B12C-01
Water isotopologues in leaves
Leaf water isotope enrichment is a cornerstone of a variety of isotopic applications. It imprints on different substances such as atmospheric CO2, O2, and plant organic matter. But different applications use enrichment in different parts of the leaf and weighted by different fluxes. For example, leaf organic matter is determined by the assimilation-weighted average bulk water enrichment. Atmospheric CO2 and O2 are determined by the enrichment near the evaporating sites, either weighted by the one-way CO2 flux from the stomata to the atmosphere or by electron transport, resp. These applications of leaf water enrichment are used from the leaf level up to global scales. It is therefore essential to understand the time course of leaf water enrichment at both the evaporating sites and in the mesophyll but also to asses the suitability of simple models such as the Craig & Gordon (1965) steady-state prediction or the Dongmann et al. (1974) non-steady-state model. We describe here advection and diffusion of water isotopologues in leaves in the non-steady state. We first show how this relates to earlier non-steady state bulk leaf water enrichment models. The adv.-diff. model compares very well with observations of bulk mesophyll water during the whole diel cycle. It compares well with the enrichment at the evaporative sites during the day but shows some deviations at night. It is clear that night-time stomatal conductance should be measured in the future. However, varying mesophyll water volume did not seem critical for a good prediction. In addition, observations of single diurnal cycles do not constrain the effective length in the mesophyll. Finally, we show when simpler models of leaf water enrichment are suitable for applications of leaf water isotopes once weighted with the appropriate gas exchange flux. We then present a two-dimensional adv.-diff. description of leaf water enrichment along monocot leaves. The model reproduces well all published measurements along monocot leaf blades, except at the leaf tip and giving the uncertainties on measurements and model parameters. Our results suggest that the observed differences between C3 and C4 plants reflect more mesophyll tortuosity rather than leaf length or interveinal distance. Using measurements of non-steady-state, spatially varying leaf water enrichment we show that spatial patterns are in steady state around midday only, just as observed for bulk leaf water, but can be easily up-scaled to the whole leaf level, independent of the degree of heterogeneity. This together suggests that regardless of the heterogeneity of leaf water enrichment, it is appropriate to take simple models of leaf water enrichment weighted with the appropriate gas exchange flux for applications involving leaf water isotope enrichment. References Cuntz M, Og\'ee J, Farquhar GD, Peylin P & Cernusak LA (2007) Modelling advection and diffusion of water isotopologues in leaves, Plant, Cell & Environment 30, 892-909 Farquhar GD & Cernusak LA (2005) On the isotopic composition of leaf water in the non-steady state, Functional Plant Biology 32, 293-303 Og\'ee J, Cuntz M, Peylin P & Bariac T (2007) Non-steady-state, non-uniform transpiration rate and leaf anatomy effects on the progressive stable isotope enrichment of leaf water along monocot leaves, Plant, Cell & Environment 30, 367-387
B12C-02
Oxygen Isotopes of Water in Evapotranspiration and at the Sites of Leaf Evaporation in a Soybean Canopy
Stable isotopes in water have the potential to diagnose changes in the Earth's hydrologic budget in response to climate change and land use change. While the isotopic composition of the liquid water phase has been monitored for over four decades, there have been far fewer measurements of the isotopic composition of water in the vapor phase. The recent development of tunable diode laser technology (TDL) now makes it possible to monitor ambient water vapor isotopolgues with high temporal frequency. Combining this technology with micrometeorological techniques, it is also possible to determine the isotopic composition of evapotranspiration. We will present an unprecedented time series of the oxygen isotopic compositions of water vapor (δv) and evapotranspiration (δET) above a soybean canopy for the entire 2006-growing season. We observed large variability in surface δv from the daily to seasonal timescales that can be largely explained by Rayleigh processes but was also increasingly influenced by local evapotranspiration (ET) in the evenings. We used δET measurements to calculate the isotopic composition at the sites of evaporative enrichment in leaves (δL,e) and compared that to the commonly used steady state prediction (δL,s). During mid-day there was fair agreement. In the evening, non-steady state conditions caused δL,s to underestimate δL,e by nearly 2‰. Several new canopy scale properties emerged from this study. The formation of dew caused a sudden change in the sign of δET providing unique evidence in support of nighttime transpiration from the lower canopy even in saturated atmospheric conditions. Isotopic equilibrium was approximated between dew water, water vapor and bulk leaf water suggesting that δv controlled the δ18O of ecosystem water pools during very humid nights. We also found that vertical humidity and temperature variability associated with canopy structure must have affected vertical gradients in the δ18O of bulk leaf water (δL,b). Finally, we examined this dataset for direct evidence of the Peclet effect and found that the existing theory did not fully explain the observed variability in δL,e and δL,b. These and other observations provide excellent tests for canopy-scale water transfer models.
