Tree-ring δ13C Research Articles

A Dual Stable Isotope Approach Unravels Common Climate Signals and Species-Specific Responses to Environmental Change Stored in Multi-Century Tree-Ring Series from the Tibetan Plateau

Tree-rings are recorders of environmental signals and are therefore often used to reconstruct past environmental conditions. In this paper, we present four annually resolved, multi-centennial tree-ring isotope series from the southeastern Tibetan plateau. The investigation site, where juniper and spruce trees jointly occur, is one of the highest known tree-stands in the world. Tree ring cellulose oxygen (δ18O) and carbon (δ13C) isotopes were analyzed for a common period of 1685–2007 AD to investigate climate–isotope relationships. Therefore, various climate parameters from a local meteorological station and from the CRU 4.02 dataset were used. Tree-ring δ18O of both species revealed highly significant sensitivities with a high degree of coherence to hydroclimate variables during the growing season. The obtained δ18O–climate relationships can even be retained using a species mean. In contrast, the individual δ13C series indicated a weaker and non-uniform response to the tested variables. Underlying species-specific responses and adaptations to the long-term trend in atmospheric CO2 bias even after a trend correction identified dominant environmental factors triggering the tree-ring δ13C at our site. However, analysis of individual intrinsic water-use efficiency in juniper and spruce trees indicated a species-specific adaptation strategy to climate change.

Tree-ring isotopes adjacent to Lake Superior reveal cold winter anomalies for the Great Lakes region of North America

Tree-ring carbon isotope discrimination (Δ13C) and oxygen isotopes (δ18O) collected from white pine (Pinus strobus) trees adjacent to Lake Superior show potential to produce the first winter-specific paleoclimate reconstruction with inter-annual resolution for this region. Isotopic signatures from 1976 to 2015 were strongly linked to antecedent winter minimum temperatures (Tmin), Lake Superior peak ice cover, and regional to continental-scale atmospheric winter pressure variability including the North American Dipole. The immense thermal inertia of Lake Superior underlies the unique connection between winter conditions and tree-ring Δ13C and δ18O signals from the following growing season in trees located near the lake. By combining these signals, we demonstrate feasibility to reconstruct variability in Tmin, ice cover, and continental-scale atmospheric circulation patterns (r ≥ 0.65, P < 0.001).

Strong and stable relationships between tree-ring parameters and forest-level carbon fluxes in a Siberian larch forest

The tree-ring width index (RWI) and satellite-derived vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), are used as long-term indicators of the past forest carbon uptake. However, fundamental questions remain with respect to what is represented by the RWI and NDVI at the ecosystem level. To address this question, we compared tree-ring parameters (RWI and the carbon isotope ratio: δ13C) and NDVI products with forest ecosystem CO2 fluxes estimated using the eddy covariance method, at a larch forest in eastern Siberia. The RWI and tree-ring δ13C correlated well with the ecosystem gross primary production (GPP), and their temporal stabilities were high during 2004–2014. However, the NDVI products did not show any temporally stable relationship with the GPP. This could be ascribed to significant changes in the understory vegetation in this forest, i.e., from dense cowberry to shrubs and moisture-tolerant grasses, because of an excessively moist environment during 2007–2008. Changes in the understory vegetation could be reflected by the NDVI products but not by the GPP. Our results indicate that it is more feasible to study forest carbon uptake using tree-ring parameters than using satellite-derived vegetation indices in such larch-dominated forest ecosystems in eastern Siberia.

Soil fertilization transiently increases radial growth in sessile oaks but does not change their resilience to severe soil water deficit

