Tree-ring Stable Isotope Research Articles

Impact of the North Atlantic and the Indian Ocean on the summer hydroclimate over East Central Asia: A case study in the central Tianshan Mountains

Observations have shown that the hydroclimatic variations in East Central Asia (ECA) are a result of the interaction of high-, middle- and low-latitude atmospheric circulation systems. This study aims to explore the potential mechanisms of hydroclimate variability in ECA over a longer timescale than in previous studies. Based on tree-ring stable oxygen isotope (δ 18 O), we reconstructed the May–July relative humidity variations in the central Tianshan Mountains from 1661 to 2016, which can represent the dry/wet variations over a large region of ECA. The reconstruction revealed several dry periods that occurred in 1674–1691, 1705–1740, 1750–1770, 1806–1826, the 1940s, and 1960–1980. The reconstructed relative humidity presented an obvious upward trajectory from the 18th to the early 20th century and humidification characteristics in the mid- to late 1980s. The summer North Atlantic Oscillation (SNAO) had a nonlinear impact on the hydroclimate of ECA by modulating the position of the subtropical westerly jet stream (SWJS). Positive sea surface temperature anomalies in the tropical Indian Ocean (TIO), which may enhance the cross-equatorial jet, transport additional water vapour into ECA, and the South Asian summer monsoon (SASM) provides favourable circulation conditions for water vapour transport. Summer hydroclimatic variations in ECA are jointly affected by the North Atlantic and the TIO. The impact of the TIO on the hydroclimate in ECA cannot be ignored. • Summer relative humidity in ECA was reconstructed by tree-ring δ 18 O during the past three centuries. • Summer hydroclimatic variations in ECA were jointly affected by the North Atlantic and the Indian Ocean. • The negative phase of the SNAO favours the transport of southerly water vapour to ECA through the SWJS. • Summer hydroclimatic variations in ECA correspond well with the climate variability of tropical Indian Ocean.

Responses of Tree Growth and Intrinsic Water Use Efficiency to Environmental Factors in Central and Northern China in the Context of Global Warming

The Loess Plateau is a fragile ecological zone that is sensitive to climate change. The response, adaptation, and feedback of tree growth in forest ecosystems to global warming and CO2 enrichment are urgent scientific issues. Intrinsic water use efficiency (iWUE) is an important indicator for understanding forest ecosystem adaptability to climate change and CO2 enrichment. In this study, tree-ring width, tree-ring stable carbon isotope ratio (δ13C), and iWUE of P. tabulaeformis Carr. were established. Climate response analysis showed that temperature was the main limiting factor affecting radial tree growth and that relative humidity significantly affected the stable carbon isotope fractionation of tree rings. During 1645–2011, the iWUE increased by 27.1%. The responses of iWUE to climate factors and atmospheric CO2 concentrations (Ca) showed that the long-term variation in iWUE was affected by Ca, which could explain 69% of iWUE variation, and temperature was the main factor causing iWUE interannual variation. The ecosystem of P. tabulaeformis showed a positive response to rising Ca, as its carbon sequestration capacity increased. In response to global warming and CO2 enrichment, rising Ca promoted increases in iWUE but ultimately failed to offset the negative impact of warming on tree growth in the study area.

Isotopic Characterization of Cedrela to Verify Species and Regional Provenance of Bolivian Timber

With increasing concerns about sustainable exploitation of tropical timber, there is a need for developing independent tools to check their origin. We evaluated the potential of tree-ring stable isotopes for identifying four Cedrela species (C. balansae, C. fissilis, C. odorata, and C. saltensis) and for identifying geographic origin of C. fissilis and C. odorata, two of the most intensively exploited species. We studied differences in δ13C and δ18O of wood among 11 forest sites (163 trees). We quantified isotope composition of 10-year bulk samples, and for a subset we also evaluated isotopic annual fluctuations for the last 10 years. Although annual isotopic variability was not correlated to precipitation or elevation, we found a significant relationship between the 10-year bulk stable-isotope composition and average precipitation and elevation. However these relationships were not consistent across all sites. We also explored isotopic site and species differentiation using Kernel Discriminant Analyses. Site discrimination was low: 30% accuracy for C. odorata, and 40% for C. fissilis sites. However, species discrimination was 57.5% for C. odorata and 95.3% for C. fissilis. These results suggest that although δ13C and δ18O isotopic analyses hold potential to verify species identification, discrimination of geographical origin within a country may still be challenging.

