Net Carbon Flux Research Articles

Dynamics of Biotic Carbon Fluxes under Different Scenarios of Forest Area Changes

This paper estimates the global biotic carbon fluxes into the atmosphere under various scenarios of changes in forest area for temperate/boreal and tropical zones. Forestry scenarios have been developed from both national forest land-cover inventories and various estimates for tree vegetation-cover areas derived from the Earth’s remote sensing data for recent decades. Three scenarios have been proposed for changes in the area of temperate/boreal forests: FAOSTAT (a growth scenario on the basis of inventories), CONST (a scenario of maintaining the present level), and LANDSAT (a scenario on the basis of satellite data from different sources). The trend of tropical deforestation is maintained for FAOSTAT and LANDSAT scenarios. Together with GEPL97 (the base scenario published earlier by the authors), these scenarios have been used to obtain model biotic carbon fluxes. A model of the global carbon cycle developed in MPEI and recently updated to take into account anthropogenic carbon emissions (both industrial and related to deforestation/land use) has been used to perform a thorough analysis of the plausible dynamics of biotic carbon sources and sinks. It has been shown that the magnitude and sign of biotic carbon fluxes into the atmosphere depends substantially on the adopted scenario—from preserving the biosphere as an effective sink of excess carbon from the atmosphere in the current century (GEPL97 and CONST) to its turning into an additional source of CO2 (LANDSAT). The FAOSTAT scenario leads to the return of biotic carbon fluxes by 2100 to the modern level. The maximum difference in net carbon fluxes from the biosphere to atmosphere for the scenarios considered here is almost 2.5 Gt C/year and falls approximately on the middle of the current century. The difference between the corresponding CO2 concentrations for these scenarios reaches 65 ppm by 2100.

C-exchange and balance following clear-cutting in hemiboreal forest ecosystem under summer drought

Growth of CO2 concentration level has strong interactions with forests. Forests are able to sequester carbon (C) through photosynthesis and can help to mitigate the effects of climate warming, as well as to reduce the CO2 concentration in the atmosphere. Drought and other extreme weather conditions play a key role in ecosystem functioning and the C-cycle. The eddy covariance (EC) method can be used to better understand forest ecosystems CO2 exchange by directly measuring net carbon and water fluxes. In our study, EC results for measurement of fluxes between the atmosphere and forest canopy are reported for the study period from May to August 2018 in Järvselja, Estonia. Stand-replacing disturbance (clear-cutting) took place in April 2013. The young forest stand is dominated by Norway spruce (Picea abies) and birch (Betula spp.). Findings so far include (1) a C-budget for the study period that showed a slight C-sink status; (2) net ecosystem exchange (NEE) was −0.0084 µmol m−2 s−1 indicating C-uptake during the measurement period; (3) in May, June, July and August, NEE was −0.027, −0.015, 0.001 and 0.006 µmol m−2 s−1, respectively; (4) NEE fluxes are lower in drought conditions and are affected by temperature that averaged 15 °C.

Offshore transport of organic carbon by upwelling filaments in the Canary Current System

A coupled physical-biogeochemical model (ROMS-PISCES) forced by climatological fields is used to examine the role of upwelling filaments in the offshore exchange of particulate (POC) and dissolved (DOC) organic carbon in the Canary Current eastern boundary upwelling system (CanC EBUS). In this region, mesoscale filaments at Capes Ghir (∼30.5 °N) and Juby (∼27.5 °N) have been frequently described using both observational and numerical data. Due to their semi-permanent presence and unique dynamical characteristics, studies focusing on filaments often provide an incomplete picture of the physical and biological processes at work, and their effects on coast-to-ocean export. The present model experiment confirms the complex three-dimensional structure of the filaments that comprises both offshore and onshore flow components. The model shows strong seasonal variability in the offshore transport mediated by the filaments. Recirculation at the edges of the filaments returns water towards the shore, especially in autumn when they are diverted northwards by the large scale boundary circulation. By contrast, offshore transport peaks during late spring - early summer when onshore recirculation is limited. Overall, the estimated net annual offshore flux of excess total organic carbon (e-TOC, the non-refractory pools of DOC and POC) averages 2.0 ×109 kg C y−1, and may increase up to 4.3 ×109 kg C y−1 during the peak upwelling season, each filament contributing to export of up to 22.6% of the organic carbon within the first 100 km from shore along the CanC EBUS (between 9.5 and 32 ° N). These results strongly support the inclusion of offshore transport estimates by coastal filaments in regional carbon budgets.

