The enigma of effective path length for (18) O enrichment in leaf water of conifers

H2 18O vapour labelling reveals evidence of radial Péclet effects, but in not all leaves

Contradictory evidence exists regarding the relevance of Péclet-like gradients in leaf water isotopes, making it difficult to accurately predict variation in isotope composition. Here, we use H2 18O vapour labelling to directly test whether leaf water isotopes diffuse back into the xylem to be carried forward to more distal leaf portions. Backward diffusion has been assumed, due to observations of increasing enrichment towards the tip and outer edges of some leaves. Further complicating the selection of leaf water isotope models is the observation that some, but not all, leaves demonstrate a radial Péclet effect in bulk leaf water and that the hydraulic design of leaves may influence the development of isotope gradients in leaves. Carry-forward of H2 18O vapour label was detected in the two monocot species assessed (oat and corn), but not in the two dicot species (foxglove and sunflower). Further, bulk leaf water measurements at differing transpiration rates indicated that a bulk leaf water Péclet effect was relevant for foxglove only. We conclude that both leaf hydraulic design and relative velocities of water within transport pathways influence leaf water isotope composition, reconciling seemingly contradictory previous results regarding the relevance of Péclet effects to leaf water isotopes.

Effects of increasing atmospheric CO2 on leaf water δ18O values are small and are attenuated in grasses and amplified in dicotyledonous herbs and legumes when transferred to cellulose δ18O values

The oxygen isotope composition of cellulose (δ18O values) has been suggested to contain information on stomatal conductance (gs) responses to rising pCO2. The extent by which pCO2 affects leaf water and cellulose δ18O values (δ18OLW and δ18OC) and the isotope processes that determine pCO2 effects on δ18OLW and δ18OC are, however, unknown. We tested the effects of pCO2 on gs, δ18OLW and δ18OC in a glasshouse experiment, where six plant species were grown under pCO2 ranging from 200 to 500 ppm. Increasing pCO2 caused a decline in gs and an increase in δ18OLW, as expected. Importantly, the effects of pCO2 on gs and δ18OLW were small and pCO2 effects on δ18OLW were not directly transferred to δ18OC but were attenuated in grasses and amplified in dicotyledonous herbs and legumes. This is likely because of functional group-specific pCO2 effects on the model parameter pxpex. Our study highlights important uncertainties when using δ18OC as a proxy for gs. Specifically, pCO2-triggered gs effects on δ18OLW and δ18OC are possibly too small to be detected in natural settings and a pCO2 effect on pxpex may render the commonly assumed negative linkage between δ18OC and gs to be incorrect, potentially confounding δ18OC based gs reconstructions.

Unenriched xylem water contribution during cellulose synthesis influenced by atmospheric demand governs the intra-annual tree-ring δ18 O signature

The oxygen isotope composition (δ18 O) of tree-ring cellulose is used to evaluate tree physiological responses to climate, but their interpretation is still limited due to the complexity of the isotope fractionation pathways. We assessed the relative contribution of seasonal needle and xylem water δ18 O variations to the intra-annual tree-ring cellulose δ18 O signature of larch trees at two sites with contrasting soil water availability in the Swiss Alps. We combined biweekly δ18 O measurements of soil water, needle water, and twig xylem water with intra-annual δ18 O measurements of tree-ring cellulose, xylogenesis analysis, and mechanistic and structural equation modeling. Intra-annual cellulose δ18 O values resembled source water δ18 O mean levels better than needle water δ18 O. Large parts of the rings were formed under high proportional exchange with unenriched xylem water (pex ). Maximum pex values were achieved in August and imprinted on sections at 50-75% of the ring. High pex values were associated with periods of high atmospheric evaporative demand (VPD). While VPD governed needle water δ18 O variability, we estimated a limited Péclet effect at both sites. Due to a variable pex , source water has a strong influence over large parts of the intra-annual tree-ring cellulose δ18 O variations, potentially masking signals coming from needle-level processes.

Explicitly accounting for needle sugar pool size crucial for predicting intra-seasonal dynamics of needle carbohydrates δ18 O and δ13 C

We explore needle sugar isotopic compositions (δ¹⁸ O and δ¹³ C) in boreal Scots pine (Pinus sylvestris) over two growing seasons. A leaf-level dynamic model driven by environmental conditions and based on current understanding of isotope fractionation processes was built to predict δ¹⁸ O and δ¹³ C of two hierarchical needle carbohydrate pools, accounting for the needle sugar pool size and the presence of an invariant pinitol pool. Model results agreed well with observed needle water δ¹⁸ O, δ¹⁸ O and δ¹³ C of needle water-soluble carbohydrates (sugars + pinitol), and needle sugar δ¹³ C (R²  = 0.95, 0.84, 0.60, 0.73, respectively). Relative humidity (RH) and intercellular to ambient CO₂ concentration ratio (C_i /C_a ) were the dominant drivers of δ¹⁸ O and δ¹³ C variability, respectively. However, the variability of needle sugar δ¹⁸ O and δ¹³ C was reduced on diel and intra-seasonal timescales, compared to predictions based on instantaneous RH and C_i /C_a , due to the large needle sugar pool, which caused the signal formation period to vary seasonally from 2 d to more than 5 d. Furthermore, accounting for a temperature-sensitive biochemical ¹⁸ O-fractionation factor and mesophyll resistance in ¹³ C-discrimination were critical. Interpreting leaf-level isotopic signals requires understanding on time integration caused by mixing in the needle sugar pool.