B12C-03
MIBA-US: Temporal and Spatial Variation of Water Isotopes in Terrestrial Ecosystems Across the United States
The oxygen and hydrogen isotope ratios of water in plants and soils are powerful tools for identifying the water sources in terrestrial ecosystems, partitioning evapotranspiration fluxes between evaporation and transpiration, and validating global climate models. To date, water isotope samples have only been collected at very few sites for any particular region or continent and often these collections are not made in coordination with important complementary observations such as eddy covariance measurements of latent and sensible heat fluxes and carbon dioxide exchange. We present data from 2005 and 2006 on the seasonal and interannual variation in the oxygen and hydrogen isotope ratios of leaf, stem and soil water across 12 eddy covariance flux sites comprising the MIBA (Moisture Isotopes in the Biosphere and Atmosphere) network within the continental United States. Values of δ18O in leaf, stem and soil water ranged from -10 to +30°, -16 to -2°, and -16 to 0°, respectively, reflecting the large variation across the major climatic and vegetation zones in the U.S. Stems were often more depleted than soils, but both approached the isotope ratios of local precipitation. As expected leaves were always more enriched than soil or stems and exhibited the greatest seasonal variation. Within each site, variation in δD and δ18O were strongly related, yet there were different "evaporation lines" for leaves, stems and soil. Differences in the slopes and intercepts of these evaporation lines appeared to be most strongly related to climatic differences among the sites. We demonstrate the utility of the MIBA data for constraining process-based models by comparing against response functions and spatial and temporal patters predicted by ISOLSM for the continental U.S. We also compare measurements against predictions by the widely used Craig-Gordon model of fractionation during evaporation.
B12C-04
Comparing three methods of NEE-flux partitioning from the same grassland ecosystem: the 13C, 18O isotope approach and using simulated Ecosystem respiration
As a change in the global climate occurs with increasing temperatures, the Carbon exchange processes of terrestrial ecosystems will change as well. However, it is difficult to quantify the degree to what ecosystem respiration will change relative to the CO2 uptake by photosynthesis. To estimate the carbon sequestration potential of terrestrial vegetation cover it is essential to know both fluxes: ecosystem respiration and the carbon uptake by the vegetation cover. Therefore the net ecosystem exchange of CO2 (NEE) was measured with the eddy covariance method and separated into assimilation and respiration flux. We applied three different approaches, 1) the conventional method, applying the nighttime relationship between soil temperature and NEE for calculating the respiration flux during the day, 2) the use of stable carbon and 3) oxygen isotopes. We compared the results of the three partitioning exercises for a temperate grassland ecosystem in the pre-Alps of Switzerland for four days in June 2004. The assimilation flux derived with the conventional NEE partitioning approach, was best represented at low PAR and low temperatures, in the morning between 5 and 9 am. With increasing temperature and PAR the assimilation for the whole canopy was underestimated. For partitioning NEE via 18O approach, correlations of temperature and radiation with assimilation and respiration flux were significantly higher for the partitioning approach with 18O than for the 13C NEE partitioning. A sensitivity analysis showed the importance of an accurate determination of the equilibrium term θ between CO2 and leaf water δ18O for the NEE partitioning with 18O. For using 13C to partition NEE, the correct magnitude of the 13C fractionation and for the respiration term is essential. The analysis of the data showed that for low light and low morning temperatures the conventional method delivers reasonably good results. When the temperatures exceeded 21°C the isotope approach provided the more realistic results, particularly the use of the oxygen isotopes. These results represent the situation for this particular grassland in the Swiss Alps while in other ecosystems the three partitioning approaches could show different results with regard to the quality and precision of the flux separation. In the presentation the potential reasons for the variation of the three approaches will be discussed.
B12C-05
Linking carbon isotopes and carbon-water exchange of plants across different scales
The anthropogenic rise in atmospheric CO2 levels may lead to increased photosynthetic uptake while transpiration rates remain constant or are reduced. Changes in plant regulation of carbon uptake and water loss also affect the carbon isotope signatures of plant material. But environmental conditions may change in addition to CO2. The resulting combination of factors can have different effects on the carbon-water balance of plants, and their carbon isotope signatures. For example, changes in evaporative demand alter the ratio of total carbon gain to water loss of a plant, the parameter of interest from the point of view of the atmosphere. Isotope values, on the other hand, also reflect physiological properties, including C:N allocation to carboxylation and internal conductance. Here, we explore how these factors shape carbon isotope signatures as well as carbon and water fluxes from leaf to ecosystem levels, and across diurnal to decadal timescales. We present new data to illustrate that a correlation between carbon isotope signatures and carbon-water exchange at the leaf level may not be passed on to the whole plant level. We then use a simple coupled model to analyse the relationships between carbon-water fluxes and isotope values. The model calculates gas exchange and carbon isotope signatures at the leaf level (for comparison with leaf samples), and propagates both gas exchange and isotope values to long- term trends in carbon-water exchange and carbon isotope signatures at the canopy scale (for comparison with samples of annual resolution). This approach is useful for exploring the sensitivity of carbon isotope ratios and carbon-water exchange of plants to simultaneous changes in external and internal factors, for example when interpreting trends in carbon isotope signatures obtained from tree rings.
