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 20^th 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.     

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.     

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.     

Volcanic activity signals in tree-rings at the treeline of the Popocatépetl,Mexico

The Popocatépetl volcano resumed its eruptive activity in 1994 and is still active. The largest eruption recorded during this new stage of activity occurred in December 2000. We traced the volcanic activity signal in tree-rings from Pinus hartwegii trees located in the north slope of the volcano, located at ∼3 km from the volcanic cone. Annually resolved tree-ring widths, elemental and stable δ¹³C and δ¹⁸O isotope composition were measured during the period 1989–2014 to study the effects of the volcanic activity on trees. Our results indicate a high increase in the concentration of metal elements (Co, Cr, Cu, Fe, Li, Mo, Ni, Pb, Rb, Sr, Ti, Zn) in tree rings following the major 2000 volcanic eruption, compared to the pre-eruption period from 1989 to 1993. Other chemical elements such as Al, K and S peaked 2 years later, in the 2003 tree ring, that matched with the formation of a very narrow ring that year. This sharp reduction of growth was probably driven by a combination of harsh climatic conditions (drought) with the lagged negative effects of the 2000 eruption. Carbon isotope discrimination (Δ¹³C) and δ¹⁸O increased from 1995 to 2006, suggesting reduced stomatal conductance, photosynthetic activity and water use efficiency due to the large dust veil covering the study zone. The variation of relevant elements (Ca, Mn) showing significant correlations with tree growth, Δ¹³C and δ¹⁸O can be attributed to the selective availability of elements following the soil acidification caused by the volcanic activity. Our findings suggest that the recent activity of the Popocatépetl might have increased tree vulnerability, as reflected in the sharp reduction of growth following the drought recorded 2 years after the large eruption of December 2000. Our results warn about the cumulative negative effects of volcanic activity and harsh climatic conditions on tree growth and functioning.     

Regional differences in the carbon isotopic compositions of teak from two monsoonal regimes of India

Inter and intra-annual carbon isotope compositions (δ¹³C) of several annual growth rings of teak trees from two monsoonal regimes from India were studied and compared with the corresponding oxygen isotopic (δ¹⁸O) variations. In teak from both the regimes, amplitudes of intra-annual δ¹³C were ∼2-3 times lower than that observed in δ¹⁸O. Seasonal cycle with lower δ¹³C values at the middle and higher at ring boundaries was observed for teak from central India, dominated by the southwest monsoon. Positive correlations of intra-annual δ¹³C values with the corresponding δ¹⁸O values of the same rings and with relative humidity (RH) of the concurrent period suggest a dominant role of RH in controlling δ¹³C values of teak from central India. Intra-annual δ¹³C variations of teak from southern India, receiving both the southwest and northeast monsoons, revealed an initial decreasing trend followed by an increasing trend before culminating in depleted ¹³C values at the end of the growing season. No correlation was observed between intra-annual δ¹³C and δ¹⁸O variations of teak trees from southern India. Regional differences in the climatology of δ¹³C of atmospheric CO₂ or the lengths of growing season could be likely reasons for differing intra-annual δ¹³C variations of teak from the two climatic regimes.     

Climatic responses of tree-ring width and δ13C signatures of sessile oak (Quercus petraea Liebl.) on soils with contrasting water supply

We investigated climate–growth relationships (in terms of tree-ring width, basal area increment (BAI), and tree-ring δ¹³C signatures) of Quercus petraea in Central Europe (Luxembourg). Tree responses were assessed for 160 years and compared for sites with contrasting water supply (i.e. Cambisols vs. Regosols with 175 and 42 mm available water capacity, respectively). Oak trees displayed very low climate sensitivity, and climatic variables explained only 24 and 21 % of variance in tree-ring width (TRW) (Cambisol and Regosol sites, respectively). Contrary to our expectations, site-related differences in growth responses (i.e. BAI, δ¹³C signatures) to climate shifts were not significant. This finding suggests a high plasticity of oak trees in the study area. Despite a distinct growth depression found for all trees in the decade 1988–1997 (attributable to increasing annual mean temperatures by 1.1 °C), oak trees completely recovered in subsequent years. This indicates a high resilience of sessile oak to climate change. Shifts in δ¹³C_corr signatures were mainly affected by temperature, and peaks in δ¹³C_corr values (corrected for the anthropogenic increase in atmospheric CO₂) coincided with decadal maximum temperatures. Correlations between δ¹³C signatures and TRW (mainly affected by precipitation) were not significant. This finding suggests that wood growth often was disconnected from carbon assimilation (e.g. due to carbon storage in the trunk or allocation to seeds). Since the selection of drought-resistant tree species gains importance within the context of adaptive forest management strategies, Q. petraea proves to be an adaptive tree species in Central Europe’s forests under shifting climatic conditions.     

