Climate extremes and grassland potential productivity

The considerable interannual variability (IAV) (~5 PgC yr−1) observed in atmospheric CO2 is dominated by variability in terrestrial productivity. Among terrestrial ecosystems, grassland productivity IAV is greatest. Relationships between grassland productivity IAV and climate drivers are poorly explained by traditional multiple-regression approaches. We propose a novel method, the perfect-deficit approach, to identify climate drivers of grassland IAV from observational data. The maximum daily value of each ecological or meteorological variable for each day of the year, over the period of record, defines the \u27perfect\u27 annual curve. Deficits of these variables can be identified by comparing daily observational data for a given year against the perfect curve. Links between large deficits of ecosystem activity and extreme climate events are readily identified. We applied this approach to five grassland sites with 26 site-years of observational data. Large deficits of canopy photosynthetic capacity and evapotranspiration derived from eddy-covariance measurements, and leaf area index derived from satellite data occur together and are driven by a local-dryness index during the growing season. This new method shows great promise in using observational evidence to demonstrate how extreme climate events alter yearly dynamics of ecosystem potential productivity and exchanges with atmosphere, and shine a new light on climate–carbon feedback mechanisms

Direct and indirect effects of climatic variations on the interannual variability in net ecosystem exchange across terrestrial ecosystems

Climatic variables not only directly affect the interannual variability (IAV) in net ecosystem exchange of CO2 (NEE) but also indirectly drive it by changing the physiological parameters. Identifying these direct and indirect paths can reveal the underlying mechanisms of carbon (C) dynamics. In this study, we applied a path analysis using flux data from 65 sites to quantify the direct and indirect climatic effects on IAV in NEE and to evaluate the potential relationships among the climatic variables and physiological parameters that represent physiology and phenology of ecosystems. We found that the maximum photosynthetic rate was the most important factor for the IAV in gross primary productivity (GPP), which was mainly induced by the variation in vapour pressure deficit. For ecosystem respiration (RE), the most important drivers were GPP and the reference respiratory rate. The biome type regulated the direct and indirect paths, with distinctive differences between forests and non-forests, evergreen needleleaf forests and deciduous broadleaf forests, and between grasslands and croplands. Different paths were also found among wet, moist and dry ecosystems. However, the climatic variables can only partly explain the IAV in physiological parameters, suggesting that the latter may also result from other biotic and disturbance factors. In addition, the climatic variables related to NEE were not necessarily the same as those related to GPP and RE, indicating the emerging difficulty encountered when studying the IAV in NEE. Overall, our results highlight the contribution of certain physiological parameters to the IAV in C fluxes and the importance of biome type and multi-year water conditions, which should receive more attention in future experimental and modelling research

Climate control of terrestrial carbon exchange across biomes and continents

Peer reviewe

The effects of CRF and urocortins on the hippocampal glutamate release

Corticotropin-releasing factor (CRF) is a hypothalamic neurohormone and an extrahypothalamic neurotransmitter that regulates the hypothalamic-pituitary-adrenal (HPA) axis. The urocortins (UCN I, UCN II and UCN III) are CRF-related peptides, which may also regulate the HPA axis directly or indirectly, by modulation of extrahypothalamic neurotransmitters, such as amygdalar GABA and hippocampal glutamate. Our previous in vitro superfusion studies have already demonstrated that CRF and UCN I stimulate the amygdalar GABA release in rats. The aim of the present study was to investigate the effects of CRF, UCN I, UCN II and UCN III on the glutamate release elicited electrically from rat hippocampal slices in similar in vitro conditions. In order to investigate the participation of CRF receptors (CRFR1 and CRFR2) in this process, hippocampal slices were pretreated with antalarmin, a selective antagonist of CRFR1 or astressin 2B, a selective antagonist of CRFR2. CRF and UCN I at 100 nM decreased significantly the hippocampal glutamate release evoked by electrical stimulation. In contrast, 100 nM of UCN II and UCN III did not affect significantly the hippocampal glutamate release enhanced by electrical stimulation. The decreasing effects of CRF and UCN I were reversed by antalarmin, but not by astressin 2B, both being administered in equimolar doses. Our results demonstrate that CRF and UCN I inhibit the glutamate release in the hippocampus via CRFR1 and that CRFR2 does not participate to this process. Based on the previous and the present results we conclude that CRFR1 agonists can activate the HPA axis not only directly, but also indirectly by increasing the amygdalar GABA release and decreasing the hippocampal glutamate release

Plant traits as predictor of ecosystem carbon fluxes - a case study across European grasslands

