Temporal instability of isotopes–climate statistical relationships – A study of black spruce trees in northeastern Canada

Climate reconstructions using stable isotopes (δ¹⁸O and δ¹³C values) in tree rings are based on relationships between present climatic conditions and isotopic series. This widely used approach relies on the assumption that correlations between stable isotopes and climatic conditions are steady over time. In this paper, we evaluate the strength of the correlations between δ¹⁸O and δ¹³C series with several climatic parameters on fourteen black spruce trees coming from three different sites, in northeastern Canada. We applied a 21-year moving window on the r Pearson calculated between stable isotopes and March–May and June–August precipitation, June–August and April–June maximal temperatures. Our results indicate that despite the large distance and differences in stand conditions between the sites, the three sites responded in the same way over time. We show that because the climatic ambiance has changed during the 1980–1990 period due to a positive North Atlantic Oscillation index the δ¹³C values are not controlled anymore by spring precipitation or summer maximal temperature in the following two decades. As opposed to δ¹³C series, the relationship between summer maximal temperature and δ¹⁸O values was stable over time, and decreased only in the last decade. All these results attest of a “divergence problem” in the last decades which is most pronounced for δ¹³C series. We conclude that the spruce δ¹⁸O series appears to be the most appropriate indicator for reconstructing June–August maximal temperature in the studied area despite the divergence issue, given that the calibration–validation tests and reconstruction can exclude the divergent last decade.     

Tree age,site and climate controls on tree ring cellulose δ18O: A case study on oak trees from south-western France

A main concern of dendroclimatic reconstruction is to distinguish in the tree ring proxy the influence of the climate variables of interest from other controlling factors. In order to investigate age, site and climate controls on tree ring width and cellulose δ¹⁸O, measurements have been performed in nearby groups of young (145 years old) and older (310–405 years old) oak trees in south-western France, covering the period 1860–2010.Within a given site, inter-tree deviations are small, pointing to a common climatic signal. Despite a similar inter-annual variability, the average level of cellulose δ¹⁸O in the young tree group is ∼0.8‰ higher than in the old trees. Such offsets might be caused by different soil properties and differences in the fraction of the source water used by trees from different depths. The δ¹⁸O of water in the top soil layer is directly related to the current growing season precipitation, while deeper water can have a lower and more constant δ¹⁸O. Local cave drip waters at 10 m depth indeed show a constant isotopic composition, which corresponds to pluri-annual mean precipitation.A 2‰ increasing trend is observed in cellulose δ¹⁸O of young trees in the first 30 years of growth, during a period when no trend is visible in older trees. This increase can be quantitatively explained by humidity gradients under the forest canopy, and a changing microclimate around the crown as trees grow higher.While relationships between tree ring width and climate appear complex, the isotopic composition of cellulose is strongly correlated with summer maximum temperature, relative humidity and evapotranspiration (r ≈ 0.70). Weaker correlations (r ≈ 0.40) are identified with precipitation δ¹⁸O from a 15-year long local record and from the REMOiso model output. These results imply that leaf water enrichment has a stronger control on the inter-annual variability of cellulose δ¹⁸O than the δ¹⁸O of precipitation.This study demonstrates the suitability of oak tree ring cellulose δ¹⁸O for reconstructing past summer climate variability in south-western France, provided that the sampling and pooling strategy accounts for the fact that trees from different sites and of different age can introduce non-climatic signals.     

