling the impact of photosynthesis and stomatal conductance on rising water-use efficiency at different altitudes on the Tibetan plateau

Tree intrinsic water-use efficiency (iWUE) has dramatically increased in recent decades in global forests. The rising iWUE can be a result of either enhanced photosynthesis rate (A), or decreased stomatal conductance (gs) or both. The underlying physiological mechanisms are still not well understood. Here, we investigated tree-ring isotopes δ13C and δ18O from two tree species in three altitudinal transects on the southeast Tibetan plateau. We found that the relationship between iWUE and ∆18Olw was negative at lower-altitude forests, whereas it was positive at the treeline, indicating that enhanced photosynthesis was the main driver of the increasing iWUE at the treeline, whereas reduction in stomatal conductance was the prime regulator at lower-altitude forests. Furthermore, vapor pressure deficit (VPD) and precipitation during the main growing season showed the highest and significant correlations with leaf water 18O (Δ18Olw) at lower-altitude forests, suggesting that gs was strongly controlled by moisture conditions in the growing season particularly in the lower-altitude forests. These findings shed light to a better understanding of the regulatory mechanism of iWUE changes under the background of rising atmospheric CO2 and climate change and serve to improve the reliability of ecophysiological modeling.

Combination of multiple stable isotope and elemental analyses in urban trees reveals air pollution and climate change effects in Central Mongolia

The Ulaanbaatar area in Mongolia has become one of the most polluted regions worldwide due to the rapid increase in urbanization, industrial activity and traffic. However, we critically lack knowledge on the impacts of air pollution on surrounding forest ecosystems that may be further amplified by the ongoing climate change. Here, we apply a novel combination of multiple stable isotope analyses (nitrogen: δ15N, carbon: δ13C, oxygen: δ18O, hydrogen: δ2H) in foliar and tree-ring samples from different tree species, including deciduous, broadleaf species (poplar and birch), a deciduous conifer (larch) and needle evergreen species (spruce and Scots pine). This was complemented by trace element analysis, to study the influence of air pollution and climate on urban, suburban and more remote forests in and around Ulaanbaatar. We found indications of pollution effects in urban and suburban sites in foliar material, particularly in δ15N, with unusually high values of > 10‰, that could be related to tree uptake of NOx. Results were similar for all species, but with a smaller effect for Scots pine. The tree-ring δ15N values were found to be clearly enriched in recent years compared to 50 years ago at the urban sites, consistent with a pollution signal. Leaves and needles at suburban and urban sites had accumulated higher concentrations of various trace elements including Al, B, Ba, Ca, Cr, Cu, Fe, Mg, Na, S and Zn compared to the more distant sites, confirmed by Principal Component Analysis. Our data on δ13C, δ18O and δ2H enabled us to infer possible physiological effects induced by air pollution. Consistently increasing tree-ring δ13C values over recent decades for all investigated species indicated increasing plant stress, like hampered stomatal conductance and photosynthesis, but this was found for all sites, suggesting climate change rather than air pollution effects. In summary, we show that our multi-isotope and -element approach provides new insights into the threats to forests in urban areas, where the occurrence of more frequent droughts acts together with air pollution.

Tree-ring δ13C of Pinus koraiensis is a better tracer of gross primary productivity than tree-ring width index in an old-growth temperate forest