The future climate is projected to become increasingly problematic for European forest ecosystems, likely leading to the decline of many tree species. Forest management must adapt to cope with the negative effects of these changes. Despite the close interactions between nutrients and water in plant physiology, very few studies have investigated the link between soil fertility and tree response to soil water availability and vapour pressure deficit. The objective of this study was to test whether fertilization would modify forest tree resilience to soil water deficit.The potential impact of fertilization on trees during soil water deficit was assessed in a fertilization experiment carried out on sessile oaks. Three fertilization treatments (NCa, NPKCaMg and an unfertilized control) were applied to 40-year-old oaks that were cored almost 40 years later. The responses to drought in the fertilized and control trees were compared by analysing the radial growth chronologies following two complementary approaches. Firstly, the relationships between radial growth and inter-annual climate variations were investigated through dendroclimatic modelling including water deficit indices computed with the daily soil water balance model Biljou©. Secondly, resistance, recovery and resilience to drought-induced crises were evaluated from tree-ring width and compared among treatments. In addition, the carbon isotope composition (δ13C) of the tree rings was measured to evaluate potential differential gas-exchange processes between fertilized and control trees.Fertilization had an immediate, strong positive effect on oak growth which disappeared after nine years. NCa and NPKCaMg fertilizations had similar effects, suggesting that adding P, K and Mg did not increase radial growth. Whatever the treatment, ring width variations appeared to be identically constrained by climate, especially by the summer water deficits of the current and previous years. Tree resistance to soil water deficit was negatively affected by the severity of the drought event but did not significantly differ between control and fertilized trees. Tree resilience was not affected by fertilization and remained stable across crises. The tree-ring δ13C clearly increased during the severe 1976 water deficit but did not vary among treatments. We discuss the possible mechanisms involved in changes in growth and the lack of any significant effect of fertilization on resilience to drought and conclude that, under the relatively dry conditions and moderately poor soil fertility of our study site, fertilization could temporarily stimulate aboveground growth without inducing imbalances that would modify resilience and resistance to severe drought episodes.

Increasing intrinsic water-use efficiency over the past 160 years does not stimulate tree growth in southeastern China

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 76:115-130 (2018) - DOI: https://doi.org/10.3354/cr01526 Increasing intrinsic water-use efficiency over the past 160 years does not stimulate tree growth in southeastern China Guoyang Guo1, Keyan Fang1,2,*, Jinbao Li3, Hans W. Linderholm2, Dawen Li1, Feifei Zhou1, Zhipeng Dong1, Yingjun Li1, Lei Wang1 1Institute of Geography, Key Laboratory of Humid Subtropical Eco-geographical Process (Ministry of Education), College of Geographical Sciences, Fujian Normal University, Fuzhou 350007, PR China 2Regional Climate Group, Department of Earth Sciences, University of Gothenburg, Box 460, 405 30 Gothenburg, Sweden 3Department of Geography, University of Hong Kong, Pokfulam, Hong Kong SAR *Corresponding author: kfang@fjnu.edu.cn ABSTRACT: Understanding the responses of tree growth and intrinsic water-use efficiency (iWUE) to anthropogenic CO2 increase and climate warming provides important benchmarks for evaluating future forest dynamics under different scenarios of warming and CO2 changes. Relative to the short period of instrumental data of the past few decades, long-term tree-ring width and stable carbon isotopic (δ13C) data are invaluable in fully comprehending their interactions during the entire industrial era, since ~1850. Here, we present a tree-ring width chronology (1856 to 2015) and δ13C series (1876-2015) of Pinus massoniana for Yongtai county of Fujian province, in humid subtropical China, a ‘green island’ relative to other dry subtropical areas of the world. Tree growth was limited by precipitation of the hydrological year (previous November to current October) (r = 0.568, p < 0.001), and the stable carbon isotope discrimination (Δ13C) was strongly correlated with relative humidity in September-October (r = -0.677, p < 0.001) of the current growing season. We found that the iWUE increased by 40.9% since 1876. Specifically, we found that the ci:ca ratio decreased during the study period whereas intercellular CO2 concentration (ci) increased. The negative relationship between basal area increment (BAI) and iWUE indicated that increasing iWUE may not lead to long-term enhancement of tree growth. Our results indicated a drought-induced limitation to tree growth in response to rising CO2, and that trees may mitigate the negative effects of a decrease in water availability through a reduction in stomatal conductance. KEY WORDS: Tree-ring · Stable carbon isotope · Intrinsic water-use efficiency · Pinus massoniana · Humid subtropical China Full text in pdf format PreviousNextCite this article as: Guo G, Fang K, Li J, Linderholm HW and others (2018) Increasing intrinsic water-use efficiency over the past 160 years does not stimulate tree growth in southeastern China. Clim Res 76:115-130. https://doi.org/10.3354/cr01526 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 76, No. 2. Online publication date: October 10, 2018 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2018 Inter-Research.