B12C-06
A Plant-Based Proxy for the Oxygen Isotope Ratio of Atmospheric Water Vapor
Atmospheric water vapor is a major component of the global hydrological cycle, but the isotopic balance of vapor is largely unknown. It is shown here that the oxygen isotope ratio of leaf water in the epiphytic Crassulacean acid metabolism (CAM) plant Tillandsia usneoides (Spanish Moss) is controlled by the oxygen isotope ratio of atmospheric water vapor in both field and lab studies. Assuming that the leaf-water isotopic signature (and hence the atmospheric water vapor signature) is recorded in plant organic material, the atmospheric water vapor oxygen isotope ratios for Miami, Florida (USA) were reconstructed for several years from 1878 to 2005 using contemporary and herbarium specimens. T. usneoides ranges from Virginia, USA southwards through the tropics to Argentina, and the CAM epiphytic lifeform is widespread in other species. Therefore, epiphytes may be used to reconstruct the isotope ratio of atmospheric water for spatial scales that span over 60° of latitude and temporal scales that cover the last century of global temperature increase.
B12C-07
Accurate reproduction of daily to interannual variations of stable water isotopes using Iso- GSM and spectral nudging technique
This study incorporates the stable water isotopes into NCEP/ECPC's global spectral model in a manner similar to other isotope AGCMs. In addition, a newly developed spectral nudging technique (Yoshimura et al., 2007) is used, which allows to reproduce more actual spatial and temporal distribution of water and isotopes distributions. Divergence, vorticity, and temperature in NCEP/NCAR Reanalysis 2 data are the base fields that are nudged for more than 1000 km scales. Specific humidity remains unnudged in order to close the water budget. A T62L28 large-scale nudging simulation for 1979-2006 has now been simulated and the results show much more realistic precipitation isotope variations in comparison to an unnudged (free-forecast) simulation. Those simulation results with 3-dimensional structure of water isotopes are directly comparable with any occasional observations, in contrast to previous isotopic AGCMs, which are only usable for climatologic analyses.
B12C-08 INVITED
Global scale water isotope observations and the impact of the terrestrial biosphere on atmospheric hydrology
Water isotope measurements are known to be extremely useful for identifying hydrologic exchange processes at both single site scales and at larger scales from networks of, for instance, precipitation. Recent advances in observational techniques have allowed the development of a global scale dataset of the HDO to H2O isotope ratio in lower troposphere from spacecraft. The HDO estimates are found though a spectroscopic retrieval based on high resolution and well calibrated infrared spectra obtained from the Tropospheric Emission Spectrometer (TES) on NASA's Aura spacecraft. With these global scale observations available almost every two days, the ability to use isotopes to understand the impact of the terrestrial biosphere on atmospheric hydrology has become a possibility at not just local scales but for large geographic regions. Simulating the isotope exchange in global climate models continues to advance, but now such models can for the first time be validated and tested with observations. Further, with models, the importance of the processes identified in the observational data can be assessed in detail. Of particular interest is identifying the terrestrial source of atmospheric water vapor, and specifically continental evapotransipration. Using a combination of the satellite observations and model simulations, we identify the terrestrial source of atmospheric water, and demonstrate its importance is larger than previously recognized. This can be deduced from the observations since the isotopic signature of transpired water reflects the isotopic composition of precipitation, while that of oceanic origin reflects the disequilibrium fractionation during evaporation from the ocean. Based on these results and guided by model simulations, we speculate that should the land use characteristics of the tropical continental regions change, and reflect more arid environments, the impacts on the atmospheric hydrology and climate is more than of just local in extent. The use of the deuterium excess parameter allows continental exchange to be partitioned between evaporation from soil and standing water, and transpiration, and therefore a highly desirable quantity to measure. While the capacity to observe the 18O isotope in H2O from the TES exists, the signal to noise ratio in the data make estimation difficult for single atmospheric profiles. Nonetheless, mean statistics show promise. We describe the issues in the measurement technique that makes estimation of 18O challenging, and outline what requirements a next generation instruments should meet were it to specifically target understanding hydrologic processes with water isotope observations.