Tree-ring carbon (C) isotope composition (δ13C) varies depending on photosynthetic capacity and stomatal conductance, and it is therefore intrinsically associated with vegetation productivity. Still, very little is known about the relationship between tree-ring δ13C values and forest gross primary productivity (GPP). Here, we investigated relationships between tree-ring δ13C, tree-ring width index (RWI) and ecosystem-level GPP in a Korean pine (Pinus koraiensis)-dominated old-growth temperate forest in northeastern China. Specifically, we generated chronologies of RWI and δ13C from early-wood (δ13CEW) and late-wood (δ13CLW) of Korean pines, and we built a 17-year continuous ecosystem-level GPP series using eddy covariance measurements, in which a GPP model was used for gap filling. RWI showed vague relationships with tree-ring δ13C and whole-forest GPP, and it was insensitive to interannual variations in climate at the study site. By contrast, both δ13CEW and δ13CLW showed significant relationships with climate variables and GPP. This suggests that isotopic signals of photosynthetic C uptake were imprinted on the formation of tree rings and that tree-ring δ13C of Korean pine is a better proxy for forest GPP than RWI. In addition, tree-ring δ13C of Korean pines was positively correlated with GPP, with an especially close relationship between δ13CEW and GPP of the current year. This implies that photosynthetic activity, rather than stomatal conductance, was the main driver of tree-ring δ13C signals of Korean pine and it was mainly influenced by the climate conditions of the current year, probably because of a limited use of previously fixed C during early-wood formation. Our findings demonstrate the great potential of tree-ring δ13C for reconstructing GPP timeseries at the centennial scale for forest ecosystems, and they could help to constrain parameters in terrestrial C cycle models to improve predictions of C fluxes.

Differences in the Responses of Tree-Ring Stable Carbon Isotopes of L. sibirica and P. schrenkiana to Climate in the Eastern Tianshan Mountains

The eastern Tianshan Mountains are located in the arid interior of Asia, where tree growth is especially sensitive to climate. The ratio of stable carbon isotopes (δ13C) in the tree rings can provide information on changes in atmospheric CO2 concentrations, water availability, and physiological processes within the tree. In particular, the use of tree-ring δ13C values as a proxy for past atmospheric CO2 concentrations has gained widespread acceptance. In this study, detrended stable carbon isotope chronologies (13Ccorr) of Larix sibirica Ledeb. and Picea schrenkiana Fisch. et Mey was established using tree-ring samples from high elevations in the eastern Tianshan Mountains of Xinjiang, China. The relationships between the tree-ring 13Ccorr and different climatic factors were explored using the correlation function and collinearity analysis. Our results demonstrate that the tree-ring δ13Ccorr of L. sibirica is significantly and negatively correlated with precipitation and relative humidity during the growing season. The main climate factor affecting the stable carbon isotope fractionation of L. sibirica during the growing season is relative humidity during the growing season. The tree-ring δ13Ccorr of P. schrenkiana is significantly and negatively correlated with the mean temperature, mean minimum temperature, precipitation, and vapor pressure deficit from the end of the previous growing season and throughout the current growing season, especially in summer. However, it is significantly and positively correlated with relative humidity, indicating that the relationship between the climate factors and the tree-ring stable carbon isotope fractionation of P. schrenkiana is more complex. Further analysis showed that summer temperature and summer precipitation jointly controlled the tree-ring stable carbon isotope fractionation of P. schrenkiana at a high elevation. This research has important implications for our understanding of past and future climate change, as well as for the development of effective strategies to mitigate and adapt to these changes. This study also contributed to the development of a more in-depth understanding of the effects of climate change on tree growth in extremely arid environments and provided evidence to support effective forest management in arid regions.

A 195-Year Growing Season Relative Humidity Reconstruction Using Tree-Ring Cellulose δ13C in the Upper Tarim River Basin, NW China

