Cellulose δ18O Research Articles

Cellulose Oxygen Isotopes of Sphagnum and Vascular Plants in a Peat Core Reveal Climate Change in Northern Japan Over the Past 2,000 Years

AbstractThe cellulose δ18O of plant tissues in peat is a potential climate proxy. However, understanding what is driving the shifts in δ18O of cellulose is required for its application. Here, we analyzed the δ18O values of Sphagnum and vascular plant cellulose, as well as the δ18O and δD values of pore water, in a 4‐m‐long peat core from the Bekanbeushi ombrotrophic bog to understand paleoclimatic changes in northern Japan over the past 2,000 years. The cellulose δ18O values of Sphagnum were lower than those of vascular plant tissues, although both draw water from the same layer. Whereas the cellulose δ18O values of Sphagnum more directly reflect those of precipitation, those of vascular plants become enriched through transpiration. Thus, the difference between vascular plants and Sphagnum (Δδ18Ovp–sp) is a potential proxy for relative humidity. Cellulose δ18O of Sphagnum revealed centennial variations with maxima around 800, 1300, and 1500 CE and minima around 500, 1000, 1700, and 1900 CE. The Δδ18Ovp–sp was inversely correlated with Sphagnum cellulose δ18O, indicating that the axis of summer westerlies was more frequently located to the north and the climate was moist in the former periods, whereas the axis of summer westerlies was generally located to the south and the climate was dry in the latter periods. These results suggest a warm and moist climate during the former periods due to frequent rainy summers driven by strong East Asian summer monsoon activity, and the opposite conditions in the latter periods.

Teak Tree-Ring Cellulose δ13C, δ18O, and Tree-Ring Width from Northwestern Thailand Capture Different Aspects of Asian Monsoon Variability

The inter-annual variability in tree-ring cellulose δ13C (δ13CTR, δ18OTR), and tree-ring chronology in teak (TRW) (Tectona grandis L.f.) trees from Northwestern Thailand during 1901–2009 AD was performed. The δ13CTR and δ18OTR have a positive correlation, significant at r =0.400, p <0.0001, and both of the stable isotopes were not significantly related to the TRW. The TRW is related to rainfall in the first half of the rainy season and has a strong relationship with the relative humidity. The δ18OTR captured moisture well throughout the rainy season, and the δ13CTR had a strong correlation with rainfall in the second half of the rainy season and had a high correlation with cloud fraction and vapor pressure. The δ13CTR and δ18OTR were associated with the stomata conductance response, but had no effect on photosynthesis. The three indices of the teak annual ring respond well to the variability in the Asian monsoon, and give us a better understanding of both the hydrological cycle and the factors that contribute to the growing of tropical broadleaf trees under changing climates.

Stable isotopes of tree rings reveal seasonal-to-decadal patterns during the emergence of a megadrought in the Southwestern US.

Recent evidence has revealed the emergence of a megadrought in southwestern North America since 2000. Megadroughts extend for at least 2 decades, making it challenging to identify such events until they are well established. Here, we examined tree-ring growth and stable isotope ratios in Pinus ponderosa at its driest niche edge to investigate whether trees growing near their aridity limit were sensitive to the megadrought climatic pre-conditions, and were capable of informing predictive efforts. During the decade before the megadrought, trees in four populations revealed increases in the cellulose δ13C content of earlywood, latewood, and false latewood, which, based on past studies are correlated with increased intrinsic water-use efficiency. However, radial growth and cellulose δ18O were not sensitive to pre-megadrought conditions. During the 2 decades preceding the megadrought, at all four sites, the changes in δ13C were caused by the high sensitivity of needle carbon and water exchange to drought trends in key winter months, and for three of the four sites during crucial summer months. Such pre-megadrought physiological sensitivity appears to be unique for trees near their arid range limit, as similar patterns were not observed in trees in ten reference sites located along a latitudinal gradient in the same megadrought domain, despite similar drying trends. Our results reveal the utility of tree-ring δ13C to reconstruct spatiotemporal patterns during the organizational phase of a megadrought, demonstrating that trees near the arid boundaries of a species' distribution might be useful in the early detection of long-lasting droughts.