Terrestrial fluxes of carbon in GCP carbon budgets.

The Global Carbon Project (GCP) has published global carbon budgets annually since 2007 (Canadell et al. [2007], Proc Natl Acad Sci USA, 104, 18866-18870; Raupach et al. [2007], Proc Natl Acad Sci USA, 104, 10288-10293). There are many scientists involved, but the terrestrial fluxes that appear in the budgets are not well understood by ecologists and biogeochemists outside of that community. The purpose of this paper is to make the terrestrial fluxes of carbon in those budgets more accessible to a broader community. The GCP budget is composed of annual perturbations from pre-industrial conditions, driven by addition of carbon to the system from combustion of fossil fuels and by transfers of carbon from land to the atmosphere as a result of land use. The budget includes a term for each of the major fluxes of carbon (fossil fuels, oceans, land) as well as the rate of carbon accumulation in the atmosphere. Land is represented by two terms: one resulting from direct anthropogenic effects (Land Use, Land-Use Change, and Forestry or land management) and one resulting from indirect anthropogenic (e.g., CO2 , climate change) and natural effects. Each of these two net terrestrial fluxes of carbon, in turn, is composed of opposing gross emissions and removals (e.g., deforestation and forest regrowth). Although the GCP budgets have focused on the two net terrestrial fluxes, they have paid little attention to the gross components, which are important for a number of reasons, including understanding the potential for land management to remove CO2 from the atmosphere and understanding the processes responsible for the sink for carbon on land. In contrast to the net fluxes of carbon, which are constrained by the global carbon budget, the gross fluxes are largely unconstrained, suggesting that there is more uncertainty than commonly believed about how terrestrial carbon emissions will respond to future fossil fuel emissions and a changing climate.

Scaling carbon fluxes from eddy covariance sites to globe: synthesis and evaluation of the FLUXCOM approach

Abstract. FLUXNET comprises globally distributed eddy-covariance-based estimates of carbon fluxes between the biosphere and the atmosphere. Since eddy covariance flux towers have a relatively small footprint and are distributed unevenly across the world, upscaling the observations is necessary to obtain global-scale estimates of biosphere–atmosphere exchange. Based on cross-consistency checks with atmospheric inversions, sun-induced fluorescence (SIF) and dynamic global vegetation models (DGVMs), here we provide a systematic assessment of the latest upscaling efforts for gross primary production (GPP) and net ecosystem exchange (NEE) of the FLUXCOM initiative, where different machine learning methods, forcing data sets and sets of predictor variables were employed. Spatial patterns of mean GPP are consistent across FLUXCOM and DGVM ensembles (R2>0.94 at 1∘ spatial resolution) while the majority of DGVMs show, for 70 % of the land surface, values outside the FLUXCOM range. Global mean GPP magnitudes for 2008–2010 from FLUXCOM members vary within 106 and 130 PgC yr−1 with the largest uncertainty in the tropics. Seasonal variations in independent SIF estimates agree better with FLUXCOM GPP (mean global pixel-wise R2∼0.75) than with GPP from DGVMs (mean global pixel-wise R2∼0.6). Seasonal variations in FLUXCOM NEE show good consistency with atmospheric inversion-based net land carbon fluxes, particularly for temperate and boreal regions (R2>0.92). Interannual variability of global NEE in FLUXCOM is underestimated compared to inversions and DGVMs. The FLUXCOM version which also uses meteorological inputs shows a strong co-variation in interannual patterns with inversions (R2=0.87 for 2001–2010). Mean regional NEE from FLUXCOM shows larger uptake than inversion and DGVM-based estimates, particularly in the tropics with discrepancies of up to several hundred grammes of carbon per square metre per year. These discrepancies can only partly be reconciled by carbon loss pathways that are implicit in inversions but not captured by the flux tower measurements such as carbon emissions from fires and water bodies. We hypothesize that a combination of systematic biases in the underlying eddy covariance data, in particular in tall tropical forests, and a lack of site history effects on NEE in FLUXCOM are likely responsible for the too strong tropical carbon sink estimated by FLUXCOM. Furthermore, as FLUXCOM does not account for CO2 fertilization effects, carbon flux trends are not realistic. Overall, current FLUXCOM estimates of mean annual and seasonal cycles of GPP as well as seasonal NEE variations provide useful constraints of global carbon cycling, while interannual variability patterns from FLUXCOM are valuable but require cautious interpretation. Exploring the diversity of Earth observation data and of machine learning concepts along with improved quality and quantity of flux tower measurements will facilitate further improvements of the FLUXCOM approach overall.