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.     

Stable isotope records (O, C) of Jurassic aragonitic shells from England and NW Poland: palaeoecologic and environmental implications

Shells of fully marine Middle to Upper Jurassic molluscs from England and north-western Poland were analysed with respect to their stable isotope (δ¹⁸O, δ¹³C) compositions, and palaeoecological and environmental life conditions of these molluscs were inferred from them. Light microscopical and SEM inspection and an analysis of the minor element content (Fe, Mn, Mg, Sr) suggest rather unaltered isotope signals. The δ¹⁸O and δ¹³C values show a characteristic distribution among three groups of co-occurring organisms. Benthic (adult) bivalves generally preserved higher δ¹⁸O and δ¹³C values than ammonites, whereas planktic bivalve larvae tend to possess the lowest δ¹⁸O but higher δ¹³C than adult bivalves. As this distribution pattern is found in numerous horizons and sections of Bathonian to Kimmeridgian age in NW Poland and England, it is thought to reflect real palaeoenvironmental parameters. All observations can be incorporated in a single model that assumes (i) seasonally induced temperature stratification of the water column, (ii) a correlation between phytoplankton blooms and reproduction season of planktic-planktotrophic bivalves, and (iii) insignificant vital effects with respect to the δ¹³C in bivalves, but strong biological control in ammonites. In addition, the δ¹⁸O evolution suggests that the Late Bajocian to Middle/Late Bathonian and Early Oxfordian to Late Kimmeridgian were considerably warmer than the latest Bathonian to Late Callovian time interval. The oxygen isotopic records from other European regions indicate a similar pattern of long-term palaeotemperature evolution. The comparatively high water temperatures during the Callovian to Oxfordian of the Isle of Skye (NW Scotland) are enigmatic, however. In the Early Oxfordian, sea surface and bottom temperatures began to rise in continental Europe and England. These changes coincide with a south-westward drift of the West European crustal plate, but a causal relationship remains to be demonstrated.     

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

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

Relevance of using whole-ring stable isotopes of black spruce trees in the perspective of climate reconstruction

Studies in dendroisotope chemistry suggested that latewood cellulose contains better climatic records than whole-ring cellulose. However, this approach has never been tested on northeastern Canadian spruce trees. This study compares dendroisotopic series of cellulose from late and whole ring, and analyses their statistical relationships with hydro-climatic variables with the aim of selecting the best suited protocol for future hydro-climatic reconstruction in the downstream sector of Churchill River basin of Labrador, Canada. To this end, δ¹³C and δ¹⁸O series from latewood (LW) and whole ring (WR) α-cellulose of black spruce trees (Picea mariana [Mill.] B.S.P.) were produced for the 1940–2010 period. The results show strong correlations between LW and WR isotopic series suggesting that there are no important variation in the isotopic ratios during the growing year and that black spruce trees use photosynthates of the current growing season to form their earlywood. Moreover, LW and WR δ¹³C and δ¹⁸O show similar relationships with both maximum temperature (T_max) and Churchill River discharge. Correlations are higher when combining δ¹³C and δ¹⁸O for LW and WR. Overall, those correlations support the indirect relationship between tree-ring isotopic series and river discharge, as they are integrators of several climatic variables and derived parameters (T_max, relative humidity, evapotranspiration, etc.). The LW and WR isotopic series give similar statistical relationships with hydro-climatic variables, and the WR treatment is faster (separation easier compared to LW). Thus, for black spruce the use of combined isotopic series in WR can be favored over LW for hydro-climatic reconstruction in the study region.     