Unexpected climate variability inferred from a 380-year tree-ring earlywood oxygen isotope record in the Karakoram, Northern Pakistan

To provide a long-term context for understanding the “Karakoram Anomaly” in recent glacier studies, we originally aimed to develop an annually resolved multi-century winter–spring precipitation record using tree-ring earlywood cellulose δ18O (δ18OEW) of Pinus wallachina from the Karakoram, northern Pakistan. Out of expectation, winter (January–May) temperature rather than precipitation is found to be the dominant climate signal (r = 0.63, p < 0.01, 1955–2013) stored in the δ18OEW chronology. Precipitation signals mainly appear at high-frequency variations, but weaker than temperature signal. We reconstructed January–May temperature back to 1631 with an explained variance of 39.7 % during the calibration period of 1955–2013. The reconstruction reveals that the Little Ice Age (LIA) was 0.94 °C warmer during 1647–1746 than the twentieth century (1900–1999). These warmer conditions are additionally validated by ice-core δ18O data from the Kunlun Mountains of High Asia and the northern North America. The eastward Polar Vortex and enhanced mid-latitude Westerlies on the Euro-Asia continent may be a possible explanation of a spatial coherency of LIA temperature between the Karakoram and the northern North America. Although not the original aim of this study, we provide evidence that the attribution of anomalous behavior of Karakoram glaciers in a long-term context may be misled when using precipitation reconstructions derived from tree-ring oxygen isotope.

Climate Response of Oxygen Isotopic Compositions in Tree-Ring Cellulose in Java: Evaluation Using a Proxy System Model

Tree-ring cellulose oxygen isotopic composition (δ18O) is controlled by several hydrological factors such as precipitation, relative humidity, and temperature. A proxy system model can reveal how these factors affect tree-ring cellulose δ18O. In this study, to identify a key control on tree-ring cellulose δ18O variations, we performed model calculation of year-to year variation of tree-ring cellulose δ18O of Javanese teak in Indonesia from 1960 to 1998. Our model results reasonably reproduce the observed δ18O values and their temporal variations (r = 0.6; p &lt; 0.001). Moreover, the sensitivity test shows that the cellulose δ18O values are sensitive to the teak growing period. The simulation result with earlier or later shifts of the growing period captured the amplitude of observed δ18O variations over 39 years. These results indicate that the tree-ring cellulose δ18O of Javanese teak might be influenced by a subtle shift of the intra-annual growing period.

A July-August relative humidity record in North China since 1765 AD reconstructed from tree-ring cellulose δ18O.

Since the late 1970s, East Asian summer monsoon (EASM) has shown a significant weakening trend, and sustained drought has occurred across North China. Placing recent climate changes in the paleoclimatic context can better understand the EASM variations. Four δ18O sequences based on tree-ring cellulose of Chinese pine were developed from Mt. Beiwudang, North China, covering a period from 1700 to 2013. Based on a climatic response analysis, a transfer function was designed to reconstruct the relative humidity from July to August (RHJA hereafter). The RHJA spans from 1765 to 2013 and explains 49% (R2adj = 48%) of the instrumental variance during the calibration period (1961-2013, r = - 0.70, p < 0.0001). The RHJA is mainly influenced by precipitation in the summer rainy season and reflect EASM variations. Spatial representation analysis indicates that RHJA represents the dry/wet variations across North China. At the interannual scale, RHJA records many extreme dry/wet events, among which the events in 1876-1878, 1900, and the 1920s are extensive droughts. Those events correspond well to ENSO events, plus further correlation and periodicity analysis indicate that RHJA contains ENSO signals. At the interdecadal scale, RHJA shows a decreasing trend and unprecedented low values from 1981 to 2013, suggesting that the weakening of EASM since the late 1970s is unprecedented in the past 249 years. Similarly, the significantly correlating region in the spatial correlation analysis, covering the Meiyu/Baiu/Changma rainfall belt and India, have also undergone a climatic shift since the late 1970s according to previous papers.