Best Practices for using tree-ring stable isotopes of Carbon in paleoclimate and ecophysiology.

Best Practices for using tree-ring stable isotopes of Carbon in paleoclimate and ecophysiology.

Towards the Third Millennium Changes in Siberian Triple Tree-Ring Stable Isotopes

Significant air temperature and precipitation changes have occurred since the 2000s in vulnerable Siberian subarctic regions and urged updates of available chronologies towards the third millennium. It is important to better understand recent climatic changes compared to the past decades, centuries and even millennia. In this study, we present the first annually resolved triple tree-ring isotope dataset (δ13C, δ18O and δ2H) for the eastern part of the Taimyr Peninsula (TAY) and northeastern Yakutia (YAK) from 1900 to 2021. We found that the novel and largely unexplored δ2H of larch tree-ring cellulose was linked significantly with δ18O for the YAK site, which was affected by averaged April–June air temperatures and evaporation. Simulated by the Land Surface Processes and Exchanges (LPX-Bern 1.0) model, the water fraction per year for soil depths at 0–20 and 20–30 cm was significantly linked with the new eco-hydrological tree-ring δ2H data. Our results suggest increasing evapotranspiration and response of trees’ water relation to rising thaw water uptake from lower (20–30 cm) soil depth. A positive effect of July air temperature on tree-ring δ18O and a negative impact of July precipitation were found, indicating dry conditions. The δ13C in larch tree-ring cellulose for both sites showed negative correlations with July precipitation and relative humidity, confirming dry environmental conditions towards the third millennium.

Investigation of age trends in tree-ring stable carbon and oxygen isotopes from northern Fennoscandia over the past millennium

Although tree-ring stable carbon (δ13C) and oxygen (δ18O) isotopes are increasingly used for climate reconstructions, it remains unclear whether isotopic ratios from the two chemical elements and different tree species exhibit age-related trends that require removal prior to any paleoclimatic interpretation. Here, we present 2,355 δ13C and 2,237 δ18O decadal measurements of living and relict Scots pines (Pinus sylvestris L.) from northern Fennoscandia to investigate the occurrence of isotope-specific age trends at both the individual tree and chronology level between 941 and 2010 CE, together with total-ring width and maximum density data. We show that δ13C values increase by ∼0.035‰ per 10 years of tree age, and therefore require detrending, which is not the case for δ18O that only contains minor changes related to age. This study provides independent evidence for the unique paleoclimatic value of stable δ18O isotopic ratios from the cellulose of living and relict pine wood to reconstruct high-to low-frequency climate variability. Conversely, caution is advised when information from diverse tree-ring parameters, species and regions is combined in multi-proxy climate reconstructions.

Tree-Ring Stable Carbon Isotope as a Proxy for Hydroclimate Variations in Semi-Arid Regions of North-Central China

Carbon and oxygen isotope ratios (δ13C and δ18O) were measured in annual tree-ring cellulose samples dated from 1756 to 2015 CE. These samples were extracted from Chinese pine (Pinus tabulaeformis Carr.) trees located in a semi-arid region of north-central China. We found that tree-ring δ13C and δ18O values both recorded similar climatic signals (e.g., temperature and moisture changes), but found that tree-ring δ13C exhibited a stronger relationship with mean temperature, precipitation, average relative humidity, self-calibrating Palmer drought severity index (scPDSI), and standard precipitation evaporation index (SPEI) than δ18O during the period 1951–2015 CE. The strongest correlation observed was between tree-ring δ13C and scPDSI (previous June to current May), which explains ~43% of the variance. The resulting 130-year reconstruction reveals severe drought events in the 1920s and a sustained drying trend since the 1980s. This hydroclimate record based on tree-ring δ13C data also reveals similar dry and wet events to other proxy data (i.e., tree-ring width and historical documentation) that have allowed reconstructions to be made across the northern fringe of the Asian summer monsoon region. Our results suggest that both large-scale modes of climate variability (e.g., El Niño-Southern Oscillation, Pacific Decadal Oscillation, and North Atlantic Oscillation) and external forcing (e.g., solar variability) may have modulated moisture variability in this region. Our results imply that the relationship between tree-ring δ18O and local climate is less well-characterized when compared to δ13C and may be affected more strongly by the influences of these different atmospheric circulation patterns. In this semi-arid region, tree-ring δ13C appears to represent a better tool with which to investigate historical moisture changes (scPDSI).

