Search

Bryophytes are abundant in tundra ecosystems, where they affect carbon and nitrogen cycling through primary production and associations with N2-fixing bacteria. Bryophyte responses to climate warming are inconclusive, likely because species-specific responses are poorly understood. Here we investigated how warming affects the growth and nitrogenase activity of 10 tundra bryophyte species in two tundra landscapes. Collected bryophyte samples were grown in temperature-controlled growth chambers for 12 weeks at five temperatures from 3 to 18 °C. We measured growth, N concentration, δ15N, and δ13C after 3 months and nitrogenase activity after 5 and 12 weeks. Bryophyte growth and associated nitrogenase activity generally increased with temperature, but species differed in their optima. Bryophyte N concentration and δ15N indicated that, for some species, increased N2-fixation could not compensate for growth-induced N limitation. High landscape coverage and large positive warming effects on feather mosses and Sphagnum species highlight their competitive advantages, confirm earlier field observations, and contribute to the mechanistic understanding of differential bryophyte growth in response to warming. We suggest that indirect effects of climate change, such as surface drying and shrub expansion, are likely main threats to slow-growing bryophytes across the Arctic, with consequences for biodiversity and C balance.

Interpreting phloem carbohydrate or xylem tissue carbon isotopic composition as measures of water-use efficiency or past tree productivity requires in-depth knowledge of the factors altering the isotopic composition within the pathway from ambient air to phloem contents and tree ring. One of least understood of these factors is mesophyll conductance (g(m)). We formulated a dynamic model describing the leaf photosynthetic pathway including seven alternative g(m) descriptions and a simple transport of sugars from foliage down the trunk. We parameterised the model for a boreal Scots pine stand and compared simulated g(m) responses with weather variations. We further compared the simulated delta C-13 of new photosynthates among the different g(m) descriptions and against measured phloem sugar delta C-13. Simulated g(m) estimates of the seven descriptions varied according to weather conditions, resulting in varying estimates of phloem delta C-13 during cold/moist and warm/dry periods. The model succeeded in predicting a drought response and a postdrought release in phloem sugar delta C-13 indicating suitability of the model for inverse prediction of leaf processes from phloem isotopic composition. We suggest short-interval phloem sampling during and after extreme weather conditions to distinguish between mesophyll conductance drivers for future model development.

Background and Aims The stomatal conductance (gs) of most plant species decreases in response to elevated atmospheric CO2 concentration. This response could have a significant impact on plant water use in a future climate. However, the regulation of the CO2-induced stomatal closure response is not fully understood. Moreover, the potential genetic links between short-term (within minutes to hours) and long-term (within weeks to months) responses of gs to increased atmospheric CO2 have not been explored. Methods We used Arabidopsis thaliana recombinant inbred lines originating from accessions Col-0 (strong CO2 response) and C24 (weak CO2 response) to study short- and long-term controls of gs. Quantitative trait locus (QTL) mapping was used to identify loci controlling short- and long-term gs responses to elevated CO2, as well as other stomata-related traits. Key Results Short- and long-term stomatal responses to elevated CO2 were significantly correlated. Both short- and long-term responses were associated with a QTL at the end of chromosome 2. The location of this QTL was confirmed using near-isogenic lines and it was fine-mapped to a 410-kb region. The QTL did not correspond to any known gene involved in stomatal closure and had no effect on the responsiveness to abscisic acid. Additionally, we identified numerous other loci associated with stomatal regulation. Conclusions We identified and confirmed the effect of a strong QTL corresponding to a yet unknown regulator of stomatal closure in response to elevated CO2 concentration. The correlation between short- and long-term stomatal CO2 responses and the genetic link between these traits highlight the importance of understanding guard cell CO2 signalling to predict and manipulate plant water use in a world with increasing atmospheric CO2 concentration. This study demonstrates the power of using natural variation to unravel the genetic regulation of complex traits.

Several publications have examined leaf-trait and carbon-cycling shifts along an Amazon-Andes transect spanning 3.5 km in elevation and 16°C in mean annual temperature. Photosynthetic capacity was previously shown to increase as temperature declines with increasing elevation, counteracting enzyme-kinetic effects. Primary production declines, nonetheless, due to decreasing light availability. We aimed to predict leaf-trait and production gradients from first principles, using published data to test an emerging theory whereby photosynthetic traits and primary production depend on optimal acclimation and/or adaptation to environment. We re-analysed published data for 210 species at 25 sites, fitting linear relationships to elevation for both predicted and observed photosynthetic traits and primary production. Declining leaf-internal/ambient CO2 ratio (χ) and increasing carboxylation (Vcmax ) and electron-transport (Jmax ) capacities with increasing elevation were predicted. Increases in leaf nitrogen content with elevation were explained by increasing Vcmax and leaf mass-per-area. Leaf and soil phosphorus covaried, but after controlling for elevation, no nutrient metric accounted for any additional variance in photosynthetic traits. Primary production was predicted to decline with elevation. This analysis unifies leaf and ecosystem observations in a common theoretical framework. The insensitivity of primary production to temperature is shown to emerge as a consequence of the optimisation of photosynthetic traits.

Artículo escrito por un elevado número de autores, solo se referencian el que aparece en primer lugar, el nombre del grupo de colaboración, si le hubiere, y los autores pertenecientes a la UAM, For more than five decades, research has been conducted at Ny-Ålesund, in Svalbard, Norway, to understand the structure and functioning of High-Arctic ecosystems and the profound impacts on them of environmental change. Terrestrial, freshwater, glacial and marine ecosystems are accessible year-round from Ny-Ålesund, providing unique opportunities for interdisciplinary obser-vational and experimental studies along physical, chemical, hydrological and climatic gradients. Here, we synthesize terrestrial and freshwater research at Ny-Ålesund and review current knowledge of biodiversity patterns, species population dynamics and interactions, ecosystem processes, biogeochemical cycles and anthropogenic impacts. There is now strong evidence of past and ongoing biotic changes caused by climate change, including negative effects on populations of many taxa and impacts of rain-on-snow events across multiple trophic levels. While species-level characteristics and responses are well under-stood for macro-organisms, major knowledge gaps exist for microbes, inverte-brates and ecosystem-level processes. In order to fill current knowledge gaps, we recommend (1) maintaining monitoring efforts, while establishing a long-term ecosystem-based monitoring programme; (2) gaining a mechanistic under-standing of environmental change impacts on processes and linkages in food webs; (3) identifying trophic interactions and cascades across ecosystems; and (4) integrating long-term data on microbial, invertebrate and freshwater com-munities, along with measurements of carbon and nutrient fluxes among soils, atmosphere, freshwaters and the marine environment. The synthesis here shows that the Ny-Ålesund study system has the characteristics needed to fill these gaps in knowledge, thereby enhancing our understanding of High-Arctic ecosystems and their responses to environmental variability and change

