Variation in the oxygen isotope ratio of phloem sap sucrose from castor bean. Evidence in support of the Péclet effect

Half of the 18O enrichment of leaf sucrose is conserved in leaf cellulose of a C3 grass across atmospheric humidity and CO2 levels

The 18O enrichment (Δ18O) of cellulose (Δ18OCel) is recognized as a unique archive of past climate and plant function. However, there is still uncertainty regarding the proportion of oxygen in cellulose (pex) that exchanges post-photosynthetically with medium water of cellulose synthesis. Particularly, recent research with C3 grasses demonstrated that the Δ18O of leaf sucrose (Δ18OSuc, the parent substrate for cellulose synthesis) can be much higher than predicted from daytime Δ18O of leaf water (Δ18OLW), which could alter conclusions on photosynthetic versus post-photosynthetic effects on Δ18OCel via pex. Here, we assessed pex in leaves of perennial ryegrass (Lolium perenne) grown at different atmospheric relative humidity (RH) and CO2 levels, by determinations of Δ18OCel in leaves, Δ18OLGDZW (the Δ18O of water in the leaf growth-and-differentiation zone) and both Δ18OSuc and Δ18OLW (adjusted for εbio, the biosynthetic fractionation between water and carbohydrates) as alternative proxies for the substrate for cellulose synthesis. Δ18OLGDZW was always close to irrigation water, and pex was similar (0.53 ± 0.02 SE) across environments when determinations were based on Δ18OSuc. Conversely, pex was erroneously and variably underestimated (range 0.02-0.44) when based on Δ18OLW. The photosynthetic signal fraction in Δ18OCel is much more constant than hitherto assumed, encouraging leaf physiological reconstructions.

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.

Modeling the response of Norway spruce tree-ring carbon and oxygen isotopes to selection harvest on a drained peatland forest

Continuous cover forestry (CCF) has gained interest as an alternative to even-aged management particularly on drained peatland forests. However, relatively little is known about the physiological response of suppressed trees when larger trees are removed as a part of CCF practices. Consequently, studies concentrating on process-level modeling of the response of trees to selection harvesting are also rare. Here, we compared, modeled and measured harvest response of previously suppressed Norway spruce (Picea abies) trees to a selection harvest. We quantified the harvest response by collecting Norway spruce tree-ring samples in a drained peatland forest site and measuring the change in stable carbon and oxygen isotopic ratios of wood formed during 2010-20, including five post-harvest years. The measured isotopic ratios were compared with ecosystem-level process model predictions for ${\kern0em }^{13}$C discrimination and ${\kern0em }^{18}$O leaf water enrichment. We found that the model predicted similar but lower harvest response than the measurements. Furthermore, accounting for mesophyll conductance was important for capturing the variation in ${\kern0em }^{13}$C discrimination. In addition, we performed sensitivity analysis on the model, which suggests that the modeled ${\kern0em }^{13}$C discrimination is sensitive to parameters related to CO2 transport through stomata to the mesophyll.

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.

Sourcing the water that makes up tree biomass

Determining the potential sources of water that eventually become organic matter that make up the bulk of tree biomass has been facilitated by using the stable isotope composition of waters. Until recently these water sources were thought to only be taken up by roots from soils and other subsurface reservoirs. However, there is a growing body of evidence that now shows that water taken up directly by leaves and stems can not only be significant but can also dominate as the water source used in organic matter synthesis. In this commentary, I review and discuss these issues and point to an important paper by Akira Kagawa in this issue of Tree Physiology that provides a new experimental method and some striking evidence that foliar water uptake can be the primary water source that makes up tree biomass.

Species variation in the hydrogen isotope composition of leaf cellulose is mostly driven by isotopic variation in leaf sucrose

Experimental approaches to isolate drivers of variation in the carbon-bound hydrogen isotope composition (δ² H) of plant cellulose are rare and current models are limited in their application. This is in part due to a lack in understanding of how ² H-fractionations in carbohydrates differ between species. We analysed, for the first time, the δ² H of leaf sucrose along with the δ² H and δ¹⁸ O of leaf cellulose and leaf and xylem water across seven herbaceous species and a starchless mutant of tobacco. The δ² H of sucrose explained 66% of the δ² H variation in cellulose (R²  = 0.66), which was associated with species differences in the ² H enrichment of sucrose above leaf water ( ε sucrose <math altimg="urn:x-wiley:01407791:media:pce14362:pce14362-math-0001" wiley:location="equation/pce14362-math-0001.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mtext>\unicode{x003B5}</mtext><mtext>sucrose</mtext></msub></mrow></math> : -126% to -192‰) rather than by variation in leaf water δ² H itself. ε sucrose <math altimg="urn:x-wiley:01407791:media:pce14362:pce14362-math-0002" wiley:location="equation/pce14362-math-0002.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mtext>\unicode{x003B5}</mtext><mtext>sucrose</mtext></msub></mrow></math> was positively related to dark respiration (R²  = 0.27), and isotopic exchange of hydrogen in sugars was positively related to the turnover time of carbohydrates (R²  = 0.38), but only when ε sucrose <math altimg="urn:x-wiley:01407791:media:pce14362:pce14362-math-0003" wiley:location="equation/pce14362-math-0003.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><msub><mi mathvariant="normal">\unicode{x003B5}</mi><mtext>sucrose</mtext></msub></mrow></mrow></math> was fixed to the literature accepted value of - 171 <math altimg="urn:x-wiley:01407791:media:pce14362:pce14362-math-0004" wiley:location="equation/pce14362-math-0004.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><mo>\unicode{x02212}</mo><mn>171</mn></mrow></mrow></math> ‰. No relation was found between isotopic exchange of hydrogen and oxygen, suggesting large differences in the processes shaping post-photosynthetic fractionation between elements. Our results strongly advocate that for robust applications of the leaf cellulose hydrogen isotope model, parameterization utilizing δ² H of sugars is needed.