Application and verification of simultaneous determination of cellulose δ13C and δ18O in Picea shrenkiana tree rings from northwestern China using the high-temperature pyrolysis method

Stable isotopes in tree-ring cellulose provide important data in ecological, archaeological, and paleoenvironmental researches, thereby, the demand for stable isotope analyses is increasing rapidly. Simultaneous measurement of cellulose δ13C and δ18O values from tree rings would reduce the cost of isotopic commodities and improve the analytical efficiency compared with conventional separate measurement. In this study, we compared the δ13C and δ18O values of tree-ring α-cellulose from Tianshan spruce (Picea schrenkiana) in an arid site in the drainage basin of the Urumqi River in Xinjiang of northwestern China based on separate and simultaneous measurements, using the combustion method (at 1050°C) and the high-temperature pyrolysis method (at 1350°C and 1400°C). We verified the results of simultaneous measurement using the outputs from separate measurement and found that both methods (separate and simultaneous) produced similar δ13C values. The two-point calibrated method improved the results (range and variation) of δ13C and δ18O values. The mean values, standard deviations, and trends of the tree-ring δ13C obtained by the combustion method were similar to those by the pyrolysis method followed by two-point calibration. The simultaneously measured δ18O from the pyrolysis method at 1400°C had a nearly constant offset with that the pyrolysis method at 1350°C due to isotopic-dependence on the reaction temperature. However, they showed similar variations in the time series. The climate responses inferred from simultaneously and separately measured δ13C and δ18O did not differ between the two methods. The tree-ring δ13C and δ18O values were negatively correlated with standardized precipitation evapotranspiration index from May to August. In addition, the δ18O was significantly correlated with temperature (positive), precipitation (negative), and relative humidity (negative) from May to August. The tree-ring δ13C and δ18O values determined simultaneously through the high-temperature pyrolysis method could produce acceptable and reliable stable isotope series. The simultaneous isotopic measurement can greatly reduce the cost and time requirement compared with the separate isotopic measurement. These results are consistent with the previous studies at humid sites, suggesting that the simultaneous determination of δ13C and δ18O in tree-ring α-cellulose can be used in wide regions.

Inter-annual variation of tree-ring width, δ13C and δ18O in juvenile trees of five plantation poplar cultivars (Populus spp.)

The 10-year juvenile records of three hybrid poplar and two aspen cultivars (Populus spp.) from a short rotation coppice (SRC) were assessed by measuring tree-ring width (annual radial increment, ir) and stable isotope ratios of carbon and oxygen (δ13C and δ18O) of α-cellulose. All cultivars showed common ‘juvenile trends’ that were modeled with nonlinear fit (NLF) functions. The ir of all cultivars culminated in the middle of the juvenile phase. Within the first ten years, δ13C showed a gradual decrease of approximately 2.5 ‰ in all cultivars and δ18O showed an asymptotic increase which was variable among the poplar cultivars and which was more pronounced in two hybrid poplars. Potential causes of the juvenile inter-annual variability of δ13C and δ18O were discussed. Likely, the maturation related changes in hydraulic architecture, the canopy closure and the resulting increase of the proportion of shaded crown segments which have lower photosynthetic capacities had an effect on δ13C. An additional effect of changes in N nutrition on δ13C is assumed at the present SRC trial because the NO3− concentration had significantly decreased after ten years. Interpretation of δ18O data remained difficult given the lack of soil water δ18O records.The maturation effect and the respective interaction have resulted in large temporal variability in the present three investigated tree ring traits. However, the impact of two drought vegetation periods (2003 and 2006) was still reflected by the juvenile tree ring records of all traits. Different juvenile trends in the records of the stable isotope ratios δ13C and δ18O, and the trend slopes, which may vary between genetically different cultivars, must be considered in tree ring investigations of SRC poplars.