Constraining parameter uncertainty for predicting oxygen and hydrogen isotope values in fruit

Understanding δ18O and δ2H values of agricultural products like fruit is of particular scientific interest in plant physiology, ecology, and forensic studies. Applications of mechanistic stable isotope models to predict δ18O and δ2H values of water and organic compounds in fruit, however, are hindered by a lack of empirical parameterizations and validations. We addressed this lack of data by experimentally evaluating model parameter values required to model δ18O and δ2H values of water and organic compounds in berries and leaves from strawberry and raspberry plants grown at different relative humidities. Our study revealed substantial differences between leaf and berry isotope values, consistent across the different relative humidity treatments. We demonstrated that existing isotope models can reproduce water and organic δ18O and δ2H values for leaves and berries. Yet, these simulations require organ-specific model parameterization to accurately predict δ18O and δ2H values of leaf and berry tissue and water pools. We quantified these organ-specific model parameters for both species and relative humidity conditions. Depending on the required model accuracy, species- and environment-specific model parameters may be justified. The parameter values determined in this study thus facilitate applications of stable isotope models where understanding δ18O and δ2H values of fruit is of scientific interest.

The unknown third - Hydrogen isotopes in tree-ring cellulose across Europe

This is the first Europe-wide comprehensive assessment of the climatological and physiological information recorded by hydrogen isotope ratios in tree-ring cellulose (δ²H_c) based on a unique collection of annually resolved 100-year tree-ring records of two genera (Pinus and Quercus) from 17 sites (36°N to 68°N). We observed that the high-frequency climate signals in the δ²H_c chronologies were weaker than those recorded in carbon (δ¹³C_c) and oxygen isotope signals (δ¹⁸O_c) but similar to the tree-ring width ones (TRW). The δ²H_c climate signal strength varied across the continent and was stronger and more consistent for Pinus than for Quercus. For both genera, years with extremely dry summer conditions caused a significant ²H-enrichment in tree-ring cellulose. The δ²H_c inter-annual variability was strongly site-specific, as a result of the imprinting of climate and hydrology, but also physiological mechanisms and tree growth. To differentiate between environmental and physiological signals in δ²H_c, we investigated its relationships with δ¹⁸O_c and TRW. We found significant negative relationships between δ²H_c and TRW (7 sites), and positive ones between δ²H_c and δ¹⁸O_c (10 sites). The strength of these relationships was nonlinearly related to temperature and precipitation. Mechanistic δ²H_c models performed well for both genera at continental scale simulating average values, but they failed on capturing year-to-year δ²H_c variations. Our results suggest that the information recorded by δ²H_c is significantly different from that of δ¹⁸O_c, and has a stronger physiological component independent from climate, possibly related to the use of carbohydrate reserves for growth. Advancements in the understanding of ²H-fractionations and their relationships with climate, physiology, and species-specific traits are needed to improve the modelling and interpretation accuracy of δ²H_c. Such advancements could lead to new insights into trees' carbon allocation mechanisms, and responses to abiotic and biotic stress conditions.

Heterogeneous isotope effects decouple conifer leaf and branch sugar δ18O and δ13C

Isotope ratios of tree-ring cellulose are a prominent tool to reconstruct paleoclimate and plant responses to environmental variation. Current models for cellulose isotope ratios assume a transfer of the environmental signals recorded in bulk leaf water to carbohydrates and ultimately into stem cellulose. However, the isotopic signal of carbohydrates exported from leaf to branch may deviate from mean leaf values if spatial heterogeneity in isotope ratios exists in the leaf. We tested whether the isotopic heterogeneity previously observed along the length of a ponderosa pine (Pinus ponderosa) leaf water was preserved in photosynthetic products. We observed an increase in both sugar and bulk tissue δ¹⁸O values along the needle, but the increase in carbohydrate δ¹⁸O values was dampened relative to the trend observed in leaf water. In contrast, δ¹³C values of both sugar and bulk organic matter were invariant along the needle. Phloem-exported sugar measured in the branch below the needles did not match whole-needle values of δ¹⁸O or δ¹³C. Instead, there was a near-constant offset observed between the branch and needle sugar δ¹³C values, while branch δ¹⁸O values were most similar to δ¹⁸O values observed for sugar at the base of the needle. The observed offset between the branch and needle sugar δ¹⁸O values likely arises from partial isotope oxygen exchange between sugars and water during phloem loading and transport. An improved understanding of the conditions producing differential δ¹³C and δ¹⁸O isotope effects between branch phloem and needle sugars could improve tree-ring-based climate reconstructions.

