Tree-rings stable isotope (δ18O and δ2H) based 368 years long term precipitation reconstruction of South Eastern Kashmir Himalaya

The hydroclimatic variability in Kashmir Himalaya is influenced by the western disturbances and the Indian Summer Monsoon. To investigate long-term hydroclimatic variability, 368 years tree-ring oxygen and hydrogen isotope ratios (δ18O and δ2H) extending from 1648 to 2015 CE were analysed. These isotopic ratios are calculated using five core samples of Himalayan silver fir (Abies pindrow) collected from the south-eastern region of Kashmir valley. The relationship between the long and short periodicity components of δ18O and δ2H suggested that physiological processes had a minimum effect on the tree-ring stable isotopes in Kashmir Himalaya. The δ18O chronology was developed based on the average of five-individual tree-ring δ18O time series covering the time span of 1648-2015 CE. The climate response analysis revealed the strongest and most significant negative correlation between tree ring δ18O and precipitation amount from the previous year's December to current year's August (D2Apre). The reconstructed D2Apre (D2Arec) explains precipitation variability from 1671 to 2015 CE and is supported by historical and other proxy-based hydroclimatic records. The reconstruction has two distinguishing features: first, it is characterized by stable wet conditions during the last phase of Little Ice Age (LIA) i.e., from 1682 to 1841 CE; and second, the southeast Kashmir Himalaya had experienced drier conditions as compared to recent and historical period with intense pluvial events since 1850. The present reconstruction shows, there have been more extreme dry events than extreme wet events since 1921. A tele-connection is observed between D2Arec and Sea Surface Temperature (SST) of the Westerly region.

Recent Progresses in Stable Isotope Analysis of Cellulose Extracted from Tree Rings

In this work, the challenges and progression in stable isotope investigation, from the analytical tools and technical sample preparation procedures to the dendroclimatological experiments, were reviewed in terms of their use to assess tree physiological responses to environmental changes. Since the isotope signature of whole wood is not always a reliable tool in studying the climate changes, cellulose is often preferred as the study material in paleoclimatic studies. Nevertheless, the isotope analysis of cellulose is challenging due to the difficulty to remove the other wood components (extractives, lignin, pectin, and hemicelluloses). Additionally, in the case of hydrogen isotope analysis, about 30% of the hydrogen atoms of cellulose are exchanged with the surrounding water, which complicates the isotope analysis. In recent years, more automated isotope analysis methods were developed based on high temperature pyrolysis of cellulose, followed by the chromatographic separation of H₂ from CO and by their individual isotope analysis using isotope ratio mass spectrometry. When used to investigate climate factors, the combined isotope analysis δ¹³C and δ¹⁸O appears to be the most promising isotope tool. In contrast, the role of δ²H values is yet to be elucidated, together with the development of new methods for hydrogen isotope analysis.

Coexisting oak species, including rear-edge populations, buffer climate stress through xylem adjustments

The ability of trees to cope with climate change is a pivotal feature of forest ecosystems, especially for rear-edge populations facing warm and dry conditions. To evaluate current and future forests threats, a multi-proxy focus on the growth, anatomical and physiological responses to climate change is needed. We examined the long-term xylem adjustments to climate variability of the temperate Quercus robur L. at its rear edge and the sub-Mediterranean Quercus pyrenaica Willd. Both species coexist at a mesic (ME, humid and warmer) and a xeric (XE, dry and cooler) site in northern Spain, the latter experiencing increasing temperatures in recent decades. We compared xylem traits at each site and assessed their trends, relationships and responses to climate (1960-2008). Traits included basal area increment, earlywood vessel hydraulic diameter, density and theoretical-specific hydraulic conductivity together with latewood oxygen (δ18O) stable isotopes and δ13C-derived water-use efficiency (iWUE). Quercus robur showed the highest growth at ME, likely through enhanced cambial activity. Quercus pyrenaica had higher iWUE at XE compared with ME, but limited plasticity of anatomical xylem traits was found for the two oak species. Similar physiological performance was found for both species. The iWUE augmented in recent years especially at XE, likely explained by stomatal closure given the increasing δ18O signal in response to drier and sunnier growing seasons. Overall, traits were more correlated at XE than at ME. The iWUE improvements were linked to higher growth up to a threshold (~85 μmol mol-1) after which reduced growth was found at XE. Our results are consistent with Q. pyrenaica and Q. robur coexisting at the central and dry edge of the climatic species distribution, respectively, showing similar responses to buffer warmer conditions. In fact, the observed adjustments found for Q. robur point towards growth stability of similar rear-edge oak populations under warmer climate conditions.