Evaluation of the effect of the 2011 Tsunami on coastal forests by means of multiple isotopic analyses of tree-rings

ABSTRACTThe March 2011 Mega-Tsunami in eastern Japan damaged at different degrees the black pine (Pinus thunbergii) forests along the coast. In order to evaluate the recovery of black pine four years later, tree-ring samples from 9 trees for the period 2002–2014 were analyzed for ring growth and stable isotopes (δ13C, δ15N and δ18O). The results showed that annual tree-ring width decreased approximately 70 % from the year 2011 to 2014 compared to the period previous to the tsunami (2002–2010). The multiple isotopic analyses showed that the reduction in growth was caused by soil salinity that prompted stomatal closure and an abrupt increase of tree-ring δ13C. Sea water deposition in the soil did not affect tree-ring δ18O values. Two years after the tsunami, decreasing tree-ring δ13C values caused by apparently photosynthetic recovery did not translate into radial tree-growth, indicating a possible shift in carbon allocation to foliage and mainly roots as a defense mechanism to sodium toxicity. The dual δ13C-δ18O model explains neither the limited growth nor the subsequent recovery in δ13C. Similarly tree-ring δ15N indicated that there was no difference in nitrogen availability before and after the tsunami, suggesting that nutrients were not a limitation but rather soil salinity.

Millennial-scale tree-ring isotope chronologies from coast redwoods provide insights on controls over California hydroclimate variability.

To understand drivers of hydroclimate variability in north-coastal California, we obtained tree cross-sections from eleven coastal redwoods (mean age of 1232years old) from the northern half of the species range. Tree rings from eight trees were cross-dated and sampled at sub-annual resolution for carbon isotope discrimination (Δ13C) and oxygen isotope composition (δ18O). Tree-ring Δ13C and δ18O, compared to modern climate data, demonstrate these signals primarily record summertime hydroclimate variability-primarily through variables associated with evaporative conditions and/or precipitation. Our 1100-year stable isotope chronologies showed that north-coastal California did not undergo the megadroughts observed elsewhere in California and the western United States. This result implicates extended periods of low winter precipitation, rather than growing season evaporation, as the primary driver of previous megadroughts across California and neighboring regions. Compared to cool conditions prevailing over the Northern Hemisphere during the Little Ice age (1301-1875 of the common era, CE), the frequency of isotopic events of a certain magnitude was greater during periods with warmer Northern Hemisphere temperatures such as the Medieval Climate Anomaly (900-1300 CE) and the modern period (1876 to present). This association between tree-ring isotopic variability and long-term shifts in temperatures is consistent with the expected patterns in mid-latitude hydroclimate variability expected from arctic amplification (i.e., shifts in equator-to-pole temperature differences that modify jet stream speed and amplitude) or amplified quasi-resonant wave activity (i.e., wave-patterns in high-altitude winds that become "trapped" within a certain pattern, thereby producing a longer-duration periods of drought or wetness) across mid-latitudes during the boreal summer.

Warming and CO2 enrichment modified the ecophysiological responses of Dahurian larch and Mongolia pine during the past century in the permafrost of northeastern China.

Tree-ring δ13C and δ18O of dominant Dahurian larch and Mongolia pine in the permafrost region of the northern Great Higgnan Mountains, China were used to elucidate species-specific ecophysiological responses to warming temperatures and increasing CO2 over the past century. Larch and pine stable carbon discrimination (Δ13C) 13C and δ18O in tree rings both showed synchronous changes during the investigated period (1901-2010), but with species-specific isotopic responses to atmospheric enriched CO2 and warming. Tree-ring Δ13C and δ18O were controlled by both maximum temperature and moisture conditions (precipitation, relative humidity and vapor pressure deficit), but with different growth periods (Δ13C in June-July and δ18O in July-August, respectively). In addition, stable isotopes of larch showed relatively greater sensitivity to moisture deficits than pine. Climatic conditions from 1920 to 1960 strongly and coherently regulated tree-ring Δ13C and δ18O through stomatal conductance. However, climatic-sensitivities of tree-ring Δ13C and δ18O recently diverged, implying substantial adjustments of stomatal conductance, photosynthetic rate and altered water sources over recent decades, which reveal the varied impacts of each factor on tree-ring Δ13C and δ18O over time. Based on expected changes in leaf gas-exchange, we isolated the impacts of atmospheric CO2 and climate change on intrinsic water-use efficiency (iWUE) over the past century. Higher intracellular CO2 in pine than larch from 1960 onwards suggests this species may be more resilient to severe droughts in the future. Our data also illustrated no weakening of the iWUE response to increasing CO2 in trees from this permafrost region. The overall pattern of CO2 enrichment and climate impacts on iWUE of pine and larch were similar, but warming increased iWUE of larch to a greater extent than that of pine over recent two decades. Taken together, our findings highlight the importance of considering how leaf gas-exchange responses to atmospheric CO2 concentration influence species-specific responses to climate and the alteration of the hydrological environment in forests growing in regions historically dominated by permafrost that will be changing rapidly in response to future warming and increased CO2.