The continuously increasing atmospheric carbon dioxide concentration ([CO2]) has substantial effects on plant growth, and on the composition and structure of forests. However, how plants respond to elevated [CO2] (e[CO2]) under intra- and interspecific competition has been largely overlooked. In this study, we employed Abies faxoniana Rehder & Wilson and Picea purpurea Mast. seedlings to explore the effects of e[CO2] (700 p.p.m.) and plant–plant competition on plant growth, physiological and morphological traits, and leaf ultrastructure. We found that e[CO2] stimulated plant growth, photosynthesis and nonstructural carbohydrates (NSC), affected morphological traits and leaf ultrastructure, and enhanced water- and nitrogen (N)- use efficiencies in A. faxoniana and P. purpurea. Under interspecific competition and e[CO2], P. purpurea showed a higher biomass accumulation, photosynthetic capacity and rate of ectomycorrhizal infection, and higher water- and N-use efficiencies compared with A. faxoniana. However, under intraspecific competition and e[CO2], the two conifers showed no differences in biomass accumulation, photosynthetic capacity, and water- and N-use efficiencies. In addition, under interspecific competition and e[CO2], A. faxoniana exhibited higher NSC levels in leaves as well as more frequent and greater starch granules, which may indicate carbohydrate limitation. Consequently, we concluded that under interspecific competition, P. purpurea possesses a positive growth and adjustment strategy (e.g. a higher photosynthetic capacity and rate of ectomycorrhizal infection, and higher water- and N-use efficiencies), while A. faxoniana likely suffers from carbohydrate limitation to cope with rising [CO2]. Our study highlights that plant–plant competition should be taken into consideration when assessing the impact of rising [CO2] on the plant growth and physiological performance.

Grain legumes are commonly used for food and feed all over the world and are the main source of protein for over a billion people worldwide, but their production is at risk from climate change. Water deficit and heat stress both significantly reduce the yield of grain legumes, and the faba bean is considered particularly susceptible. The genetic improvement of faba bean for drought adaptation (water deficit tolerance) by conventional methods and molecular breeding is time-consuming and laborious, since it depends mainly on selection and adaptation in multiple sites. The lack of high-throughput screening methodology and low heritability of advantageous traits under environmental stress challenge breeding progress. Alternatively, selection based on secondary characters in a controlled environment followed by field trials is successful in some crops, including faba beans. In general, measured features related to drought adaptation are shoot and root morphology, stomatal characteristics, osmotic adjustment and the efficiency of water use. Here, we focus on the current knowledge of biochemical and physiological markers for legume improvement that can be incorporated into faba bean breeding programs for drought adaptation.

Long-term tree growth is significantly affected by climate change, which have become a global concern. Tree-ring width and isotopic information can show how trees respond to climate change on a long-term scale and reveal some phenomena of tree decline or death. In this study, we used isotopic techniques and investigated annual changes in carbon isotope composition and tree-ring width of Populus simonii Carr. in Zhangbei, as well as trends in tree-ring carbon discrimination (&Delta, 13C) and iWUE in normal, mildly declining and severely declining trees, in order to make a retrospective analysis and further understand the process of tree decline. We found that there were significant differences (p &lt, 0.01**) in &delta, 13C, &Delta, 13C, ci and iWUE at different decline stages, meaning that the &delta, 13C and iWUE could be new indicators of tree health. The iWUE of all groups increased significantly, while the growth rate of declined P. simonii was much higher than that of normal growth P. simonii. According to the analysis, there may be a threshold of iWUE for healthy trees, which once the threshold value is exceeded, it indicates that trees are resistant to adversity and their growth is under stress. Similarly, the changing trend of BAI supports our conclusion with its changes showed that tree growth became slower and slower as degradation progressed. iWUE inferred from tree-ring stable carbon isotope composition is a strong modulator of adaptation capacity in response to environmental stressors under climate change. Elevated annual temperatures and increased groundwater depth are all contributing to the decline of P. simonii in north China.

© 2018 The Authors. New Phytologist © 2018 New Phytologist Trust The ratio of leaf intercellular to ambient CO 2 (χ) is modulated by stomatal conductance (g s ). These quantities link carbon (C) assimilation with transpiration, and along with photosynthetic capacities (V cmax and J max ) are required to model terrestrial C uptake. We use optimization criteria based on the growth environment to generate predicted values of photosynthetic and water-use efficiency traits and test these against a unique dataset. Leaf gas-exchange parameters and carbon isotope discrimination were analysed in relation to local climate across a continental network of study sites. Sun-exposed leaves of 50 species at seven sites were measured in contrasting seasons. Values of χ predicted from growth temperature and vapour pressure deficit were closely correlated to ratios derived from C isotope (δ 13 C) measurements. Correlations were stronger in the growing season. Predicted values of photosynthetic traits, including carboxylation capacity (V cmax ), derived from δ 13 C, growth temperature and solar radiation, showed meaningful agreement with inferred values derived from gas-exchange measurements. Between-site differences in water-use efficiency were, however, only weakly linked to the plant's growth environment and did not show seasonal variation. These results support the general hypothesis that many key parameters required by Earth system models are adaptive and predictable from plants’ growth environments.

Plant functional ecology requires the quantification of trait variation and its controls. Field measurements on 483 species at 48 sites across China were used to analyse variation in leaf traits, and assess their predictability. Principal components analysis (PCA) was used to characterize trait variation, redundancy analysis (RDA) to reveal climate effects, and RDA with variance partitioning to estimate separate and overlapping effects of site, climate, life-form and family membership. Four orthogonal dimensions of total trait variation were identified: leaf area (LA), internal-to-ambient CO2 ratio (χ), leaf economics spectrum traits (specific leaf area (SLA) versus leaf dry matter content (LDMC) and nitrogen per area (Narea )), and photosynthetic capacities (Vcmax , Jmax at 25°C). LA and χ covaried with moisture index. Site, climate, life form and family together explained 70% of trait variance. Families accounted for 17%, and climate and families together 29%. LDMC and SLA showed the largest family effects. Independent life-form effects were small. Climate influences trait variation in part by selection for different life forms and families. Trait values derived from climate data via RDA showed substantial predictive power for trait values in the available global data sets. Systematic trait data collection across all climates and biomes is still necessary.