Insight into Canary Island pine physiology provided by stable isotope patterns of water and plant tissues along an altitudinal gradient

The Canary Islands, an archipelago east of Morocco's Atlantic coast, present steep altitudinal gradients covering various climatic zones from hot deserts to subalpine Mediterranean, passing through fog-influenced cloud forests. Unlike the majority of the Canarian flora, Pinus canariensis C. Sm. ex DC. in Buch grow along most of these gradients, allowing the study of plant functioning in contrasting ecosystems. Here we assess the water sources (precipitation, fog) of P. canariensis and its physiological behavior in its different natural environments. We analyzed carbon and oxygen isotope ratios of water and organics from atmosphere, soil and different plant organs and tissues (including 10-year annual time series of tree-ring cellulose) of six sites from 480 to 1990 m above sea level on the Canary Island La Palma. We found a decreasing δ18O trend in source water that was overridden by an increasing δ18O trend in needle water, leaf assimilates and tree-ring cellulose with increasing altitude, suggesting site-specific tree physiological responses to relative humidity. Fog-influenced and fog-free sites showed similar δ13C values, suggesting photosynthetic activity to be limited by stomatal closure and irradiance at certain periods. In addition, we observed an 18O-depletion (fog-free and timberline sites) and 13C-depletion (fog-influenced and fog-free sites) in latewood compared with earlywood caused by seasonal differences in: (i) water uptake (i.e., deeper ground water during summer drought, fog water frequency and interception) and (ii) meteorological conditions (stem radial growth and latewood δ18O correlated with winter precipitation). In addition, we found evidence for foliar water uptake and strong isotopic gradients along the pine needle axis in water and assimilates. These gradients are likely the reason for an unexpected underestimation of pine needle water δ18O when applying standard leaf water δ18O models. Our results indicate that soil water availability and air humidity conditions are the main drivers of the physiological behavior of pine along the Canary Island's altitudinal gradients.

Can hydraulic design explain patterns of leaf water isotopic enrichment in C3 plants?

H₂ ¹⁸ O enrichment develops when leaves transpire, but an accurate generalized mechanistic model has proven elusive. We hypothesized that leaf hydraulic architecture may affect the degree to which gradients in H₂ ¹⁸ O develop within leaves, influencing bulk leaf stable oxygen isotope enrichment (Δ_L ) and the degree to which the Péclet effect is relevant in leaves. Leaf hydraulic design predicted the relevance of a Péclet effect to Δ_L in 19 of the 21 species tested. Leaves with well-developed hydraulic connections between the vascular tissue and the epidermal cells through bundle sheath extensions and clear distinctions between palisade and spongy mesophyll layers (while the mesophyll is hydraulically disconnected) may have velocities of the transpiration stream such that gradients in H₂ ¹⁸ O develop and are expressed in the mesophyll. In contrast, in leaves where the vascular tissue is hydraulically disconnected from the epidermal layers, or where all mesophyll cells are well connected to the transpiration stream, velocities within the liquid transport pathways may be low enough that gradients in H₂ ¹⁸ O are very small. Prior knowledge of leaf hydraulic design allows informed selection of the appropriate Δ_L modelling framework.

Effects of soil moisture, needle age and leaf morphology on carbon and oxygen uptake, incorporation and allocation: a dual labeling approach with 13CO2 and H218O in foliage of a coniferous forest

The carbon and oxygen isotopic composition of water and assimilates in plants reveals valuable information on plant responses to climatic conditions. Yet, the carbon and oxygen uptake, incorporation and allocation processes determining isotopic compositions are not fully understood. We carried out a dual-isotope labeling experiment at high humidity with 18O-enriched water (H218O) and 13C-enriched CO2 (13CO2) with attached Scots pine (Pinus sylvestris L.) branches and detached twigs of hemiparasitic mistletoes (Viscum album ssp. austriacum) in a naturally dry coniferous forest, where also a long-term irrigation takes place. After 4 h of label exposure, we sampled previous- and recent-year leaves, twig phloem and twig xylem over 192 h for the analysis of isotope ratios in water and assimilates. For both species, the uptake into leaf water and the incorporation of the 18O-label into leaf assimilates was not influenced by soil moisture, while the 13C-label incorporation into assimilates was significantly higher under irrigation compared with control dry conditions. Species-specific differences in leaf morphology or needle age did not affect 18O-label uptake into leaf water, but the incorporation of both tracers into assimilates was two times lower in mistletoe than in pine. The 18O-label allocation in water from pine needles to twig tissues was two times higher for phloem than for xylem under both soil moisture conditions. In contrast, the allocation of both tracers in pine assimilates were similar and not affected by soil moisture, twig tissue or needle age. Soil moisture effects on 13C-label but not on 18O-label incorporation into assimilates can be explained by the stomatal responses at high humidity, non-stomatal pathways for water and isotope exchange reactions. Our results suggest that non-photosynthetic 18O-incorporation processes may have masked prevalent photosynthetic processes. Thus, isotopic variation in leaf water could also be imprinted on assimilates when photosynthetic assimilation rates are low.

