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The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such asmore » gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.« less

Two ideas regarding the structure of turbulence near a clear air-water interface are used to derive a waterside gas transfer velocity kL for sparingly and slightly soluble gases. The first is that k_L is proportional to the turnover velocity described by the vertical velocity structure function D_ww(r), where r is separation distance between two points. The second is that the scalar exchange between the air-water interface and the waterside turbulence can be suitably described by a length scale proportional to the Batchelor scale lB=ηSc^-1/2, where Sc is the molecular Schmidt number and η is the Kolmogorov microscale defining the smallestmore » scale of turbulent eddies impacted by fluid viscosity. Using an approximate solution to the von Kármán-Howarth equation predicting D_ww(r) in the inertial and viscous regimes, prior formulations for k_L are recovered including (i) k_L=$$\sqrt{2/15}$$Sc^-1/2$$v$$, v_K is the Kolmogorov velocity defined by the Reynolds number v_Kη/ν = 1 and ν is the kinematic viscosity of water; (ii) surface divergence formulations; (iii) k_L ∝ SC^-1/2u_*, where u_* is the waterside friction velocity; (iv) k_L ∝ Sc^-1/2$$\sqrt{gv/u_*}$$ for Keulegan numbers exceeding a threshold needed for long-wave generation, where the proportionality constant varies with wave age, g is the gravitational acceleration; and (v) k_L = $$\sqrt{2/15}$$Sc^-1/2(vgβ_oq_o)^1/4 in free convection, where qo is the surface heat flux and β_o is the thermal expansion of water. The work demonstrates that the aforementioned k_L formulations can be recovered from a single structure function model derived for locally homogeneous and isotropic turbulence.« less

This paper describes the formation of, and initial results for, a new FLUXNET coordination network for ecosystem-scale methane (CH₄) measurements at 60 sites globally, organized by the Global Carbon Project in partnership with other initiatives and regional flux tower networks. The objectives of the effort are presented along with an overview of the coverage of eddy covariance (EC) CH₄ flux measurements globally, initial results comparing CH4 fluxes across the sites, and future research directions and needs. Annual estimates of net CH₄ fluxes across sites ranged from -0.2 ± 0.02 g C m^–2 yr^–1 for an upland forest site to 114.9more » ± 13.4 g C m^–2 yr^–1 for an estuarine freshwater marsh, with fluxes exceeding 40 g C m^–2 yr^–1 at multiple sites. Average annual soil and air temperatures were found to be the strongest predictor of annual CH₄ flux across wetland sites globally. Water table position was positively correlated with annual CH₄ emissions, although only for wetland sites that were not consistently inundated throughout the year. The ratio of annual CH4 fluxes to ecosystem respiration increased significantly with mean site temperature. Uncertainties in annual CH₄ estimates due to gap-filling and random errors were on average ±1.6 g C m^–2 yr^–1 at 95% confidence, with the relative error decreasing exponentially with increasing flux magnitude across sites. Through the analysis and synthesis of a growing EC CH₄ flux database, the controls on ecosystem CH₄ fluxes can be better understood, used to inform and validate Earth system models, and reconcile differences between land surface model- and atmospheric-based estimates of CH₄ emissions.« less

While the role of highly oxygenated molecules (HOMs) in new particle formation (NPF) and secondary organic aerosol (SOA) formation is not in dispute, the interplay between HOM chemistry and atmospheric conditions continues to draw significant research attention. During the Influence of Biosphere-Atmosphere Interactions on the Reactive Nitrogen budget (IBAIRN) campaign in September 2016, profile measurements of neutral HOMs below and above the forest canopy were performed for the first time at the boreal forest SMEAR II station. The HOM concentrations and composition distributions below and above the canopy were similar during daytime, supporting a well-mixed boundary layer approximation. However, muchmore » lower nighttime HOM concentrations were frequently observed at ground level, which was likely due to the formation of a shallow decoupled layer below the canopy. Near the ground HOMs were influenced by the changes in the precursors and oxidants and enhancement of the loss on surfaces in this layer, while the HOMs above the canopy top were not significantly affected. Our findings clearly illustrate that near-ground HOM measurements conducted under stably stratified conditions at this site might only be representative of a small fraction of the entire nocturnal boundary layer. This could, in turn, influence the growth of newly formed particles and SOA formation below the canopy where the large majority of measurements are typically conducted.« less