To improve our understanding of the global carbon balance and its representation in terrestrial biosphere models, we present here a first dual-species application of the CarbonTracker Data Assimilation System (CTDAS). The system's modular design allows for assimilating multiple atmospheric trace gases simultaneously to infer exchange fluxes at the Earth surface. In the prototype discussed here, we interpret signals recorded in observed carbon dioxide (CO2) along with observed ratios of its stable isotopologues (CO2)-C-13/(CO2)-C-12 (delta C-13). The latter is in particular a valuable tracer to untangle CO2 exchange from land and oceans. Potentially, it can also be used as a proxy for continent-wide drought stress in plants, largely because the ratio of (CO2)-C-13 and (CO2)-C-12 molecules removed from the atmosphere by plants is dependent on moisture conditions. The dual-species CTDAS system varies the net exchange fluxes of both (CO2)-C-13 and CO2 in ocean and terrestrial biosphere models to create an ensemble of (CO2)-C-13 and CO2 fluxes that propagates through an atmospheric transport model. Based on differences between observed and simulated (CO2)-C-13 and CO2 mole fractions (and thus delta C-13) our Bayesian minimization approach solves for weekly adjustments to both net fluxes and isotopic terrestrial discrimination that minimizes the difference between observed and estimated mole fractions.With this system, we are able to estimate changes in terrestrial delta C-13 exchange on seasonal and continental scales in the Northern Hemisphere where the observational network is most dense. Our results indicate a decrease in stomatal conductance on a continent-wide scale during a severe drought. These changes could only be detected after applying combined atmospheric CO2 and delta C-13 constraints as done in this work. The additional constraints on surface CO2 exchange from delta C-13 observations neither affected the estimated carbon fluxes nor compromised our ability to match observed CO2 variations. The prototype presented here can be of great benefit not only to study the global carbon balance but also to potentially function as a data-driven diagnostic to assess multiple leaf-level exchange parameterizations in carbon-climate models that influence the CO2, water, isotope, and energy balance.

Fungi and plants have engaged in intimate symbioses that are globally widespread and have driven terrestrial biogeochemical processes since plant terrestrialisation >500 Mya. Recently, hitherto unknown nutritional mutualisms involving ancient lineages of fungi and non-vascular plants have been discovered. However, their extent and functional significance in vascular plants remains uncertain. Here, we provide first evidence of abundant carbon-for-nitrogen exchange between an early-diverging vascular plant (Lycopodiaceae) and Mucoromycotina (Endogonales) fine root endophyte regardless of changes in atmospheric CO2 concentration. Furthermore, we provide evidence that the same fungi also colonize neighbouring non-vascular and flowering plants. These findings fundamentally change our understanding of the evolution, physiology, interrelationships and ecology of underground plant-fungal symbioses in terrestrial ecosystems by revealing an unprecedented nutritional role of Mucoromycotina fungal symbionts in vascular plants.

The ratio of leaf internal (ci ) to ambient (ca ) partial pressure of CO2 , defined here as χ, is an index of adjustments in both leaf stomatal conductance and photosynthetic rate to environmental conditions. Measurements and proxies of this ratio can be used to constrain vegetation model uncertainties for predicting terrestrial carbon uptake and water use. We test a theory based on the least-cost optimality hypothesis for modelling historical changes in χ over the 1951-2014 period, across different tree species and environmental conditions, as reconstructed from stable carbon isotopic measurements across a global network of 103 absolutely dated tree-ring chronologies. The theory predicts optimal χ as a function of air temperature, vapour pressure deficit, ca and atmospheric pressure. The theoretical model predicts 39% of the variance in χ values across sites and years, but underestimates the intersite variability in the reconstructed χ trends, resulting in only 8% of the variance in χ trends across years explained by the model. Overall, our results support theoretical predictions that variations in χ are tightly regulated by the four environmental drivers. They also suggest that explicitly accounting for the effects of plant-available soil water and other site-specific characteristics might improve the predictions.