Evaluating climate signal recorded in tree-ring δ13 C and δ18 O values from bulk wood and α-cellulose for six species across four sites in the northeastern US

RATIONALE

We evaluated the applicability of tree-ring δ¹³ C and δ¹⁸ O values in bulk wood - instead of the more time and lab-consuming α-cellulose δ¹³ C and δ¹⁸ O values, to assess climate and physiological signals across multiple sites and for six tree species along a latitudinal gradient (35°97'N to 45°20'N) of the northeastern United States.

METHODS

Wood cores (n = 4 per tree) were sampled from ten trees per species. Cores were cross-dated within and across trees at each site, and for the last 30 years. Seven years, including the driest on record, were selected for this study. The δ¹³ C and δ¹⁸ O values were measured on two of the ten trees from the bulk wood and the α-cellulose. The offsets between materials in δ¹³ C and δ¹⁸ O values were assessed. Correlation and multiple regression analyses were used to evaluate the strength of the climate signal across sites. Finally the relationship between δ¹³ C and δ¹⁸ O values in bulk wood vs α-cellulose was analyzed to assess the consistency of the interpretation, in terms of CO₂ assimilation and stomatal conductance, from both materials.

RESULTS

We found offsets of 1.1‰ and 5.6‰ between bulk and α-cellulose for δ¹³ C and δ¹⁸ O values, respectively, consistent with offset values reported in the literature. Bulk wood showed similar or stronger correlations to climate parameters than α-cellulose for the investigated sites. In particular, temperature and vapor pressure deficit and standard precipitation-evaporation index (SPEI) were the most visible climate signals recorded in δ¹³ C and δ¹⁸ O values, respectively. For most of the species, there was no relationship between δ¹³ C and δ¹⁸ O values, regardless of the wood material considered.

CONCLUSIONS

Extraction of α-cellulose was not necessary to detect climate signals in tree rings across the four investigated sites. Furthermore, the physiological information inferred from the dual isotope approach was similar for most of the species regardless of the material considered.

Comparison of δ(13)C and δ(18)O from cellulose, whole wood, and resin-free whole wood from an old high elevation Pinus uncinata in the Spanish central Pyrenees

δ(13)C and δ(18)O values from sapwood of a single Pinus uncinata tree, from a high elevation site in the Spanish Pyrenees, were determined to evaluate the differences between whole wood and resin-free whole wood. This issue is addressed for the first time with P. uncinata over a 38-year long period. Results are also compared with published isotope values of α-cellulose samples from the same tree. The differences in δ(13)C and δ(18)O between whole wood and resin-free whole wood vary within the analytical uncertainty of 0.3 and 0.5 ‰, respectively, indicating that resin extraction is not necessary for sapwood of P. uncinata. Mean differences between cellulose and whole wood are 0.9 ‰ (δ(13)C) and 5.0 ‰ (δ(18)O), respectively. However, further analyses of different species and other sites are needed to evaluate whether the findings reported here are coherent more generally.

Comparison of δ(18)O and δ(13)C values between tree-ring whole wood and cellulose in five species growing under two different site conditions

RATIONALE

We investigated the applicability of tree-ring whole-wood material for δ(18)O and δ(13)C analysis in comparison with the more time- and resource-intensive use of cellulose, by considering possible variability between (i) five different tree species (Fagus sylvatica, Quercus robur, Picea abies, Abies alba, Pseudotsuga menziesii), (ii) two sites that differ in soil moisture, and (iii) climate conditions within a 10-year period.