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.

Strongly Active Responses of Pinus tabuliformis Carr. and Sophora viciifolia Hance to CO2 Enrichment and Drought Revealed by Tree-Ring Isotopes on the Central China Loess Plateau

Understanding the water-use strategy of human-planted species used in response to climate change is essential to optimize afforestation programs in dry regions. Since 2000, trees on the central Loess Plateau have experienced a shift from strengthening drought to weakening drought. In this study, we combined tree-ring δ13C and δ18O records from Pinus tabuliformis (syn. tabulaeformis) Carr. (a tree) and Sophora viciifolia Hance (a shrub) on the central Loess Plateau to investigate species-specific responses to rising atmospheric CO2 (Ca) and drought. We found summer relative humidity controlled the fractionation of tree-ring δ18O, but the magnitude of the climate influence on δ13C differed between the species. The intrinsic water-use efficiency (iWUE) trends of both species suggested a strongly active response to maintain constant intercellular CO2 concentrations as Ca rose. The tree-ring δ13C and δ18O of both species using first-difference data were significantly and positively correlated, with stronger relationships for the shrub. This indicated the dominant regulation of iWUE by stomatal conductance in both species, but with greater stomatal control for the shrub. Moreover, the higher mean iWUE value of S. viciifolia indicated a more conservative water-use strategy than P. tabuliformis. Based on our commonality analysis, the main driver of the increased iWUE was the joint effect of Ca and vapor-pressure deficit (25.51%) for the tree, while it was the joint effect of Ca and the self-calibrated Palmer drought severity index (39.13%) for the shrub. These results suggest S. viciifolia will be more drought-tolerant than P. tabuliformis and as Ca continually rises, we should focus more on the effects of soil drought than atmospheric drought on the water-use strategy of S. viciifolia.

Seasonal variations in water flux compositions controlled by leaf development: isotopic insights at the canopy-atmosphere interface

Water-stable isotopes provide a valuable tool for tracing plant-water interactions, particularly evapotranspiration (ET) partitioning and leaf water dynamics at the plant-atmosphere interface. However, process-based investigations of plant/leaf development and the associated isotopic dynamics of water fluxes involving isotope enrichment at plant-atmosphere interfaces at the ecosystem scale remain challenging. In this study, in situ isotopic measurements and tracer-aided models were used to study the dynamic interactions between vegetation growth and the isotopic dynamics of water fluxes (ET, soil evaporation, and transpiration) involving isotope enrichment in canopy leaves in a multispecies grassland ecosystem. The day-to-day variations in the isotopic compositions of ET flux were mainly controlled by plant growth, which could be explained by the significant logarithmic relationship determined between the leaf area index and transpiration fraction. Leaf development promoted a significant increase in the isotopic composition of ET and led to a slight decrease in the isotopic composition of water in canopy leaves. The transpiration (evaporation) isoflux acted to increase (decrease) the δ¹⁸O of water vapor, and the total isoflux impacts depended on the seasonal tradeoffs between transpiration and evaporation. The isotopic evidence in ET fluxes demonstrates the biotic controls on day-to-day variations in water/energy flux partitioning through transpiration activity. This study emphasizes that stable isotopes of hydrogen and oxygen are effective tools for quantitative evaluations of the hydrological component partitioning of ecosystems and plant-climate interactions.

Temperature-sensitive biochemical 18 O-fractionation and humidity-dependent attenuation factor are needed to predict δ18 O of cellulose from leaf water in a grassland ecosystem

We explore here our mechanistic understanding of the environmental and physiological processes that determine the oxygen isotope composition of leaf cellulose (δ¹⁸ O_cellulose ) in a drought-prone, temperate grassland ecosystem. A new allocation-and-growth model was designed and added to an ¹⁸ O-enabled soil-vegetation-atmosphere transfer model (MuSICA) to predict seasonal (April-October) and multi-annual (2007-2012) variation of δ¹⁸ O_cellulose and ¹⁸ O-enrichment of leaf cellulose (Δ¹⁸ O_cellulose ) based on the Barbour-Farquhar model. Modelled δ¹⁸ O_cellulose agreed best with observations when integrated over c. 400 growing-degree-days, similar to the average leaf lifespan observed at the site. Over the integration time, air temperature ranged from 7 to 22°C and midday relative humidity from 47 to 73%. Model agreement with observations of δ¹⁸ O_cellulose (R²  = 0.57) and Δ¹⁸ O_cellulose (R²  = 0.74), and their negative relationship with canopy conductance, was improved significantly when both the biochemical ¹⁸ O-fractionation between water and substrate for cellulose synthesis (ε_bio , range 26-30‰) was temperature-sensitive, as previously reported for aquatic plants and heterotrophically grown wheat seedlings, and the proportion of oxygen in cellulose reflecting leaf water ¹⁸ O-enrichment (1 - p_ex p_x , range 0.23-0.63) was dependent on air relative humidity, as observed in independent controlled experiments with grasses. Understanding physiological information in δ¹⁸ O_cellulose requires quantitative knowledge of climatic effects on p_ex p_x and ε_bio .

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.

Plant's-eye view of temperature governs elevational distributions

Explaining species geographic distributions by macroclimate variables is the most common approach for getting mechanistic insights into large-scale diversity patterns and range shifts. However, species' traits influencing biophysical processes can produce a large decoupling from ambient air temperature, which can seriously undermine biogeographical inference. We combined stable oxygen isotope theory with a trait-based approach to assess leaf temperature during carbon assimilation (T_L ) and its departure (ΔT) from daytime free air temperature during the growing season (T_gs ) for 158 plant species occurring from 3,400 to 6,150 m a.s.l. in Western Himalayas. We uncovered a general extent of temperature decoupling in the region. The interspecific variation in ΔT was best explained by the combination of plant height and δ¹³ C, and leaf dry matter content partly captured the variation in T_L . The combination of T_L and ΔT, with ΔT contributing most, explained the interspecific difference in elevational distributions. Stable oxygen isotope theory appears promising for investigating how plants perceive temperatures, a pivotal information to species biogeographic distributions.