Wetlands are one of the most significant natural sources of methane (CH₄) to the atmosphere. They emit CH₄ because decomposition of soil organic matter in waterlogged anoxic conditions produces CH₄, in addition to carbon dioxide (CO₂). Production of CH₄ and how much of it escapes to the atmosphere depend on a multitude of environmental drivers. Models simulating the processes leading to CH₄ emissions are thus needed for upscaling observations to estimate present CH₄ emissions and for producing scenarios of future atmospheric CH₄ concentrations. Aiming at a CH₄ model that can be added to models describing peatland carbon cycling, we composedmore » a model called HIMMELI that describes CH₄ build-up in and emissions from peatland soils. It is not a full peatland carbon cycle model but it requires the rate of anoxic soil respiration as input. Driven by soil temperature, leaf area index (LAI) of aerenchymatous peatland vegetation, and water table depth (WTD), it simulates the concentrations and transport of CH₄, CO₂, and oxygen (O₂) in a layered one-dimensional peat column. Here, we present the HIMMELI model structure and results of tests on the model sensitivity to the input data and to the description of the peat column (peat depth and layer thickness), and demonstrate that HIMMELI outputs realistic fluxes by comparing modeled and measured fluxes at two peatland sites. As HIMMELI describes only the CH₄-related processes, not the full carbon cycle, our analysis revealed mechanisms and dependencies that may remain hidden when testing CH₄ models connected to complete peatland carbon models, which is usually the case. Our results indicated that (1) the model is flexible and robust and thus suitable for different environments; (2) the simulated CH₄ emissions largely depend on the prescribed rate of anoxic respiration; (3) the sensitivity of the total CH₄ emission to other input variables is mainly mediated via the concentrations of dissolved gases, in particular, the O₂ concentrations that affect the CH₄ production and oxidation rates; (4) with given input respiration, the peat column description does not significantly affect the simulated CH₄ emissions in this model version.« less

Research infrastructures play a key role in launching a new generation of integrated long-term, geographically distributed observation programmes designed to monitor climate change, better understand its impacts on global ecosystems, and evaluate possible mitigation and adaptation strategies. The pan-European Integrated Carbon Observation System combines carbon and greenhouse gas (GHG; CO₂, CH₄, N₂O, H₂O) observations within the atmosphere, terrestrial ecosystems and oceans. High-precision measurements are obtained using standardised methodologies, are centrally processed and openly available in a traceable and verifiable fashion in combination with detailed metadata. The Integrated Carbon Observation System ecosystem station network aims to sample climate and land-cover variabilitymore » across Europe. In addition to GHG flux measurements, a large set of complementary data (including management practices, vegetation and soil characteristics) is collected to support the interpretation, spatial upscaling and modelling of observed ecosystem carbon and GHG dynamics. The applied sampling design was developed and formulated in protocols by the scientific community, representing a trade-off between an ideal dataset and practical feasibility. Finally, the use of open-access, high-quality and multi-level data products by different user communities is crucial for the Integrated Carbon Observation System in order to achieve its scientific potential and societal value.« less

Abstract. For the past decade, observations of carbonyl sulfide (OCS or COS)have been investigated as a proxy for carbon uptake by plants. OCS is destroyed by enzymes that interact with CO₂ during photosynthesis, namely carbonic anhydrase (CA) and RuBisCO, where CA is the more important one. The majority of sources of OCS to the atmosphere are geographically separated from this large plant sink,whereas the sources and sinks of CO₂ are co-located in ecosystems. The draw down of OCS can therefore be related to the uptake of CO₂ without the added complication of co-located emissions comparable in magnitude. Here we review themore » state of our understanding of the global OCS cycle and its applications to ecosystem carbon cycle science. OCS uptake is correlated well toplant carbon uptake, especially at the regional scale. OCS can be used in conjunction with other independent measures of ecosystem function, like solar-induced fluorescence and carbon and water isotope studies. More work needs to be done to generate global coverage for OCS observations and to link this powerful atmospheric tracer to systems where fundamental questions concerning the carbon and water cycle remain.« less