A 2600-year summer climate reconstruction in central Japan by integrating tree-ring stable oxygen and hydrogen isotopes

Abstract. Oxygen isotope ratios (δ18O) of tree-ring cellulose are a novel proxy for summer hydroclimate in monsoonal Asia. In central Japan, we collected 67 conifer wood samples, mainly Chamaecyparis obtusa, with ages encompassing the past 2600 years. The samples were taken from living trees, archeological wood, architectural wood, and buried logs. We analyzed stable isotope ratios of oxygen (δ18O) and hydrogen (δ2H) in tree-ring cellulose in these samples (more than 15 000 rings in total) without using a pooling method and constructed a statistically reliable tree-ring cellulose δ18O time series for the past 2500 years. However, there were distinct age trends and level offsets in the δ18O record, and cellulose δ18O values showed a gradual decrease as an individual tree matures. This suggested it is difficult to establish a cellulose δ18O chronology for low-frequency signals by simple averaging of all the δ18O time series data. In addition, there were opposite age trends in the cellulose δ2H, and δ2H gradually increased with tree age. There were clear positive correlations in the short-periodicity variations between δ18O and δ2H, probably indicating a common climate signal. A comparison of the δ18O and δ2H time series in individual trees with tree-ring width suggested that the opposite age trends of δ18O and δ2H are caused by temporal changes in the degree of post-photosynthetic isotope exchange with xylem water (physiological effect), accompanied by changes in stem growth rate that are influenced by human activity in the forests of central Japan. Based on the assumptions that cellulose δ18O and δ2H vary positively and negatively with constant proportional coefficients due to climatological and physiological effects, respectively, we solved simultaneous equations for the climatological and physiological components of variations in tree-ring cellulose δ18O and δ2H in order to remove the age trend. This enabled us to evaluate the climatic record from cellulose δ18O variations. The extracted climatological component in the cellulose δ18O for the past 2600 years in central Japan was well correlated with numerous instrumental, historical, and paleoclimatological records of past summer climate at various spatial and temporal scales. This indicates that integration of tree-ring cellulose δ18O and δ2H data is a promising method to reconstruct past summer climate variations on annual to millennial timescales, irrespective of the growth environment. However, analytical and statistical methods need to be improved for further development of this climate proxy.

Tree-ring δ18O from Southeast China reveals monsoon precipitation and ENSO variability

Considerable advances have been made in use of tree-ring δ18O for climate reconstructions in Southeast China, the East Asian summer monsoon region. However, the relationships among tree-ring cellulose δ18O (δ18Ocell), local hydroclimate, precipitation δ18O (δ18Opre) and El Niño–Southern Oscillation (ENSO) have not yet been fully resolved. The usual interpretation has been that local hydroclimate influences δ18Ocell through both the “amount effect” on δ18Opre and evaporative enrichment, but it cannot fully explain the high inter-site correlation of δ18Ocell chronologies or their response pattern to ENSO. In this study, we use a newly-developed δ18Ocell chronology of Pinus massoniana from a water-stressed site in Zhejiang province, in combination with another three δ18Ocell chronologies in Southeast China, to investigate their climatic implications from a regional perspective. The results show that besides local hydroclimate, δ18Opre is also significantly correlated with δ18Ocell, but with different effects. Spatially homogeneous δ18Opre variation causes high spatial correlations among δ18Ocell from different sites. Analyses show that ENSO variations are responsible for the large-scale common signals in δ18Ocell by modulating δ18Opre. Therefore, combining δ18Ocell chronologies from different locations can enhance ENSO signals, which provides us new opportunities to reconstruct paleo-ENSO activities. Furthermore, eliminating the part of ENSO signal that is not related to the growing season hydroclimate from δ18Ocell can increase the explained variance of observed values. These findings provide a guide for future optimized ENSO and local paleoclimate reconstructions using δ18Ocell.