δ15 N values in plants are determined by both nitrate assimilation and circulation

Nitrogen (N) assimilation is associated with ¹⁴ N/¹⁵ N fractionation such that plant tissues are generally ¹⁵ N-depleted compared to source nitrate. In addition to nitrate concentration, the δ¹⁵ N value in plants is also influenced by isotopic heterogeneity amongst organs and metabolites. However, our current understanding of δ¹⁵ N values in nitrate is limited by the relatively small number of compound-specific data. We extensively measured δ¹⁵ N in nitrate at different time points, in sunflower and oil palm grown at fixed nitrate concentration, with nitrate circulation being varied using potassium (K) conditions and waterlogging. There were strong interorgan δ¹⁵ N differences for contrasting situations between the two species, and a high ¹⁵ N-enrichment in root nitrate. Modelling shows that this ¹⁵ N-enrichment can be explained by nitrate circulation and compartmentalisation whereby despite a numerically small flux value, the backflow of nitrate to roots via the phloem can lead to a c. 30‰ difference between leaves and roots. Accordingly, waterlogging and low K conditions, which down-regulate sap circulation, cause a decrease in the leaf-to-root isotopic difference. Our study thus suggests that plant δ¹⁵ N can be used as a natural tracer of N fluxes between organs and highlights the potential importance of δ¹⁵ N of circulating phloem nitrate.

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.

Investigating old-growth ponderosa pine physiology using tree-rings, δ13 C, δ18 O, and a process-based model

In dealing with predicted changes in environmental conditions outside those experienced today, forest managers and researchers rely on process-based models to inform physiological processes and predict future forest growth responses. The carbon and oxygen isotope ratios of tree-ring cellulose (δ13 Ccell , δ18 Ocell ) reveal long-term, integrated physiological responses to environmental conditions. We incorporated a submodel of δ18 Ocell into the widely used Physiological Principles in Predicting Growth (3-PG) model for the first time, to complement a recently added δ13 Ccell submodel. We parameterized the model using previously reported stand characteristics and long-term trajectories of tree-ring growth, δ13 Ccell , and δ18 Ocell collected from the Metolius AmeriFlux site in central Oregon (upland trees). We then applied the parameterized model to a nearby set of riparian trees to investigate the physiological drivers of differences in observed basal area increment (BAI) and δ13 Ccell trajectories between upland and riparian trees. The model showed that greater available soil water and maximum canopy conductance likely explain the greater observed BAI and lower δ13 Ccell of riparian trees. Unexpectedly, both observed and simulated δ18 Ocell trajectories did not differ between the upland and riparian trees, likely due to similar δ18 O of source water isotope composition. The δ18 Ocell submodel with a Peclet effect improved model estimates of δ18 Ocell because its calculation utilizes 3-PG growth and allocation processes. Because simulated stand-level transpiration (E) is used in the δ18 O submodel, aspects of leaf-level anatomy such as the effective path length for transport of water from the xylem to the sites of evaporation could be estimated.

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.

Generating Plants with Improved Water Use Efficiency

To improve sustainability of agriculture, high yielding crop varieties with improved water use efficiency (WUE) are needed. Despite the feasibility of assessing WUE using different measurement techniques, breeding for WUE and high yield is a major challenge. Factors influencing the trait under field conditions are complex, including different scenarios of water availability. Plants with C3 photosynthesis are able to moderately increase WUE by restricting transpiration, resulting in higher intrinsic WUE (iWUE) at the leaf level. However, reduced CO2 uptake negatively influences photosynthesis and possibly growth and yield as well. The negative correlation of growth and WUE could be partly disconnected in model plant species with implications for crops. In this paper, we discuss recent insights obtained for Arabidopsis thaliana (L.) and the potential to translate the findings to C3 and C4 crops. Our data on Zea mays (L.) lines subjected to progressive drought show that there is potential for improvements in WUE of the maize line B73 at the whole plant level (WUEplant). However, changes in iWUE of B73 and Arabidopsis reduced the assimilation rate relatively more in maize. The trade-off observed in the C4 crop possibly limits the effectiveness of approaches aimed at improving iWUE but not necessarily efforts to improve WUEplant.

Oxygen isotope enrichment in rice (Oryza sativa L.) grain organic matter captures signature of relative humidity

The oxygen isotopic composition (δ¹⁸O) of plant organic matter (OM) is primarily governed by the δ¹⁸O of source water (δ¹⁸O_SW) and climatic factor of relative humidity (RH). Among the cereals, the growth of rice plants is critically dependent on the water availability in the growth-environment. In the present study, we investigated the sensitivity of δ¹⁸O in the bulk organic matter of rice grains to RH of their growth-environment. Our experimental setup consisted of both glasshouse and field experiments, where eight genotypes were grown at RH levels ranging from 67% to 87%. The δ¹⁸O measured in bulk grain OM and source water was used to calculate the net oxygen isotopic enrichment (Δ¹⁸O_OM). Regression analysis of Δ¹⁸O_OM with RH demonstrated a significant relationship (r² = 0.96; p < 0.0001), thereby implying that the isotopic signature of evaporative conditions gets recorded in the rice grain OM. In addition, our study involved a separate experiment that monitored the degree of oxygen isotope enrichment in water samples extracted from different parts of the rice plant. For this purpose, we sampled four of the above eight genotypes along with three other rice genotypes that were grown in both open cultivation fields and glasshouse. Water present in the culms, leaves, and grains were extracted quantitatively. Isotopic analyses revealed progressive ¹⁸O enrichment of the water in the culms and leaves and intermediate enrichment values of that in the grains. Based on the isotope data, we validated mechanistic models for prediction of δ¹⁸O of the leaf water and that of the plant carbohydrates. The model predictions were in close agreement with the experimental observations. The study provides insights into the rice plant's oxygen isotope systematics that build the foundation for future applications of the stable isotope technique to study the interactions between rice and environment.