Climate Impacts on Tree-Ring Stable Isotopes Across the Northern Hemispheric Boreal Zone

Climate Impacts on Tree-Ring Stable Isotopes Across the Northern Hemispheric Boreal Zone

Tree-ring stable isotopes indicate mass wasting processes at Radicofani in the upper Orcia Valley (Tuscany, Italy)

Tree-ring carbon (C) and oxygen (O) stable isotope (SI) chronologies spanning the period 1983–2012 were analysed at three Pinus nigra Arn. sites located in the upper Orcia Valley (Tuscany, Italy) in a badlands landscape. The goal of the study was to determine the extent to which tree-ring stable isotopes (SI) can serve as a proxy for mass wasting processes. To this end, we applied an established dual-isotope model to detect physiological changes between trees growing at three sites, one along the upper border of a well-studied shallow landslide, one on the landslide, and one in a stable area, as control. We further analysed whether trees at the three sites showed different δ18O responses to climate and to precipitation δ18O. Tree-ring δ13C and δ18O variations and trends revealed impairments of the photosynthetic process at the landslide site. We found that trees growing on the landslide show signs of reduced photosynthetic capacity since the onset of the landslide in 1993, whereas since 2000, while producing compression wood during periods of landslide activity, the trees show trends of higher average maximum net photosynthesis. The correlation analysis performed between the SI chronologies and the climatic variables revealed that the climatic signals at the site located on the landslide are masked by growth stress induced by the mass wasting processes. The most distinct differences in climate responses between sites were found in tree-ring δ13C in response to mean temperature and to mean temperature range, and in tree-ring δ18O in response to precipitation δ18O. Our research confirms that it is possible to reconstruct mass-wasting processes on forested slopes and to date geomorphological events by considering the trees' physiological conditions as recorded by stable C and O isotopes in tree rings, and by comparing affected with unaffected sites.

Short-term resin tapping activities had a minor influence on physiological responses recorded in the tree-ring isotopes of Chinese pine (Pinus tabuliformis)

Resin tapping might affect tree-ring growth, but details on the physiological responses of trees to resin tapping are still lacking, particularly for long-term responses. This study aimed to explore the physiological processes underlying resin-tapping of Chinese pine (Pinus tabuliformis) by using tree-ring stable isotopes. We compared tree-ring earlywood and latewood stable carbon (δ13C) and oxygen (δ18O) isotopes in the pre-resin tapping and post-resin tapping period for tapped trees and compared their values between tapped and untapped trees and their responses to climate variables in a forest stand from 1984 to 2017. Furthermore, we used a dual isotope model to distinguish between the effects of the photosynthetic assimilation rate and stomatal conductance. Results indicated that tapped and untapped trees showed similar inter-annual variation for two isotopes, while the absolute values of tapped trees were slightly (P > 0.05) lower than tapped trees in the two years following resin tapping. Climate response analysis indicated that resin tapping had no significant effect on climatic sensitivity for either stable isotope. Earlywood stable isotopes were mainly influenced by temperature, relative humidity, and Palmer Drought Severity Index (PDSI) from May to July, while latewood isotopes were mainly influence by relative humidity form July to August and PDSI from July to September. The conceptual model results indicated that resin tapping lead to a slight, but not significant, decrease in the intrinsic water-use efficiency caused by increased stomatal conductance for the first two to three years following resin tapping. We conclude that tree-ring physiological responses could be less affected by short-term resin tapping activities.

Wood anatomy and tree-ring stable isotopes indicate a recent decline in water-use efficiency in the desert tree Moringa peregrina.