International audiencePredicting ecosystem responses to global change has become a major challenge, particularly as terrestrial ecosystems contribute to the mitigation of global climate change through carbon sequestration. Plant traits are major surrogates of ecosystem physiology may thus help to predict carbon (C) fluxes and their consequences for the delivery of ecosystem services (e.g. C sequestration) across climatic gradients and in changing environments. However, linkages between community abundance-weighted means (CWM) of plant functional traits and ecosystem C fluxes have rarely been tested. It is also not known to what degree traits, which are typically measured at a defined point in time, are suitable for predicting annual C fluxes. We analysed the relationships between ecosystem fluxes and community level plant traits for 13 European grasslands under contrasting climate and management regimes, using multiyear eddy covariance data. Plant traits (specific leaf area SLA, leaf dry matter content LDMC, specific root length SLR) were determined at peak biomass. Analyses showed that GPPmax (at maximum radiation) was related to SLA, SRL and LDMC across sites and management, where GPPmax was an excellent indicator for annual GPP. Similar relations were found between for root density (and -diameter) and ecosystem respiration. Ecosystems respiration at GPPmax was also in line with annual respiration, indicating the strong predictive potential of plant community traits. Our study therefore suggests that above- and belowground community level plant traits are well suited surrogates for predicting ecosystem C fluxes at peak biomass and at annual scale

The Neurokinin-1 Receptor Contributes to the Early Phase of Lipopolysaccharide-Induced Fever via Stimulation of Peripheral Cyclooxygenase-2 Protein Expression in Mice

Neurokinin (NK) signaling is involved in various inflammatory processes. A common manifestation of systemic inflammation is fever, which is usually induced in animal models with the administration of bacterial lipopolysaccharide (LPS). A role for the NK1 receptor was shown in LPS-induced fever, but the underlying mechanisms of how the NK1 receptor contributes to febrile response, especially in the early phase, have remained unknown. We administered LPS (120 µg/kg, intraperitoneally) to mice with the Tacr1 gene, i.e., the gene encoding the NK1 receptor, either present (Tacr1+/+) or absent (Tacr1−/−) and measured their thermoregulatory responses, serum cytokine levels, tissue cyclooxygenase-2 (COX-2) expression, and prostaglandin (PG) E2 concentration. We found that the LPS-induced febrile response was attenuated in Tacr1−/− compared to their Tacr1+/+ littermates starting from 40 min postinfusion. The febrigenic effect of intracerebroventricularly administered PGE2 was not suppressed in the Tacr1−/− mice. Serum concentration of pyrogenic cytokines did not differ between Tacr1−/− and Tacr1+/+ at 40 min post-LPS infusion. Administration of LPS resulted in amplification of COX-2 mRNA expression in the lungs, liver, and brain of the mice, which was statistically indistinguishable between the genotypes. In contrast, the LPS-induced augmentation of COX-2 protein expression was attenuated in the lungs and tended to be suppressed in the liver of Tacr1−/− mice compared with Tacr1+/+ mice. The Tacr1+/+ mice responded to LPS with a significant surge of PGE2 production in the lungs, whereas Tacr1−/− mice did not. In conclusion, the NK1 receptor is necessary for normal fever genesis. Our results suggest that the NK1 receptor contributes to the early phase of LPS-induced fever by enhancing COX-2 protein expression in the periphery. These findings advance the understanding of the crosstalk between NK signaling and the “cytokine-COX-2-prostaglandin E2” axis in systemic inflammation, thereby open up the possibilities for new therapeutic approaches

Direct and indirect effects of climatic variations on the interannual variability in net ecosystem exchange across terrestrial ecosystems

Climatic variables not only directly affect the interannual variability (IAV) in net ecosystem exchange of CO2 (NEE) but also indirectly drive it by changing the physiological parameters. Identifying these direct and indirect paths can reveal the underlying mechanisms of carbon (C) dynamics. In this study, we applied a path analysis using flux data from 65 sites to quantify the direct and indirect climatic effects on IAV in NEE and to evaluate the potential relationships among the climatic variables and physiological parameters that represent physiology and phenology of ecosystems. We found that the maximum photosynthetic rate was the most important factor for the IAV in gross primary productivity (GPP), which was mainly induced by the variation in vapour pressure deficit. For ecosystem respiration (RE), the most important drivers were GPP and the reference respiratory rate. The biome type regulated the direct and indirect paths, with distinctive differences between forests and non-forests, evergreen needleleaf forests and deciduous broadleaf forests, and between grasslands and croplands. Different paths were also found among wet, moist and dry ecosystems. However, the climatic variables can only partly explain the IAV in physiological parameters, suggesting that the latter may also result from other biotic and disturbance factors. In addition, the climatic variables related to NEE were not necessarily the same as those related to GPP and RE, indicating the emerging difficulty encountered when studying the IAV in NEE. Overall, our results highlight the contribution of certain physiological parameters to the IAV in C fluxes and the importance of biome type and multi-year water conditions, which should receive more attention in future experimental and modelling research