Temperature-Dependent Oxygen Isotope Fractionation in Plant Cellulose Biosynthesis Revealed by a Global Dataset of Peat Mosses

The oxygen isotope composition (δ18O) of plant cellulose has been widely used to study ecohydrological processes of ecosystems as well as to reconstruct past climate conditions in terrestrial climate archives. These applications are grounded on a key assumption that the biochemical fractionation during cellulose synthesis is a constant around +27‰ and is not affected by environmental factors. Here we revisit the influence of temperature on biochemical fractionation factor during cellulose synthesis using a global compilation of Sphagnum cellulose δ18O data. Sphagnum (peat mosses) are known for inhabiting waterlogged peatlands and possessing unique physiological strategy in that their cellulose δ18O could closely reflect growing-season precipitation δ18O. Although within-site cellulose δ18O variability shows a median standard deviation of 0.7–0.8‰ resulting from different degree of evaporative enrichment of 18O in metabolic leaf water, this evaporative enrichment is a small quantity due to the external capillary “water buffer” in Sphagnum mosses. Using site-specific minimum cellulose δ18O data that most likely reflect the signal of unevaporated source water, we show that the apparent enrichment factor between cellulose and precipitation δ18O increases with decreasing air temperature. In particular, the apparent enrichment factor could reach as high as 32‰ when growth temperature is below 5 °C. This observational dataset extends the support for the temperature-dependent oxygen isotope fractionation in plant cellulose synthesis previously demonstrated in laboratory experiment, with implications for paleoclimate and plant physiology studies that employ cellulose δ18O measurements particularly in alpine regions.

Age‐Related Climate Response of Tree‐Ring δ13C and δ18O From Spruce in Northwestern China, With Implications for Relative Humidity Reconstructions

AbstractUnderstanding varying climate responses in tree‐ring data across tree ages is important, but little is known about tree‐age effects on climate responses in tree‐ring stable isotopes. To detect whether age differences in tree‐ring δ13C and δ18O could lead to differing climate responses, we measured tree‐ring cellulose δ13C and δ18O (1901–2010) from Schrenk spruce (Picea schrenkiana) trees in northwestern China with ages ranging from 110 to 470 years, which we binned into three age groups. Tree‐ring δ13C (pin‐corrected) and δ18O exhibited similar year‐to‐year variability between age groups and did not feature age‐related trends. δ13C series from old trees (270–470 years) showed stronger legacy effects, reflecting influences from the antecedent period (due to carbohydrate reserves and climate), compared to young trees (110–125 years). Both tree‐ring δ13C and δ18O values decreased with increasing relative humidity (RH) and precipitation and with decreasing mean and maximum temperatures during the main growing season (May–August). δ13C and δ18O exhibited age‐dependent climate responses: Young trees had a stronger climate response in δ13C but a weaker or similar climate response in δ18O compared to old trees. We developed multiple growing‐season RH reconstructions based on composite chronologies using δ13C and δ18O series from different age groups. In particular, we found that including δ13C from young trees improved the skill of RH reconstructions because of the age‐specific mechanisms driving the δ13C‐climate relationship, but that caution is warranted with regard to extreme values. We therefore suggest that young trees should be considered when using stable isotopes, particularly in δ13C, for climate reconstruction.

A tree-ring δ18O based reconstruction of East Asia summer monsoon over the past two centuries.