Influence of seasonal temperature on tree-ring δ13C in different-aged temperate pine forests

Tree growth rings from three specimens in two different aged (14- and 77-year old) white pine plantation forests were analyzed for stable carbon isotope ratios to identify both short- and long-term variations in physiological response to changing environmental conditions. Three isotopic (δ13Ccorr) time series records were constructed from whole wood samples extracted from paths parallel to the growth rings in each forest. These δ13Ccorr records were corrected for the long-term anthropogenically induced CO2 and compared to historical climate (temperature, precipitation) data from 1935 to 2016. High resolution inter-annual variations in trees in each stand displayed similar intra-annual cycles in δ13Ccorr, demonstrating the seasonal physiological response of these forests to environmental stressors. In both stands, growing season temperature acted as a significant control (p < 0.05) on δ13Ccorr values and hence carbon uptake through enhanced physiological discrimination against 13CO2. Wavelet transform methods applied to the carbon isotope composition extracted from tree-rings to investigate isotope-climate linkages revealed phase relationships linked to changes in stand structure and microclimate due to thinning treatment applied in the older stand in 1983. Cross wavelet transform (XWT) of mean annual temperature with latewood tree-ring isotope time series demonstrated a persistent 12- to 16-year phase-locked relationship, which suggests long-term temperature variations influenced stomatal conductance behavior in the post-thinning period (1983–2012). Overall, air temperature acted as a multi-year control on tree-ring carbon isotope compositions in these different-age forests and variations in δ13Ccorr time series provided a potential record of past temperature changes in the region.

Climatic signals in stable carbon isotope ratios of Juniper and Oak tree rings from northern Iran

Abstract Stable isotope ratios in tree rings are increasingly used as palaeoclimatic archive and ecophysiological indicator. We used cross-dated tree-ring series from different tree functional types, i.e. the evergreen conifer Juniperus polycarpus and the deciduous broadleaved Quercus macranthera. The samples were collected from Chaharbagh Gorgon forest in northern Iran, where oaks and junipers grow on north-facing and south-facing slopes, respectively. We extracted α-cellulose from the whole wood of annually separated tree rings and evaluated their potential for palaeoclimate reconstructions by examining the relationships between variations of δ13C in tree-ring cellulose and climate parameters. Based on tree-ring δ13C, we calculated intrinsic water use efficiency (iWUE) and evaluated changes in water availability at the study site over the past 50 years. We found significant negative relationships between tree-ring δ13C values in oak and juniper and precipitation in April and spring (only in junipers), while no significant correlations of tree-ring δ13C with temperature occurred. A strong negative relationship between tree-ring δ13C and summer Palmer Drought Severity Index (PDSI) was found at the drier site, indicating that juniper growing on south-facing slopes is a better indicator of drought stress in semiarid environments. A continuous increase in iWUE in both species was observed. δ13C variations in both species reflect the combined influence of climate and local site conditions. The interplay between climatic conditions and species behavior determines the inter-annual δ13C patterns of oak and juniper trees. Due to its more enriched mean δ13C (i.e., less negative average δ13C) and its higher climatic sensitivity, juniper is a more suitable proxy for paleoclimatic reconstructions in northern Iran.