Correlating δ13C and δ18O in cellulose of trees

We measured the carbon and oxygen isotopic composition of stem cellulose of Pinus sylvestris, Picea abies, Fagus sylvatica and Fraxinus excelsior. Several sites along a transect of a small valley in Switzerland were selected which differ in soil moisture conditions. At every site, six trees per species were sampled, and a sample representing a mean value for the period from 1940 to 1990 was analysed. For all species, the mean site δ¹³C and δ¹⁸O of stem cellulose are related to the soil moisture availability, whereby higher isotope ratios are found at drier sites. This result is consistent with isotope fractionation models when assuming enhanced stomatal resistance (thus higher δ¹³C of incorporated carbon) and increased oxygen isotope enrichment in the leaf water (thus higher δ¹⁸O) at the dry sites. δ¹⁸ O-δ¹³C plots reveal a linear relationship between the carbon and oxygen isotopes in cellulose. To interpret this relationship we developed an equation which combines the above-mentioned fractionation models. An important new parameter is the degree to which the leaf water enrichment is reflected in the stem cellulose. In the combined model the slope of the δ¹⁸O-δ¹³C plot is related to the sensitivity of the p_i/p_a of a plant to changing relative humidity.     

More than climate: Hydrogen isotope ratios in tree rings as novel plant physiological indicator for stress conditions

The analysis of stable carbon and oxygen isotopes in tree-rings is a widely applied tool which allows to retrieve information about past climatic conditions, as well as tree physiological responses to environmental changes. This is based on well-established mechanistic models and firm statistical relationships with climate variables. In contrast, the hydrogen isotopic signature (δ²H) of tree-rings has been reported to be poorly correlated to climate or difficult to explain, and as a consequence, hydrogen isotopes are far less utilized. However, recent plant-physiological experiments have highlighted the role of autotrophic versus heterotrophic processes affecting δ²H values, i.e. use of fresh assimilates versus stored carbohydrates, and have much improved our understanding of the role of post-photosynthetic ²H-fractionation. Using unpublished and literature δ²H data of tree-ring cellulose (δ²H_C) of 5 study sites in Europe and Asia, we systematically investigated the relationships between δ²H_C and tree-ring width (TRW), which, in contrast to previous research, could now be explained through post-photosynthetic ²H-fractionation. In most cases, these relationships were found to be negative (r² = 0.23 to 0.51, all P < 0.05) when the main growth limiting factors are precipitation and light, while in temperature-limited sites we observed a positive trend (r² = 0.14, P < 0.05). Our results suggest that, under stress conditions, trees use a surplus of carbon from reserves for wood formation. Therefore, in combination with TRW chronologies, δ²H_C may allow to infer about physiological information on stressful time periods independently of biotic and abiotic origin. Here, we discuss implications of these findings for tree-ring research, summarize them in a conceptual framework and suggest future research directions.     

Exploring climate forcing of growth depression in subfossil South Swedish bog pines using stable isotopes

Comparison between growth variability, based on ring-width (RW) analysis, and moisture-sensitive signals in tree-ring carbon and oxygen stable-isotope composition provides increased understanding of how climate and hydrology influenced bog pines (Pinus sylvestris L.) at two sites in southern Sweden during the mid- and late Holocene. Tree-ring sequences from two subfossil trees collected at raised bogs having different hydrology and catchment size were analyzed to probe the stable-isotope signals associated with two bog-wide episodes of growth depression, one during the Holocene Thermal Maximum and the other during the Neoglacial Transition. The occurrence of lower whole-wood δ¹³C and cellulose δ¹³C and δ¹⁸O values immediately prior to the onset of growth depression in both trees, suggesting increased atmospheric relative humidity, is consistent with the notion that excessive effective moisture impeded tree growth. Correlation analysis indicates that the growth response lagged about three years behind the decline in δ¹³C and δ¹⁸O values in each tree, possibly reflecting relatively slow rise in the local water table in response to wetter climate.     