Seasonal and diurnal trends in progressive isotope enrichment along needles in two pine species

The Craig-Gordon type (C-G) leaf water isotope enrichment models assume a homogeneous distribution of enriched water across the leaf surface, despite observations that Δ¹⁸ O can become increasingly enriched from leaf base to tip. Datasets of this 'progressive isotope enrichment' are limited, precluding a comprehensive understanding of (a) the magnitude and variability of progressive isotope enrichment, and (b) how progressive enrichment impacts the accuracy of C-G leaf water model predictions. Here, we present observations of progressive enrichment in two conifer species that capture seasonal and diurnal variability in environmental conditions. We further examine which leaf water isotope models best capture the influence of progressive enrichment on bulk needle water Δ¹⁸ O. Observed progressive enrichment was large and equal in magnitude across both species. The magnitude of this effect fluctuated seasonally in concert with vapour pressure deficit, but was static in the face of diurnal cycles in meteorological conditions. Despite large progressive enrichment, three variants of the C-G model reasonably successfully predicted bulk needle Δ¹⁸ O. Our results thus suggest that the presence of progressive enrichment does not impact the predictive success of C-G models, and instead yields new insight regarding the physiological and anatomical mechanisms that cause progressive isotope enrichment.

How does varying water supply affect oxygen isotope variations in needles and tree rings of Scots pine?

In many regions, drought is suspected to be a cause of Scots pine decline and mortality, but the underlying physiological mechanisms remain unclear. Because of their relationship to ecohydrological processes, δ18O values in tree rings are potentially useful for deciphering long-term physiological responses and tree adaptation to increasing drought. We therefore analyzed both needle- and stem-level isotope fractionations in mature trees exposed to varying water supply. In a first experiment, we investigated seasonal δ18O variations in soil and needle water of Scots pine in a dry inner Alpine valley in Switzerland, comparing drought-stressed trees with trees that were irrigated for more than 10 years. In a second experiment, we analyzed twentieth-century δ18O variations in tree rings of the same forest, including a group of trees that had recently died. We observed less 18O enrichment in needle water of drought-stressed compared with irrigated trees. We applied different isotope fractionation models to explain these results, including the Péclet and the two-pool correction, which considers the ratio of unenriched xylem water in the needles to total needle water. Based on anatomical measurements, we found this ratio to be unchanged in drought-stressed needles, although they were shorter. The observed lower 18O enrichment in needles of stressed trees was therefore likely caused by increased effective path length for water movement within the leaf lamina. In the tree-ring study, we observed lower δ18O values in tree rings of dead trees compared with survivors during several decades prior to their death. These lower values in declining trees are consistent with the lower needle water 18O enrichment observed for drought-stressed compared with irrigated trees, suggesting that this needle-level signal is reflected in the tree rings, although changes in rooting depth could also play a role. Our study demonstrates that long-term effects of drought are reflected in the tree-ring δ18O values, which helps to provide a better understanding of past tree physiological changes of Scots pine.

Reduction in lumen area is associated with the δ18 O exchange between sugars and source water during cellulose synthesis

High temporal resolution measurements of wood anatomy and the isotopic composition in tree-rings have the potential to enhance our interpretation of climate variability, but the sources of variation within the growing season are still not well understood. Here we test the response of wood anatomical features in Pinus ponderosa and Pseudotsuga menziesii, including cell-wall thickness (CWT) and lumen area (LA), along with the oxygen isotopic composition of α-cellulose (δ¹⁸ O_cell ) to shifts in relative humidity (RH) in two treatments, one from high-low RH and the second one form low-high RH. We observed a significant decrease in LA and a small increase in CWT within the experimental growing season in both treatments. The measured δ¹⁸ O_cell along the ring was responsive to RH variations in both treatments. However, estimated δ¹⁸ O_cell did not agree with measured δ¹⁸ O_cell when the proportion of exchangeable oxygen during cellulose synthesis (P_ex ) was kept constant. We found that P_ex increased throughout the ring as LA decreased. Based on this varying P_ex within an annual ring, we propose a targeted sampling strategy for different hydroclimate signals: earlier season cellulose is a better recorder of RH while late-season cellulose is a better recorder of the source water.

Air moisture signals in a stable oxygen isotope chronology of dwarf shrubs from the central Tibetan Plateau

BACKGROUND AND AIMS

Annually resolved biological climate proxies beyond the altitudinal and latitudinal distribution limit of trees are rare. In such regions, several studies have demonstrated that annual growth rings of dwarf shrubs are suitable proxies for palaeoclimatic investigations. In High Asia, the pioneer work of Liang et al. (Liang E, Lu X, Ren P, Li X, Zhu L, Eckstein D, 2012. Annual increments of juniper dwarf shrubs above the tree line on the central Tibetan Plateau: a useful climatic proxy. Annals of Botany109: 721-728) confirmed the suitability of shrub growth-ring chronologies for palaeoclimatic research. This study presents the first sensitivity study of an annually resolved δ18O time series inferred from Wilson juniper (Juniperus pingii var. wilsonii) from the northern shoreline of lake Nam Co (Tibetan Plateau).