The ability of desert plants to adapt to future climate changes and maximize their water-use efficiency will determine their survival. This study uses wood anatomy and δ13C and δ18O isotope analyses to investigate how Moringa peregrina trees in the Egyptian desert have responded to the environment over the last 10years. Our results show that M. peregrina tree-ring widths (TRWs) have generally declined over the last decade, although individual series are characterized by high variability and low Rbars. Vessel lumen area percentages (VLA%) are low in wet years but increase significantly in dry years, such as the period 2017-2020. Stable δ13C isotope values decrease between 2010 (- 23.4‰) and 2020 (- 24.9‰), reflecting an unexpected response to an increase in drought conditions. The mean δ18O value (± standard error, SE) for the first ten rings of each tree from bark to pith (2020-2010) is 33.0 ‰ ± 0.85 with a range of 29.2-36.3‰, which indicates a common drought signal. The intrinsic water-use efficiency (iWUE) declines gradually with time, from 130.0µmolmol-1 in 2010 to 119.4µmolmol-1 in 2020. The intercellular carbon concentration (Ci) and Ci/Ca ratio increase over the same period, likely as a result of decreasing iWUE. The results show that M. peregrina trees seem to cool their leaves and the boundary air at the cost of saving water.

Intra-annual high resolution stable isotope records of tree ring: Methods, progress and prospects

Stable isotope ratios of tree ring can effectively record climate and environmental changes during tree growth and the physiological responses of trees to such changes. Intra-annual high resolution stable isotope ratios of tree ring can provide more detailed climatic and environmental information, reveal the physiological and ecological response mechanism of trees to seasonal climatic variation, and thus with great potential in the study of paleoclimate and global change ecology. Based on the intra-annual high resolution stable isotope ratios related literature since 1990, we reviewed the research progress of intra-annual high resolution tree ring stable isotope records in the aspects of sample stripping method, chemical extraction method of α-cellulose and application, and further proposed the potential and future development direction of intra-annual high resolution tree ring stable isotope records.

Response of Temperature-limited Forests to Recent Moisture Changes Derived from Tree-ring Stable Carbon Isotopes

Response of Temperature-limited Forests to Recent Moisture Changes Derived from Tree-ring Stable Carbon Isotopes

Changes in Root-Shoot Allometric Relations in Alpine Norway Spruce Trees After Strip Cutting.

Silvicultural interventions such as strip cuttings can change the resource availability of the edge trees. This may alter tree allometry, as light regime, water, and nutrient availability can change at the forest edge. Increased root growth may optimize resource uptake and/or enhance tree anchorage to withstand the altered wind regime. However, little is known about the patterns of the root–shoot allometric responses to strip cuttings. In three alpine stands differing in climate, site productivity, and stand characteristics, we selected 71 Norway spruce trees and took increment cores from stems, root collars, and main roots. This enabled us to study changes in the long-term root-stem allometry for 46 years and short-term allometric responses to intervention. The effects of cutting were compared between edge trees and trees from the stand interior in 10 years before and after the intervention. The long-term allocation to roots increased with stem diameter, with the strongest effects on the regularly managed stand with the tallest and largest trees. These results support the allometric biomass partitioning theory, which postulates resource allocation patterns between different plant organs to depend on plant size. Strip cutting on north-facing slopes boosted edge-tree growth in all plant compartments and enhanced allocation to roots. This change in allometry started 2 years after cutting but disappeared 7–8 years later. In the post-cutting period, the highest root–shoot increase was observed in the small trees independent of the site. This indicates the change in growing conditions to have the strongest effects in formerly suppressed trees. Thus, the effect of such acclimation on the wind firmness of subdominant spruce trees is a question with high importance for optimizing cutting layouts in lowering post-cutting vulnerability to disturbance. The results from this case study contribute to a better understanding of the structural acclimation of spruce trees from high-elevation forests to new forest edges. However, for a more mechanistic understanding of environmental drivers, further analyses of tree-ring stable isotopes are recommended.