Oxygen isotope fractionations across individual leaf carbohydrates in grass and tree species

Almost no δ¹⁸O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ¹⁸O relationship between leaf water and cellulose. We measured δ¹⁸O values of bulk leaf water (δ¹⁸O_LW) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ¹⁸O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally ¹⁸O‐enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (ε_bio) of more than 27‰ relative to δ¹⁸O_LW, which might be explained by isotopic leaf water and sucrose synthesis gradients. δ¹⁸O_LW and δ¹⁸O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (ε_bio = 25.1‰). Interestingly, damping factor p_exp_x, which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ¹⁸O_LW but not if modelled directly from δ¹⁸O of individual carbohydrates. We conclude that δ¹⁸O_LW is not always a good substitute for δ¹⁸O of synthesis water due to isotopic leaf water gradients. Thus, compound‐specific δ¹⁸O analyses of individual carbohydrates are helpful to better constrain (post‐)photosynthetic isotope fractionation processes in plants.     

Drought response of five conifer species under contrasting water availability suggests high vulnerability of Norway spruce and European larch

The ability of tree species to cope with anticipated decrease in water availability is still poorly understood. We evaluated the potential of Norway spruce, Scots pine, European larch, black pine, and Douglas‐fir to withstand drought in a drier future climate by analyzing their past growth and physiological responses at a xeric and a mesic site in Central Europe using dendroecological methods. Earlywood, latewood, and total ring width, as well as the δ¹³C and δ¹⁸O in early‐ and latewood were measured and statistically related to a multiscalar soil water deficit index from 1961 to 2009. At the xeric site, δ¹³C values of all species were strongly linked to water deficits that lasted longer than 11 months, indicating a long‐term cumulative effect on the carbon pool. Trees at the xeric site were particularly sensitive to soil water recharge in the preceding autumn and early spring. The native species European larch and Norway spruce, growing close to their dry distribution limit at the xeric site, were found to be the most vulnerable species to soil water deficits. At the mesic site, summer water availability was critical for all species, whereas water availability prior to the growing season was less important. Trees at the mesic were more vulnerable to water deficits of shorter duration than the xeric site. We conclude that if summers become drier, trees growing on mesic sites will undergo significant growth reductions, whereas at their dry distribution limit in the Alps, tree growth of the highly sensitive spruce and larch may collapse, likely inducing dieback and compromising the provision of ecosystem services. However, the magnitude of these changes will be mediated strongly by soil water recharge in winter and thus water availability at the beginning of the growing season.     

Isotopic heterogeneity of water in transpiring leaves: identification of the component that controls the δ18O of atmospheric O2 and CO2

Two direct but independent approaches were developed to identify the average δ¹⁸O value of the water fraction in the chloroplasts of transpiring leaves. In the first approach, we used the δ¹⁸O value of CO₂ in isotopic equilibrium with leaf water to reconstruct the δ¹⁸O value of water in the chloroplasts. This method was based on the idea that the enzyme carbonic anhydrase facilitates isotopic equilibrium between CO₂ and H₂O predominantly in the chloroplasts, at a rate that is several orders of magnitude faster than the non-catalysed exchange in other leaf water fractions. In the second approach, we measured the δ¹⁸O value of O₂ from photosynthetic water oxidation in the chloroplasts of intact leaves. Since O₂ is produced from chloroplast water irreversibly and without discrimination, the δ¹⁸O value of the O₂ should be identical to that of chloroplast water. In intact, transpiring leaves of sunflower (Helianthus annuus cv. giant mammoth) under the experimental conditions used, the average δ¹⁸O value of chloroplasts water was displaced by 3—10 % (depending on relative humidity and atmospheric composition) below the value predicted by the conventional Craig & Gordon model. Furthermore, this δ¹⁸O value was always lower than the δ¹⁸O value that was measured for bulk leaf water. Our results have implications for a variety of environmental studies since it is the δ¹⁸O value of water in the chloroplasts that is the relevant quantity in considering terrestrial plants influence on the δ¹⁸O values of atmospheric CO₂ and O₂, as well as in influencing the δ¹⁸O of plant organic matter.     