METHODS

Based on five individual dwarf shrub discs, a statistically reliable δ18O chronology covering the period 1957-2009 was achieved (expressed population signal = 0.80). Spearman's correlation analysis between the δ18O chronology and climate variables from different sources was applied. In a first step, the suitability of various climate data was evaluated.

KEY RESULTS

Examinations of climate-proxy relationships revealed significant negative correlations between the δ18O shrub chronology and summer season moisture variability of the previous and current year. In particular, relative humidity of the previous and current vegetation period significantly determined the proxy variability (ρ = -0.48, P < 0.01). Furthermore, the δ18O variability of the developed shrub chronology significantly coincided with a nearby tree-ring δ18O chronology of the same genus (r = 0.62, P < 0.01).

CONCLUSIONS

The δ18O shrub chronology reliably recorded humidity variations in the Nam Co region. The chronology was significantly correlated with a nearby moisture-sensitive tree-ring δ18O chronology, indicating a common climate signal in the two chronologies. This climate signal was likely determined by moisture variations of the Asian summer monsoon. Local climate effects were superimposed on the supra-regional climate signature of the monsoon circulation. Opposing δ18O values between the two chronologies were interpreted as plant-physiological differences during isotopic fractionation processes.

Water fluxes mediated by vegetation: emerging isotopic insights at the soil and atmosphere interfaces

Plants mediate water fluxes within the soil-vegetation-atmosphere continuum. This water transfer in soils, through plants, into the atmosphere can be effectively traced by stable isotopologues of water. However, rapid dynamic processes have only recently gained attention, such as adaptations in root water uptake depths (within hours to days) or the imprint of transpirational fluxes on atmospheric moisture, particularly promoted by the development of real-time in-situ water vapour stable isotope observation techniques. We focus on open questions and emerging insights at the soil-plant and plant-atmosphere interfaces, as we believe that these are the controlling factors for ecosystem water cycling. At both interfaces, complex pictures of interacting ecophysiological and hydrological processes emerge: root water uptake dynamics depend on both spatiotemporal variations in water availability and species-specific regulation of adaptive root conductivity within the rooting system by, for example, modulating soil-root conductivity in response to water and nutrient demands. Similarly, plant water transport and losses are a fine-tuned interplay between species-specific structural and functional strategies of water use and atmospheric processes. We propose that only by explicitly merging insights from distinct disciplines - for example, hydrology, plant physiology and atmospheric sciences - will we gain a holistic picture of the impact of vegetation on processes governing the soil-plant-atmosphere continuum.

Growth and wood isotopic signature of Norway spruce (Picea abies) along a small-scale gradient of soil moisture

Among the environmental factors that have an effect on the isotopic signature of tree rings, the specific impact of soil moisture on the Δ13C and, in particular, the δ18O ratios has scarcely been investigated. We studied the effects of soil type and soil moisture (from moderately moist [Cambisol] to wet [Gleysol]) on the growth and isotopic signature of tree rings of Norway spruce (Picea abies [L.] H. Karst.), a widely distributed forest tree species in Central Europe, at a small spatial scale in a typical mature forest plantation in the low mountain ranges of Western Germany. The δ18O ratios were lower in rings of trees growing at the wettest microsite (Gleysol) than in tree rings from the microsite with moderately moist soil (Cambisol). This indicates higher uptake rates of 18O-unenriched soil water at the Gleysol microsite and corresponds to less negative soil water potentials and higher transpiration rates on the Gleysol plots. Contrary to our expectations, the basal area increments, the Δ13C ratios and the intrinsic water-use efficiency (calculated on the basis of δ13C) did not differ significantly between the Cambisol and the Gleysol microsites. For average values of each microsite and year investigated, we found a significantly positive correlation between δ13C and δ18O, which indicates a consistent stomatal control over gas exchange along the soil moisture gradient at comparable relative air humidity in the stand. As δ18O ratios of tree rings integrate responses of wood formation to soil moisture over longer periods of time, they may help to identify microsites differing in soil water availability along small-scale gradients of soil moisture under homogeneous climatic conditions and to explain the occurrence of particular tree species along those gradients in forest stands.