Increasing carbon discrimination rates and depth of water uptake favor the growth of Mediterranean evergreen trees in the ecotone with temperate deciduous forests

Tree populations at the low‐altitudinal or ‐latitudinal limits of species' distributional ranges are predicted to retreat toward higher altitudes and latitudes to track the ongoing changes in climate. Studies have focused on the climatic sensitivity of the retreating species, whereas little is known about the potential replacements. Competition between tree species in forest ecotones will likely be strongly influenced by the ecophysiological responses to heat and drought. We used tree‐ring widths and δ¹³C and δ¹⁸O chronologies to compare the growth rates and long‐term ecophysiological responses to climate in the temperate‐Mediterranean ecotone formed by the deciduous Fagus sylvatica and the evergreen Quercus ilex at the low altitudinal and southern latitudinal limit of F. sylvatica (NE Iberian Peninsula). F. sylvatica growth rates were similar to those of other southern populations and were surprisingly not higher than those of Q. ilex, which were an order of magnitude higher than those in nearby drier sites. Higher Q. ilex growth rates were associated with high temperatures, which have increased carbon discrimination rates in the last 25 years. In contrast, stomatal regulation in F. sylvatica was proportional to the increase in atmospheric CO₂. Tree‐ring δ¹⁸O for both species were mostly correlated with δ¹⁸O in the source water. In contrast to many previous studies, relative humidity was not negatively correlated with tree‐ring δ¹⁸O but had a positive effect on Q. ilex tree‐ring δ¹⁸O. Furthermore, tree‐ring δ¹⁸O decreased in Q. ilex over time. The sensitivity of Q. ilex to climate likely reflects the uptake of deep water that allowed it to benefit from the effect of CO₂ fertilization, in contrast to the water‐limited F. sylvatica. Consequently, Q. ilex is a strong competitor at sites currently dominated by F. sylvatica and could be favored by increasingly warmer conditions.     

Mother–embryo isotope fractionation in the pygmy devilray Mobula kuhlii cf. eregoodootenkee

We determined stable-isotope ratios for replicate muscle tissues in 13 gravid Mobula kuhlii cf. eregoodootenkee (110.4–120.4 cm disc width; W_D) and their embryos (7.0–42.3 cm W_D) and also yolks and histrotroph, to assess the potential implications for juvenile nutrition and habitat use. Irrespective of their development in the uterus, embryos had similar δ¹³C values in their muscle tissue as the mothers and both had greater values than in the histotroph. During gestation, δ¹³C values increased across all sample types. However, while embryo muscle tissue and the histotroph were associated with increasing ¹⁵N levels during embryonic development, this was depleted in the mothers’ muscle tissue and yolk. Although speculative, the observed variation in stable-isotope ratios might imply a dietary shift among gravid females during their early gestation. Irrespective of the underlying mechanisms, the results indicate neonates will have relatively greater δ¹⁵N values than post-partum females, which would probably confound juvenile foraging-ecology estimates.     

Does resin tapping affect the tree-ring growth and climate sensitivity of the Chinese pine (Pinus tabuliformis) in the Loess Plateau,China?

Resin tapping could affect water and nutrient transport processes in Chinese pine trees, rendering them more vulnerable to extreme climatic events, such as drought, and affecting the ecological function of forests in semi-arid regions. This study evaluated how resin tapping affects the tree-ring growth and climate sensitivity of Chinese pine in the Loess Plateau. We compared tree-ring growth patterns between the tapped and untapped faces of tapped trees, and investigated tree-ring growth and its response to climate between tapped and untapped trees in a forest stand during the 1967–2017 period. Tapped trees showed asymmetrical growth patterns after resin tapping, with narrower rings near the tapped face and wider ones near the untapped face. Furthermore, tapped trees had inter-annual variations consistent with those of untapped trees except for the years 2000 and 2001, with significantly lower values following resin tapping, and tree-ring growth then returning to normal. The climate response analysis indicated that the tree-ring growth of both tapped and untapped trees was negatively affected by monthly mean temperatures during the early growing season (May to July) in the post-resin-tapping period. Furthermore, tree-ring growth in tapped trees also revealed significant correlation with water vapour deficit and the Palmer drought index, which indicates that tapped trees are more vulnerable to drought. Further studies based on stable isotopes (i.e. δ¹³C, δ¹⁸O, and δ¹⁵N) could improve our understanding of the physiological mechanisms that regulate the effects of resin tapping on tree-ring growth.     