Non-climatic variations in the oxygen isotopic compositions of plants

The ¹⁸O content of leaf water strongly influences the ¹⁸O contents of atmospheric CO₂ and O₂. The ¹⁸O signatures of these atmospheric gases, in turn, emerge as important indicators of large-scale gas exchange processes. Better understanding of the factors that influence the isotopic composition of leaf water is still required, however, for the quantitative utilization of these tracers. The ¹⁸O enrichment of leaf water relative to local meteoric water, is known to reflect climatic conditions. Less is known about the extent variations in the ¹⁸O content of leaf water are influenced by nonclimatic, species-specific characteristics. In a collection of 90 plant species from all continents grown under the same climatic conditions in the Jerusalem Botanical Garden we observed variations of about 9‰ in the δ¹⁸O values of stem water, δ_s, and of about 14‰ in the mid-day δ¹⁸O enrichment of bulk leaf water, δ_LW–δ_s. Differences between δ¹⁸O values predicted by a conventional evaporation model, δ_M, and δ_LW ranged between – 3.3‰ and + 11.8‰. The δ¹⁸O values of water in the chloroplasts (δ_ch) in leaves of 10 selected plants were estimated from on-line CO₂ discrimination measurements. Although much uncertainty is still involved in these estimates, the results indicated that δ_ch can significantly deviate from δ_M in species with high leaf peclet number. The δ¹⁸O values of bulk leaf water significantly correlated with δ¹⁸O values of leaf cellulose (directly) and with instantaneous water use efficiency (A/E, inversely). Differences in isotopic characteristics among conventionally defined vegetation types were not significant, except for conifers that significantly differed from shrubs in δ¹⁸O and δ¹³C values of cellulose and in their peclet numbers, and from deciduous woodland species in their δ¹⁸O and δ¹³C values of cellulose. The results indicated that predictions of the δ¹⁸O values of leaf water (δ_LW, δ_M and δ_ch) could be improved by considering plant species-specific characteristics.     

Stand structure modulates the long-term vulnerability of Pinus halepensis to climatic drought in a semiarid Mediterranean ecosystem

We investigated whether stand structure modulates the long-term physiological performance and growth of Pinus halepensis Mill. in a semiarid Mediterranean ecosystem. Tree radial growth and carbon and oxygen stable isotope composition of latewood (δ(13)C(LW) and δ(18)O(LW), respectively) from 1967 to 2007 were measured in P. halepensis trees from two sharply contrasting stand types: open woodlands with widely scattered trees versus dense afforested stands. In both stand types, tree radial growth, δ(13)C(LW) and δ(18)O(LW) were strongly correlated with annual rainfall, thus indicating that tree performance in this semiarid environment is largely determined by inter-annual changes in water availability. However, trees in dense afforested stands showed consistently higher δ(18)O(LW) and similar δ(13)C(LW) values compared with those in neighbouring open woodlands, indicating lower stomatal conductance and photosynthesis rates in the former, but little difference in water use efficiency between stand types. Trees in dense afforested stands were more water stressed and showed lower radial growth, overall suggesting greater vulnerability to drought and climate aridification compared with trees in open woodlands. In this semiarid ecosystem, the negative impacts of intense inter-tree competition for water on P. halepensis performance clearly outweigh potential benefits derived from enhanced infiltration and reduced run-off losses in dense afforested stands.     

Aerial decay influence on the stable oxygen and carbon isotope ratios in tree ring cellulose