Long-term decrease in Asian monsoon rainfall and abrupt climate change events over the past 6,700 years

Asian summer monsoon (ASM) variability and its long-term ecological and societal impacts extending back to Neolithic times are poorly understood due to a lack of high-resolution climate proxy data. Here, we present a precisely dated and well-calibrated tree-ring stable isotope chronology from the Tibetan Plateau with 1- to 5-y resolution that reflects high- to low-frequency ASM variability from 4680 BCE to 2011 CE. Superimposed on a persistent drying trend since the mid-Holocene, a rapid decrease in moisture availability between ∼2000 and ∼1500 BCE caused a dry hydroclimatic regime from ∼1675 to ∼1185 BCE, with mean precipitation estimated at 42 ± 4% and 5 ± 2% lower than during the mid-Holocene and the instrumental period, respectively. This second-millennium-BCE megadrought marks the mid-to late Holocene transition, during which regional forests declined and enhanced aeolian activity affected northern Chinese ecosystems. We argue that this abrupt aridification starting ∼2000 BCE contributed to the shift of Neolithic cultures in northern China and likely triggered human migration and societal transformation.

A triple tree-ring constraint for tree growth and physiology in a global land surface model

Abstract. Annually resolved tree-ring records extending back to pre-industrial conditions have the potential to constrain the responses of global land surface models at interannual to centennial timescales. Here, we demonstrate a framework to simultaneously constrain the representation of tree growth and physiology in the ORCHIDEE global land surface model using the simulated variability of tree-ring width and carbon (Δ13C) and oxygen (δ18O) stable isotopes in six sites in boreal and temperate Europe. We exploit the resulting tree-ring triplet to derive integrative constraints for leaf physiology and growth from well-known mechanistic relationships among the variables. ORCHIDEE simulates Δ13C (r=0.31–0.80) and δ18O (r=0.36–0.74) better than tree-ring width (r<0.55), with an overall skill similar to that of a tree-ring model (MAIDENiso) and another isotope-enabled global vegetation model (LPX-Bern). The comparison with tree-ring data showed that growth variability is not well represented in ORCHIDEE and that the parameterization of leaf-level physiological responses (stomatal control) to drought stress in the temperate region can be constrained using the interannual variability of tree-ring stable isotopes. The representation of carbon storage and remobilization dynamics emerged as a critical process to improve the realism of simulated growth variability, temporal carryover, and recovery of forest ecosystems after climate extremes. Simulated forest gross primary productivity (GPP) correlates with simulated tree-ring Δ13C and δ18O variability, but the origin of the correlations with tree-ring δ18O is not entirely physiological. The integration of tree-ring data and land surface models as demonstrated here should guide model improvements and contribute towards reducing current uncertainties in forest carbon and water cycling.

Tree-Ring Isotopes Provide Clues for Sink Limitation on Treeline Formation on the Tibetan Plateau

Identifying what determines the high elevation limits of tree growth is crucial for predicting how treelines may shift in response to climate change. Treeline formation is either explained by a low-temperature restriction of meristematic activity (sink limitation) or by the photosynthetic constraints (source limitation) on the trees at the treeline. Our study of tree-ring stable isotopes in two Tibetan elevational transects showed that treeline trees had higher iWUE than trees at lower elevations. The combination of tree-ring δ13C and δ18O data further showed that photosynthesis was higher for trees at the treeline than at lower elevations. These results suggest that carbon acquisition may not be the main determinant of the upper limit of trees; other processes, such as immature tissue growth, may be the main cause of treeline formation. The tree-ring isotope analysis (δ13C and δ18O) suggests that Tibetan treelines have the potential to benefit from ongoing climate warming, due to their ability to cope with co-occurring drought stress through enhanced water use efficiency.