Tree-ring stable carbon isotope-based June–September maximum temperature reconstruction since AD 1788, north-west Thailand

The first study of tree-ring stable carbon isotopes in Thailand has demonstrated that stable carbon isotope in northwestern Thailand represents a promising proxy for the temperature reconstruction of core-monsoon periods. A tree-ring δ13C chronology was constructed based on four cores covering the period of 1788–2013. After removing the long-term decreasing trend reflecting atmospheric CO2 concentrations, the ∆13C chronology was able to capture both temperature and hydro-climate signals. ∆13C chronology showed particularly strong and significant negative correlation (r = –0.62, p < 0.0001) with June–September maximum temperature (CRU TS 3.24). The maximum temperature was reconstructed, which explained 37.8% of the variance in the instrumental maximum temperatures over the period of 1901–2013. The maximum temperature reconstruction revealed that four cooler and three warmer periods, as well as a slightly increasing temperature trend, occurred during the late seventeenth to mid-eighteenth centuries, which were followed by severe temperature fluctuations during the twentieth century century. While the sea surface temperature anomaly in the Indian Ocean might not affect the maximum temperature, its unstable relationship with the El Niño-Southern Oscillation (ENSO) was detected. In addition, a close relationship was observed between the maximum temperature and ENSO during the negative phase of the Pacific Decadal Oscillation (PDO), but this relationship was lost during the positive phase of the PDO.

Species-specific tree growth and intrinsic water-use efficiency of Dahurian larch (Larix gmelinii) and Mongolian pine (Pinus sylvestris var. mongolica) growing in a boreal permafrost region of the Greater Hinggan Mountains, Northeastern China

Increasing air temperature and atmospheric CO2 concentrations (Ca) can profoundly affect photosynthesis and intrinsic water-use efficiency (iWUE). However, the response of trees in boreal permafrost regions to rapid warming and Ca increases is poorly constrained by prior research. Here, we evaluated long-term changes in growth (using regional curve standardization [RCS]) and iWUE of Dahurian larch (Larix gmelinii) and Mongolian pine (Pinus sylvestris var. mongolica) in the boreal permafrost region of northeastern China and species-specific responses to increasing Ca and temperature. From 1930–2010, RCS growth of Dahurian larch and Mongolian pine decreased, while iWUE increased by 25.5 and 21.1%, respectively. RCS growth of both species was negatively correlated with winter temperatures, but Mongolian pine depended most strongly on previous December to current February temperatures and Dahurian larch depended most strongly on March temperatures. Moisture conditions only weakly influenced growth.We found similar long-term changes of tree-ring δ13C in the two species. Carbon isotopic discrimination of Dahurian larch and Mongolian pine was determined mainly by growing season temperature (positive) and moisture (negative), but with different signal strengths, suggesting that stomatal conductance influenced tree-ring δ13C. Commonality analysis showed that RCS growth was affected mainly by temperature, but also by the combined effect (interaction) of iWUE and temperature. However, the contribution of iWUE alone was lower for Mongolian pine. Our results suggest that the increased iWUE caused by increasing Ca will not improve tree growth sufficiently to compensate for temperature-induced water stress. The rate of temperature increase has slowed around 1990, which would have stabilized the degree of temperature-induced water stress, and this could be helpful to tree growth recovery in the permafrost region of northeastern China.

Tree-ring δ13C and water use efficiency of Platycladus orientalis in mountains of Beijing

Water use efficiency (WUE) is different among species and regions. Few literatures have been reviewed related to long-term WUE of Platycladus orientalis in mountainous areas of Beijing, China. Tree-ring δ13C of P. orientalis was used to determine the long-term variation of annual intrinsic water use efficiency (WUEi) and its response to environmental change. Combining with quantification of tree-ring width, the relationship between net carbon sequestration and WUEi of P. orientalis was eventually explored. The results showed that mean annual temperature increased with the increase of time from 1918 to 2013, whereas annual precipitation fiercely fluctuated. Tree-ring δ13C decreased and WUEi increased over time. WUEi was positively related and more sensitive to air temperature increasing than temperature decreasing. Correlation between WUEi and fluctuated annualprecipitation was ambiguous, which indicated the precipitation was not the main factor affecting WUEi. The de-trend tree-ring width of P. orientalis increased initially and then decreased, especially in recent 20 years. According to the correlation between WUEi and environmental factors, temperature resulted in stomatal conductance (gs) decreasing, which caused a reduction in evapotranspiration and an increase in respiratory loss, leading to the increase of WUEi and a down trend in net carbon sequestration and tree growth.