An assessment of oil palm plantation aboveground biomass stocks on tropical peat using destructive and non-destructive methods

The recent expansion of oil palm (OP, Elaeis guineensis) plantations into tropical forest peatlands has resulted in ecosystem carbon emissions. However, estimates of net carbon flux from biomass changes require accurate estimates of the above ground biomass (AGB) accumulation rate of OP on peat. We quantify the AGB stocks of an OP plantation on drained peat in Malaysia from 3 to 12 years after planting using destructive harvests supported by non-destructive surveys of a further 902 palms. Peat specific allometric equations for palm (R2 = 0.92) and frond biomass are developed and contrasted to existing allometries for OP on mineral soils. Allometries are used to upscale AGB estimates to the plantation block-level. Aboveground biomass stocks on peat accumulated at ~6.39 ± 1.12 Mg ha−1 per year in the first 12 years after planting, increasing to ~7.99 ± 0.95 Mg ha−1 yr−1 when a ‘perfect’ plantation was modelled. High inter-palm and inter-block AGB variability was observed in mature classes as a result of variations in palm leaning and mortality. Validation of the allometries defined and expansion of non-destructive inventories across alternative plantations and age classes on peat would further strengthen our understanding of peat OP AGB accumulation rates.

The Australian Earth System Model: ACCESS-ESM1.5

The Australian Community Climate and Earth System Simulator (ACCESS) has been extended to include land and ocean carbon cycle components to form an Earth System Model (ESM). The current version, ACCESS-ESM1.5, has been mainly developed to enable Australia to participate in the Coupled Model Intercomparison Project Phase 6 (CMIP6) with an ESM version. Here we describe the model components and changes to the previous version, ACCESS-ESM1. We use the 500-year pre-industrial control run to highlight the stability of the physical climate and the carbon cycle. The long spin-up, negligible drift in temperature and small pre-industrial net carbon fluxes (0.02 and 0.08 PgC year−1 for land and ocean respectively) highlight the suitability of ACCESS-ESM1.5 to explore modes of variability in the climate system and coupling to the carbon cycle. The physical climate and carbon cycle for the present day have been evaluated using the CMIP6 historical simulation by comparing against observations and ACCESS-ESM1. Although there is generally little change in the climate simulation from the earlier model, many aspects of the carbon simulation are improved. An assessment of the climate response to CO2 forcing indicates that ACCESS-ESM1.5 has an equilibrium climate sensitivity of 3.87°C.

Interannual variation of terrestrial carbon cycle: Issues and perspectives

With accumulation of carbon cycle observations and model developments over the past decades, exploring interannual variation (IAV) of terrestrial carbon cycle offers the opportunity to better understand climate-carbon cycle relationships. However, despite growing research interest, uncertainties remain on some fundamental issues, such as the contributions of different regions, constituent fluxes and climatic factors to carbon cycle IAV. Here we overviewed the literature on carbon cycle IAV about current understanding of these issues. Observations and models of the carbon cycle unanimously show the dominance of tropical land ecosystems to the signal of global carbon cycle IAV, where tropical semiarid ecosystems contribute as much as the combination of all other tropical ecosystems. Vegetation photosynthesis contributes more than ecosystem respiration to IAV of the global net land carbon flux, but large uncertainties remain on the contribution of fires and other disturbance fluxes. Climatic variations are the major drivers to the IAV of net land carbon flux. Although debate remains on whether the dominant driver is temperature or moisture variability, their interaction,that is, the dependence of carbon cycle sensitivity to temperature on moisture conditions, is emerging as key regulators of the carbon cycle IAV. On timescales from the interannual to the centennial, global carbon cycle variability will be increasingly contributed by northern land ecosystems and oceans. Therefore, both improving Earth system models (ESMs) with the progressive understanding on the fast processes manifested at interannual timescale and expanding carbon cycle observations at broader spatial and longer temporal scales are critical to better prediction on evolution of the carbon-climate system.