Tree mortality is a key driver of forest dynamics and its occurrence is projected to increase in the future due to climate change. Despite recent advances in our understanding of the physiological mechanisms leading to death, we still lack robust indicators of mortality risk that could be applied at the individual tree scale. Here, we build on a previous contribution exploring the differences in growth level between trees that died and survived a given mortality event to assess whether changes in temporal autocorrelation, variance, and synchrony in time-series of annual radial growth data can be used as early warning signals of mortality risk. Taking advantage of a unique global ring-width database of 3065 dead trees and 4389 living trees growing together at 198 sites (belonging to 36 gymnosperm and angiosperm species), we analyzed temporal changes in autocorrelation, variance, and synchrony before tree death (diachronic analysis), and also compared these metrics between trees that died and trees that survived a given mortality event (synchronic analysis). Changes in autocorrelation were a poor indicator of mortality risk. However, we found a gradual increase in inter-annual growth variability and a decrease in growth synchrony in the last ∼20 years before mortality of gymnosperms, irrespective of the cause of mortality. These changes could be associated with drought-induced alterations in carbon economy and allocation patterns. In angiosperms, we did not find any consistent changes in any metric. Such lack of any signal might be explained by the relatively high capacity of angiosperms to recover after a stress-induced growth decline. Our analysis provides a robust method for estimating early-warning signals of tree mortality based on annual growth data. In addition to the frequently reported decrease in growth rates, an increase in inter-annual growth variability and a decrease in growth synchrony may be powerful predictors of gymnosperm mortality risk, but not necessarily so for angiosperms. Fil: Cailleret, Maxime. Swiss Federal Institute for Forest, Snow and Landscape Research; Suiza. Eidgenössische Technische Hochschule Zürich; Suiza Fil: Dakos, Vasilis. Université Montpellier II; Francia. Centre National de la Recherche Scientifique; Francia Fil: Jansen, Steven. Universitat Ulm; Alemania Fil: Robert, Elisabeth M. R.. Royal Museum For Central Africa; Bélgica. Vrije Unviversiteit Brussel; Bélgica. Consejo Superior de Investigaciones Científicas. Centre de Recerca Ecológica I Aplicacions Forestals; España Fil: Aakala, Tuomas. University of Helsinski; Finlandia Fil: Amoroso, Mariano Martin. Universidad Nacional de Río Negro. Sede Andina. Instituto de Investigaciones en Recursos Naturales, Agroecología y Desarrollo Rural; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina Fil: Antos, Joe A.. University of Victoria; Canadá Fil: Bigler, Christof. Eidgenössische Technische Hochschule Zürich; Suiza Fil: Bugmann, Harald. Eidgenössische Technische Hochschule Zürich; Suiza Fil: Caccianaga, Marco. Università degli Studi di Milano; Italia Fil: Camarero, Jesus Julio. Instituto Pirenaico de Ecología; España. Consejo Superior de Investigaciones Científicas; España Fil: Cherubini, Paolo. Swiss Federal Institute for Forest, Snow and Landscape Research; Suiza Fil: Coyea, Marie R.. Laval University; Canadá Fil: Cufar, Katarina. University Of Ljubljana; Eslovenia Fil: Das, Adrian J.. United States Geological Survey; Estados Unidos Fil: Davi, Hendrik. Unité de Recherche des Forets Méditerranéennes; Francia Fil: Gea Izquierdo, Guillermo. Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria. Centro de Investigación Forestal; España Fil: Gillner, Sten. Institute of Forest Botany and Forest Zoology; Alemania Fil: Haavik, Laurel J.. USDA Forest Service, Forest Health Protection; Estados Unidos. University of Arkansas for Medical Sciences; Estados Unidos Fil: Hartmann, Henrik. Max Planck Institute Of Biochemistry.; Alemania Fil: Heres, Ana Maria. Universitatea Transilvania Din Brasov; Rumania. Basque Centre For Climate Change; España Fil: Hultine, Kevin R.. Desert Botanical Garden. Conservation and Collections. Department of Research; Estados Unidos Fil: Janda, Pavel. Czech University of Life Sciences; República Checa Fil: Kane, Jeffrey M.. Humboldt State University; Estados Unidos Fil: Kharuk, Viachelsav I.. Siberian Federal University; Rusia Fil: Kitzberger, Thomas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; Argentina Fil: Klein, Tamir. Weizmann Institute of Science; Israel Fil: Levanic, Tom. Slovenian Forestry Institute; Eslovenia Fil: Linares, Juan Carlos. Universidad Pablo de Olavide; España Fil: Lombardi, Fabio. Universita Della Calabria.; Italia Fil: Mäkinen, Harri. Natural Resources Institute Finland; Finlandia Fil: Mészáros, Ilona. Debreceni Egyetem Természettudományi És Technológiai Kar; Hungría Fil: Metsaranta, Juha M.. Northern Forestry Centre; Canadá Fil: Oberhuber, Walter. Universidad de Innsbruck; Austria Fil: Papadopoulos, Andreas. Technological Educational Institute of Stereas Elladas; Grecia Fil: Petritan, Any Mary. Eidgenössische Forschungsanstalt Fur Wald, Schnee Und Landschaft Eth-bereichs; Alemania Fil: Rohner, Brigitte. Eidgenössische Forschungsanstalt Fur Wald, Schnee Und Landschaft Eth-bereichs; Alemania Fil: Sangüesa Barreda, Gabriel. Universidad de Valladolid; España Fil: Smith, Jeremy M.. State University of Colorado at Boulder; Estados Unidos Fil: Stan, Amanda B.. Northern Arizona University; Estados Unidos Fil: Stojanovic, Dejan B.. University Of Novi Sad; Serbia Fil: Suarez, Maria Laura. Instituto Nacional de Tecnologia Agropecuaria Bariloche; Argentina Fil: Svoboda, Miroslav. Czech University Of Life Sciences Prague; República Checa Fil: Trotsiuk, Volodymyr. Czech University of Life Sciences Prague; República Checa. Eidgenössische Forschungsanstalt Fur Wald, Schnee Und Landschaft Eth bereichs; Alemania. Eidgenössische Technische Hochschule Zürich; Alemania Fil: Villalba, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Cientifícas y Tecnológicas; Argentina Fil: Westwood, Alana R.. University of Alberta; Canadá Fil: Wyckoff, Peter H.. University Of Minnesota System; Estados Unidos Fil: Martínez Vilalta, Jordi. Universitat Autònoma de Barcelona; España. Consejo Superior de Investigaciones Científicas. Centre de Recerca Ecológica I Aplicacions Forestals; España

It remains unclear how global climate change affects dioecious plants that may be especially vulnerable to climate drivers, because they often exhibit skewed sex ratios and eco-physiological specialization in certain microhabitats. In this study, female and male saplings of Populus cathayana were employed to explore sex-specific responses and the effects of sexual competition under elevated temperature (ET), elevated CO2 (EC) and combination of elevated temperature and CO2 (ETC). The results demonstrated that elevated temperature and CO2 interactively modulated sexual competition and responses of P. cathayana. Moreover, competition patterns affected the eco-physiological responses of P. cathayana to climate change treatments. Under both intra- and inter-sexual competition, biomass components, photosynthetic parameters and carbon-related metabolites of females were most strongly affected by ET, while males exhibited a higher photosynthesis and resource use efficiency, and a better biomass accumulation and carbon balance mechanism when compared to females when experiencing intra-sexual competition under EC. The competitive pressure of females on males in inter-sexual competition was intensified by ET, while it was alleviated by ETC. We conclude that climate change drivers and competition patterns differently regulate the sex-specific responses and competitive intensity of males and females, which may have a crucial effect on sex ratios, spatial sexual segregation, biomass production and carbon sequestration in dioecious species in the future.