Recent and historical discharge of a large European river system - oxygen isotopic composition of river water and skeletal aragonite of Unionidae in the Rhine

Seasonal variations in the oxygen isotopic composition of Rhine River water were analyzed in detail and compared with the oxygen isotopic record from recent and historical specimens of freshwater bivalves (Unionidae). The purpose of this study was to investigate the potential use of these aragonitic bivalves as proxy recorders for varying amounts and sources of discharge, and thereby infer climate change. Seasonal variations in the river water δ¹⁸O are on the order of 1-2‰. During summer, Alpine melt-water contributes significantly to the total discharge, resulting in average values of −10 to −10.5‰, whereas the non-Alpine contribution is higher during winter, as indicated by mean δ¹⁸O values of −8.5 to −9‰. The basic pattern of the modern seasonal variation of δ¹⁸O of river water can be described by a numerical mass balance approximation of the various contributions from the Alpine and non-Alpine catchments with their average δ¹⁸O composition. The δ¹⁸O of growth increments in the prismatic shell layer of Anodonta corresponds perfectly to what is predicted by known fractionation of ¹⁸O between water and aragonite. Shell growth is restricted to water temperatures above 8-10°C, so variations in δ¹⁸O and the river water temperature are faithfully recorded by relatively large growth increments during summer. The distinctive isotopic signatures of individual flood events during summer and autumn are also recorded in the shells.     

A retrospective,dual‐isotope approach reveals individual predispositions to winter‐drought induced tree dieback in the southernmost distribution limit of Scots pine

Winter‐drought induced forest diebacks in the low‐latitude margins of species' distribution ranges can provide new insights into the mechanisms (carbon starvation, hydraulic failure) underlying contrasting tree reactions. We analysed a winter‐drought induced dieback at the Scots pine's southern edge through a dual‐isotope approach (Δ¹³C and δ¹⁸O in tree‐ring cellulose). We hypothesized that a differential long‐term performance, mediated by the interaction between CO₂ and climate, determined the fates of individuals during dieback. Declining trees showed a stronger coupling between climate, growth and intrinsic water‐use efficiency (WUEi) than non‐declining individuals that was noticeable for 25 years prior to dieback. The rising stomatal control of water losses with time in declining trees, indicated by negative Δ¹³C‐δ¹⁸O relationships, was likely associated with their native aptitude to grow more and take up more water (suggested by larger tracheid lumen widths) than non‐declining trees and, therefore, to exhibit a greater cavitation risk. Freeze‐thaw episodes occurring in winter 2001 unveiled such physiological differences by triggering dieback in those trees more vulnerable to hydraulic failure. Thus, WUEi tightly modulated growth responses to long‐term warming in declining trees, indicating that co‐occurring individuals were differentially predisposed to winter‐drought mortality. These different performances were unconnected to the depletion of stored carbohydrates.     

Seasonal stable isotope variations of the modern Amazonian freshwater bivalve Anodontites trapesialis

In a floodplain lake of the Amazon River near the city of Iquitos, northeastern Peru, a one-year monitoring experiment was conducted during which water samples and living bivalves (Anodontites trapesialis) were collected with the aim to investigate seasonal δ¹⁸O variation in and fractionation between bivalve aragonite and host water. Both host water and molluscan growth increments show more than 8‰ seasonal variation in δ¹⁸O. In the floodplain lake under study the δ¹⁸O variation of the water is controlled by contrasting dry and wet season evaporation-precipitation regimes. Molluscan δ¹⁸O appears to be in equilibrium with the host water. Although an approximately 4.0‰ offset occurs, δ¹³C records of water and bivalves are in good agreement, suggesting that both δ¹⁸O and δ¹³C of the shells of freshwater bivalve A. trapesialis are good recorders of (palaeo-)environmental conditions. The δ¹³C of Dissolved Inorganic Carbon (DIC) is governed by plant growth and/or by changes in aquatic chemistry, affecting the DIC pool.     

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.     

Tree-ring responses in Araucaria araucana to two major eruptions of Lonquimay Volcano (Chile)

Palaeoclimate proxies have demonstrated links between climate changes and volcanic activity. However, not much is known about the impact of volcanic eruptions on forest productivity. Here we used tree-ring width and annually resolved carbon and oxygen isotopic records from tree rings of Araucaria araucana (Molina) K. Koch, providing a centennial-scale reconstruction of tree ecophysiological processes in forest stands nearby the Lonquimay Volcano (Chile). We observed a mean decrease in tree-ring width following the major eruption of 1988–1990 (with aerosol emission), most probably caused by the modified ecological conditions due to acid rain and ash deposition, while a generally negative relationship between δ¹³C and δ¹⁸O would point to a decline in humidity and precipitation. More negative δ¹³C and lower δ¹⁸O values (positive correlation) following the major eruption of 1887–1890 (without aerosol emission) would suggest high stomatal conductance and moisture availability, though tree-ring width (and probably photosynthetic rate) was unaltered. At least for this sample of trees, in the case of eruption with large tephra emission, the beneficial effect of aerosol light scattering on tree productivity appears to be outweighed by the detrimental effect of eruption-induced toxic deposition. Signals of the two major eruptions of the past 200?years at Lonquimay were present in tree rings of nearby A. araucana. No unique response of tree functions to volcanic eruptions can be expected, but rather (1) the variable volcanic properties and (2) the complex interplay of diffuse light increase (aerosol scattering), air temperature decrease (cloud shading), and toxic deposition impact (volcanic ash), makes any prediction of tree growth and ecophysiological response very challenging.     