Recent Warming Has Resulted in Smaller Gains in Net Carbon Uptake in Northern High Latitudes

AbstractCarbon balance of terrestrial ecosystems in the northern high latitudes (NHL) is sensitive to climate change. It remains uncertain whether current regional carbon uptake capacity can be sustained under future warming. Here the atmospheric CO2 drawdown rate (CDR) between 1974 and 2014, defined as the CO2 decrease in ppm over the number of days in spring or summer, is estimated using atmospheric CO2 observations at Barrow (now known as Utqiaġvik), Alaska. We found that the sensitivity of CDR to interannual seasonal air temperature anomalies has trended toward less carbon uptake for a given amount of warming over this period. Changes in interannual temperature sensitivity of CDR suggest that relatively warm springs now result in less of a carbon uptake enhancement. Similarly, relatively warm summers now result in greater carbon release. These results generally agree with the sensitivity of net carbon exchange (NCE) estimated by atmospheric CO2 inversion. When NCE was aggregated over North America (NA) and Eurasia (EA), separately, the temperature sensitivity of NCE in NA has changed more than in EA. To explore potential mechanisms of this signal, we also examine trends in interannual variability of other climate variables (soil temperature and precipitation), satellite-derived gross primary production (GPP), and Trends in Net Land–Atmosphere Carbon Exchanges (TRENDY) model ensemble results. Our analysis suggests that the weakened spring sensitivity of CDR may be related to the slowdown in seasonal soil thawing rate, while the summer sensitivity change may be caused by the temporally coincident decrease in temperature sensitivity of photosynthesis. This study suggests that the current NHL carbon sink may become unsustainable as temperatures warm further. We also found that current carbon cycle models do not represent the decrease in temperature sensitivity of net carbon flux. We argue that current carbon–climate models misrepresent important aspect of the carbon–climate feedback and bias the estimation of warming influence on NHL carbon balance.

Role of Eelgrass in the Coastal Filter of Contrasting Baltic Sea Environments

Coastal ecosystems act as filters of nutrients from land to the open sea. We investigated the role of eelgrass (Zostera marina) metabolism in the coastal filter transforming nitrogen, phosphorus, and organic carbon. Field campaigns following identical methodologies were carried out at two contrasting coastal locations: the mesohaline and nutrient-rich Roskilde Fjord, Denmark, and the mesotrophic brackish Tvärminne archipelago, Finland. Over the 24-h in situ benthic incubations, we measured oxygen concentrations continuously and assessed changes in DOM characteristics and net fluxes of carbon, nitrogen, and phosphorus. Ecosystem metabolism modeled on the basis of the O2 data showed that the systems were either net heterotrophic (Roskilde Fjord; − 1.6 and − 2.4 g O2 m−2 day−1 in eelgrass meadow and bare sand, respectively) or had balanced primary production and respiration (Tvärminne; 0.0 and 0.2 g O2 m−2 day−1). Overall, initial nutrient stoichiometry was a key factor determining benthic–pelagic fluxes of nutrients, which exacerbated the deviations from Redfield ratios of N and P, indicating an efficient use of the limiting nutrient. A net diel uptake of dissolved inorganic N was observed at both locations (− 2.3 μmol l−1 day−1 in Roskilde Fjord and − 0.1 μmol l−1 day−1 in Tvärminne). Despite minor changes in dissolved organic carbon concentrations during the incubations, a marked increase of fluorescent DOM was observed at both locations, suggesting rapid heterotrophic processing of the DOM pool. Our results underline that the biogeochemical role of eelgrass in the coastal filter is not inherent, but strongly dependent on the environmental conditions.

Key Species Superpose the Effect of Species Richness and Species Interaction on Carbon Fluxes in a Restored Minerotrophic Peatland

In a controlled field experiment, we tested the effect of plant biodiversity on carbon accumulation in a formerly-extracted peatland that was rewetted and re-vegetated. We monitored carbon fluxes in experimental units re-vegetated with different numbers and types of characteristic minerotrophic peatland species, planted in monoculture and mixed treatments. Using measured environmental variables, we modelled the different components of carbon flux (photosynthesis, respiration and methane flux) and simulated the Net Seasonal Carbon Flux for three standard wetness scenarios of each planting treatment. We tested the effect of species and functional group richness, species and functional group identity and species interactions on Net Seasonal Carbon Flux. Our findings did not indicate any significant effect of the richness of species or plant functional groups on carbon accumulation. However, the presence of key species was important; Carex aquatilis, particularly when planted alone, significantly increased carbon accumulation; other graminoid species were not associated with the same effect. Mixed-species treatments did not accumulate more carbon than was expected of the same species when planted in monoculture, providing no evidence for an overall species interaction effect. Our results suggest that in the context of restored minerotrophic peatlands, species identity is more important than species/functional group richness or species-interaction for driving carbon accumulation.