Plant water-use efficiency (WUE, the carbon gained through photosynthesis per unit of water lost through transpiration) is a tracer of the plant physiological controls on the exchange of water and carbon dioxide between terrestrial ecosystems and the atmosphere. At the leaf level, rising CO2 concentrations tend to increase carbon uptake (in the absence of other limitations) and to reduce stomatal conductance, both effects leading to an increase in leaf WUE. At the ecosystem level, indirect effects (e.g. increased leaf area index, soil water savings) may amplify or dampen the direct effect of CO2 . Thus, the extent to which changes in leaf WUE translate to changes at the ecosystem scale remains unclear. The differences in the magnitude of increase in leaf versus ecosystem WUE as reported by several studies are much larger than would be expected with current understanding of tree physiology and scaling, indicating unresolved issues. Moreover, current vegetation models produce inconsistent and often unrealistic magnitudes and patterns of variability in leaf and ecosystem WUE, calling for a better assessment of the underlying approaches. Here, we review the causes of variations in observed and modelled historical trends in WUE over the continuum of scales from leaf to ecosystem, including methodological issues, with the aim of elucidating the reasons for discrepancies observed within and across spatial scales. We emphasize that even though physiological responses to changing environmental drivers should be interpreted differently depending on the observational scale, there are large uncertainties in each data set which are often underestimated. Assumptions made by the vegetation models about the main processes influencing WUE strongly impact the modelled historical trends. We provide recommendations for improving long-term observation-based estimates of WUE that will better inform the representation of WUE in vegetation models.

To improve our understanding of the global carbon balance and its representation in terrestrial biosphere models, we present here a first dual-species application of the CarbonTracker Data Assimilation System (CTDAS). The system's modular design allows for assimilating multiple atmospheric trace gases simultaneously to infer exchange fluxes at the Earth surface. In the prototype discussed here, we interpret signals recorded in observed carbon dioxide (CO2) along with observed ratios of its stable isotopologues (CO2)-C-13/(CO2)-C-12 (delta C-13). The latter is in particular a valuable tracer to untangle CO2 exchange from land and oceans. Potentially, it can also be used as a proxy for continent-wide drought stress in plants, largely because the ratio of (CO2)-C-13 and (CO2)-C-12 molecules removed from the atmosphere by plants is dependent on moisture conditions. The dual-species CTDAS system varies the net exchange fluxes of both (CO2)-C-13 and CO2 in ocean and terrestrial biosphere models to create an ensemble of (CO2)-C-13 and CO2 fluxes that propagates through an atmospheric transport model. Based on differences between observed and simulated (CO2)-C-13 and CO2 mole fractions (and thus delta C-13) our Bayesian minimization approach solves for weekly adjustments to both net fluxes and isotopic terrestrial discrimination that minimizes the difference between observed and estimated mole fractions. With this system, we are able to estimate changes in terrestrial delta C-13 exchange on seasonal and continental scales in the Northern Hemisphere where the observational network is most dense. Our results indicate a decrease in stomatal conductance on a continent-wide scale during a severe drought. These changes could only be detected after applying combined atmospheric CO2 and delta C-13 constraints as done in this work. The additional constraints on surface CO2 exchange from delta C-13 observations neither affected the estimated carbon fluxes nor compromised our ability to match observed CO2 variations. The prototype presented here can be of great benefit not only to study the global carbon balance but also to potentially function as a data-driven diagnostic to assess multiple leaf-level exchange parameterizations in carbon-climate models that influence the CO2, water, isotope, and energy balance.

Source at https://doi.org/10.5194/cp-14-473-2018. Reanalysis data show an increasing trend in Arctic precipitation over the 20th century, but changes are not homogenous across seasons or space. The observed hydroclimate changes are expected to continue and possibly accelerate in the coming century, not only affecting pan-Arctic natural ecosystems and human activities, but also lower latitudes through the atmospheric and ocean circulations. However, a lack of spatiotemporal observational data makes reliable quantification of Arctic hydroclimate change difficult, especially in a long-term context. To understand Arctic hydroclimate and its variability prior to the instrumental record, climate proxy records are needed. The purpose of this review is to summarise the current understanding of Arctic hydroclimate during the past 2000 years. First, the paper reviews the main natural archives and proxies used to infer past hydroclimate variations in this remote region and outlines the difficulty of disentangling the moisture from the temperature signal in these records. Second, a comparison of two sets of hydroclimate records covering the Common Era from two data-rich regions, North America and Fennoscandia, reveals inter- and intra-regional differences. Third, building on earlier work, this paper shows the potential for providing a high-resolution hydroclimate reconstruction for the Arctic and a comparison with last-millennium simulations from fully coupled climate models. In general, hydroclimate proxies and simulations indicate that the Medieval Climate Anomaly tends to have been wetter than the Little Ice Age (LIA), but there are large regional differences. However, the regional coverage of the proxy data is inadequate, with distinct data gaps in most of Eurasia and parts of North America, making robust assessments for the whole Arctic impossible at present. To fully assess pan-Arctic hydroclimate variability for the last 2 millennia, additional proxy records are required.

Estimating summer temperature fluctuations over long timescales in southern South America is essential for better understanding the past climate variations in the Southern Hemisphere. Here we developed robust 212 year long basal area increment (BAI) and δ13C chronologies from living temperature-sensitive Fitzroya cupressoides on the eastern slope of the northern Patagonian Andes (41°S). After removing the increasing trend from the growth records likely due to the CO2 fertilization effect, we tested the potential to reconstruct past summer temperature variations using BAI and δ13C as predictors. The reconstruction based on δ13C records has the strongest predictive skills and explains as much as 62% of the total variance in instrumental summer temperature (n = 81, p

Severe droughts in the Northern Hemisphere cause a widespread decline of agricultural yield, the reduction of forest carbon uptake, and increased CO2 growth rates in the atmosphere. Plants respond to droughts by partially closing their stomata to limit their evaporative water loss, at the expense of carbon uptake by photosynthesis. This trade-off maximizes their water-use efficiency (WUE), as measured for many individual plants under laboratory conditions and field experiments. Here we analyse the C-13/C-12 stable isotope ratio in atmospheric CO2 to provide new observational evidence of the impact of droughts on the WUE across areas of millions of square kilometres and spanning one decade of recent climate variability. We find strong and spatially coherent increases in WUE along with widespread reductions of net carbon uptake over the Northern Hemisphere during severe droughts that affected Europe, Russia and the United States in 2001-2011. The impact of those droughts on WUE and carbon uptake by vegetation is substantially larger than simulated by the land-surface schemes of six state-of-the-art climate models. This suggests that drought-induced carbon-climate feedbacks may be too small in these models and improvements to their vegetation dynamics using stable isotope observations can help to improve their drought response.