Sub-fossil wood is often affected by the decaying process that introduces uncertainties in the measurement of oxygen and carbon stable isotope composition in cellulose. Although the cellulose stable isotopes are widely used as climatic proxies, our understanding of processes controlling their behavior is very limited. We present here a comparative study of stable oxygen and carbon isotope ratios in tree ring cellulose in decayed and non-decayed wood samples of Swiss stone pine (Pinus cembra) trees. The intra-ring stable isotope variability (around the circumference of a single ring) was between 0.1 and 0.5‰ for δ¹⁸O values and between 0.5 and 1.6‰ for δ¹³C values for both decayed and non-decayed wood. Observed intra-tree δ¹⁸O variability is less than that reported in the literature (0.5–1.5‰), however, for δ¹³C it is larger than the reported values (0.7–1.2‰). The inter-tree variability for non-decayed wood ranges between 1.1 and 2.3‰ for δ¹⁸O values, and between 2 and 4.7‰ for δ¹³C values. The inter-tree differences for δ¹⁸O values are similar to those reported in the literature (1–2‰ for oxygen and 1–3‰ for carbon) but are larger for δ¹³C values. We have found that the differences for δ¹⁸O and δ¹³C values between decayed and non-decayed wood are smaller than the variation among different trees from the same site, suggesting that the decayed wood can be used for isotopic paleoclimate research.     

Cellulose oxygen isotopic composition of teak (Tectona grandis) collected from Java Island: a tool for dendrochronological and dendroclimatological analysis

The oxygen isotopic composition (δ¹⁸O) of tree-ring cellulose has been recognized as a powerful tool for dendroclimatological and dendrochronological investigations in Asia. Only a few studies of cellulose δ¹⁸O so far published, however, have been conducted in Indonesia, and the spatial coherence of cellulose δ¹⁸O has yet to be clarified. In this study we measured cellulose δ¹⁸O of teak (Tectona grandis) collected from four sites on Java Island to evaluate the similarity between the different teak δ¹⁸O values and the potential for using teak δ¹⁸O both as a climate proxy and as a tool for cross-dating. Cellulose δ¹⁸O time series of Javanese teaks were found to be significantly correlated in all of the comparisons between sites, even between sites separated by a distance as great as 400 km. While significant correlations did not appear in the ring width data between our samples (expressed population signal (EPS) = 0.64; Rbar = 0.23; sample depth = 10), they were found between the cellulose δ¹⁸O values (EPS = 0.89; Rbar = 0.58; sample depth = 10). These results suggest that teak δ¹⁸O variations have higher spatial coherence and might be a useful tool for cross-dating. A significant negative correlation was observed between cellulose δ¹⁸O and the relative humidity/precipitation during the rainy season, indicating that the cellulose δ¹⁸O of Javanese teak is an effective proxy for relative humidity/precipitation during the rainy season. Cellulose δ¹⁸O was found to be positively correlated with precipitation during the dry season preceding the growing season, whereas it showed no correlations with the temperature and Palmer Drought Severity Index (PDSI), the key constraints of δ¹⁸O on the Indochina Peninsula.     

Interpreting species‐specific variation in tree‐ring oxygen isotope ratios among three temperate forest trees

Although considerable variation has been documented in tree‐ring cellulose oxygen isotope ratios (δ¹⁸O_cell) among co‐occurring species, the underlying causes are unknown. Here, we used a combination of field measurements and modelling to investigate the mechanisms behind variations in late‐wood δ¹⁸O_cell (δ¹⁸O_lc) among three co‐occurring species (chestnut oak, black oak and pitch pine) in a temperate forest. For two growing seasons, we quantified among‐species variation in δ¹⁸O_lc, as well as several variables that could potentially cause the δ¹⁸O_lc variation. Data analysis based on the δ¹⁸O_cell model rules out leaf water enrichment (Δ¹⁸O_lw) and tree‐ring formation period (Δt), but highlights source water δ¹⁸O (δ¹⁸O_sw) as an important driver for the measured difference in δ¹⁸O_lc between black and chestnut oak. However, the enriched δ¹⁸O_lc in pitch pine relative to the oaks could not be sufficiently explained by consideration of the above three variables only, but rather, we show that differences in the proportion of oxygen exchange during cellulose synthesis (p_ex) is most likely a key mechanism. Our demonstration of the relevance of some species‐specific features (or lack thereof) to δ¹⁸O_cell has important implications for isotope based ecophysiological/paleoclimate studies.     