Leaf water 18 O and 2 H maps show directional enrichment discrepancy in Colocasia esculenta

Spatial patterns of leaf water isotopes are challenging to predict because of the intricate link between vein and lamina water. Many models have attempted to predict these patterns, but to date, most have focused on monocots with parallel veins. These provide a simple system to study, but do not represent the majority of plant species. Here, a new protocol is developed using a Picarro induction module coupled to a cavity ringdown spectrometer to obtain maps of the leaf water isotopes (¹⁸ O and ² H). The technique is applied to Colocasia esculenta leaves. The results are compared with isotope ratio mass spectrometry. In C. esculenta, a large enrichment in the radial direction is observed, but not in the longitudinal direction. The string-of-lakes model fails to predict the observed patterns, while the Farquhar-Gan model is more successful, especially when enrichment is accounted for along the radial direction. Our results show that reticulate-veined leaves experience a larger enrichment along the axis of the secondary veins than along the midrib. We hypothesize that this is due to the lower major/minor vein ratio that leads to longer pathways between major veins and sites of evaporation.

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

Almost no δ¹⁸ O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ¹⁸ O relationship between leaf water and cellulose. We measured δ¹⁸ O values of bulk leaf water (δ¹⁸ O_LW ) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ¹⁸ O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally ¹⁸ O-enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (ε_bio ) of more than 27‰ relative to δ¹⁸ O_LW , which might be explained by isotopic leaf water and sucrose synthesis gradients. δ¹⁸ O_LW and δ¹⁸ O 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_ex p_x , which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ¹⁸ O_LW but not if modelled directly from δ¹⁸ O of individual carbohydrates. We conclude that δ¹⁸ O_LW is not always a good substitute for δ¹⁸ O of synthesis water due to isotopic leaf water gradients. Thus, compound-specific δ¹⁸ O analyses of individual carbohydrates are helpful to better constrain (post-)photosynthetic isotope fractionation processes in plants.

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.

The δ18 O and δ2 H of water in the leaf growth-and-differentiation zone of grasses is close to source water in both humid and dry atmospheres

The oxygen and hydrogen isotope composition of water in the leaf growth-and-differentiation zone, LGDZ, (δ¹⁸ O_LGDZ , δ² H_LGDZ ) of grasses influences the isotopic composition of leaf cellulose (oxygen) and wax (hydrogen) - important for understanding (paleo)environmental and physiological information contained in these biological archives - but is presently unknown. This work determined δ¹⁸ O_LGDZ and δ² H_LGDZ , ¹⁸ O- and ² H-enrichment of LGDZ (∆¹⁸ O_LGDZ and ∆² H_LGDZ ), and the ¹⁸ O- and ² H-enrichment of leaf blade water (∆¹⁸ O_LW, ∆² H_LW ) in two C₃ and three C₄ grasses grown at high and low vapor pressure deficit (VPD). The proportion of unenriched water (p_x ) in the LGDZ ranged from 0.9 to 1.0 for ¹⁸ O and 1.0 to 1.2 for ² H. VPD had no effect on the proportion of ¹⁸ O- and ² H-enriched water in the LGDZ, and species effects were small or nonsignificant. Deuterium discrimination caused depletion of ² H in LGDZ water, increasing (apparent) p_x -values > 1.0 in some cases. The isotopic composition of water in the LGDZ was close to that of source water, independent of VPD and much less enriched than previously supposed, but similar to reported xylem water in trees. The well-constrained p_x will be useful in future investigations of oxygen and hydrogen isotopic fractionation during cellulose and wax synthesis, respectively.

The Sites of Evaporation within Leaves

The sites of evaporation within leaves are unknown, but they have drawn attention for decades due to their perceived implications for many factors, including patterns of leaf isotopic enrichment, the maintenance of mesophyll water status, stomatal regulation, and the interpretation of measured stomatal and leaf hydraulic conductances. We used a spatially explicit model of coupled water and heat transport outside the xylem, MOFLO 2.0, to map the distribution of net evaporation across leaf tissues in relation to anatomy and environmental parameters. Our results corroborate earlier predictions that most evaporation occurs from the epidermis at low light and moderate humidity but that the mesophyll contributes substantially when the leaf center is warmed by light absorption, and more so under high humidity. We also found that the bundle sheath provides a significant minority of evaporation (15% in darkness and 18% in high light), that the vertical center of amphistomatous leaves supports net condensation, and that vertical temperature gradients caused by light absorption vary over 10-fold across species, reaching 0.3°C. We show that several hypotheses that depend on the evaporating sites require revision in light of our findings, including that experimental measurements of stomatal and hydraulic conductances should be affected directly by changes in the location of the evaporating sites. We propose a new conceptual model that accounts for mixed-phase water transport outside the xylem. These conclusions have far-reaching implications for inferences in leaf hydraulics, gas exchange, water use, and isotope physiology.

Nitrogen fertilization and δ18 O of CO2 have no effect on 18 O-enrichment of leaf water and cellulose in Cleistogenes squarrosa (C4 ) - is VPD the sole control?