It has been known for a long time that drought intensity is a critical variable in determining water stress of Mediterranean tree species. However, not as much attention has been paid to other drought characteristics, for example the timing of the dry periods.We investigated the impact of the timing and intensity of extreme droughts on growing season length, growth and water-use efficiency of three tree species, Pinus nigra ssp. Salzmannii J.F. Arnold, Quercus ilex ssp. ballota (Desf.) Samp. and Quercus faginea Lam. coexisting in a continental Mediterranean ecosystem. Over the study period (2009-13), intense droughts were observed at annual and seasonal scales, particularly during 2011 and 2012. In 2012, an atypically dry winter and spring was followed by an intense summer drought.Quercus faginea growth was affected more by drought timing than by drought intensity, probably because of its winter-deciduous leaf habit. Pinus nigra showed a lower decrease in secondary growth than observed in the two Quercus species in extremely dry years. Resilience to extreme droughts was different among species, with Q. faginea showing poorer recovery of growth after very dry years. The highest intra-and inter-annual plasticity in water-use efficiency was observed in P. nigra, which maintained a more water-saving strategy. Our results revealed that the timing of extreme drought events can affect tree function to a larger extent than drought intensity, especially in deciduous species.Legacy effects of drought over months and years significantly strengthened the impact of drought timing and intensity on tree function.

Drought is a major limitation to crop productivity worldwide. Plants lose most of their water through stomata, thus making stomata an important organ in the control of transpiration and photosynthesis. This study assessed the stomatal behavior of four cowpea genotypes grown under four moisture levels under hot semi-arid conditions. Stomatal conductance (gs) was measured at 47, 54, 70 and 77 days after planting (DAP). Biomass and carbon isotope composition (C-13) were also determined at flowering. Genotype and moisture level significantly influenced gs. Genotypes varied in gs at vegetative stages (47 and 54 DAP) only. TVu4607 had higher gs under severe drought conditions at both 47 and 54 DAP. On the other hand, moisture level influenced gs at 54 and 70 DAP only. Stomatal conductance was severely restricted in cowpea under both moderate and severe drought conditions as gs was mostly below the threshold 0.10 mol m(-2) s(-1). Relationships between: biomass and gs, and C-13 and gs were positive under severe drought only. The findings revealed that cowpea genotypes vary in gs under dry conditions and that the variation is more prominent at vegetative stage, suggesting that cowpea productivity in dry areas could be improved through selection of genotypes that maintain higher gs under dry conditions.

Water scarcity and the increasing severity of water deficit stress are major challenges to sustaining irrigated rice (Oryza sativa) production. Despite the technologies developed to reduce the water requirement, rice growth is seriously constrained under water deficit stress compared with other dryland cereals such as wheat (Triticum aestivum). We exposed rice cultivars with contrasting responses to water deficit stress and wheat cultivars well adapted to water-limited conditions to the same moisture stress during vegetative growth to unravel the whole-plant (shoot and root morphology) and organ/tissue (root anatomy) responses. Wheat cultivars followed a water-conserving strategy by reducing specific leaf area and developing thicker roots and moderate tillering. In contrast, rice ‘IR64’ and ‘Apo’ adopted a rapid water acquisition strategy through thinner roots under water deficit stress. Root diameter, stele and xylem diameter, and xylem number were more responsive and varied with different positions along the nodal root under water deficit stress in wheat, whereas they were relatively conserved in rice cultivars. Increased metaxylem diameter and lower metaxylem number near the root tips and exactly the opposite phenomena at the root-shoot junction facilitated the efficient use of available soil moisture in wheat. Tolerant rice ‘Nagina 22’ had an advantage in root morphological and anatomical attributes over cultivars IR64 and Apo but lacked plasticity, unlike wheat cultivars exposed to water deficit stress. The key traits determining the adaptation of wheat to dryland conditions have been summarized and discussed.

We present an enhanced version of the SiBCASA terrestrial biosphere model that is extended with (a) biomass burning emissions from the SiBCASA carbon pools using remotely sensed burned area from the Global Fire Emissions Database (GFED), (b) an isotopic discrimination scheme that calculates 13C signatures of photosynthesis and autotrophic respiration, and (c) a separate set of 13C pools to carry isotope ratios into heterotrophic respiration. We quantify in this study the terrestrial exchange of CO2 and 13CO2 as a function of environmental changes in humidity and biomass burning. The implementation of biomass burning yields similar fluxes as CASA-GFED both in magnitude and spatial patterns. The implementation of isotope exchange gives a global mean discrimination value of 15.2‰, ranges between 4 and 20‰ depending on the photosynthetic pathway in the plant, and compares favorably (annually and seasonally) with other published values. Similarly, the isotopic disequilibrium is similar to other studies that include a small effect of biomass burning as it shortens the turnover of carbon. In comparison to measurements, a newly modified starch/sugar storage pool propagates the isotopic discrimination anomalies to respiration much better. In addition, the amplitude of the drought response by SiBCASA is lower than suggested by the measured isotope ratios. We show that a slight increase in the stomatal closure for large vapor pressure deficit would amplify the respired isotope ratio variability. Our study highlights the importance of isotope ratio observations of 13C to assess and improve biochemical models like SiBCASA, especially with regard to the allocation and turnover of carbon and the responses to drought.

Heat and drought stress frequently occur together, however, their impact on plant growth and photosynthesis (PN ) is unclear. The frequency, duration and severity of heat and drought stress events are predicted to increase in the future, having severe implications for agricultural productivity and food security. To assess the impact on plant gas exchange, physiology and morphology we grew drought tolerant and sensitive varieties of C3 sunflower (Helianthus annuus) and C4 maize (Zea mays) under conditions of elevated temperature for 4 weeks prior to the imposition of water deficit. The negative impact of temperature on PN was most apparent in sunflower. The drought tolerant sunflower retained ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) activity under heat stress to a greater extent than its drought sensitive counterpart. Maize exhibited no varietal difference in response to increased temperature. In contrast to previous studies, where a sudden rise in temperature induced an increase in stomatal conductance (Gs ), we observed no change or a reduction in Gs with elevated temperature, which alongside lower leaf area mitigated the impact of drought at the higher temperature. The drought tolerant sunflower and maize varieties exhibited greater investment in root-systems, allowing greater uptake of the available soil water. Elevated temperatures associated with heat-waves will have profound negative impacts on crop growth in both sunflower and maize, but the deleterious effect on PN was less apparent in the drought tolerant sunflower and both maize varieties. As C4 plants generally exhibit water use efficiency (WUE) and resistance to heat stress, selection on the basis of tolerance to heat and drought stress would be more beneficial to the yields of C3 crops cultivated in drought prone semi-arid regions.