Remote sensing and ecosystem modeling to simulate terrestrial carbon fluxes

<p>The main goal of this thesis is the establishment of a framework to analyze the forest ecosystems in peninsular Spain in terms of their role in the carbon cycle. In particular, the carbon fluxes that they exchange with atmosphere are modeled to evaluate their potential as carbon sinks and biomass reservoirs. The assessment of gross and net carbon fluxes is performed at 1-km spatial scale and on a daily basis using two different ecosystem models, Monteith and BIOME-BGC, respectively. These models are driven by a combination of satellite and ground data, part of the latter being also employed as a complementary data source and in the validation process.</p>

Impact of Disturbances on the Carbon Cycle of Forest Ecosystems in Ukrainian Polissya

Climate change continues to threaten forests and their ecosystem services while substantially altering natural disturbance regimes. Land cover changes and consequent management entail discrepancies in carbon sequestration provided by forest ecosystems and its accounting. Currently there is a lack of sufficient and harmonized data for Ukraine that can be used for the robust and spatially explicit assessment of forest provisioning and regulation of ecosystem services. In the frame of this research, we established an experimental polygon (area 45 km2) in Northern Ukraine aiming at estimating main forest carbon stocks and fluxes and determining the impact caused by natural disturbances and harvest for the study period of 2010–2015. Coupled field inventory and remote sensing data (RapidEye image for 2010 and SPOT 6 image for 2015) were used. Land cover classification and estimation of biomass and carbon pools were carried out using Random Forest and k-Nearest Neighbors (k-NN) method, respectively. Remote sensing data indicates a ca. 16% increase of carbon stock, while ground-based computations have shown only a ca. 1% increase. Net carbon fluxes for the study period are relatively even: 5.4 Gg C·year−1 and 5.6 Gg C C·year−1 for field and remote sensing data, respectively. Stand-replacing wildfires, as well as insect outbreaks and wind damage followed by salvage logging, and timber harvest have caused 21% of carbon emissions among all C sources within the experimental polygon during the study period. Hence, remote sensing data and non-parametric methods coupled with field data can serve as reliable tools for the precise estimation of forest carbon cycles on a regional spatial scale. However, featured land cover changes lead to unexpected biases in consistent assessment of forest biophysical parameters, while current management practices neglect natural forest dynamics and amplify negative impact of disturbances on ecosystem services.

The physics and ecology of mining carbon dioxide from the atmosphere by ecosystems.

Reforesting and managing ecosystems have been proposed as ways to mitigate global warming and offset anthropogenic carbon emissions. The intent of our opinion piece is to provide a perspective on how well plants and ecosystems sequester carbon. The ability of individual plants and ecosystems to mine carbon dioxide from the atmosphere, as defined by rates and cumulative amounts, is limited by laws of physics and ecological principles. Consequently, the rates and amount of net carbon uptake are slow and low compared to the rates and amounts of carbon dioxide we release by fossil fuels combustion. Managing ecosystems to sequester carbon can also cause unintended consequences to arise. In this paper, we articulate a series of key take-home points. First, the potential amount of carbon an ecosystem can assimilate on an annual basis scales with absorbed sunlight, which varies with latitude, leaf area index and available water. Second, efforts to improve photosynthesis will come with the cost of more respiration. Third, the rates and amount of net carbon uptake are relatively slow and low, compared to the rates and amounts and rates of carbon dioxide we release by fossil fuels combustion. Fourth, huge amounts of land area for ecosystems will be needed to be an effective carbon sink to mitigate anthropogenic carbon emissions. Fifth, the effectiveness of using this land as a carbon sink will depend on its ability to remain as a permanent carbon sink. Sixth, converting land to forests or wetlands may have unintended costs that warm the local climate, such as changing albedo, increasing surface roughness or releasing other greenhouse gases. We based our analysis on 1,163 site-years of direct eddy covariance measurements of gross and net carbon fluxes from 155 sites across the globe.