Disentangling the effects of acidic air pollution,atmospheric CO2, and climate change on recent growth of red spruce trees in the Central Appalachian Mountains

In the 45 years after legislation of the Clean Air Act, there has been tremendous progress in reducing acidic air pollutants in the eastern United States, yet limited evidence exists that cleaner air has improved forest health. Here, we investigate the influence of recent environmental changes on the growth and physiology of red spruce (Picea rubens Sarg.) trees, a key indicator species of forest health, spanning three locations along a 100 km transect in the Central Appalachian Mountains. We incorporated a multiproxy approach using 75‐year tree ring chronologies of basal tree growth, carbon isotope discrimination (?¹³C, a proxy for leaf gas exchange), and δ¹⁵N (a proxy for ecosystem N status) to examine tree and ecosystem level responses to environmental change. Results reveal the two most important factors driving increased tree growth since ca. 1989 are reductions in acidic sulfur pollution and increases in atmospheric CO₂, while reductions in pollutant emissions of NO_x and warmer springs played smaller, but significant roles. Tree ring ?¹³C signatures increased significantly since 1989, concurrently with significant declines in tree ring δ¹⁵N signatures. These isotope chronologies provide strong evidence that simultaneous changes in C and N cycling, including greater photosynthesis and stomatal conductance of trees and increases in ecosystem N retention, were related to recent increases in red spruce tree growth and are consequential to ecosystem recovery from acidic pollution. Intrinsic water use efficiency (iWUE) of the red spruce trees increased by ~51% across the 75‐year chronology, and was driven by changes in atmospheric CO₂ and acid pollution, but iWUE was not linked to recent increases in tree growth. This study documents the complex environmental interactions that have contributed to the recovery of red spruce forest ecosystems from pervasive acidic air pollution beginning in 1989, about 15 years after acidic pollutants started to decline in the United States.     

Tree-ring isotopes from Araucaria araucana as useful proxies for climate reconstructions

Tree-ring width (TRW) chronologies have been widely and long-time used to reconstruct past climate variations in the Andes in South America. The use of tree-ring isotopic chronologies is still not widespread in this region although they have proved to be very efficient climate proxies. Araucaria araucana (Molina) K. Koch is a conifer tree species with some multi-century-old individuals that offers an excellent opportunity to measure stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotopes in cellulose from long tree-ring records. Here, we explore whether current or stored carbohydrates are used for A. araucana radial growth and we assess the potential of a tree-ring isotopic record of to study past climate variability. Eleven A. araucana cores from a dry and high-elevation forest at the northern border of Patagonia, Argentina (38°55’S, 70°44’W) were selected for stable isotopes analyses. The strong correlation between the isotopic composition of the first and second parts of the same ring, but also the strong relationships between δ¹³C and δ¹⁸O records with climate parameters of the current growing season such as temperature, show that tree-rings are built mostly with carbohydrates produced during the current growing season with little or no supply from storage or reserves. This finding leads to reconsidering the interpretation of the legacy effect (i.e. ecological memory effects) based on the previously described strong negative correlation between A. araucana TRW chronologies and previous growing season temperature and suggests a dependence of radial tree growth on the level of development of organs. Regarding climate sensitivity, the A. araucana tree-ring δ¹³C chronology is strongly related to current summer temperature (r = 0.82, p < 0.001), vapour pressure deficit (VPD; r = 0.79, p < 0.001), precipitation (r = −0.53, p < 0.001) and SPEI2 (r = −0.73, p < 0.001). These strong relationships support the use of δ¹³C of A. araucana tree-ring cellulose to reconstruct past temperature variations at regional scale in relation with large-atmospheric drivers of climate variability such as the Southern Annular Mode. The A. araucana tree-ring δ¹⁸O chronology is also correlated with temperature (r = 0.42, p < 0.01) and VPD (r = 0.45, p < 0.01) of the winter preceding the growing season. This suggests that trees are using water from precipitation infiltrated in the soil during the previous recharge period (autumn-winter). The weak correlations of δ¹⁸O with current summer atmospheric conditions and the decoupling between δ¹⁸O and δ¹³C, may be due to a high rate of oxygen exchange between sugars and xylem water (P_ex) during cellulose synthesis, which dampens evaporative isotopic fractionation.     