Reconstruction of relative humidity changes in the upper Tarim River using carbon isotopic tree-ring chronology bridges the gap in historical observations on the Tarim River Basin in Arid Central Asia. Populus euphratica Olivier (P. euphratica), growing in the Tarim River Basin of Xinjiang, is an excellent record of past climate change. Based on precise dating, we analysed alpha-cellulose stable carbon isotopes in four cores of P. euphratica taken from the Alaer region of the upper Tarim River Basin. The four stable carbon isotope series records were corrected by the “pin method” and then combined into a carbon isotopic discrimination (Δ13C) series by the “numerical mix method”. The discrimination (Δ13C) series were clearly correlated with the mean relative humidity (RHAS) in April–September of the growing season (n = 60, r = −0.78, p < 0.001), and according to the climate response analysis, we designed a simple regression equation to reconstruct the mean relative humidity (RHAS) in April–September from 1824 to 2018 on the Alaer region. The reconstructed sequence showed mainly dry periods in the last 195 years, 1857–1866 and 1899–1907, while primarily wet periods from 1985 to 2016. Due to increased global warming and human activities, the climate shifted from “warm–dry” to “warm–wet” in the mid-to-late 1980s, when there were signs of a shift from “warm–wet” to “warm–dry” in the 2010s, with an increasing trend towards aridity. The RHAS series of Alaer compares well to other hydroclimate series’ surrounding the research area, and the spatial correlation analysis indicates that the reconstructed series has good regional representativeness. On an interdecadal scale, the revamped RHAS series is positively correlated with the Atlantic Multidecadal Oscillation (AMO) and negatively correlated with the North Atlantic Oscillation (NAO), reflecting the influence of westerly circulation on regional wet and dry variability. At the same time, the RHAS may also be influenced by The Pacific Decadal Oscillation (PDO).

Responses of Picea meyeri to Climatic Factors Revealed by Tree Ring Isotopes and Water Use Efficiency on Luya Mountain of North-Central China

Along with tree ring width, carbon isotopes are also good proxies for climate change. Water use efficiency (WUE) can be calculated more quickly and accurately based on carbon isotopes. In this study, according to the principle of dendroclimatology, the sequence of δ13C and WUE of tree rings of Picea meyeri are built. Pearson correlation analysis and multiple regression analysis are used to explore the response of carbon stable isotopes of Picea meyeri to climate change, which revealed the relationship between δ13C of Picea meyeri and climatic factors. Based on δ13C, we calculated the WUE of Picea meyeri and analyzed its response to climate change. The main conclusions are as follows: (1) The δ13C of Picea meyeri decreases year-by-year from 1957 to 2020, in the range from −23.89‰~−21.67‰, and the average value is −22.67‰. The water use efficiency of Picea meyeri increases in the range from 17.26~61.31, with an average of 39.45. (2) The δ13C of Picea meyeri is negatively correlated with temperature, which has the highest correlation with the temperature of the growing season (c5–c9), and its coefficient is higher than that of the mean temperature of each month. (3) There is a significant positive correlation between WUE sequence and temperature. Meanwhile, due to the effect of precipitation and temperature, the Picea meyeri is subject to drought stress to some extent. Above all, temperature is the main climatic factor affecting the δ13C and WUE of Picea meyeri on Luya mountain.

Urban CO2 imprints on carbon isotope and growth of Chinese pine in the Beijing metropolitan region

Rapid urbanization has occurred globally and resulted in increasing CO2 emissions from urban areas. Compared to natural forests, urban forests are subject to higher atmospheric CO2 concentrations in view of strong urban-periurban-rural gradients of CO2 emissions. However, relevant insights in the CO2-associated urban imprints on the physiology and growth of regional forests remain lacking. By sampling foliage and tree rings of Chinese pine (Pinus tabuliformis) in the Beijing metropolitan region, China, we explored whether and how urban CO2 emissions affect stable carbon isotope ratios (δ13C) and tree growth spatially and/or temporally. The results indicate a significant decrease in foliar δ13C values towards the urban center and this pattern was mainly explained by the urban-periurban-rural gradients of CO2 emissions as surrogated by trunk road density. Tree-ring δ13C values showed a significant decrease over last four decades and this trend was mainly explained by rising levels of CO2 and secondarily mediated by the variations of aridity index during growing season. Moreover, annual basal area increment of Chinese pine was significantly accelerated during last two decades, being mainly driven by increasing CO2 emissions and secondarily mediated by climate variations. These findings reveal significant CO2-associated imprints of urbanization on plant growth and provide empirical evidences of significant CO2-induced alteration of carbon cycles in urban forests.