Tree-ring stable carbon isotope-based April–June relative humidity reconstruction since ad 1648 in Mt. Tianmu, China

Based on accurate dating, we have determined the stable carbon isotope ratios (δ13C) of five Cryptomeria fortunei specimens from Mt. Tianmu, a subtropical area in southern China. The five δ13C time series records are combined into a single representative δ13C time series using a “numerical mix method.” These are normalized to remove temporal variations of δ13 C in atmospheric CO2 to obtain a carbon isotopic discrimination (Δ13C) time series, in which we observe a distinct correlation between Δ13C and local April to June mean relative humidity (RH AMJ ) (n = 64, r = 0.858, p < 0.0001). We use this relationship to reconstruct RH AMJ variations from ad 1648 to 2014 at Mt. Tianmu. The reconstructed sequence show that over the past 367 years, Mt. Tianmu area was relatively wet, but in the latter part of the twentieth century, under the influence of increasing global warming, it has experienced a sharp reduction in relative humidity. Spatial correlation analysis reveals a significant negative correlation between RH AMJ at Mt. Tianmu and Sea Surface Temperature (SSTs) in the western equatorial Pacific and Indian Ocean. In other words, there is a positive correlation between tree-ring δ13C in Mt. Tianmu and SSTs. Both observed and reconstructed RH AMJ show significant positive correlations with East Asian and South Asian monsoons from 1951 to 2014, which indicate that RH AMJ from Mt. Tianmu reflects the variability of the Asian summer monsoon intensity to a great extent. The summer monsoon has weakened since 1960. However, an increase in relative humidity since 2003 implies a recent enhancement in the summer monsoon.

Long-term effects of drought on tree-ring growth and carbon isotope variability in Scots pine in a dry environment.

Drought frequency is increasing in many parts of the world and may enhance tree decline and mortality. The underlying physiological mechanisms are poorly understood, however, particularly regarding chronic effects of long-term drought and the response to increasing temperature and vapor pressure deficit (VPD). We combined analyses of radial growth and stable carbon isotope ratios (δ13C) in tree rings in a mature Scots pine (Pinus sylvestris L.) forest over the 20th century to elucidate causes of tree mortality in one of the driest parts of the European Alps (Pfynwald, Switzerland). We further compared trees that have recently died with living trees in a 10-year irrigation experiment, where annual precipitation was doubled. We found a sustained growth increase and immediate depletion of δ13C values for irrigated trees, indicating higher stomatal conductance and thus indeed demonstrating that water is a key limiting factor for growth. Growth of the now-dead trees started declining in the mid-1980s, when both mean temperature and VPD increased strongly. But growth of these trees was reduced to some extent already several decades earlier, while intrinsic water-use efficiency derived from δ13C values was higher. This indicates a more conservative water-use strategy compared with surviving trees, possibly at the cost of low carbon uptake and long-term reduction of the needle mass. We observed reduced climatic sensitivity of raw tree-ring δ13C for the now-dead in contrast to surviving trees, indicating impaired stomatal regulation, although this difference between the tree groups was smaller after detrending the data. Higher autocorrelation and a lower inter-annual δ13C variability of the now-dead trees further indicates a strong dependence on (low) carbon reserves. We conclude that the recent increase in atmospheric moisture demand in combination with insufficient soil water supply was the main trigger for mortality of those trees that were weakened by long-term reduced carbon uptake.