METHODS

Stem cores of 30 individual trees (n = 3 trees per each species and site) were sampled from two sites in south Germany (Bavaria), and tree rings within sapwood of the years 2001-2010 were separated. The δ(18)O and δ(13)C values from homogenized tree-ring whole wood and from extracted cellulose were measured by mass spectrometry. Species-specific offsets in isotope values were analyzed and the responses in isotopic signature to climate variability including a single drought event were compared between whole-wood and cellulose.

RESULTS

A constant offset in δ(18)O values of ca 5‰ between wood and cellulose was observed for most species independent of site conditions, with a significant difference between beech and Douglas-fir, while inter-annual variability was only observed in oak. The offset in δ(13)C values ranged between 1.45 and 1.84‰ across species, sites and years. Both materials generally showed similar strength in responses to temperature, precipitation and soil water availability, particularly for conifers. Resistance to severe drought stress--partly more strongly reflected in the δ(13)C values of cellulose--was lower for conifers than for the deciduous species.

CONCLUSIONS

Wood material from the sapwood of the studied tree species is as useful as cellulose for studying environmental effects on tree-ring δ(18)O and δ(13)C values at a short-term scale as considered in most ecophysiological studies. The more variable response of oak may require further investigations.

δ²H, δ¹³C and δ¹⁸O from whole wood, α-cellulose and lignin methoxyl groups in Pinus sylvestris: a multi-parameter approach

Novel tree ring parameters - δ(13)C and δ(2)H from methoxyl groups - have been developed to reconstruct palaeoclimate. Tests with δ(13)C and δ(18)O derived from whole wood and cellulose samples, however, indicated differences in the isotopic composition and climate signal, depending on the extracted wood component. We assess this signal dependency by analysing (i) δ(13)C and δ(18)O from whole wood and cellulose and (ii) δ(13)C and δ(2)H from methoxyl groups, using Pinus sylvestris L. growing near Altenkirchen (Germany). Results indicate significant correlations among the time series derived from whole wood, cellulose, and lignin methoxyl groups. Compared with the whole wood samples, δ(13)C from methoxyl groups showed a different and overall lower response to climate parameters. On the other hand, δ(2)H from methoxyl groups showed high correlations with temperature and was also correlated with ring width, indicating its potential as a temperature proxy. Isotope time series with the highest correlation with climatic parameter were: (i) whole wood and cellulose δ(13)C with growing season precipitation and summer temperature; (ii) methoxyl groups with spring precipitation; (iii) whole wood and cellulose δ(18)O correlates with annual evapotranspiration and water balance; and (iv) methoxyl group δ(2)H with spring temperatures. These findings reveal that multiple climate elements can be reconstructed from different wood components and that whole wood proxies perform comparably to cellulose time series.

Stable isotopes in tree rings: towards a mechanistic understanding of isotope fractionation and mixing processes from the leaves to the wood

The mechanistic understanding of isotope fractionation processes is increasing but we still lack detailed knowledge of the processes that determine the isotopic composition of the tree-ring archive over the long term. Especially with regard to the path from leaf photosynthate production to wood formation, post-assimilation fractionations/processes might cause at least a partial decoupling between the leaf isotope signals that record processes such as stomatal conductance, transpiration and photosynthesis, and the wood or cellulose signals that are stored in the paleophysiological record. In this review, we start from the rather well understood processes at the leaf level such as photosynthetic carbon isotope fractionation, leaf water evaporative isotope enrichment and the issue of the isotopic composition of inorganic sources (CO2 and H2O), though we focus on the less explored 'downstream' processes related to metabolism and transport. We further summarize the roles of cellulose and lignin as important chemical constituents of wood, and the processes that determine the transfer of photosynthate (sucrose) and associated isotopic signals to wood production. We cover the broad topics of post-carboxylation carbon isotope fractionation and of the exchange of organic oxygen with water within the tree. In two case studies, we assess the transfer of carbon and oxygen isotopic signals from leaves to tree rings. Finally we address the issue of different temporal scales and link isotope fractionation at the shorter time scale for processes in the leaf to the isotopic ratio as recorded across longer time scales of the tree-ring archive.