The oxygen isotope composition of cellulose (δ¹⁸ O_Cel ) archives hydrological and physiological information. Here, we assess previously unexplored direct and interactive effects of the δ¹⁸ O of CO₂ (δ¹⁸ O_CO2 ), nitrogen (N) fertilizer supply and vapour pressure deficit (VPD) on δ¹⁸ O_Cel , ¹⁸ O-enrichment of leaf water (Δ¹⁸ O_LW ) and cellulose (Δ¹⁸ O_Cel ) relative to source water, and p_ex p_x , the proportion of oxygen in cellulose that exchanged with unenriched water at the site of cellulose synthesis, in a C₄ grass (Cleistogenes squarrosa). δ¹⁸ O_CO2 and N supply, and their interactions with VPD, had no effect on δ¹⁸ O_Cel , Δ¹⁸ O_LW , Δ¹⁸ O_Cel and p_ex p_x . Δ¹⁸ O_Cel and Δ¹⁸ O_LW increased with VPD, while p_ex p_x decreased. That VPD-effect on p_ex p_x was supported by sensitivity tests to variation of Δ¹⁸ O_LW and the equilibrium fractionation factor between carbonyl oxygen and water. N supply altered growth and morphological features, but not ¹⁸ O relations; conversely, VPD had no effect on growth or morphology, but controlled ¹⁸ O relations. The work implies that reconstructions of VPD from Δ¹⁸ O_Cel would overestimate amplitudes of VPD variation, at least in this species, if the VPD-effect on p_ex p_x is ignored. Progress in understanding the relationship between Δ¹⁸ O_LW and Δ¹⁸ O_Cel will require separate investigations of p_ex and p_x and of their responses to environmental conditions.

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.

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.

The Impact of Climate, Sulfur Dioxide, and Industrial Dust on δ18O and δ13C in Glucose from Pine Tree Rings Growing in an Industrialized Area in the Southern Part of Poland

The mass spectrometric analysis of the impact of sulfur dioxide and dust emission on carbon and oxygen stable isotopic compositions of glucose hydrolysed from α-cellulose samples extracted from Scots pine growing in the vicinity of "Huta Katowice" steelworks was the main aim of this study. The annual rings covered the time span from 1975 to 2012 AD. The relationships between climatic conditions, sulfur dioxide, and industrial dust emission and oxygen and carbon isotopic compositions were analyzed using correlation function methods. This study shows the first analysis of carbon and oxygen stable isotopes in glucose as the bio-indicators of CO₂, sulfur dioxide, and industrial dust emission. The anticoincidence trend of δ¹⁸O and δ¹³C and dust and sulfur dioxide confirms that the decreases of dust and sulfur dioxide industrial emission increase δ¹⁸O and δ¹³C values in glucose.

Life in the clouds: are tropical montane cloud forests responding to changes in climate?

The humid tropics represent only one example of the many places worldwide where anthropogenic disturbance and climate change are quickly affecting the feedbacks between water and trees. In this article, we address the need for a more long-term perspective on the effects of climate change on tropical montane cloud forests (TMCF) in order to fully assess the combined vulnerability and long-term response of tropical trees to changes in precipitation regimes, including cloud immersion. We first review the ecophysiological benefits that cloud water interception offers to trees in TMCF and then examine current climatological evidence that suggests changes in cloud base height and impending changes in cloud immersion for TMCF. Finally, we propose an experimental approach to examine the long-term dynamics of tropical trees in TMCF in response to environmental conditions on decade-to-century time scales. This information is important to assess the vulnerability and long-term response of TMCF to changes in cloud cover and fog frequency and duration.

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

The Péclet correction is often used to predict leaf evaporative enrichment and requires an estimate of effective path length (L). Studies to estimate L in conifer needles have produced unexpected patterns based on Péclet theory and leaf anatomy. We exposed seedlings of six conifer species to different vapour pressure deficits (VPD) in controlled climate chambers to produce steady-state leaf water enrichment (in (18) O). We measured leaf gas exchange, stable oxygen isotopic composition (δ(18) O) of input and plant waters as well as leaf anatomical characteristics. Variation in bulk needle water δ(18) O was strongly related to VPD. Conifer needles had large amounts of water within the vascular strand that was potentially unenriched (up to 40%). Both standard Craig-Gordon and Péclet models failed to accurately predict conifer leaf water δ(18) O without taking into consideration the unenriched water in the vascular strand and variable L. Although L was linearly related to mesophyll thickness, large within-species variation prevented the development of generalizations that could allow a broader use of the Péclet effect in predictive models. Our results point to the importance of within needle water pools and isolating mechanisms that need further investigation in order to integrate Péclet corrections with 'two compartment' leaf water concepts.

Modelling non-steady-state isotope enrichment of leaf water in a gas-exchange cuvette environment

The combined use of a gas-exchange system and laser-based isotope measurement is a tool of growing interest in plant ecophysiological studies, owing to its relevance for assessing isotopic variability in leaf water and/or transpiration under non-steady-state (NSS) conditions. However, the current Farquhar & Cernusak (F&C) NSS leaf water model, originally developed for open-field scenarios, is unsuited for use in a gas-exchange cuvette environment where isotope composition of water vapour (δv ) is intrinsically linked to that of transpiration (δE ). Here, we modified the F&C model to make it directly compatible with the δv -δE dynamic characteristic of a typical cuvette setting. The resultant new model suggests a role of 'net-flux' (rather than 'gross-flux' as suggested by the original F&C model)-based leaf water turnover rate in controlling the time constant (τ) for the approach to steady sate. The validity of the new model was subsequently confirmed in a cuvette experiment involving cotton leaves, for which we demonstrated close agreement between τ values predicted from the model and those measured from NSS variations in isotope enrichment of transpiration. Hence, we recommend that our new model be incorporated into future isotope studies involving a cuvette condition where the transpiration flux directly influences δv . There is an increasing popularity among plant ecophysiologists to use a gas-exchange system coupled to laser-based isotope measurement for investigating non-steady state (NSS) isotopic variability in leaf water (and/or transpiration); however, the current Farquhar & Cernusak (F&C) NSS leaf water model is unsuited for use in a gas-exchange cuvette environment due to its implicit assumption of isotope composition of water vapor (δv ) being constant and independent of that of transpiration (δE ). In the present study, we modified the F&C model to make it compatible with the dynamic relationship between δv and δE as is typically associated with a cuvette setting. Using an experiment conducted on cotton leaves, we show that the modified NSS model performed well in predicting the time constant for the exponential approach of leaf water toward steady state under cuvette conditions. Such a result demonstrates the applicability of this new model to gas-exchange cuvette conditions where the transpiration flux directly influences δv , and therefore suggests the need to incorporate this model into future isotope studies that employ a laser-cuvette coupled system.