Carbon and oxygen isotope fractionations in tree rings reveal interactions between cambial phenology and seasonal climate

We developed novel approaches for using the isotope composition of tree-ring subdivisions to study seasonal dynamics in tree-climate relations. Across a 30-year time series, the δ¹³ C and δ¹⁸ O values of the earlywood (EW) cellulose in the annual rings of Pinus ponderosa reflected relatively high intrinsic water-use efficiencies and high evaporative fractionation of ¹⁸ O/¹⁶ O, respectively, compared with the false latewood (FLW), summerwood (SW), and latewood (LW) subdivisions. This result is counterintuitive, given the spring origins of the EW source water and midsummer origins of the FLW, SW, and LW. With the use of the Craig-Gordon (CG), isotope-climate model revealed that the isotope ratios in all of the ring subdivision are explained by the existence of seasonal lags, lasting several weeks, between the initial formation of tracheids and the production of cellulosic secondary cell walls during maturation. In contrast to some past studies, modification of the CG model according to conventional methods to account for mixing of needle water between fractionated and nonfractionated sources did not improve the accuracy of predictions. Our results reveal new potential in the use of tree-ring isotopes to reconstruct past intra-annual tree-climate relations if lags in cambial phenology are reconciled with isotope ratio observations and included in theoretical treatments.

Leaf anatomical traits determine the 18 O enrichment of leaf water in coastal halophytes

Anatomical adaptations to high-salinity environments in mangrove leaves may be recorded in leaf water isotopes. Recent studies observed lower ¹⁸ O enrichment (Δ_L ) of leaf water with respect to source water in three mangrove species relative to adjacent freshwater trees, but the factors that govern this phenomenon remain unclear. To resolve this issue, we investigated leaf traits and Δ_L in 15 species of true mangrove plants, 14 species of adjacent freshwater trees, and 4 species of semi-mangrove plants at five study sites along south-eastern coast of China. Our results confirm that Δ_L was generally 3-4‰ lower for mangrove species than for adjacent freshwater or semi-mangrove species. We hypothesized that higher leaf water content (LWC) and lower leaf stomatal density (LS) both played important roles in reducing Δ_L in mangroves relative to nearby freshwater plants. Both differences acted to elongate effective leaf mixing length (L) in mangroves by about 200% and lower stomatal conductance by about 30%. Péclet models based on both LWC and LS could accurately predict Δ_L . Our findings highlight the potential species-specific anatomical determinants of Δ_L (or L), which has important implications for the interpretation of environmental information from metabolites produced by leaf water isotopes in palaeoclimate research.

Disentangling seasonal and interannual legacies from inferred patterns of forest water and carbon cycling using tree-ring stable isotopes

Tree-ring carbon and oxygen isotope ratios have been used to understand past dynamics in forest carbon and water cycling. Recently, this has been possible for different parts of single growing seasons by isolating anatomical sections within individual annual rings. Uncertainties in this approach are associated with correlated climate legacies that can occur at a higher frequency, such as across successive seasons, or a lower frequency, such as across years. The objective of this study was to gain insight into how legacies affect cross-correlation in the δ¹³ C and δ¹⁸ O isotope ratios in the earlywood (EW) and latewood (LW) fractions of Pinus ponderosa trees at thirteen sites across a latitudinal gradient influenced by the North American Monsoon (NAM) climate system. We observed that δ¹³ C from EW and LW has significant positive cross-correlations at most sites, whereas EW and LW δ¹⁸ O values were cross-correlated at about half the sites. Using combined statistical and mechanistic models, we show that cross-correlations in both δ¹³ C and δ¹⁸ O can be largely explained by a low-frequency (multiple-year) mode that may be associated with long-term climate change. We isolated, and statistically removed, the low-frequency correlation, which resulted in greater geographical differentiation of the EW and LW isotope signals. The remaining higher-frequency (seasonal) cross-correlations between EW and LW isotope ratios were explored using a mechanistic isotope fractionation-climate model. This showed that lower atmospheric vapor pressure deficits associated with monsoon rain increase the EW-LW differentiation for both δ¹³ C and δ¹⁸ O at southern sites, compared to northern sites. Our results support the hypothesis that dominantly unimodal precipitation regimes, such as near the northern boundary of the NAM, are more likely to foster cross-correlations in the isotope signals of EW and LW, potentially due to greater sharing of common carbohydrate and soil water resource pools, compared to southerly sites with bimodal precipitation regimes.

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.

Leaf water 18 O and 2 H enrichment along vertical canopy profiles in a broadleaved and a conifer forest tree

Distinguishing meteorological and plant-mediated drivers of leaf water isotopic enrichment is prerequisite for ecological interpretations of stable hydrogen and oxygen isotopes in plant tissue. We measured input and leaf water δ² H and δ¹⁸ O as well as micrometeorological and leaf morpho-physiological variables along a vertical gradient in a mature angiosperm (European beech) and gymnosperm (Douglas fir) tree. We used these variables and different enrichment models to quantify the influence of Péclet and non-steady state effects and of the biophysical drivers on leaf water enrichment. The two-pool model accurately described the diurnal variation of leaf water enrichment. The estimated unenriched water fraction was linked to leaf dry matter content across the canopy heights. Non-steady state effects and reduced stomatal conductance caused a higher enrichment of Douglas fir compared to beech leaf water. A dynamic effect analyses revealed that the light-induced vertical gradients of stomatal conductance and leaf temperature outbalanced each other in their effects on evaporative enrichment. We conclude that neither vertical canopy gradients nor the Péclet effect is important for estimates and interpretation of isotopic leaf water enrichment in hypostomatous trees. Contrarily, species-specific non-steady state effects and leaf temperatures as well as the water vapour isotope composition need careful consideration.

