Plant Carbon Isotope Discrimination Research Articles

Global decadal variability of plant carbon isotope discrimination and its link to gross primary production.

Carbon isotope discrimination (Δ13C) in C3 woody plants is a key variable for the study of photosynthesis. Yet how Δ13C varies at decadal scales, and across regions, and how it is related to gross primary production (GPP), are still incompletely understood. Here we address these questions by implementing a new Δ13C modelling capability in the land‐surface model JULES incorporating both photorespiratory and mesophyll‐conductance fractionations. We test the ability of four leaf‐internal CO2 concentration models embedded in JULES to reproduce leaf and tree‐ring (TR) carbon isotopic data. We show that all the tested models tend to overestimate average Δ13C values, and to underestimate interannual variability in Δ13C. This is likely because they ignore the effects of soil water stress on stomatal behavior. Variations in post‐photosynthetic isotopic fractionations across species, sites and years, may also partly explain the discrepancies between predicted and TR‐derived Δ13C values. Nonetheless, the “least‐cost” (Prentice) model shows the lowest biases with the isotopic measurements, and lead to improved predictions of canopy‐level carbon and water fluxes. Overall, modelled Δ13C trends vary strongly between regions during the recent (1979–2016) historical period but stay nearly constant when averaged over the globe. Photorespiratory and mesophyll effects modulate the simulated global Δ13C trend by 0.0015 ± 0.005‰ and –0.0006 ± 0.001‰ ppm−1, respectively. These predictions contrast with previous findings based on atmospheric carbon isotope measurements. Predicted Δ13C and GPP tend to be negatively correlated in wet‐humid and cold regions, and in tropical African forests, but positively related elsewhere. The negative correlation between Δ13C and GPP is partly due to the strong dominant influences of temperature on GPP and vapor pressure deficit on Δ13C in those forests. Our results demonstrate that the combined analysis of Δ13C and GPP can help understand the drivers of photosynthesis changes in different climatic regions.

Isotopic fingerprints of mountain uplift and global cooling in paleoclimatic and paleoecological records from the northern Tibetan Plateau

Post-middle Miocene changes in climate and ecology in the northern Tibetan Plateau have been attributed to tectonic uplift and global climate change. However, the relative roles of tectonism and global climate change have been a long-standing debate. To untangle the complex influences of global climate change versus the tectonism on regional climate and ecology in the northern Tibetan Plateau, we studied carbon isotopes (δ13Cwax) of leaf wax long-chain n-alkanes and carbon (δ13C) and oxygen isotopes (δ18O) of carbonates from three stratigraphic successions (ca. 16 Ma to 2 Ma) in the Hexi Corridor (foreland basin) and compare them with paleoclimate records in the Qaidam Basin (intermontane basin). Our isotopic results can be divided into two stages. In Stage 1 (16–12 Ma), a decrease of δ13Cwax values in the Hexi Corridor and Qaidam Basin is synchronous with global cooling after the Mid-Miocene Climate Optimum (MMCO). We interpret the 2‰ decrease in δ13Cwax values to be associated with the decline in C4 plants due to the global cooling. In Stage 2 (since 12 Ma), plant carbon isotope discrimination (ƐCO2-wax) and δ18O values in the Hexi Corridor reflect relatively stable hydroclimate condition, which contracts with the enhanced aridity in Qaidam Basin during the same interval. We attribute the difference in paleohydrological conditions to the uplift-induced basin isolation and the enhancement of water deficiency in the Qaidam Basin. The effects of late Miocene uplift also manifest as persistent intra-basinal differences in the Hexi Corridor, indicating changes associated with orographic precipitation and ecology.

C3 plant carbon isotope discrimination does not respond to CO2 concentration on decadal to centennial timescales.

Plant carbon isotope discrimination is complex, and could be driven by climate, evolution and/or edaphic factors. We tested the climate drivers of carbon isotope discrimination in modern and historical plant chemistry, and focus in particular on the relationship between rising [CO2 ] over Industrialization and carbon isotope discrimination. We generated temporal records of plant carbon isotopes from museum specimens collected over a climo-sequence to test plant responses to climate and atmospheric change over the past 200yr (including Pinus strobus, Platycladus orientalis, Populus tremuloides, Thuja koraiensis, Thuja occidentalis, Thuja plicata, Thuja standishii and Thuja sutchuenensis). We aggregated our results with a meta-analysis of a wide range of C3 plants to make a comprehensive study of the distribution of carbon isotope discrimination and values among different plant types. We show that climate variables (e.g. mean annual precipitation, temperature and, key to this study, CO2 in the atmosphere) do not drive carbon isotope discrimination. Plant isotope discrimination is intrinsic to each taxon, and could link phylogenetic relationships and adaptation to climate quantitatively and over ecological to geological time scales.