Stable Isotopes in Tree Rings of Pinus heldreichii Can Indicate Climate Variability over the Eastern Mediterranean Region

A long-term context is important for understanding past climatic variability. Although tree-ring widths (TRWs) are widely used as a proxy for reconstructing past climate, the use of annually-resolved values of δ13C and δ18O tree-ring stable isotopes (TRSIs) is increasing and may provide further valuable information. Here, we present a 487-year-long TRW chronology and 240-year-long TRSI chronology for Bosnian pine (Pinus heldreichii H. Christ) and compare them to each other. We demonstrate that both δ13C and δ18O values are better proxies for temperature, precipitation, and drought than TRW. The correlations between these climate parameters and TRSIs are strongest for the combined summer (JJA) period. The results of temporal and spatial field correlation indicate that TRSI chronologies are stable, reliable proxies for JJA precipitation reconstruction over the whole Balkan Peninsula and surrounding eastern Mediterranean region. However, the stability of the temperature signal of the both δ13C and δ18O chronologies declines after the 1950s. Our work supports the emerging evidence that TRSI data track climate variability more accurately than a conventional TRW approach and can be subsequently used for the reconstruction of past climate.

Different climate sensitivity for radial growth, but uniform for tree-ring stable isotopes along an aridity gradient in Polylepis tarapacana, the world's highest elevation tree species.

Tree growth is generally considered to be temperature limited at upper elevation treelines, yet climate factors controlling tree growth at semiarid treelines are poorly understood. We explored the influence of climate on stem growth and stable isotopes for Polylepis tarapacana Philipi, the world's highest elevation tree species, which is found only in the South American Altiplano. We developed tree-ring width index (RWI), oxygen (δ18O) and carbon (δ13C) chronologies for the last 60years at four P. tarapacana stands located above 4400 m in elevation, along a 500 km latitude aridity gradient. Total annual precipitation decreased from 300 to 200mm from the northern to the southern sites. We used RWI as a proxy of wood formation (carbon sink) and isotopic tree-ring signatures as proxies of leaf-level gas exchange processes (carbon source). We found distinct climatic conditions regulating carbon sink processes along the gradient. Current growing-season temperature regulated RWI at northern-wetter sites, while prior growing-season precipitation determined RWI at arid southern sites. This suggests that the relative importance of temperature to precipitation in regulating tree growth is driven by site water availability. By contrast, warm and dry growing seasons resulted in enriched tree-ring δ13C and δ18O at all study sites, suggesting that similar climate conditions control carbon-source processes along the gradient. Site-level δ13C and δ18O chronologies were significantly and positively related at all sites, with the strongest relationships among the southern drier stands. This indicates an overall regulation of intercellular carbon dioxide via stomatal conductance for the entire P. tarapacana network, with greater stomatal control when aridity increases. This manuscript also highlights a coupling (decoupling) between physiological processes at leaf level and wood formation as a function of similarities (differences) in their climatic sensitivity. This study contributes to a better understanding and prediction of the response of high-elevation Polylepis woodlands to rapid climate changes and projected drying in the Altiplano.

The dendroclimatic value of oak stable isotopes

Tree-ring stable oxygen and carbon isotope ratios (δ18O and δ13C) are an important archive for climate reconstructions. However, it remains unclear whether the polyvinyl acetate emulsion, often used for the preservation and fixation of wood samples, influences δ18O and δ13C signals. Further uncertainties are associated with the possible effects of geographical origin and cambial age of historical samples. Here, we present annually-resolved and absolutely-dated δ18O and δ13C measurements of 21 living oaks (Quercus robur and Q. petraea) from the Czech Republic. We find that the δ18O and δ13C signals in the extracted alpha-cellulose are not affected by polyvinyl acetate treatment. Covering the entire 20th century and reaching until 2018 CE, our dataset reveals spatial and temporal coherency within and between the individual δ18O and δ13C chronologies of different oak species, sample locations, and tree ages. Highly significant (p < 0.01) Pearson’s correlation coefficients of the site-specific δ13C and δ18O chronologies range from 0.48–0.77 and 0.36–0.56, respectively. The isotopic inter-series correlations of Q. robur and Q. petraea from the same site are 0.75 and 0.43 for the mean δ13C and δ18O values, respectively. Significant (p < 0.01) correlations of 0.49 and 0.84 are found for δ13C and δ18O, respectively, when all measurements from all sampling locations and tree ages are included. Our study shows that non-pooled oak δ18O and δ13C measurements from both species, different locations, and diverse tree ages can be combined into robust isotopic chronologies for climate reconstructions.