We present an enhanced version of the SiBCASA terrestrial biosphere model that is extended with (a) biomass burning emissions from the SiBCASA carbon pools using remotely sensed burned area from the Global Fire Emissions Database (GFED), (b) an isotopic discrimination scheme that calculates 13C signatures of photosynthesis and autotrophic respiration, and (c) a separate set of 13C pools to carry isotope ratios into heterotrophic respiration. We quantify in this study the terrestrial exchange of CO2 and 13CO2 as a function of environmental changes in humidity and biomass burning. The implementation of biomass burning yields similar fluxes as CASA-GFED both in magnitude and spatial patterns. The implementation of isotope exchange gives a global mean discrimination value of 15.2‰, ranges between 4 and 20‰ depending on the photosynthetic pathway in the plant, and compares favorably (annually and seasonally) with other published values. Similarly, the isotopic disequilibrium is similar to other studies that include a small effect of biomass burning as it shortens the turnover of carbon. In comparison to measurements, a newly modified starch/sugar storage pool propagates the isotopic discrimination anomalies to respiration much better. In addition, the amplitude of the drought response by SiBCASA is lower than suggested by the measured isotope ratios. We show that a slight increase in the stomatal closure for large vapor pressure deficit would amplify the respired isotope ratio variability. Our study highlights the importance of isotope ratio observations of 13C to assess and improve biochemical models like SiBCASA, especially with regard to the allocation and turnover of carbon and the responses to drought.

[1] Previous studies suggest that a large part of the variability in the atmospheric ratio of 13CO2/12CO2originates from carbon exchange with the terrestrial biosphere rather than with the oceans. Since this variability is used to quantitatively partition the total carbon sink, we here investigate the contribution of interannual variability (IAV) in biospheric exchange to the observed atmospheric 13C variations. We use the Simple Biosphere ‐ Carnegie‐Ames‐Stanford Approach biogeochemical model, including a detailed isotopic fractionation scheme, separate 12C and 13C biogeochemical pools, and satellite‐observed fire disturbances. This model of 12CO2 and 13CO2 thus also produces return fluxes of 13CO2from its differently aged pools, contributing to the so‐called disequilibrium flux. Our simulated terrestrial 13C budget closely resembles previously published model results for plant discrimination and disequilibrium fluxes and similarly suggests that variations in C3 discrimination and year‐to‐year variations in C3and C4 productivity are the main drivers of their IAV. But the year‐to‐year variability in the isotopic disequilibrium flux is much lower (1σ=±1.5 PgC ‰ yr−1) than required (±12.5 PgC ‰ yr−1) to match atmospheric observations, under the common assumption of low variability in net ocean CO2 fluxes. This contrasts with earlier published results. It is currently unclear how to increase IAV in these drivers suggesting that SiBCASA still misses processes that enhance variability in plant discrimination and relative C3/C4productivity. Alternatively, 13C budget terms other than terrestrial disequilibrium fluxes, including possibly the atmospheric growth rate, must have significantly different IAV in order to close the atmospheric 13C budget on a year‐to‐year basis.

Previous studies suggest that a large part of the variability in the atmospheric ratio of (CO2)-C-13/(12)CO(2)originates from carbon exchange with the terrestrial biosphere rather than with the oceans. Since this variability is used to quantitatively partition the total carbon sink, we here investigate the contribution of interannual variability (IAV) in biospheric exchange to the observed atmospheric C-13 variations. We use the Simple Biosphere - Carnegie-Ames-Stanford Approach biogeochemical model, including a detailed isotopic fractionation scheme, separate C-12 and C-13 biogeochemical pools, and satellite-observed fire disturbances. This model of (CO2)-C-12 and (CO2)-C-13 thus also produces return fluxes of (13)CO(2)from its differently aged pools, contributing to the so-called disequilibrium flux. Our simulated terrestrial C-13 budget closely resembles previously published model results for plant discrimination and disequilibrium fluxes and similarly suggests that variations in C-3 discrimination and year-to-year variations in C(3)and C-4 productivity are the main drivers of their IAV. But the year-to-year variability in the isotopic disequilibrium flux is much lower (1 sigma=1.5PgCyr(-1)) than required (12.5PgCyr(-1)) to match atmospheric observations, under the common assumption of low variability in net ocean CO2 fluxes. This contrasts with earlier published results. It is currently unclear how to increase IAV in these drivers suggesting that SiBCASA still misses processes that enhance variability in plant discrimination and relative C-3/C(4)productivity. Alternatively, C-13 budget terms other than terrestrial disequilibrium fluxes, including possibly the atmospheric growth rate, must have significantly different IAV in order to close the atmospheric C-13 budget on a year-to-year basis.

Miscanthus is one of the most promising bioenergy crops with high photosynthetic nitrogen-use efficiency (PNUE). It is unclear how nitrogen (N) influences the photosynthesis in Miscanthus. Among three Miscanthus genotypes, the net photosynthetic rate (P (N)) under the different light intensity and CO2 concentration was measured at three levels of N: 0, 100, and 200 kg ha(-1). The concentrations of chlorophyll, soluble protein, phosphoenolpyruvate carboxylase (PEPC), ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) large subunit, leaf anatomy and carbon isotope discrimination (Delta) in the leaf were analyzed to probe the response of photosynthesis in Miscanthus genotypes to N levels. P (N) in all genotypes rose significantly as N application increased. The initial slope of response curves of P (N) to C (i) was promoted by N application in all genotypes. Both stomatal conductance and C (i) were increased with increased N supply, indicating that stomatal factors played an important role in increasing P (N). At a given C (i), P (N) in all genotypes was enhanced by N, implying that nonstomatal factors might also play an important role in increasing P (N). Miscanthus markedly regulated N investment into PEPC rather than the Rubisco large subunit under higher N conditions. Bundle sheath leakiness of CO2 was constant at about 0.35 for all N levels. Therefore, N enhanced the photosynthesis of Miscanthus mainly by increasing stomatal conductance and PEPC concentration.