Carbon, hydrogen and oxygen stable isotope ratios of whole wood, cellulose and lignin methoxyl groups of Picea abies as climate proxies

RATIONALE

Carbon, hydrogen and oxygen (C, H and O) stable isotope ratios of whole wood and components are commonly used as paleoclimate proxies. In this work we consider eight different proxies in order to discover the most suitable wood component and stable isotope ratio to provide the strongest climate signal in Picea abies in a southeastern Alpine region (Trentino, Italy).

METHODS

δ(13)C, δ(18)O and δ(2)H values in whole wood and cellulose, and δ(13)C and δ(2)H values in lignin methoxyl groups were measured. Analysis was performed using an Isotopic Ratio Mass Spectrometer coupled with an Elemental Analyser for measuring (13)C/(12)C and a Pyrolyser for measuring (2)H/(1)H and (18)O/(16)O. The data were evaluated by Principal Component Analysis, and a simple Pearson's correlation between isotope chronologies and climatic features, and multiple linear regression were performed to evaluate the data.

RESULTS

Each stable isotope ratio in cellulose and lignin methoxyl differs significantly from the same stable isotope ratio in whole wood, the values begin higher in cellulose and lignin except for the lignin δ(2)H values. Significant correlations were found between the whole wood and the cellulose fractions for each isotope ratio. Overall, the highest correlations with temperature were found with the δ(18)O and δ(2)H values in whole wood, whereas no significant correlations were found between isotope proxies and precipitation.

CONCLUSIONS

δ(18)O and δ(2)H values in whole wood provide the best temperature signals in Picea abies in the northern Italian study area. Extraction of cellulose and lignin and analysis of other isotopic ratios do not seem to be necessary.

Physiological and isotopic (delta(13)C and delta(18)O) responses of three tropical tree species to water and nutrient availability

Water-use efficiency and stable isotope composition were studied in three tropical tree species. Seedlings of Tectona grandis, Swietenia macrophylla and Platymiscium pinnatum were grown at either high or low water supply, and with or without added fertilizer. These three species previously exhibited low, intermediate and high whole-plant water-use efficiency (TE) when grown at high water supply in unfertilized soil. Responses of TE to water and nutrient availability varied among species. The TE was calculated as experiment-long dry matter production divided by cumulative water use. Species-specific offsets were observed in relationships between TE and whole-plant (13)C discrimination (Delta(13)C(p)). These offsets could be attributed to a breakdown in the relationship between Delta(13)C(p) and the ratio of intercellular to ambient CO(2) partial pressures (c(i)/c(a)) in P. pinnatum, and to variation among species in the leaf-to-air vapour pressure difference (v). Thus, a plot of v.TE against c(i)/c(a) showed a general relationship among species. Relationships between delta(18)O of stem dry matter and stomatal conductance ranged from strongly negative for S. macrophylla to no relationship for T. grandis. Results suggest inter-specific variation among tropical tree species in relationships between stable isotope ratios (delta(13)C and delta(18)O) and the gas exchange processes thought to affect them.

Conifers, angiosperm trees, and lianas: growth, whole-plant water and nitrogen use efficiency, and stable isotope composition ({delta}13C and {delta}18O) of seedlings grown in a tropical environment