Testing the accuracy of new paleoatmospheric CO2 proxies based on plant stable carbon isotopic composition and stomatal traits in a range of simulated paleoatmospheric O2:CO2 ratios

A drive to improve long-term estimates of atmospheric CO 2 change through Earths history has led to the development of novel paleoproxy CO 2 methods applicable to fossil plants. This paper compares two of these paleoproxy CO 2 methods (1) the empirical model of Schubert and Jahren (2012, 2015) termed the C3 plant proxy, which was developed using the hyperbolic relationship observed between plant carbon isotope discrimination (Δ 13 C) and p CO 2 and (2) the mechanistic model of Franks et al. (2014), which utilizes stomatal anatomy measurements and plant carbon isotopic composition and is based on established equations for leaf gas exchange and photosynthesis. To date both models lack detailed experimental testing of the robustness and accuracy of their p CO 2 predictions in relation to phylogenetic differences in Δ 13 C between C3 plant groups and/or species and atmospheric O 2 concentration, which has co-fluctuated with CO 2 in the geological past. Here, we investigate if these novel paleoproxy CO 2 approaches can produce phylogenetically independent estimates of p CO 2 that are not influenced by variations in atmospheric oxygen. To address this, model estimates of p CO 2 were compared with measured CO 2 values for ten plant species representing four major vascular plant groups (lycophytes, monilophytes, gymnosperms and angiosperms) grown for 6 months in walk-in plant growth chambers under varying O 2 :CO 2 ratios. Results from the mechanistic model reveal that species-specific plant responses to atmospheric CO 2 accounted for the large variability in CO 2 predictions between species and overestimations of p CO 2 by ∼+232 ppm to 940 ppm. Adjustments to the model that involved: (1) corrections to the photorespiratory compensation point (to account for fluctuating oxygen) and (2) removal of the phylogenetic effect on Δ 13 C, reduced between-species variability (by 50%) and led to better p CO 2 estimates within 58–229 ppm of measured values. The C3 plant proxy (empirical approach) produced accurate CO 2 estimates within +37 to +71 ppm of measured values, however it was affected by species-specific differences in Δ 13 C and for some species resulted in negative estimates of p CO 2 . Sub-ambient O 2 (16%) resulted in erroneously high CO 2 estimates (∼100 ppm higher than the control) for a number of species, as plant responses to decreasing O 2 mimicked those of increasing CO 2 . We conclude that both models (with phylogenetic corrections to Franks et al. (2014)) can produce accurate estimates of paleo-CO 2 when a mix of three to four species, preferably containing representatives from both pteridophytes and spermatophytes, is used to obtain a consensus p CO 2 estimate. We advocate a fossil assemblage rather than a single-species approach to paleo-CO 2 estimation in future application of either method.

The key factor limiting plant growth in cold and humid alpine areas also plays a dominant role in plant carbon isotope discrimination

Many environmental factors affect carbon isotope discrimination in plants, yet the predominant factor influencing this process is generally assumed to be the key growth-limiting factor. However, to our knowledge this hypothesis has not been confirmed. We therefore determined the carbon isotope composition (δ13C) of plants growing in two cold and humid mountain regions where temperature is considered to be the key growth-limiting factor. Mean annual temperature (MAT) showed a significant impact on variation in carbon isotope discrimination value (Δ) irrespective of study area or plant functional type with either partial correlation or regression analysis, but the correlation between Δ and soil water content (SWC) was usually not significant. In multiple stepwise regression analysis, MAT was either the first or the only variable selected into the prediction model of Δ against MAT and SWC, indicating that the effect of temperature on carbon isotope discrimination was predominant. The results therefore provide evidence that the key growth-limiting factor is also crucial for plant carbon isotope discrimination. Changes in leaf morphology, water viscosity and carboxylation efficiency with temperature may be responsible for the observed positive correlation between Δ and temperature.