Relationship of leaf oxygen and carbon isotopic composition with transpiration efficiency in the C4 grasses Setaria viridis and Setaria italica

Leaf carbon and oxygen isotope ratios can potentially provide a time-integrated proxy for stomatal conductance (gs) and transpiration rate (E), and can be used to estimate transpiration efficiency (TE). In this study, we found significant relationships of bulk leaf carbon isotopic signature (δ13CBL) and bulk leaf oxygen enrichment above source water (Δ18OBL) with gas exchange and TE in the model C4 grasses Setaria viridis and S. italica. Leaf δ13C had strong relationships with E, gs, water use, biomass, and TE. Additionally, the consistent difference in δ13CBL between well-watered and water-limited plants suggests that δ13CBL is effective in separating C4 plants with different availability of water. Alternatively, the use of Δ18OBL as a proxy for E and TE in S. viridis and S. italica was problematic. First, the oxygen isotopic composition of source water, used to calculate leaf water enrichment (Δ18OLW), was variable with time and differed across water treatments. Second, water limitations changed leaf size and masked the relationship of Δ18OLW and Δ18OBL with E. Therefore, the data collected here suggest that δ13CBL but not Δ18OBL may be an effective proxy for TE in C4 grasses.

Leaf water stable isotopes and water transport outside the xylem

How water moves through leaves, and where the phase change from liquid to vapour occurs within leaves, remain largely mysterious. Some time ago, we suggested that the stable isotope composition of leaf water may contain information on transport pathways beyond the xylem, through differences in the development of gradients in enrichment within the various pathways. Subsequent testing of this suggestion provided ambiguous results and even questioned the existence of gradients in enrichment within the mesophyll. In this review, we bring together recent theoretical developments in understanding leaf water transport pathways and stable isotope theory to map a path for future work into understanding pathways of water transport and leaf water stable isotope composition. We emphasize the need for a spatially, anatomically and isotopically explicit model of leaf water transport.

Leaf vein fraction influences the Péclet effect and 18 O enrichment in leaf water

The process of evaporation results in the fractionation of water isotopes such that the lighter ¹⁶ O isotope preferentially escapes the gas phase leaving the heavier ¹⁸ O isotope to accumulate at the sites of evaporation. This applies to transpiration from a leaf with the degree of fractionation dependent on a number of environmental and physiological factors that are well understood. Nevertheless, the ¹⁸ O enrichment of bulk leaf water is often less than that predicted for the sites of evaporation. The advection of less enriched water in the transpiration stream has been suggested to limit the back diffusion of enriched evaporative site water (Péclet effect); however, evidence for this effect has been varied. In sampling across a range of species with different vein densities and saturated water contents, we demonstrate the importance of accounting for the relative 'pool' sizes of the vascular and mesophyll water for the interpretation of a Péclet effect. Further, we provide strong evidence for a Péclet signal within the xylem that if unaccounted for can lead to confounding of the estimated enrichment within the mesophyll water. This has important implications for understanding variation in the effective path length of the mesophyll and hence potentially the δ¹⁸ O of organic matter.

Site-specific water-use strategies of mountain pine and larch to cope with recent climate change

We aim to achieve a mechanistic understanding of the eco-physiological processes in Larix decidua and Pinus mugo var. uncinata growing on north- and south-facing aspects in the Swiss National Park in order to distinguish the short- and long-term effects of a changing climate. To strengthen the interpretation of the δ(18)O signal in tree rings and its coherence with the main factors and processes driving evaporative δ(18)O needle water enrichment, we analyzed the δ(18)O in needle, xylem and soil water over the growing season in 2013 and applied the mechanistic Craig-Gordon model (1965) for the short-term responses. We found that δ(18)O needle water strongly reflected the variability of relative humidity mainly for larch, while only δ(18)O in pine xylem water showed a strong link to δ(18)O in precipitation. Larger differences in offsets between modeled and measured δ(18)O needle water for both species from the south-facing aspects were detected, which could be explained by the high transpiration rates. Different soil water and needle water responses for the two species indicate different water-use strategies, further modulated by the site conditions. To reveal the long-term physiological response of the studied trees to recent and past climate changes, we analyzed δ(13)C and δ(18)O in wood chronologies from 1900 to 2013. Summer temperatures as well as summer and annual amount of precipitations are important factors for growth of both studied species from both aspects. However, mountain pine trees reduced sensitivity to temperature changes, while precipitation changes come to play an important role for the period from 1980 to 2013. Intrinsic water-use efficiency (WUEi) calculated for larch trees since the 1990s reached a saturation point at elevated CO2 Divergent trends between pine WUEi and δ(18)O are most likely indicative of a decline of mountain pine trees and are also reflected in decoupling mechanisms in the isotope signals between needles and tree-rings.

