Discrimination against 13C during photosynthesis is a well-characterised phenomenon. It leaves behind distinct signatures in organic matter of plants and in the atmosphere. The former is depleted in 13C, the latter is enriched during periods of preponderant photosynthetic activity of terrestrial ecosystems. The intra-annual cycle and latitudinal gradient in atmospheric 13C resulting from photosynthetic and respiratory activities of terrestrial plants have been exploited for the reconstruction of sources and sinks through deconvolution by inverse modelling. Here, we compile evidence for widespread post-photosynthetic fractionation that further modifies the isotopic signatures of individual plant organs and consequently leads to consistent differences in delta13C between plant organs. Leaves were on average 0.96 per thousand and 1.91 per thousand more depleted than roots and woody stems, respectively. This phenomenon is relevant if the isotopic signature of CO2-exchange fluxes at the ecosystem level is used for the reconstruction of individual sources and sinks. It may also modify the parameterization of inverse modelling approaches if it leads to different isotopic signatures of organic matter with different residence times within the ecosystems and to a respiratory contribution to the average difference between the isotopic composition of plant organic matter and the atmosphere. We discuss the main hypotheses that can explain the observed inter-organ differences in delta13C.

The acclimation responses of walnut leaf photosynthesis to the irradiance microclimate were investigated by characterizing the photosynthetic properties of the leaves sampled on young trees (Juglans nigraxregia) grown in simulated sun and shade environments, and within a mature walnut tree crown (Juglans regia) in the field. In the young trees, the CO(2) compensation point in the absence of mitochondrial respiration (Gamma*), which probes the CO(2) versus O(2) specificity of Rubisco, was not significantly different in sun and shade leaves. The maximal net assimilation rates and stomatal and mesophyll conductances to CO(2) transfer were markedly lower in shade than in sun leaves. Dark respiration rates were also lower in shade leaves. However, the percentage inhibition of respiration by light during photosynthesis was similar in both sun and shade leaves. The extent of the changes in photosynthetic capacity and mesophyll conductance between sun and shade leaves under simulated conditions was similar to that observed between sun and shade leaves collected within the mature tree crown. Moreover, mesophyll conductance was strongly correlated with maximal net assimilation and the relationships were not significantly different between the two experiments, despite marked differences in leaf anatomy. These results suggest that photosynthetic capacity is a valuable parameter for modelling within-canopies variations of mesophyll conductance due to leaf acclimation to light.

Little is known about the role of plant functional diversity for ecosystem-level carbon (C) fluxes. To fill this knowledge gap, we translocated monoliths hosting communities with four and 16 sown species from a long-term grassland biodiversity experiment ('The Jena Experiment') into a controlled environment facility for ecosystem research (Ecotron). This allowed quantifying the effects of plant diversity on ecosystem C fluxes as well as three parameters of C uptake efficiency (water and nitrogen use efficiencies and apparent quantum yield). By combining data on ecosystem C fluxes with vegetation structure and functional trait-based predictors, we found that increasing plant species and functional diversity led to higher gross and net ecosystem C uptake rates. Path analyses and light response curves unravelled the diversity of leaf nitrogen concentration in the canopy as a key functional predictor of C fluxes, either directly or indirectly via LAI and aboveground biomass.

Extreme climatic events (ECEs) such as droughts and heat waves are predicted to increase in intensity and frequency and impact the terrestrial carbon balance. However, we lack direct experimental evidence of how the net carbon uptake of ecosystems is affected by ECEs under future elevated atmospheric CO 2 concentrations (eCO 2 ). Taking advantage of an advanced controlled environment facility for ecosystem research (Ecotron), we simulated eCO 2 and extreme cooccurring heat and drought events as projected for the 2050s and analyzed their effects on the ecosystem-level carbon and water fluxes in a C3 grassland. Our results indicate that eCO 2 not only slows down the decline of ecosystem carbon uptake during the ECE but also enhances its recovery after the ECE, as mediated by increases of root growth and plant nitrogen uptake induced by the ECE. These findings indicate that, in the predicted near future climate, eCO 2 could mitigate the effects of extreme droughts and heat waves on ecosystem net carbon uptake.climate change | extreme events | elevated CO 2 | carbon fluxes | grassland ecosystem I ncreased aridity and heat waves are projected to increase in the 21st century for most of Africa, southern and central Europe, the Middle East, and parts of the Americas, Australia, and southeast Asia (1-3). These regions have a large fraction of their land covered by grasslands and rangelands, a biome covering approximately one-quarter of the Earth's land area and contributing to the livelihoods of more than 800 million people (4). There is mounting evidence that extreme climatic events (ECEs) may significantly affect the regional and global carbon (C) fluxes (3, 5-9) and potentially feed back on atmospheric CO 2 concentrations and the climate system (7). However, our knowledge concerning the outcome of the interaction between future ECEs and elevated atmospheric CO 2 concentrations (eCO 2 ) for ecosystem C stocks is equivocal (10-12). Studies focusing on plant physiological responses have shown that eCO 2 has the potential to mitigate future drought-related stress on plant growth by reducing stomatal conductance, thereby increasing water use efficiency (WUE) (13-15) and preserving soil moisture (16)(17)(18). However, to date, little is known on whether and how eCO 2 alters the consequences of ECEs for ecosystem net C uptake. Because the capacity of ecosystems to act as a C sink depends on the relative effects of eCO 2 , ECE, and their potential interaction on both plant and soil processes, an integrated assessment of all C fluxes during and after the ECEs is important if we are to estimate the overall C balance.Using the Montpellier CNRS Ecotron facility (www.ecotron. cnrs.fr), we tested with 12 large controlled environment units (macrocosms, SI Appendix, Fig. S1) whether (i) an ECE (severe drought and heat wave) predicted for the 2050s reduces ecosystem net C uptake by reducing ecosystem photosynthesis relative to ecosystem respiration (R eco ), (ii) eCO 2 buffers the negative effects of the ECE on ecosystem CO 2 fluxes ...

The spatial variations in the stable carbon isotope composition ( and water-use efficiency resulted from the short-term response of leaf gas exchange to variations in local irradiance and, to a much lesser extent, from the long-term acclimation of leaf characteristics to the local light regime.