The East Asian summer monsoon (EASM) exhibits considerable decadal variations since the late 20th century. Efforts to examine long-term behaviors and dynamics of the EASM are impeded largely due to the shortness of instrumental meteorological records. So far, reconstructions of the EASM with annual resolution from its core regions remain limited. We conduct the first 200-year robust EASM reconstruction based on tree-ring cellulose δ18O records derived from Pinus massoniana trees growing in the middle Yangtze River basin, one of the core EASM areas. The δ18O chronology accounts for 46.2% of the actual variation in an index of the EASM from 1948 to 2014. The reconstructed EASM indicates that the monsoon intensity was below average before the 1950s, peaked in the 1950s-1970s, and then began to decline. The reconstructed EASM is negatively correlated with the El Niño-Southern Oscillation (ENSO), but this teleconnection is dynamic through time, i.e. enhanced (reduced) ENSO variability coheres with strong (weak) EASM-ENSO connections. In addition, despite high ENSO variability since the 1980s, the EASM-ENSO relationship weakened possibly due to anthropogenic impact, particularly aerosol emissions.

Compression wood has a minor effect on the climate signal in tree-ring stable isotope records of montane Norway spruce.

Compression wood (CW) is a common tissue present in the trunk, branches and roots of mechanically stressed coniferous trees. Its main role is to increase the mechanical strength and regain the vertical orientation of a leaning stem. Compression wood is thought to influence the climate signal in different tree-ring measures. Hence trees containing CW are mostly excluded from tree-ring studies reconstructing past climate variability. There is a large gap of systematic work testing the potential effect of CW on the strength of the climate signal in different tree-ring parameters, especially stable isotope records. Here we test for the first time the effect of CW contained in montane Norway spruce (Picea abies L. Karst) on both δ13C and δ18O tree-ring cellulose records by analyzing compression and opposite wood radii from several disturbed trees together with samples from undisturbed reference trees. We selected four trees tilted by geomorphic processes that were felled by wind and four undisturbed reference trees in the Tatra Mountains, Poland. We qualitatively classified the strength of CW using wood cell anatomical characteristics (tracheid shape, cell wall thickness and presence of intercellular spaces). Then we developed tree-ring width and δ13C and δ18O chronologies from the CW radii, from the opposite radii of the tilted trees and from the reference radii. We tested the effect of CW on tree-ring cellulose δ13C and δ18O variability and on the climate signal strength. We found only minor differences in the means of δ13C and δ18O of compression (δ13C: -22.81‰, δ18O: 28.29‰), opposite (δ13C: -23.02‰; δ18O: 28.05‰) and reference (δ13C: -22.78‰; δ18O: 27.61‰) radii. The statistical relationships between climate variables, δ13C and δ18O, remained consistent among all chronologies. Our findings suggest that moderately tilted trees containing CW can be used to reconstruct past geomorphic activity and for stable isotope-based dendroclimatology.

Environmental controls on the carbon and water (H and O) isotopes in peatland Sphagnum mosses

We conducted a modern process study on Sphagnum magellanicum from southern Patagonian peat bogs to improve our understanding of environmental controls on the carbon and water (hydrogen and oxygen) isotope compositions of peat mosses. We found that moisture availability gradients in peat bogs, measured by Sphagnum water content, could explain the intra-site variability in both Sphagnum cellulose δ13C and δ18O. Also, there was a site-specific significant negative correlation between cellulose δ13C and δ18O across microtopographical gradient. This new finding suggests that Sphagnum moisture availability influences cellulose δ13C via water film effect on discrimination against 13CO2 and, similarly, can imprint on cellulose δ18O via evaporative enrichment of 18O in metabolic leaf water. Sphagnum leaf wax n-alkane δ13C also responded to the water film effect as cellulose did, but n-alkane δ2H appears less sensitive than cellulose δ18O to the moisture availability gradient likely because hydrogen isotopes have a more complex biochemical fractionation pathway and a smaller kinetic fractionation during leaf water evaporation. Furthermore, we used long strands of Sphagnum to explore if isotopic signals in moss strand increments were sensitive to recent growing season conditions. We demonstrated that rapidly growing Sphagnum strand increments could document the isotopic composition of precipitation and moisture conditions at sub-annual scale. Altogether, these findings highlight the sensitivity of stable isotopes in Sphagnum to environmental conditions. On the basis of these results, we propose that paired measurements of carbon and water isotopes in Sphagnum cellulose or leaf wax biomarker provide an improved approach in peat-based paleoclimate reconstructions.