The pull of water from the soil to the leaves causes water in the transpiration stream to be under negative pressure decreasing the water potential below zero. The osmotic concentration also contributes to the decrease in leaf water potential but with much lesser extent. Thus, the surface tension force is approximately balanced by a force induced by negative water potential resulting in concavely curved water-air interfaces in leaves. The lowered water potential causes a reduction in the equilibrium water vapor pressure in internal (sub-stomatal/intercellular) cavities in relation to that over water with the potential of zero, i.e., over the flatmore » surface. The curved surface causes a reduction also in the equilibrium vapor pressure of dissolved CO₂, thus enhancing its physical solubility to water. Although the water vapor reduction is acknowledged by plant physiologists its consequences for water vapor exchange at low water potential values have received very little attention. Consequences of the enhanced CO₂ solubility to a leaf water-carbon budget have not been considered at all before this study. We use theoretical calculations and modeling to show how the reduction in the vapor pressures affects transpiration and carbon assimilation rates. Here, our results indicate that the reduction in vapor pressures of water and CO₂ could enhance plant water use efficiency up to about 10% at a leaf water potential of -2 MPa, and much more when water potential decreases further. The low water potential allows for a direct stomatal water vapor uptake from the ambient air even at sub-100% relative humidity values. This alone could explain the observed rates of foliar water uptake by e.g., the coastal redwood in the fog belt region of coastal California provided the stomata are sufficiently open. Lastly, the omission of the reduction in the water vapor pressure causes a bias in the estimates of the stomatal conductance and leaf internal CO₂ concentration based on leaf gas exchange measurements. Manufactures of leaf gas exchange measurement systems should incorporate leaf water potentials in measurement set-ups.« less

Current climate models disagree on how much carbon dioxide land ecosystems take up for photosynthesis. Tracking the stronger carbonyl sulfide signal could help.

Abstract Earth observing systems are now routinely used to infer leaf area index ( LAI ) given its significance in spatial aggregation of land surface fluxes. Whether LAI is an appropriate scaling parameter for daytime growing season energy budget, surface conductance ( G _s ), water‐ and light‐use efficiency and surface–atmosphere coupling of European boreal coniferous forests was explored using eddy‐covariance ( EC ) energy and CO ₂ fluxes. The observed scaling relations were then explained using a biophysical multilayer soil–vegetation–atmosphere transfer model as well as by a bulk G _s representation. The LAI variations significantlymore » alter radiation regime, within‐canopy microclimate, sink/source distributions of CO ₂ , H ₂ O and heat, and forest floor fluxes. The contribution of forest floor to ecosystem‐scale energy exchange is shown to decrease asymptotically with increased LAI , as expected. Compared with other energy budget components, dry‐canopy evapotranspiration ( ET ) was reasonably ‘conservative’ over the studied LAI range 0.5–7.0 m ² m ⁻² . Both ET and G _s experienced a minimum in the LAI range 1–2 m ² m ⁻² caused by opposing nonproportional response of stomatally controlled transpiration and ‘free’ forest floor evaporation to changes in canopy density. The young forests had strongest coupling with the atmosphere while stomatal control of energy partitioning was strongest in relatively sparse ( LAI ~2 m ² m ⁻² ) pine stands growing on mineral soils. The data analysis and model results suggest that LAI may be an effective scaling parameter for net radiation and its partitioning but only in sparse stands ( LAI <3 m ² m ⁻² ). This finding emphasizes the significance of stand‐replacing disturbances on the controls of surface energy exchange. In denser forests, any LAI dependency varies with physiological traits such as light‐saturated water‐use efficiency. The results suggest that incorporating species traits and site conditions are necessary when LAI is used in upscaling energy exchanges of boreal coniferous forests.« less