Do centennial tree-ring and stable isotope trends of Larix gmelinii (Rupr.) Rupr. indicate increasing water shortage in the Siberian north?

Tree-ring width of Larix gmelinii (Rupr.) Rupr., ratios of stable isotopes of C (δ¹³C) and O (δ¹⁸O) of whole wood and cellulose chronologies were obtained for the northern part of central Siberia (Tura, Russia) for the period 1864–2006. A strong decrease in the isotope ratios of O and C (after atmospheric δ¹³C corrections) and tree-ring width was observed for the period 1967–2005, while weather station data show a decrease in July precipitation, along with increasing July air temperature and vapor pressure deficit (VPD). Temperature at the end of May and the whole month of June mainly determines tree radial growth and marks the beginning of the vegetation period in this region. A positive correlation between tree-ring width and July precipitation was found for the calibration period 1929–2005. Positive significant correlations between C isotope chronologies and temperatures of June and July were found for whole wood and cellulose and negative relationships with July precipitation. These relationships are strengthened when the likely physiological response of trees to increased CO₂ is taken into account (by applying a recently developed δ¹³C correction). For the O isotope ratios, positive relationships with annual temperature, VPD of July and a negative correlation with annual precipitation were observed. The δ¹⁸O in tree rings may reflect annual rather than summer temperatures, due to the late melting of the winter snow and its contribution to the tree water supply in summer. We observed a clear change in the isotope and climate trends after the 1960s, resulting in a drastic change in the relationship between C and O isotope ratios from a negative to a positive correlation. According to isotope fractionation models, this indicates reduced stomatal conductance at a relatively constant photosynthetic rate, as a response of trees to water deficit for the last half century in this permafrost region.     

Seeing the forest for the trees: New approaches and challenges for dendroarchaeology in the 21st century

The application of tree-ring research to the study of cultural heritage has seen important conceptual and methodological developments in the 21 st century. Following the breakthrough discovery in the 1980s of the importation of timber from the south-eastern Baltic to the Low Countries for panel paintings, the historical timber trade acquired paramount relevance in European dendrochronology. The improvement of methods and tools to locate the area of origin of the wood has since become a focal line of research. Reference chronologies of different variables (ring width, earlywood, latewood, earlywood vessel size in oak, latewood density in conifers, stable isotope chronologies of δ¹³C, δ¹⁸O) are now being developed in areas formerly (and currently) exploited for timber production, and isotopic signatures of ⁸⁷Sr/⁸⁶Sr are being mapped to provide a geochemical reference. In parallel, novel techniques to identify wood species (automated wood identification, chemical biomarkers, DNA barcoding) and their application on historical and ancient wood are being explored, given that this could sometimes help narrow down the timber source area. Modern technology is playing a key role in the study of wooden objects through non-invasive methods, and collaboration with (art) historians, mathematicians, engineers and conservators has proven essential in current achievements. Tree-ring series can now be retrieved from high resolution X-ray computed tomography images, allowing the research of otherwise inaccessible pieces. This paper reviews recent advances in those fields (tree-ring based dendroprovenancing, wood species identification, chemical fingerprinting, use of genetic markers, isotopic signatures, and non-invasive methods), and discusses their implementation and challenges in dendroarchaeological studies.     