Eco-Physiological Response of Conifers from High-Latitude and -Altitude Eurasian Regions to Stratospheric Volcanic Eruptions

Stratospheric volcanic eruptions have had significant impacts on the radiation budget, atmospheric and surface temperatures, precipitation and regional weather patterns, resulting in global climatic changes. The changes associated with such eruptions most commonly result in cooling during several years after events. This study aimed to reveal eco-physiological response of larch trees from northeastern Yakutia (YAK), eastern Taimyr (TAY) and Altai (ALT) regions to climatic anomalies after major volcanic eruptions CE 535, 540, 1257, 1641, 1815 and 1991 using new multiple tree-ring parameters: tree-ring width (TRW), maximum latewood density (MXD), cell wall thicknesses (CWT), δ13C and δ18O in tree-ring cellulose. This investigation showed that TRW, CWT, MXD and δ18O chronologies recorded temperature signal, while information about precipitation and vapor pressure deficit was captured by δ13C chronologies. Sunshine duration was well recorded in δ18O from YAK and ALT. Tree-ring parameters recorded cold, wet and cloudy summer anomalies during the 6th and 13th centuries. However, significant summer anomalies after Tambora (1815) and Pinatubo (1991) eruptions were not captured by any tree-ring parameters

Climate variability over the past 100 years in Myanmar derived from tree-ring stable oxygen isotope variations in Teak

We present a 100-year oxygen isotope record from teak tree-ring cellulose (δ18O), originating from a site in southern Myanmar that preserves the isotopic ratios of the regional wet season’s rainfall. Tree-ring δ18O correlates strongly with regional rainfall during the months of May to October (r = − 0.353, p < 0.01). We found the tree-ring δ18O had clear signals of the June to September Indian Summer Monsoon (ISM) over the years 1948–1998 (r = − 0.53, p < 0.01). The δ18O has a significant and negative correlation with the minimum temperature in September and has a significant positive correlation with maximum temperature in November and December. The study found that δ18O has a significant positive correlation with the difference between the maximum temperature and the minimum temperature (DTR) in August to October. Based on our results, it can be concluded that tree-ring δ18O in teak in southern Myanmar is controlled by the amount of rainfall during the monsoon season and the temperature in the November and December. Spatial correlation and spectral analyses revealed a strong impact of the El Nino-Southern Oscillation (ENSO) on tree-ring δ18O of teak. In addition, tree-ring δ18O also captures the signal of the Indian Ocean Dipole (IOD).

Effect of changes in precipitation amounts and moisture sources on inter- and intra-annual stable oxygen isotope ratios (δ18O) of teak trees from northern Thailand

Thailand is situated in a transition zone of overlap between the Indian monsoon (IM) and the western North Pacific (WNP) monsoon. The region serves as an important area to study the influences of different moisture sources. This study presents the first stable oxygen isotope analysis on seven teak (Tectona grandis Linn.) trees collected from Phrae Province in northern Thailand at different temporal resolutions. Isotopic analyses of teak tree rings at both annual and sub-annual timescales reflected the variations in their source water oxygen isotope ratios (δ18O, i.e., rainfall) to different degrees (Annual scale: r = 0.31, p < 0.05, Monthly scale: r = 0.63, p < 0.01). A 146-year-long annually resolved tree-ring cellulose δ18O series, spanning between AD 1871 and 2016, exhibited moderate negative relationships with both local (r = −0.58, p < 0.01) and regional rainfall amounts, representing the climatology of rainfall totals during the entire summer monsoon period from May to October. A spatial correlation analysis between monthly resolved tree-ring cellulose δ18O with large-scale Climate Prediction Center (CPC), Outgoing Longwave Radiation (OLR), and Merged Analysis of Precipitation (CMAP) datasets, as well as an air mass backward trajectory analysis, indicated that the intraseasonal tree-ring cellulose δ18O values reflected changes in moisture originating from the Indian and Pacific Oceans, rather than the amount of local precipitation. These results imply that variations in northern Thai teak cellulose δ18O values are modulated not only by the local rainfall amount but also by large-scale convection, which varies between different seasons and over time. Advance knowledge of the regional monsoons in the transition area can therefore lead to a broad and complete understanding of the entire Asian monsoon circulation.