Variations in bone collagen δC and δN values of fauna from Northwest Europe over the last 40?000 years

We report here a signal in the temporal variation of stable isotopes in protein from surviving animal bone in Northwest Europe over the past glacial cycle. There is a change in the average δ¹³C values of fauna in the Holocene, and there is also a significant reduction in δ¹⁵N values of herbivore bone collagen towards the end of the last glaciation, with a subsequent recovery soon after the start of the Holocene. This change is observed for several species and is restricted to those regions most affected by the glacial advance. Comparison with ice core data shows that there is a strong correlation between the average δ¹³C values of three herbivore species and ice core CO₂ concentration. The data presented here show how readily available faunal bone collagen δ¹³C and δ¹⁵N measurements provide a record of past climate and environmental change on a regional to continental scale.     

Foliar and litter needle carbon and oxygen isotope compositions relate to tree growth of an exotic pine plantation under different residue management regimes in subtropical Australia

Background and aim

Significant differences in tree growth were observed in an exotic pine plantation under different harvest residue management regimes at ages 2–10 years. However, the variations in tree growth between residue management treatments could not be explained by soil and foliar nutrient analyses, except by potassium (K) concentration. Therefore, this study determined the carbon isotope composition (δ¹³C) and oxygen isotope composition (δ¹⁸O) of current and archived foliar samples from the exotic pine plantation to establish relationships with foliar K concentration and tree growth indices as a means to determine changes in stomatal conductance (g_s) and photosynthetic rate (A_max) or water use efficiency (WUE), and therefore understand the variations in tree growth across treatments.

Methods

The harvest residue treatments were: (1) residue removal, RR₀; (2) single level residue retention, RR₁; and (3) double level residue retention, RR₂. Foliar δ¹³C and δ¹⁸O were determined for samples at ages 2, 4, 6 and 10 years, and the atmospheric ¹³C discrimination (Δ¹³C), intercellular CO₂ concentration (C_i) and WUE were determined from the δ¹³C data. Litter needle δ¹³C and δ¹⁸O were also determined over 15 months between ages 9 and 10 years. These parameters or variables where correlated to each other as well as to the periodic mean annual increment of basal area (PAIB) and the periodic mean annual increment of tree diameter at breast height (PAID) across the treatments and over time. Foliar δ¹³C and δ¹⁸O were also related to published data of foliar K concentrations of the same trees.

Results

Significant variations of foliar δ¹³C, and therefore WUE and Δ¹³C, across treatments were only observed at ages 4 and 10 years old, and foliar δ¹⁸O at age 4 years old only. The results showed increasing foliar δ¹³C, δ¹⁸O and WUE, and decreasing Δ¹³C and Ci, from RR₀ to RR₂ treatments. However, while the WUE was positively related to the PAID and PAIB at age 4 years, it was negatively related to PAID and PAIB at age 10 years old. Litter needle δ¹³C, indicative of WUE, was also negatively related to the PAID at age 10 years old. . At age 4 years, foliar δ¹³C and δ¹⁸O were positively related with a steep slope of 7.70 ‰ across treatments, and that both isotopes were positively related to foliar K concentrations. Similarly, δ¹⁸O was negatively related to the Δ¹³C. No significant relationship can be determined between foliar δ¹³C, or Δ¹³C, and δ¹⁸O at age 10 years old. In addition, WUE was increasing (p?<?0.001) and Δ¹³C and C_i decreasing (p?<?0.001) with decreasing PAID over time.

Conclusions

The variations at age 4 years in foliar δ¹³C or Δ¹³C and δ¹⁸O and increasing WUE with increasing growth rate suggest growth induced water-stress with increasing residue-loading rate as a result of the nutritional effect of the harvest residues on tree growth. At age 10 years, the negative relationships between WUE and PAID indicate nutrient limitation has an over-riding effect on δ¹³C variations rather than g_s. This was due to the lack of a significant relationship between foliar Δ¹³C and δ¹⁸O at this age, as well as over time.