Measurements of transpiration isotopologues and leaf water to assess enrichment models in cotton

The two-pool and Péclet effect models represent two theories describing mechanistic controls underlying leaf water oxygen isotope composition at the whole-leaf level (δ(18) OL ). To test these models, we used a laser spectrometer coupled to a gas-exchange cuvette to make online measurements of δ(18) O of transpiration (δ(18) Otrans ) and transpiration rate (E) in 61 cotton (Gossypium hirsutum) leaves. δ(18) Otrans measurements permitted direct calculation of δ(18) O at the sites of evaporation (δ(18) Oe ) which, combined with values of δ(18) OL from the same leaves, allowed unbiased estimation of the proportional deviation of enrichment of δ(18) OL from that of δ(18) Oe (f) under both steady-state (SS) and non-steady-state (NSS) conditions. Among all leaves measured, f expressed relative to both δ(18) O of transpired water (ftrans ) and source water (fsw ) remained relatively constant with a mean ± SD of 0.11 ± 0.05 and 0.13 ± 0.05, respectively, regardless of variation in E spanning 0.8-9.1 mmol m(-2)  s(-1) . Neither ftrans nor fsw exhibited a significant difference between the SS and NSS leaves at the P < 0.05 level. Our results suggest that the simpler two-pool model is adequate for predicting cotton leaf water enrichment at the whole-leaf level. We discuss the implications of adopting a two-pool concept for isotopic applications in ecological studies.

Observed relationships between leaf H218O Péclet effective length and leaf hydraulic conductance reflect assumptions in Craig-Gordon model calculations

Stable oxygen isotope techniques may be a useful tool to investigate the pathways of water movement within leaves. However, implementation of such methods is limited due to uncertainty in the effective path length (L) for the Péclet effect in leaf water enrichment models. Previous studies have found relationships between L and physiological parameters such as transpiration rate (E) and leaf hydraulic conductance (k(leaf)) both within and between species. However, these studies relied on assumptions in their calculation of L, which were not directly tested. Eucalyptus paniculata Smith was used to evaluate the relationships between L, k(leaf) and E under differing water availability and a range of leaf temperatures. Coupled gas exchange and transpiration isotope measurements allowed previous assumptions to be directly tested. L was significantly and negatively related to both k(leaf) and E when the isotopic signature of water vapour was assumed to be in equilibrium with source water, was equivalent to the room vapour or equal to source water. However, the relationship between L and k(leaf) was non-significant when measured δ( 18)O of transpired vapour was used and disappeared entirely when non-steady-state leaves were excluded, and when evaporation site water was calculated from coupled gas exchange and transpiration isotope values. These results suggest that great care must be taken when calculating L, particularly regarding assumptions of isotopic steady state and δ( 18)O of vapour. Previous suggestions of changes in pathways for water movement as transpiration rate varied need to be reassessed in light of these observations.

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 (δ(18)O(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 δ(18) O(cell) (δ(18)O(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 δ(18)O(lc), as well as several variables that could potentially cause the δ(18)O(lc) variation. Data analysis based on the δ(18) O(cell) model rules out leaf water enrichment (Δ(18)O(lw)) and tree-ring formation period (Δt), but highlights source water δ(18) O (δ(18) O(sw)) as an important driver for the measured difference in δ(18)O(lc) between black and chestnut oak. However, the enriched δ(18)O(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 δ(18)O(cell) has important implications for isotope based ecophysiological/paleoclimate studies.

Reduced climate sensitivity of carbon, oxygen and hydrogen stable isotope ratios in tree-ring cellulose of silver fir (Abies alba Mill.) influenced by background SO2 in Franconia (Germany, Central Europe)

The climate sensitivity of carbon (δ(13)C), oxygen (δ(18)O) and hydrogen (δ(2)H) isotope signatures in tree-ring cellulose of Abies alba Mill. from a marginally industrialized area of Franconia (Germany) was analysed for the last 130 years. All isotopes preserve climatic signals up to c. 1950 AD. After 1950 we observe a clear reduction in climate sensitivity of δ(13)C and δ(2)H while δ(18)O - climate relations remain well pronounced. Nevertheless statistical tests implied that SO2 background emissions of West Germany had influenced isotope signatures long before 1950. The relationships between isotope values and concentrations of SO2, dust, O3 and NO2 at the regional level during the period 1979-2006 indicate that δ(13)C and δ(18)O were influenced primarily by SO2. The impact of SO2 on δ(2)H was negligible, but the observed reduction of climate sensitivity may be caused by synergic influences. The results have significant implications if isotope signatures from tree-rings from anthropogenic influenced regions are used to reconstruct past climate.