Altered nitrogen and precipitation along urban gradients affect harvester ants and seed sources

We investigated the effects of nitrogen deposition and precipitation on Messor pergandei (Mayr) harvester ants and plants to identify alterations in the desert food web in California. We measured ant colony attributes and shrub fruit densities, as well as nitrogen and carbon concentrations and stable isotopes, at 18 sites along a nitrogen deposition gradient. Ant nest density increased from low to high deposition sites; however, ant nest mound size and the density of abandoned nests decreased as deposition increased. Nest mound size was positively correlated with the size and age of the colony; therefore, these results suggest that colonization has been more frequent with increased inter-colony competition in areas of high deposition. Nitrogen and carbon isotope values of perennial plant leaves and seeds, annual plant seeds, and ants were significantly enriched in the heavy isotopes from low to high nitrogen deposition regions, indicating the possibility of plants assimilating different sources of both elements, including anthropogenically-produced compounds. Plant carbon isotope discrimination also differed with the decrease in precipitation across the gradient. Considering that deserts are limited by both nitrogen and precipitation, our results suggest that altered nitrogen inputs in conjunction with precipitation may result in cascading effects through trophic levels and drive arid ecosystem change.

Evaluation of the Use of Carbon Isotope Discrimination as a Selection Tool of Perennial Fodder Species for Temporary Pastures

Estimation of the relationship between carbon isotope discrimination (Δ13C) in perennial fodder plants and various factors such as climate, fertilization and soil could contribute to a better selection and/or improvement of genotype of fodder species. Theoretical models of carbon isotope discrimination in plants indicate a linear relationship between Δ13C and CO2 partial pressures ratio (pi/pa) from the intercellular spaces (pi) of CO2 in leaves and from the atmosphere (pa). It is also known that between pi parameter and water-use efficiency (WUE) there is a negative correlation. Eight perennial fodder legume and grass mixtures were cultivated on three levels of fertilization in order to assess mixtures for pastures that can provide sustainable quantitative and qualitative yields. The species, which composed the mixtures, were also cultivated as pure crops on the same fertilization conditions. The results of this study indicated that in five of the six species, which composed the mixture with the highest yield, pi/pa values decreased while fertilization doses increased, most likely as a result of variation in water-use efficiency.

Correlations of foliar Δ with K concentration and ash content in sand-fixing plants in the Tengger Desert of China: patterns and implications

In different-aged sand-fixing zones and shifting sand dunes, the seasonal variations of foliar carbon isotope discrimination (Δ), potassium (K) concentration and ash content were investigated in three dominated deserted plants (Artemisia ordosica, Hedysarum scoparium and Caragana korshinskii) during growth season in Shapotou, which is in the southeast margin of the Tengger Desert. The correlations of foliar Δ with foliar K concentration and ash content were examined to evaluate the foliar K and ash content as surrogates of Δ in those deserted plants. Results showed that there were significant effects of plant species, micro-habitation and growth season on foliar Δ, K concentration and ash content. Foliar Δ of C. korshinskii was significantly lower than those of A. ordosica and H. scoparium, and K concentrations in A. ordosica were 2.14 and 2.36 times those of C. korshinskii and H. scoparium. At the same time, micro-habitation and the conditions in growth seasons had significant effects on foliar Δ, K concentration and ash content. Ash content and K concentration were positively correlated to Δ in A. ordosica and H. scoparium, while there was significantly negative relationship between foliar K concentration and Δ in C. korshinskii. Thus, those findings suggest that foliar ash content and K concentration can serve as surrogates of carbon isotope discrimination (Δ) in A. ordosica and H. scoparium, while they do not in C. korshinskii. This result implied that the correlations of foliar Δ with ash content and K concentration were various due to the physiological features of plant species, and species differences should be fully considered when evaluating the surrogates of carbon isotope discrimination in plants.