International audience; Although there is increasing awareness of the importance of food legumes in human, animal and soil health, adoption of improved production technologies for food legume crops is not proceeding at the same pace as for cereal crops. Over the previous decade, the only food legumes to have shown significant production increases have been chickpea, lentil and faba bean in North America, chickpea in Australia, and faba bean in Europe. In smallholder farming in developing countries, production trends have mostly been static or have declined over the past decade despite the existence of technology that should permit higher and more stable yields. Ability to reverse negative trends is jeopardized by climate change as food legumes are mostly grown rainfed and are being exposed to increasingly variable and extreme weather. This review examines recent innovations in cultivation technology for the major food legumes-chickpea, lentil, dry pea, faba bean, lupin, common bean, mung bean, black gram, cowpea, and pigeonpea-and explores constraints to their adoption, particularly by resource-poor smallholder farmers. Conservation agriculture, involving minimum soil disturbance, maximum soil cover, and diverse rotations, has contributed to sustainable cropping system production in large-scale commercial farming systems in the Americas, Europe, Australia, and Turkey. Temperate food legumes have been incorporated into such systems. Adoption of conservation agriculture is only just beginning for smallholder farming in Asia and Africa, catalyzed by the development of low-cost implements suitable for minimum tillage. Water use efficiency improves with conservation agriculture as it allows for earlier planting, reduced soil evaporation, better weed management, and increased access to nutrients. Ecosystem-based approaches to plant nutrition are evolving which place more reliance on accessing organic and mineral reservoirs than in replenishing the immediately available pool with chemical fertilizers, leading to enhanced nutrient use efficiency of cropping systems. Ecosystem-based approaches are also being applied to management of weeds, diseases, and insect pests of food legumes, again with decreased reliance on synthetic chemicals. In achieving sustainable agricultural production systems, there is increasing realization of the need to move towards the tenets of organic agriculture, as exemplified in conservation agriculture and ecosystem-based approaches to plant nutrition and pest management. This does not necessarily imply a desire to qualify for organic product certification but more a realization of the need for sustainable agriculture. The movement towards conservation and organic agriculture encourages greater inclusion of food legumes, and legumes generally, in cropping systems. Unfortunately, however, technology transfer to resource-poor farming situations, where most food legumes are produced, remains a major bottleneck to meeting global demand. More participatory approaches to technology development, testing, and dissemination are required than hitherto practiced. It is suggested that this process could be enhanced by better focusing on major constraints within the value addition chain for food legumes.

Electron flux from water via photosystem II (PSII) and PSI to oxygen (water–water cycle) may provide a mechanism for dissipation of excess excitation energy in leaves when CO2 assimilation is restricted. Mass spectrometry was used to measure O2 uptake and evolution together with CO2 uptake in leaves of French bean and maize at CO2 concentrations saturating for photosynthesis and the CO2 compensation point. In French bean at high CO2 and low O2 concentrations no significant water–water cycle activity was observed. At the CO2 compensation point and 3% O2 a low rate of water–water cycle activity was observed, which accounted for 30% of the linear electron flux from water. In maize leaves negligible water–water cycle activity was detected at the compensation point. During induction of photosynthesis in maize linear electron flux was considerably greater than CO2 assimilation, but no significant water–water cycle activity was detected. Miscanthus × giganteus grown at chilling temperature also exhibited rates of linear electron transport considerably in excess of CO2 assimilation; however, no significant water–water cycle activity was detected. Clearly the water–water cycle can operate in leaves under some conditions, but it does not act as a major sink for excess excitation energy when CO2 assimilation is restricted.

Most of the water in the River Nile originates from monsoonal rainfall over the Ethiopian Highlands. Despite warnings of future climate change, little is known about the historical variability in this supply, particularly at annual resolution. Development of tree-ring records in this region has been limited by the occurrence of bi- or multimodal rainfall regimes, causing the development of multiple growth rings that cannot be dated with confidence. In this study, we identified annual rings in 30 Juniperus procera trees from northwest Ethiopia by dendrochronology and AMS radiocarbon dating. Carbon isotope ratios (4 series) and ring widths (73 series) were measured.The carbon isotope series did not contain strong trends possibly attributable to increased anthropogenic atmospheric CO2 concentrations or the juvenile effect. Both carbon isotope values and ring widths were strongly correlated with Blue Nile baseflow, and from composite chronology indices ( r=0.75, p < 0.01), a preliminary reconstruction of Blue Nile baseflow back to AD 1836 was developed. Subsample signal strength remained above 0.85 for most of the reconstruction. Uncertainty bands were relatively narrow and the reliability of the preliminary reconstruction was confirmed by correspondence with reported years of drought and famine. The preliminary reconstruction is characterized by an exceptional decline in baseflow during the late AD 1960s. Flows recovered during the late 1990s. Additional sampling is advised to increase replication, spatial coverage and length of the preliminary reconstruction.

The ability to concentrate CO2 around Rubisco allows C-4 crops to suppress photorespiration. However, as phosphoenolpyruvate regeneration requires ATP, the energetic efficiency of the C-4 pathway at low photosynthetic flux densities (PFD) becomes a balancing act between primary fixation and concentration of CO2 in mesophyll (M) cells, and CO2 reduction in bundle sheath (BS) cells. At low PFD, retro-diffusion of CO2 from BS cells, relative to the rate of bicarbonate fixation in M cells (termed leakiness phi), is known to increase. This paper investigates whether this increase in phi could be explained by incomplete inhibition of photorespiration. The PFD response of phi was measured at various O-2 partial pressures in young Zea mays plants grown at 250 (LL) and 750 mu mol m-2 s-1 PFD (HL). phi increased at low PFD and was positively correlated with O-2 partial pressure. Low PFD during growth caused BS conductance and interveinal distance to be lower in the LL plants, compared to the HL plants, which correlated with lower phi. Model analysis showed that incomplete inhibition of photorespiration, especially in the HL plants, and an increase in the relative contribution of mitochondrial respiration at low PFD could explain the observed increases in phi.