Isotopic composition of transpiration and rates of change in leaf water isotopologue storage in response to environmental variables

During daylight hours, the isotope composition of leaf water generally approximates steady-state leaf water isotope enrichment model predictions. However, until very recently there was little direct confirmation that isotopic steady-state (ISS) transpiration in fact exists. Using isotope ratio infrared spectroscopy (IRIS) and leaf gas exchange systems we evaluated the isotope composition of transpiration and the rate of change in leaf water isotopologue storage (isostorage) when leaves were exposed to variable environments. In doing so, we developed a method for controlling the absolute humidity entering the gas exchange cuvette for a wide range of concentrations without changing the isotope composition of water vapour. The measurement system allowed estimation of (18)O enrichment both at the evaporation site and for bulk leaf water, in the steady state and the non-steady state. We show that non-steady-state effects dominate the transpiration isoflux even when leaves are at physiological steady state. Our results suggest that a variable environment likely prevents ISS transpiration from being achieved and that this effect may be exacerbated by lengthy leaf water turnover times due to high leaf water contents.

The oxygen isotope enrichment of leaf-exported assimilates--does it always reflect lamina leaf water enrichment?

The oxygen stable isotope composition of plant organic matter (OM) (particularly of wood and cellulose in the tree ring archive) is valuable in studies of plant-climate interaction, but there is a lack of information on the transfer of the isotope signal from the leaf to heterotrophic tissues. We studied the oxygen isotopic composition and its enrichment above source water of leaf water over diel courses in five tree species covering a broad range of life forms. We tracked the transfer of the isotopic signal to leaf water-soluble OM and further to phloem-transported OM. Observed leaf water evaporative enrichment was consistent with values predicted from mechanistic models taking into account nonsteady-state conditions. While leaf water-soluble OM showed the expected (18)O enrichment in all species, phloem sugars were less enriched than expected from leaf water enrichment in Scots pine (Pinus sylvestris), European larch (Larix decidua) and Alpine ash (Eucalyptus delegatensis). Oxygen atom exchange with nonenriched water during phloem loading and transport, as well as a significant contribution of assimilates from bark photosynthesis, can explain these phloem (18)O enrichment patterns. Our results indicate species-specific uncoupling between the leaf water and the OM oxygen isotope signal, which is important for the interpretation of tree ring data.

Are leaf physiological traits related to leaf water isotopic enrichment in restinga woody species?

During plant-transpiration, water molecules having the lighter stable isotopes of oxygen and hydrogen evaporate and diffuse at a faster rate through the stomata than molecules having the heavier isotopes, which cause isotopic enrichment of leaf water. Although previous models have assumed that leaf water is well-mixed and isotopically uniform, non-uniform stomatal closure, promoting different enrichments between cells, and different pools of water within leaves, due to morpho-physiological traits, might lead to inaccuracies in isotopic models predicting leaf water enrichment. We evaluate the role of leaf morpho-physiological traits on leaf water isotopic enrichment in woody species occurring in a coastal vegetation of Brazil known as restinga. Hydrogen and oxygen stable isotope values of soil, plant stem and leaf water and leaf traits were measured in six species from restinga vegetation during a drought and a wet period. Leaf water isotopic enrichment relative to stem water was more homogeneous among species during the drought in contrast to the wet period suggesting convergent responses to deal to temporal heterogeneity in water availability. Average leaf water isotopic enrichment relative to stem water during the drought period was highly correlated with relative apoplastic water content. We discuss this observation in the context of current models of leaf water isotopic enrichment as a function of the Péclet effect. We suggest that future studies should include relative apoplastic water content in isotopic models.

Transpiration rate relates to within- and across-species variations in effective path length in a leaf water model of oxygen isotope enrichment

Stable oxygen isotope ratio of leaf water (δ(18)O(L)) yields valuable information on many aspects of plant-environment interactions. However, current understanding of the mechanistic controls on δ(18)O(L) does not provide complete characterization of effective path length (L) of the Péclet effect,--a key component of the leaf water model. In this study, we collected diurnal and seasonal series of leaf water enrichment and estimated L in six field-grown angiosperm and gymnosperm tree species. Our results suggest a pivotal role of leaf transpiration rate (E) in driving both within- and across-species variations in L. Our observation of the common presence of an inverse scaling of L with E in the different species therefore cautions against (1) the conventional treatment of L as a species-specific constant in leaf water or cellulose isotope (δ(18)O(p)) modelling; and (2) the use of δ(18)O(p) as a proxy for gs or E under low E conditions. Further, we show that incorporation of a multi-species L-E scaling into the leaf water model has the potential to both improve the prediction accuracy and simplify parameterization of the model when compared with the conventional approach. This has important implications for future modelling of oxygen isotope ratios.

Elevated CO₂ increases tree-level intrinsic water use efficiency: insights from carbon and oxygen isotope analyses in tree rings across three forest FACE sites

Elevated CO₂ increases intrinsic water use efficiency (WUE(i) ) of forests, but the magnitude of this effect and its interaction with climate is still poorly understood. We combined tree ring analysis with isotope measurements at three Free Air CO₂ Enrichment (FACE, POP-EUROFACE, in Italy; Duke FACE in North Carolina and ORNL in Tennessee, USA) sites, to cover the entire life of the trees. We used δ¹³C to assess carbon isotope discrimination and changes in water-use efficiency, while direct CO₂ effects on stomatal conductance were explored using δ¹⁸O as a proxy. Across all the sites, elevated CO₂ increased ¹³C-derived water-use efficiency on average by 73% for Liquidambar styraciflua, 77% for Pinus taeda and 75% for Populus sp., but through different ecophysiological mechanisms. Our findings provide a robust means of predicting water-use efficiency responses from a variety of tree species exposed to variable environmental conditions over time, and species-specific relationships that can help modelling elevated CO₂ and climate impacts on forest productivity, carbon and water balances.