Acid treatment effects on the stable isotopic signatures of fossils

Abstract: Prior to geochemical analyses, fossil bones and teeth are often extracted from any surrounding lithified sediments using chemical techniques such as immersion in acid. As stable isotope analysis becomes more commonplace in palaeoecological investigations, it is important to consider what effects these chemical preparation techniques may have on any subsequent isotopic data and to constrain these effects as quantitatively as possible. This study aims to elucidate these effects, as it is vital that variability in a data set should not be introduced as a result of protocols used during sample preparation; in addition, it defines the most effective and viable method of carbonate removal for processing bulk fossil samples without causing alteration of their stable isotopic signatures. Various strengths of two weak acids commonly used during palaeontological preparation were tested to evaluate their effects on the δ¹⁵N and δ¹³C_org isotopic signatures of the vertebrae of a large Eocene fossil fish. Changes in the isotopic values occurred over time regardless of which acid was used, each causing a variable response in both δ¹⁵N and δ¹³C_org isotopic values. Without careful monitoring of the acidification process in a controlled environment, any resulting data could therefore confound interpretation. Based on these experiments, it is recommended that 2 m acetic acid be used for the pretreatment of fossils prior to the acquisition of N and C isotope data where carbonate removal is necessary.     

Stable carbon isotope analysis reveals widespread drought stress in boreal black spruce forests

Unprecedented rates of climate warming over the past century have resulted in increased forest stress and mortality worldwide. Decreased tree growth in association with increasing temperatures is generally accepted as a signal of temperature‐induced drought stress. However, variations in tree growth alone do not reveal the physiological mechanisms behind recent changes in tree growth. Examining stable carbon isotope composition of tree rings in addition to tree growth can provide a secondary line of evidence for physiological drought stress. In this study, we examined patterns of black spruce growth and carbon isotopic composition in tree rings in response to climate warming and drying in the boreal forest of interior Alaska. We examined trees at three nested scales: landscape, toposequence, and a subsample of trees within the toposequence. At each scale, we studied the potential effects of differences in microclimate and moisture availability by sampling on northern and southern aspects. We found that black spruce radial growth responded negatively to monthly metrics of temperature at all examined scales, and we examined ?¹³C responses on a subsample of trees as representative of the wider region. The negative ?¹³C responses to temperature reveal that black spruce trees are experiencing moisture stress on both northern and southern aspects. Contrary to our expectations, ?¹³C from trees on the northern aspect exhibited the strongest drought signal. Our results highlight the prominence of drought stress in the boreal forest of interior Alaska. We conclude that if temperatures continue to warm, we can expect drought‐induced productivity declines across large regions of the boreal forest, even for trees located in cool and moist landscape positions.     

Nitrogen Isotope Patterns in Alaskan Black Spruce Reflect Organic Nitrogen Sources and the Activity of Ectomycorrhizal Fungi

Global patterns in soil, plant, and fungal stable isotopes of N (δ¹⁵N) show promise as integrated metrics of N cycling, particularly the activity of ectomycorrhizal (ECM) fungi. At small spatial scales, however, it remains difficult to differentiate the underlying causes of plant δ¹⁵N variability and this limits the application of such measurements to better understand N cycling. We conducted a landscape-scale analysis of δ¹⁵N values from 31 putatively N-limited monospecific black spruce (Picea mariana) stands in central Alaska to assess the two main hypothesized sources of plant δ¹⁵N variation: differing sources and ECM fractionation. We found roughly 20% of the variability in black spruce foliar N and δ¹⁵N values to be correlated with the concentration and δ¹⁵N values of soil NH₄ ⁺ and dissolved organic N (DON) pools, respectively. However, ¹⁵N-based mixing models from 24 of the stands suggested that fractionation by ECM fungi obscures the ¹⁵N signature of soil N pools. Models, regressions, and N abundance data all suggested that increasing dependence on soil DON to meet black spruce growth demands predicates increasing reliance on ECM-derived N and that black spruce, on average, received 53% of its N from ECM fungi. Future research should partition the δ¹⁵N values within the soil DON pool to determine how choice of soil δ¹⁵N values influence modeled ECM activity. The C balance of boreal forests is tightly linked to N cycling and δ¹⁵N values may be useful metrics of changes to these connections.