Integrated metabolic strategy: A framework for predicting the evolution of carbon‐water tradeoffs within plant clades

Abstract The fundamental tradeoff between carbon gain and water loss has long been predicted as an evolutionary driver of plant strategies across environments. Nonetheless, challenges in measuring carbon gain and water loss in ways that integrate over leaf lifetime have limited our understanding of the variation in and mechanistic bases of this tradeoff. Furthermore, the microevolution of plant traits within species versus the macroevolution of strategies among closely related species may not be the same, and accordingly, the latter must be addressed using comparative phylogenetic analyses. Here we introduce the concept of ‘integrated metabolic strategy’ (IMS) to describe the ratio between carbon isotope composition (δ13C) and oxygen isotope composition above source water (Δ18O) of leaf cellulose. IMS is a measure of leaf‐level conditions that integrate several mechanisms contributing to carbon gain (δ13C) and water loss (Δ18O) over leaf lifespan, with larger values reflecting higher metabolic efficiency and hence less of a tradeoff. We tested how IMS evolves among closely related yet ecologically diverse milkweed species, and subsequently addressed phenotypic plasticity in response to water availability in species with divergent IMS. Integrated metabolic strategy varied strongly among 20 Asclepias species when grown under controlled conditions, and phylogenetic analyses demonstrate species‐specific tradeoffs between carbon gain and water loss. Larger IMS values were associated with species from dry habitats, with larger carboxylation capacity, smaller stomatal conductance and smaller leaves; smaller IMS was associated with wet habitats, smaller carboxylation capacity, larger stomatal conductance and larger leaves. The evolution of IMS was dominated by changes in species’ demand for carbon (δ13C) more so than water conservation (Δ18O). Although some individual physiological traits showed phylogenetic signal, IMS did not. In response to experimental decreases in soil moisture, three species maintained similar IMS across levels of water availability because of proportional increases in δ13C and Δ18O (or little change in either), while one species increased IMS due to disproportional changes in δ13C relative to Δ18O. Synthesis. IMS is a broadly applicable mechanistic tool; IMS variation among and within species may shed light on unresolved questions relating to the evolution and ecology of plant ecophysiological strategies.

Stable Oxygen Isotope Records (δ18O) of a High-Andean Cushion Peatland in NW Argentina (24° S) Imply South American Summer Monsoon Related Moisture Changes During the Late Holocene

High-elevation cushion peatlands are promising archives for paleoenvironmental studies in their extreme habitat of the Central Andean highlands between ~4000 and 4800m a.s.l. The Cerro Tuzgle cushion peatland (CTP, 24°09’ S, 66°24’ W), located in the NW Argentine Andes, is formed by the vascular cushion plants Oxychloe andina (O. andina) and Zameioscirpus muticus (Z. muticus). To extend the knowledge base on the modern ecology of these peatlands, we investigated the stable isotope composition of bulk material and cellulose (δ18O, δ13C, δ15N) of the dominant cushion-forming species O. andina (Juncaceae) and Z. muticus (Cyperaceae) as well as water samples (δ18O, δ2H) of several pools interspersed within the peatland. We further applied a multiproxy approach for a peat core from CTP spanning the last 2900 years with XRF scanning, bulk geochemistry and stable isotope analyses on bulk peat and cellulose size fractions. Modern samples of O. andina and Z. muticus expose significant differences in cellulose δ18O e.g. between leaves and rhizomes of O. andina (Δδ18Ol-r = 4.11‰) and between leaves of O. andina and Z. muticus (Δδ18Ol-l = 2.8‰). Modern water samples exhibit strong isotopic differences between single water pools (max. Δδ18O = 13.09‰) due to local variable evaporative enrichment. Within the peat core, we observe considerable multi-centennial variations in δ18O composition of cellulose confirmed by all size fractions. Based on the regional relation between decreasing δ18Oprec values with increasing precipitation amounts and 18O enrichment in the peatland waters due to evaporation, we suggest an interpretation of our δ18O cellulose record as moisture proxy for CTP. This interpretation is corroborated by a high correlation between oxygen isotopes, peat growth and geochemical data. Accordingly, CTP indicates dryer conditions between 2190 and 2120, 1750 and 1590, 1200 and 1080 and since 130 cal. yr BP, whereas periods with increased humidity prevailed from 2750 to 2250 and from 600 to 130 cal. yr BP. Temporal changes in the match to South American Summer Monsoon (SASM) reconstructions suggest impacts of other large-scale atmospheric variability modes or a different SASM expression at our southerly location.