Unravelling δ13C signal in Scots pine trees for climate change and tree physiology studies

Stable carbon isotope composition (δ13C) recorded in trees responds sensitively to the changing environmental conditions and thus provides a powerful tool for paleoenvironmental reconstructions. The retrospective interpretation of tree δ13C signal depends on comprehensive understanding of how environmental and physiological signals are recorded in δ13C during photosynthesis and how the δ13C signal is modified after photosynthesis. This thesis aims to improve the understanding of photosynthetic and post-photosynthetic isotope fractionation processes, and to examine the suitability of tree ring δ13C for intra-seasonal reconstructions of intrinsic water use efficiency (iWUE). The former goal was studied by combining compound-specific isotope analysis of organic matter with isotope discrimination models and online δ13C measurements of leaf CO2 fluxes for field-grown mature Scots pine (Pinus sylvestris L.). The latter was achieved via comparing 18-year-long intra-seasonal iWUE chronologies estimated from laser ablation derived tree ring δ13C, gas exchange and eddy covariance data. Mesophyll conductance and time-integral effect of leaf assimilates had a clear impact on the intra-seasonal dynamics of leaf sugar δ13C. No significant use of reserves was observed for biomass growth of needles, stem or roots of Scots pine. Unlike sucrose, leaf bulk matters had a significant δ13C offset from new assimilates, leading to a distorted environmental signal documented in their δ13C. The reliability of tree ring δ13C data for intra-seasonal iWUE reconstructions was supported by an agreement of intra-seasonal patterns across the iWUE estimation methods. These results broaden our knowledge of the less well-known photosynthetic and post-photosynthetic isotopic fractionation processes, demonstrate the benefits of analysing sucrose δ13C for understanding plant physiological responses, and show that the tree ring δ13C based iWUE reconstructions can be extended to intra-seasonal scale. This information not only helps to better unravel δ13C signal in trees, but also improves reliable reconstructions of environmental and physiological signals from tree ring δ13C.

Impacts of climate and tree morphology on tree-ring stable isotopes in central Mongolia.

Recent climate extremes in Mongolia have ignited a renewed interest in understanding past climate variability over centennial and longer time scales across north-central Asia. Tree-ring width records have been extensively studied in Mongolia as proxies for climate reconstruction, however, the climate and environmental signals of tree-ring stable isotopes from this region need to be further explored. Here, we evaluated a 182-year record of tree-ring δ13C and δ18O from Siberian Pine (Pinus sibirica Du Tour) from a xeric site in central Mongolia (Khorgo Lava) to elucidate the environmental factors modulating these parameters. First, we analyzed the climate sensitivity of tree-ring δ13C and δ18O at Khorgo Lava for comparison with ring-width records, which have been instrumental in reconstructing hydroclimate in central Mongolia over two millennia. We also compared stable isotope records of trees with partial cambial dieback ('strip-bark morphology'), a feature of long-lived conifers growing on resource-limited sites, and trees with a full cambium ('whole-bark morphology'), to assess the inferred leaf-level physiological behavior of these trees. We found that interannual variability in tree-ring δ13C and δ18O reflected summer hydroclimatic variability, and captured recent, extreme drought conditions, thereby complementing ring-width records. The tree-ring δ18O records also had a spring temperature signal and thus expanded the window of climate information recorded by these trees. Over longer time scales, strip-bark trees had an increasing trend in ring-widths, δ13C (and intrinsic water-use efficiency, iWUE) and δ18O, relative to whole-bark trees. Our results suggest that increases in iWUE at this site might be related to a combination of leaf-level physiological responses to increasing atmospheric CO2, recent drought, and stem morphological changes. Our study underscores the potential of stable isotopes for broadening our understanding of past climate in north-central Asia. However, further studies are needed to understand how stem morphological changes might impact stable isotopic trends.