LIFE15 ENV/IT/000392 − LIFE VITISOM Project, viticulture innovation technology and GHG emission monitoring

The main aim of the LIFE VITISOM Project is to promote an innovative solution for the management of the organic fertilisation in the viticultural sector. In parallel, different activities of monitoring of impacts have been planned. Specifically, a deep study about GHG emissions has been organised. In this context, different studies are being carried out: a continuous monitoring of net carbon fluxes (NEE) through the Eddy Covariance method, followed by University of Padua which allows data to be obtained at vineyard ecosystem level; a spatial monitoring of CH4, N2O and CO2, through a mobile instrument for measuring the variation of GHG developed by West Systems within the LIFE+ IPNOA Project. In the first case, two Eddy Covariance towers have been installed, one at Guido Berlucchi (Franciacorta, Lombardy) and one at Bosco del Merlo (Lison, Veneto). Additionally, spatial monitoring is being carried out in five testing sites involved in the LIFE VITISOM project. In this case, different organic fertilisation managements are compared.

Quantifying the effect of forest age in annual net forest carbon balance

Forests dominate carbon (C) exchanges between the terrestrial biosphere and the atmosphere on land. In the long term, the net carbon flux between forests and the atmosphere has been significantly impacted by changes in forest cover area and structure due to ecological disturbances and management activities. Current empirical approaches for estimating net ecosystem productivity (NEP) rarely consider forest age as a predictor, which represents variation in physiological processes that can respond differently to environmental drivers, and regrowth following disturbance. Here, we conduct an observational synthesis to empirically determine to what extent climate, soil properties, nitrogen deposition, forest age and management influence the spatial and interannual variability of forest NEP across 126 forest eddy-covariance flux sites worldwide. The empirical models explained up to 62% and 71% of spatio-temporal and across-site variability of annual NEP, respectively. An investigation of model structures revealed that forest age was a dominant factor of NEP spatio-temporal variability in both space and time at the global scale as compared to abiotic factors, such as nutrient availability, soil characteristics and climate. These findings emphasize the importance of forest age in quantifying spatio-temporal variation in NEP using empirical approaches.

Litterfall and the effects of thinning and seed cutting on carbon input into the soil in Scots pine stands in Turkey

Litterfall is one of the most important fractions of forest net primary production (NPP) and important carbon flux into soil, a large carbon pool in forests worldwide, and believed to account for about one-third of forest NPP. The aim of the study was to determine litterfall amount and carbon input into the soil via litterfall as well as to understand the effects of thinning (removing 20–25% of the initial basal area) and seed cutting (i.e. regeneration cutting with removing 60% of the initial basal area) on litterfall in Scots pine (Pinus sylvestris L.) stands. Litterfall samples were collected three times in a year from 32 sample plots for 5 years in Turkey. Carbon concentrations of the litterfall components were determined by dry combustion. Data were evaluated by one-way ANOVA and multiple regression analysis. As a result, litterfall varied from 1389 kg ha−1 year−1 in young stands to 4488 kg ha−1 year−1 in mature stands. Carbon inputs into the soil changed between 714 kg C ha−1 year−1 and 2289 kg C ha−1 year−1 depending on the development stage and silvicultural treatment applied to the stands. Thinnings reduced litterfall amount at a ratio of 22% in mature stands while had no significant effect in overmature stands with a decrease ratio of 8%. Seed cutting reduced considerably the litterfall amount both in mature and overmature stands. A combination of basal area, site index, and stand age accounted for 75% of the variation in needle litterfall. In conclusion, seed cutting was recommended to do preferably in mature stands instead of overmature ones and thinning to be applied lightly in mature stands in order not to reduce carbon input into the soil via litterfall dramatically.

Assessing the dynamics of vegetation productivity in circumpolar regions with different satellite indicators of greenness and photosynthesis