A controlled test of the dual-isotope approach for the interpretation of stable carbon and oxygen isotope ratio variation in tree rings

Seedlings of a conifer (Pinus radiata D. Don) and a broad leaf angiosperm (Eucalyptus globulus Labill.) were grown for 100 days in two growth cabinets at 45 or 65% relative humidity. The seedlings were exposed to treatments designed to modify carbon assimilation rates and capacities, stomatal conductance and transpiration to test conceptual models that attempt to clarify the interpretation of carbon isotope discrimination (Δ(13)C) by using oxygen isotope enrichment (Δ(18)O). Differences in relative humidity and within-cabinet treatments (including lower irradiance, lower nitrogen inputs, higher leaf temperature and lower moisture status than control seedlings) produced significant differences in assimilation rates, photosynthetic capacities, stomatal conductance, leaf transpiration rates and leaf evaporative enrichment. The dual-isotope approach accurately interpreted the cause of variation in wood cellulose Δ(13)C for some of the treatments, but not for others. We also tested whether we could use Δ(13)C variation to constrain the interpretation of δ(18)O variation. Carbon isotope discrimination appears to be influenced by transpiration (providing information on leaf evaporative enrichment), but the results did not provide a clear way to interpret such variation. The dual-isotope approach appears to be valid conceptually, but more work is needed to make it operational under different scenarios.

The Péclet effect on leaf water enrichment correlates with leaf hydraulic conductance and mesophyll conductance for CO(2)

Leaf water gets isotopically enriched through transpiration, and diffusion of enriched water through the leaf depends on transpiration flow and the effective path length (L). The aim of this work was to relate L with physiological variables likely to respond to similar processes. We studied the response to drought and vein severing of leaf lamina hydraulic conductance (K(lamina) ), mesophyll conductance for CO(2) (g(m) ) and leaf water isotope enrichment in Vitis vinifera L cv. Grenache. We hypothesized that restrictions in water pathways would reduce K(lamina) and increase L. As a secondary hypothesis, we proposed that, given the common pathways for water and CO(2) involved, a similar response should be found in g(m) . Our results showed that L was strongly related to mesophyll variables, such as K(lamina) or g(m) across experimental drought and vein-cutting treatments, showing stronger relationships than with variables included as input parameters for the models, such as transpiration. Our findings were further supported by a literature survey showing a close link between L and leaf hydraulic conductance (K(leaf) = 31.5 × L(-0.43) , r(2) = 0.60, n = 24). The strong correlation found between L, K(lamina) and g(m) supports the idea that water and CO(2) share an important part of their diffusion pathways through the mesophyll.

High but not dry: diverse epiphytic bromeliad adaptations to exposure within a seasonally dry tropical forest community

• Vascular epiphytes have developed distinct lifeforms to maximize water uptake and storage, particularly when delivered as pulses of precipitation, dewfall or fog. The seasonally dry forest of Chamela, Mexico, has a community of epiphytic bromeliads with Crassulacean acid metabolism showing diverse morphologies and stratification within the canopy. We hypothesize that niche differentiation may be related to the capacity to use fog and dew effectively to perform photosynthesis and to maintain water status. • Four Tillandsia species with either 'tank' or 'atmospheric' lifeforms were studied using seasonal field data and glasshouse experimentation, and compared on the basis of water use, leaf water δ(18) O, photosynthetic and morphological traits. • The atmospheric species, Tillandsia eistetteri, with narrow leaves and the lowest succulence, was restricted to the upper canopy, but displayed the widest range of physiological responses to pulses of precipitation and fog, and was a fog-catching 'nebulophyte'. The other atmospheric species, Tillandsia intermedia, was highly succulent, restricted to the lower canopy and with a narrower range of physiological responses. Both upper canopy tank species relied on tank water and stomatal closure to avoid desiccation. • Niche differentiation was related to capacity for water storage, dependence on fog or dewfall and physiological plasticity.

Examining the large-scale convergence of photosynthesis-weighted tree leaf temperatures through stable oxygen isotope analysis of multiple data sets

The idea that photosynthesis-weighted tree canopy leaf temperature (T(canδ)) can be resolved through analysis of oxygen isotope composition in tree wood cellulose (δ(18) O(wc)) has led to the observation of boreal-to-subtropical convergence of T(canδ) to c. 20°C. To further assess the validity of the large-scale convergence of T(canδ), we used the isotope approach to perform calculation of T(canδ) for independent δ(18) O(wc) data sets that have broad coverage of climates. For the boreal-to-subtropical data sets, we found that the deviation of T(canδ) from the growing season temperature systemically increases with the decreasing mean annual temperature. Across the whole data sets we calculated a mean T(canδ) of 19.48°C and an SD of 2.05°C, while for the tropical data set, the mean T(canδ) was 26.40 ± 1.03°C, significantly higher than the boreal-to-subtropical mean. Our study thus offers independent isotopic support for the concept that boreal-to-subtropical trees display conserved T(canδ) near 20°C. The isotopic analysis cannot distinguish between the possibility that leaf temperatures are generally elevated above ambient air temperatures in cooler environments and the possibility that leaf temperature equals air temperature, whereas the leaf/air temperature at which photosynthesis occurs has a weighted average of near 20°C in cooler environments. Future work will separate these potential explanations.

Anthropogenic NOx emissions alter the intrinsic water-use efficiency (WUEi) for Quercus cerris stands under Mediterranean climate conditions

We investigated the effect of N deposition (Ndep) on intrinsic water-use efficiency (WUEi), the ratio of photosynthesis (A) to stomatal conductance (gs), for two Quercus cerris stands at different distances to an oil refinery in Southern Italy. We used delta13C in tree rings for assessing changes in WUEi; while the influence of climate and NOx emission was explored through delta18O and delta15N, respectively. Differences in WUEi between the two sites were significant, with trees exposed to different degrees of NOx emissions showing an abrupt increase with the onset of pollution. Assuming similar gs at the two sites, as inferred through delta18O, the higher N availability at the polluted site caused the shift of the A/gs ratio in favour of A. Overall, our result suggests that an increase of Ndep may enhance tree WUE under a scenario of reduction of precipitation predicted for Mediterranean area.