The influence of (photo)respiration on carbon isotope discrimination in plants

ABSTRACTThe contribution which (photo)respiration makes to carbon isotope discrimination (Δ13C) was examined by conducting simultaneous gas exchange measurements and isotopic analysis of carbon dioxide passing over leaves of Triticum aestivum and Phaseolus vulgaris, via manipulations of the carbon isotope composition (δ13C) of source CO2 during growth and measurement. Dark respiration only altered net Δ13C (Δobs) at low CO2 assimilation, and was sensitive to source CO2δ13C during measurement. Photorespiration reduced Δobs relative to Δ13C predicted from pi/pa (Δi) over the full range of CO2 assimilation, to a greater degree under elevated oxygen partial pressure (pO2), indicating fractionation during photorespiration (f) in T. aestivum. For P. vulgaris, Δobs was insensitive to elevated pO2 at higher assimilation rates, suggesting that f was minimal. A model was developed to calculate gross discrimination (Δps), independent of (photo)respiration, from which estimates of f were obtained for T. aestivum (3.3‰) and P. vulgaris (0.5‰). Because photorespiratory fractionation varies interspecifically, and influences net Δ13C which is directly reflected in leaf δ13C, consideration of (photo)respiratory fractionation is necessary when interpreting δ13C of leaf material, especially under conditions where (photo)respiratory CO2 losses make a large relative contribution to total plant carbon budgets.

Shoot biomass, δ13C, nitrogen and chlorophyll responses of two arctic dwarf shrubs to in situ shading, nutrient application and warming simulating climatic change.

As climatic change might induce ecophysiological changes in plants which affect their long-term performance, we investigated responses in above-ground biomass, δ13C, nitrogen and chlorophyll of two evergreen arctic dwarf shrubs, Cassiope tetragona and Empetrum hermaphroditum, to 5 (biomass, N) or 6 years of shading, nutrient application and air/soil warming at a dwarf shrub dominated tree-line heath (450 m a.s.l) and a high altitude fellfield (1100 m a.s.l.) in Swedish Lapland. Warming enhanced the green biomass (equivalent to the last 3-4 years of leaf production) and the ratio of green to brown biomass of C. tetragona at the fellfield, and diluted the shoot N concentration. Fertilizer application led to higher shoot N concentration and larger green-to-brown biomass ratio at both sites, and fertilizer application and warming generally had an additive effect on the green biomass. We conclude that both warming and increased soil nutrient availability stimulated the growth of C. tetragona at the fellfield whereas at the heath there was a clear increase in production only if enhanced temperature was combined with nutrient application. Across treatments C. tetragona at the fellfield had 0.6‰ higher δ13C and 1.4 mg g-1 more leaf N, and the soil organic matter δ13C was 1.0‰ higher at the fellfield than at the heath. However, an increase in shoot N concentration with altitude does not necessarily lead to higher δ13C as no differences in δ13C were observed when leaf N of the two dwarf shrubs was increased by fertilizer application c. tetragona in non-warmed plots had higher δ13C values than those from warmed plots at the same altitude, which provides the first in situ experimental validation of the theory that temperature partly is responsible for altitudinal trends in plant carbon isotope discrimination. Increased biomass and chlorophyll concentration of C. tetragona in warmed plots points to increased assimilation, at least at the fellfield. As the δ13C-based and, therefore, time-integrated estimate of the ratio of CO2 concentration in the leaf intercellular spaces to that in the atmosphere (C i/C a) also increased, warming probably enhanced the stomatal conductance relatively more than the C assimilation, which may be harmful if climatic change leads to reduced soil moisture content and increased plant competition for water. At both sites C. tetragona and E. hermaphroditum responded to shade by increasing the concentration of shoot N and photosynthetic pigments whereas biomass production (and therefore also net photosynthesis) did not decline. Shade was accompanied by a 0.6-1.3‰ (E. hermaphroditum) or 1.2-2.2‰ (C. tetragona) decrease in δ13C. This could be due to enhanced stomatal conductance with shading, and perhaps to shade reducing the ericoid mycorrhizal uptake of soil organic C, a factor which has been overlooked as an influence on plant δ13C.

A global survey of carbon isotope discrimination in plants from high altitude

Carbon 13/12 isotope ratios have been determined from leaves of a hundred C3 plant species (or ecotypes) from all major mountain ranges of the globe, avoiding drought stressed areas. A general increase in 13C content was found with increasing altitude, i.e. overall discrimination against the heavy isotope is reduced at high elevation. The steepest decline of discrimination is observed in taxa typically ranging to highest elevations (e.g. the genus Ranunculus). Mean δ 13C for all samples collected between 2500 and 5600 m altitude is-26.15‰ compared to the lowland average of-28.80‰ (P<0.001). Forbs from highest elevations reach-24‰. According to theory of 13C discrimination this indicates decreasing relative limitation of carbon uptake by carboxylation. In other words, we estimate that the ratio of internal to external partial pressure of CO2 (p i /p a )in leaves of high elevation plants is lower than in leaves of low altitude. These results confirm recent gas exchange analyses in high and low elevation plants.