Leaf water oxygen isotope measurement by direct equilibration

The oxygen isotope composition of leaf water imparts a signal to a range of molecules in the atmosphere and biosphere, but has been notoriously difficult to measure in studies requiring a large number of samples as a consequence of the labour-intensive extraction step. We tested a method of direct equilibration of water in fresh leaf samples with CO2 , and subsequent oxygen isotope analysis on an optical spectrometer. The oxygen isotope composition of leaf water measured by the direct equilibration technique was strongly linearly related to that of cryogenically extracted leaf water in paired samples for a wide range of species with differing anatomy, with an R(2) of 0.95. The somewhat more enriched values produced by the direct equilibration method may reflect lack of full equilibration with unenriched water in the vascular bundles, but the strong relationship across a wide range of species suggests that this difference can be adequately corrected for using a simple linear relationship.

Stable isotopes in leaf water of terrestrial plants

Leaf water contains naturally occurring stable isotopes of oxygen and hydrogen in abundances that vary spatially and temporally. When sufficiently understood, these can be harnessed for a wide range of applications. Here, we review the current state of knowledge of stable isotope enrichment of leaf water, and its relevance for isotopic signals incorporated into plant organic matter and atmospheric gases. Models describing evaporative enrichment of leaf water have become increasingly complex over time, reflecting enhanced spatial and temporal resolution. We recommend that practitioners choose a model with a level of complexity suited to their application, and provide guidance. At the same time, there exists some lingering uncertainty about the biophysical processes relevant to patterns of isotopic enrichment in leaf water. An important goal for future research is to link observed variations in isotopic composition to specific anatomical and physiological features of leaves that reflect differences in hydraulic design. New measurement techniques are developing rapidly, enabling determinations of both transpired and leaf water δ(18) O and δ(2) H to be made more easily and at higher temporal resolution than previously possible. We expect these technological advances to spur new developments in our understanding of patterns of stable isotope fractionation in leaf water.

Effects of mistletoe removal on growth, N and C reserves, and carbon and oxygen isotope composition in Scots pine hosts

Most mistletoes are xylem-tapping hemiparasites, which derive their resources from the host's xylem solution. Thus, they affect the host's water relations and resource balance. To understand the physiological mechanisms underlying the mistletoe-host relationship, we experimentally removed Viscum album ssp. austriacum (Wiesb.) Vollmann from adult Pinus sylvestris L. host trees growing in a Swiss dry valley. We analyzed the effects of mistletoe removal over time on host tree growth and on concentrations of nonstructural carbohydrates (NSC) and nitrogen (N) in needles, fine roots and sapwood. In addition, we assessed the δ(13)C and δ(18)O in host tree rings. After mistletoe removal, δ(13)C did not change in newly produced tree rings compared with tree rings in control trees (still infected with mistletoe), but δ(18)O values increased. This pattern might be interpreted as a decrease in assimilation (A) and stomatal conductance (gs), but in our study, it most likely points to an inadequacy of the dual isotope approach. Instead, we interpret the unchanged δ(13)C in tree rings upon mistletoe removal as a balanced increase in A and gs that resulted in a constant intrinsic water use efficiency (defined as A/gs). Needle area-based concentrations of N, soluble sugars and NSC, as well as needle length, single needle area, tree ring width and shoot growth, were significantly higher in trees from which mistletoe was removed than in control trees. This finding suggests that mistletoe removal results in increased N availability and carbon gain, which in turn leads to increased growth rates of the hosts. Hence, in areas where mistletoe is common and the population is large, mistletoe management (e.g., removal) may be needed to improve the host vigor, growth rate and productivity, especially for relatively small trees and crop trees in xeric growth conditions.

The impact of an inverse climate-isotope relationship in soil water on the oxygen-isotope composition of Larix gmelinii in Siberia

Stable oxygen isotope ratios (δ(18) O) in trees from high latitude ecosystems are valuable sources of information for recent and past environmental changes, but the interpretation is hampered by the complex hydrology of forests growing under permafrost conditions, where only a shallow layer of soil thaws in summer. We investigated larch trees (Larix gmelinii) at two sites with contrasting soil conditions in Siberia and determined δ(18) O of water from different soil depths, roots, twigs, and needles as well as δ(18) O of soluble carbohydrates regularly over two growing seasons. A comparison of results from the 2 yrs revealed an unexpected 'inverse' climate-isotope relationship, as dry and warm summer conditions resulted in lower soil and root δ(18) O values. This was due to a stronger uptake of isotopically depleted water pools originating from melted permafrost or previous winter snow. We developed a conceptual framework that considers the dependence of soil water profiles on climatic conditions for explaining δ(18) O in needle water, needle soluble carbohydrates and stem cellulose. The negative feedback of drought conditions on the source isotope value could explain decreasing tree-ring δ(18) O trends in a warming climate and is likely relevant in many ecosystems, where a soil isotope gradient with depth is observed.