Abstract. High-latitude treeless ecosystems represent spatially highly heterogeneous landscapes with small net carbon fluxes and a short growing season. Reliable observations and process understanding are critical for projections of the carbon balance of the climate-sensitive tundra. Space-borne remote sensing is the only tool to obtain spatially continuous and temporally resolved information on vegetation greenness and activity in remote circumpolar areas. However, confounding effects from persistent clouds, low sun elevation angles, numerous lakes, widespread surface inundation, and the sparseness of the vegetation render it highly challenging. Here, we conduct an extensive analysis of the timing of peak vegetation productivity as shown by satellite observations of complementary indicators of plant greenness and photosynthesis. We choose to focus on productivity during the peak of the growing season, as it importantly affects the total annual carbon uptake. The suite of indicators are as follows: (1) MODIS-based vegetation indices (VIs) as proxies for the fraction of incident photosynthetically active radiation (PAR) that is absorbed (fPAR), (2) VIs combined with estimates of PAR as a proxy of the total absorbed radiation (APAR), (3) sun-induced chlorophyll fluorescence (SIF) serving as a proxy for photosynthesis, (4) vegetation optical depth (VOD), indicative of total water content and (5) empirically upscaled modelled gross primary productivity (GPP). Averaged over the pan-Arctic we find a clear order of the annual peak as APAR ≦ GPP<SIF<VIs/VOD. SIF as an indicator of photosynthesis is maximised around the time of highest annual temperatures. The modelled GPP peaks at a similar time to APAR. The time lag of the annual peak between APAR and instantaneous SIF fluxes indicates that the SIF data do contain information on light-use efficiency of tundra vegetation, but further detailed studies are necessary to verify this. Delayed peak greenness compared to peak photosynthesis is consistently found across years and land-cover classes. A particularly late peak of the normalised difference vegetation index (NDVI) in regions with very small seasonality in greenness and a high amount of lakes probably originates from artefacts. Given the very short growing season in circumpolar areas, the average time difference in maximum annual photosynthetic activity and greenness or growth of 3 to 25 days (depending on the data sets chosen) is important and needs to be considered when using satellite observations as drivers in vegetation models.

Study of the Carbon Budget of a Temperate-Climate Vineyard: Inter-Annual Variability of CO2 Flux

Agriculture can improve the global carbon budget by reducing emissions and increasing soil carbon sequestration. Woody crops like vineyards present biological, structural, and management peculiarities, such as perennial structure, abundant pruning debris, and interrow cover cropping, which could potentially sequester a significant amount of CO2. However, such ecosystems are still poorly understood. Here, we present results from a long-term study from northeastern Italy on the carbon budget of a vineyard (cv. Sauvignon blanc, Guyot pruned, vertical shoot-positioned trellised, with grass-covered alleys). We used the eddy covariance technique to continuously monitor the net ecosystem CO2 exchange (NEE), and here we present the first three years of data (May 2014 to April 2017). The vineyard acted as a moderate carbon sink, with an average annual net carbon uptake of 134 g C/ m2. However, the inter-annual variability of NEE was high, and environmental conditions during the growing season had a strong impact. The annual carbon uptake was greatest in 2014 (207 g C/m2), when there was abundant rain in summer. In contrast, in 2015, several heat waves decreased the annual carbon uptake to 69 g C/m2. Elevated air temperature and low soil moisture in summer significantly reduced net carbon flux during times when the vineyard activity was at maximum. Nevertheless, we show that a vineyard can behave as a carbon sink in the medium-to-long term, which should be factored into agricultural and environmental policies, complementing the standard life cycle assessment of the wine production chain.

Effects of extreme changes in precipitation on the physiology of C4 grasses.

Climatic patterns are expected to become more extreme, with changes in precipitation characterized by heavier rainfall and prolonged dry periods. Yet, most studies focus on persistent moderate changes in precipitation, limiting our understanding of how ecosystems will function in the future. We examined the effects of extreme changes in precipitation on leaf-level and ecosystem CO2 and H2O exchange of three native C4 bunchgrasses (Andropogon gerardii, Panicum virgatum, and Sorghastrum nutans) over 3years. Grasses were grown in three precipitation treatments: extreme dry, mean, and extreme wet based on historical rainfall records. After 3years, plants were 45% smaller in the extreme dry treatment relative to the mean and extreme high treatment, which did not differ. We also found that an extreme decrease in precipitation caused reductions of 55, 40, and 40% in leaf-level photosynthesis (Anet), stomatal conductance (gs), and water use efficiency (WUE), respectively. Extreme increases in precipitation inhibited leaf-level WUE, with a 44% reduction relative to the mean treatment. At the ecosystem level, both an extreme increase and decrease in precipitation reduced net CO2 and water fluxes relative to plants grown with mean levels of precipitation. Net water fluxes (ET) were reduced by an average of 74% in the extreme dry and extreme wet treatment relative to mean treatment; net carbon fluxes followed a similar trend, with average reductions of 68% (NEE) and 100% (Re). Unlike moderate climate change, extreme increases in precipitation may be just as detrimental as extreme decreases in precipitation in shifting grassland physiology.