Tree-ring δ13C of archeological charcoals as indicator of past climatic seasonality. A case study from the Neolithic settlements of Lake Chalain (Jura, France)

Charcoal fragments from the Neolithic settlements of Lake Chalain (Jura Mountains, France) were characterized by dendro-anthracology (charcoal-pith distance, tree-ring width, earlywood/latewood proportion) and ring-scale isotope geochemistry (13C) to assess the relevance of this combined approach for paleoclimate reconstructions. Two differing climatic periods were investigated: (i) a climatic deterioration period characterized by cool and moist conditions and (ii) a climatic improvement period characterized by slightly less precipitation and warmer temperature. Latewood proportion in charcoal tree-rings was similar for the two studied climatic periods. However, the charcoal tree-rings exhibited width and 13C-content significantly different between the two studied periods, in agreement with previously inferred climatic difference. Monitoring ring-to-ring 13C variation within each charcoal fragment revealed no noticeable climatic trend, for none of the studied periods. However, calculation of the difference in 13C-content between earlywood and latewood of a given tree-ring suggested that the cool and moist climatic period also corresponded to higher seasonal contrast than the dryer climatic period. Although this exploratory study needs further confirmation, it opens promising developments for paleoclimatic reconstructions based on the stable carbon isotope composition of archeological charcoals: the potential for recording subtle paleoclimatic variations and seasonal contrasts.

Comparisons of radial growth and tree-ring cellulose δ13C for Pinus sylvestris var. mongolica in natural and plantation forests on sandy lands

ABSTRACTTo reveal the effects of water stress on the decline in introduced Mongolian pine (Pinus sylvestris var. mongolica) plantations, radial growth (basal area increment, BAI) and tree-ring cellulose δ13C (carbon isotope composition) were compared in an introduced Mongolian pine plantation and a natural Mongolian pine forest during 1965–2009. Results showed that the BAI of plantation trees increased until 1990, followed by an abrupt decline from 1990 to 1996 and maintained a stable level thereafter. In contrast, no sign of decreased growth was found for natural trees. Tree-ring δ13Ccorr of plantation trees, corrected for atmospheric changes in δ13C, significantly increased before and after 1990. However, no significant variations in tree-ring δ13Ccorr for natural trees before and after 1990 indicated no obviously water stress. Intrinsic water-use efficiency (iWUE, calculated from tree-ring δ13C) significantly increased before and after 1990 for both forests. Significant negative relationship between BAI and δ13Ccorr or iWUE was found for plantation trees after 1990 when precipitation, Palmer Drought Severity Index (PDSI) and groundwater level decreased, suggesting water stress impact on growth. For plantation trees, PDSI was positively and negatively correlated with BAI and δ13Ccorr, respectively, after 1990. For natural trees, BAI had no relationship with PDSI after 1990. These findings suggested that a decrease in PDSI (e.g. decreased precipitation and higher temperature) and groundwater level had made plantation trees face a more serious water stress compared with that of natural trees, which decreased tree growth. However, other stressors such as nutrient limitation may also contribute to growth decline.

Atmospheric pCO2 control on speleothem stable carbon isotope compositions

The stable carbon isotope compositions of C3 plants are controlled by the carbon isotope composition of atmospheric CO2 (δ13Ca) and by the stomatal response to water stress. These relationships permit the reconstruction of ancient environments and assessment of the water use efficiency of forests. It is currently debated whether the δ13C values of C3 plants are also controlled by atmospheric pCO2. Here I show that globally-averaged speleothem δ13C values closely track atmospheric pCO2 over the past 90 kyr. After accounting for other possible effects, this coupling is best explained by a C3 plant δ13C sensitivity of −1.6±0.3‰/100 ppmV CO2 during the Quaternary. This is consistent with 20th century European forest tree ring δ13C records, providing confidence in the result and suggesting that the modest pCO2-driven increase in water use efficiency determined for those ecosystems and simulated by land surface models accurately approximates the global average response. The δ13C signal from C3 plants is transferred to speleothems relatively rapidly. Thus, the effect of atmospheric pCO2 should be subtracted from new and existing speleothem δ13C records so that residual δ13C shifts can be interpreted in light of the other factors known to control spleleothem δ13C values. Furthermore, global average speleothem δ13C shifts may be used to develop a continuous radiometric chronology for Pleistocene atmospheric pCO2 fluctuations and, by correlation, ice core climate records.