Tree-ring δ18O from an Alpine catchment reveals changes in glacier stream water inputs between 1980 and 2010

ABSTRACTThe tree-ring stable carbon and oxygen isotope chronologies from two forest sites located in the Forni Glacier forefield (Italy)—one along the glacier stream (GL) and the other toward the valley slope (SL)—were analyzed with the aim of disentangling the precipitation and glacier meltwater inputs in source water δ18O, as reflected by the tree-ring cellulose δ18O. The cellulose δ18O from the GL trees has a negative correlation with winter and summer temperatures, whereas the cellulose δ18O from the SL trees has a positive correlation with precipitation δ18O. The isotopic signature of the source water at the GL site is also influenced by waters of glacial origin, as confirmed by the 18O-depleted glacier meltwater inputs (GMWI_δ18O) estimated by means of an isotope model. The GMWI_δ18O values are consistent with the mean difference measured between the δ18O in the glacier stream and in the precipitation and the winter and summer temperature explains up to 37 percent of the GMWI_δ18O variance. Our results show an increasing influence of glacier meltwater throughout the past decade for the GL site. Our analysis opens new opportunities to reconstruct changes in water regimes of the glacier streams by means of the tree-ring cellulose δ18O.

East Asian Summer Monsoon moisture sustains summer relative humidity in the southwestern Gobi Desert, China: evidence from δ18O of tree rings

The dominant climate-controlling system of the southwestern Gobi Desert and its surrounding areas in China at interannual to centennial time scale has not been determined. It is necessary to improve our understanding of the humidity variations in the past and forcing mechanisms in the southwestern Gobi Desert to enhance the accuracy of climate model predictions and cope with future desert advance/retreat crises resulting from climate change. We measured the annual stable oxygen isotope ratio (δ18O) in the cellulose of four Picea crassifolia trees growing on the fringe of the southwestern Gobi Desert from 1806 to 2011. Our tree-ring cellulose δ18O values (34.49‰) were considerably higher than those reported in other regions of Asia, suggesting that the southwestern Gobi Desert has been extremely arid over the past two centuries. The chronology of tree-ring cellulose δ18O is significantly (p < 0.01) negatively correlated with summer (June, July, and August) relative humidity (RH) records from 12 meteorological stations nearby our study region (Table 1). Based on the high correlation (r = − 0.740, p < 0.0001, n = 49) between tree-ring cellulose δ18O and instrumental summer RH from meteorological stations nearby our study region, we reconstructed summer RH variations during the past 206 years. Spatial correlation patterns between our reconstruction and several hydroclimate-related gridded data sets indicate that the reconstruction is representative of hydroclimate variations over a large area. Close agreement was observed between our reconstruction and several East Asian Summer Monsoon-related hydroclimate records. Our study indicates that the moisture from the east has introduced the RH to the southwestern Gobi Desert during the summer months over the past two centuries.