Seedlings of several species of gymnosperm trees, angiosperm trees, and angiosperm lianas were grown under tropical field conditions in the Republic of Panama; physiological processes controlling plant C and water fluxes were assessed across this functionally diverse range of species. Relative growth rate, r, was primarily controlled by the ratio of leaf area to plant mass, of which specific leaf area was a key component. Instantaneous photosynthesis, when expressed on a leaf-mass basis, explained 69% of variation in r (P < 0.0001, n = 94). Mean r of angiosperms was significantly higher than that of the gymnosperms; within angiosperms, mean r of lianas was higher than that of trees. Whole-plant nitrogen use efficiency was also significantly higher in angiosperm than in gymnosperm species, and was primarily controlled by the rate of photosynthesis for a given amount of leaf nitrogen. Whole-plant water use efficiency, TE(c), varied significantly among species, and was primarily controlled by c(i)/c(a), the ratio of intercellular to ambient CO(2) partial pressures during photosynthesis. Instantaneous measurements of c(i)/c(a) explained 51% of variation in TE(c) (P < 0.0001, n = 94). Whole-plant (13)C discrimination also varied significantly as a function of c(i)/c(a) (R(2) = 0.57, P < 0.0001, n = 94), and was, accordingly, a good predictor of TE(c). The (18)O enrichment of stem dry matter was primarily controlled by the predicted (18)O enrichment of evaporative sites within leaves (R(2) = 0.61, P < 0.0001, n = 94), with some residual variation explained by mean transpiration rate. Measurements of carbon and oxygen stable isotope ratios could provide a useful means of parameterizing physiological models of tropical forest trees.

Consequences of a rapid cellulose extraction technique for oxygen isotope and radiocarbon analyses

We use infrared, radiocarbon, and stable isotope analyses to investigate the purity of cellulose extracted from wood using a rapid processing technique. Replicate laboratory standards processed using the standard Brendel method are not significantly different with respect to delta(18)O from those prepared using traditional techniques, although the process does result in a slight acetylation of the wood samples. Radiocarbon comparisons, however, show significant differences. We conclude that the standard Brendel method is appropriate for developing stable isotope time series for high-resolution isotope dendroclimatology but must be used with caution for precision radiocarbon measurements.

Large variation in whole-plant water-use efficiency among tropical tree species

It is well known that whole-plant water-use efficiency (transpiration efficiency of carbon gain, TE(C)) varies among plant species with different photosynthetic pathways. However, less is known of such variation among tree species within the C(3) group. Here we measured the TE(C) of seven C(3) tropical tree species. Isotopic analyses (delta(13)C, delta(18)O, and delta(15)N) and elemental analyses (carbon and nitrogen) were undertaken to provide insight into sources of variation in TE(C). Plants were grown over several months in approx. 80% full sunlight in individual 38-l containers in the Republic of Panama. Soil moisture content was nonlimiting. Significant variation was observed in TE(C) among the C(3) tree species. Values ranged from 1.6 mmol C mol(-1) H(2)O for teak (Tectona grandis) to 4.0 mmol C mol(-1) H(2)O for a legume, Platymiscium pinnatum. Variation in TE(C) was correlated with both leaf N concentration, a proxy for photosynthetic capacity, and oxygen-isotope enrichment, a proxy for stomatal conductance. The TE(C) varied with C-isotope discrimination within species, but the relationship broke down among species, reflecting the existence of species-specific offsets.

Stable oxygen isotopes (delta18(O)) in Austrocedrus chilensis tree rings reflect climate variability in northwestern Patagonia, Argentina

The stable oxygen isotope (delta (18)O) composition of Austrocedrus chilensis (D. Don) Endl. (Cupressaceae) tree rings potentially provide retrospective views of changes in environment and climate in the semi-arid lands of Patagonia. We report the development of the first annually resolved delta (18)O tree-ring chronology obtained from natural forests of the foothills of the northwestern Patagonian Andes. The isotope record spans between 1890 and 1994 AD. We explore the probable links between this record and the climate of the region. Air temperatures during summer conditions are significantly, but not strongly, inversely correlated with annual delta (18)O values from Austrocedrus tree rings. The strongest correlations are between the southern oscillation index (SOI) and the tree rings. The existence of millennial-age Austrocedrus trees in northern Patagonia provides interesting possibilities for examining these climate-related isotopic signals over most of the last 1,000 years.