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Abstract: Eddy covariance measurements were carried out at the Wetzstein site in Thuringia, Germany since December 2001. Soon after the start of the measurements night-time fluxes well above average CO2-fluxes measured in temperate forest ecosystems were detected which could not be explained by biological processes but were valid with respect to standard quality criteria. The Wetzstein site is part of the CarboEurope-IP flux-network and the CO2-exchange of this spruce forest is of general ecological interest as the site is typical for central European spruce forest ecosystems at mountainous elevation. Additional investigations were made in order to identify the causes for the large difference between the flux balance and the inventory based NEP. Specific weather patterns and micrometeorological situations were identified during which a decoupling of the flows above and below the canopy leads to additional CO2-effluxes at the tower site which are not part of the net ecosystem exchange (NEE) at night. Rejecting data from these periods and gap-filling thereafter results in yearly sums of NEE, GPP and TER which are in better agreement with the biometric measurements at the tower site and comparable to other spruce forest sites. In this process ecosystem respiration was determined not only from extrapolation of night-time data but also from flux partitioning based on day-time data using the hyperbolic light response function. It can be shown that flux measurements at this complex site need to be treated in a modified procedure compared to what is generally performed, namely extrapolating ecosystem respiration from night-time data. Using multiple data sources and applying a careful filtering of the data, confidence in the estimates of the carbon balance components increased. [Copyright &y& Elsevier]

Abstract: Horizontal and vertical advective fluxes of CO2 measured during the CarboEurope-IP advection experiment (ADVEX) at the Wetzstein spruce forest site in Thuringia, Germany, were related to wind direction, stratification regime and friction velocity u *. Measurements of wind speed and direction carried out at one of the slopes of the ridge revealed the existence of reverse flow below the canopy on the downwind side. This uphill flow occurred concurrently with the advective fluxes measured at the top of the hill. Such result is in agreement with recent modeling works that support the existence of advection at low hills covered with a canopy. Another experimental evidence that suggest a link between advection at this site with the flow over the hill came from the analysis of the horizontal gradient of CO2 inside the volume formed by the ADVEX towers. It was observed that CO2 accumulated near the downwind side of the crest for cross-ridge flows, what is consistent with another modeling work of the transport of scalars across a low hill covered with a canopy. [Copyright &y& Elsevier]

Abstract: The adoption of the eddy covariance technique to estimate surface exchange is based on the assumption that certain meteorological conditions are valid. The most important of these are horizontal homogeneity, steady-state, and non-advective conditions. Since such conditions are often violated under complex terrain conditions, e.g. at flux monitoring sites over forests, this study aims to evaluate the influence of surface heterogeneity to permit a correct interpretation of the measurement results. Quality assessment tools for eddy covariance measurements have been combined with footprint modelling. This makes it possible to define the spatial context of the fluxes, and to include land use features of the surrounding terrain in the analysis. The quality of the flux data for different wind sectors and varying meteorological conditions is also determined, so that the most suitable situations for the collection of high-quality data sets can be identified. Additionally, the flux contribution of the different land use types present in the footprint area is calculated. The results are presented as two-dimensional graphs, which show the spatial distribution of the quality of different fluxes. These graphs identify terrain influences affecting the flux data quality. The evaluation is especially useful for checking to what extent the measured fluxes at a site are representative of a specific type of land use. [Copyright &y& Elsevier]

• Understory, leaf angles and sun angle were the main drivers of near-infrared reflectance. • The influence of leaf albedo and leaf area on near-infrared reflectance was negligible. • Simulated seasonal forest canopy reflectances matched Sentinel 2 observations. • Corresponding sun angle effects depended on the viewing angle. • The link between GPP and NIRv may be weaker than expected. The physical mechanisms behind correlations of earth observations and remote sensing products are of vital importance. The so-called 'near-infrared reflectance of vegetation' (NIR V) and gross primary production (GPP) show high correlations among different ecosystems and temporal scales but the underlying relationship is still poorly understood. NIR V is defined as the product of normalized difference vegetation index (NDVI) and near-infrared (NIR) canopy reflectance (R NIR ). We examined this relationship in the case of a temperate deciduous forest in Germany. GPP, R NIR and NIR V all exhibited a strong rise during leaf development in spring and a continual decline after the maximum in early summer. The decline of NIR V in late summer was mainly driven by the decline of R NIR , since NDVI remained saturated. Here we tested the R NIR decline attributions to changes in leaf area index, leaf optical properties, canopy structure, sun-sensor geometry, or understory vegetation by measuring seasonal variations of those factors of the temperate deciduous forest. Leaf area was nearly constant between May and mid September, leaf albedo decreased slightly, leaf angles increased over time towards more vertical leaves, and understory reflectance decreased considerably. We simulated the seasonal R NIR decline of the forest using the radiative transfer model FRT and quantified the sensitivity of the decline to variations in the measured parameters. FRT captured well the observed seasonal R NIR decline by Sentinel 2 using the measured optical and structural properties. Decreasing understory reflectance alone explained 43% of the simulated R NIR decrease, while leaf angle variations explained 31%, the solar zenith angle (SZA) 21%, leaf albedo 7%, and LAI 0%. The effect size of the SZA depended on the viewing angle and would hence be different for different satellites and for local instruments. The results may help to better understand and help to track seasonal changes in forest structure and leaf optical properties using remote sensing techniques. They also suggest that the proposed link between the seasonal evolution of GPP and NIR V may be weaker than expected. [ABSTRACT FROM AUTHOR]

Abstract: Soil respiration is a combination of CO2 fluxes derived from a diversity of belowground sources, many depending directly on the input of carbon from living plants. Here we present data from two different forest ecosystems, a beech and a spruce forest, where a partitioning of soil respiration was carried out. We used soil cores inside micro-pore meshes together with periodic chamber-based measurements to estimate rhizosphere, mycorrhizal fungal and microbial heterotrophic respiration. Calculated mycorrhizal mycelium respiration was 8% at the spruce forest and 3% at the beech forest. Given the nature of the partitioning method these values represent minimum estimates. The ratio of root-derived carbon respiration to heterotrophic respiration was about 1:1 at both forest types. The relationship of each source with temperature and photosynthesis, measured as gross primary productivity derived from eddy covariance measurements, was subsequently explored. Both factors revealed effects specific to the respiration source and the forest type. A response to temperature was evident in all cases except for mycorrhizal mycelium respiration at the spruce forest (R 2 =0.06, p =0.41). Significant correlations of photosynthesis with rhizosphere and mycorrhizal fungal respiration were found in all cases. Peaks in correlation values showed time lags between photosynthetic activity and a respiration response ranging from 1 day for the fungal component and 4 days for the rhizosphere component at the beech forest (R 2 =0.70, p <0.01 and R 2 =0.42, p <0.05, respectively) to 5 days for both fluxes at the spruce forest (R 2 =0.44, p <0.01 and R 2 =0.72, p <0.01, respectively). Results show that respiration of the mycorrhizal component cannot be predicted by common temperature driven models in some ecosystems. They also indicate a strong influence of forest canopy processes on the activity of roots and associated organisms. The specific response in each vegetation type should be ideally explained by physiological mechanisms inherent to different species as a next step towards understanding belowground carbon dynamics. [Copyright &y& Elsevier]

Complying with several assumption and simplifications, most of the carbon budget studies based on eddy covariance (EC) measurements quantify the net ecosystem exchange (NEE) by summing the flux obtained by EC ( FC ) and the storage flux ( SC ). SC is the rate of change of a scalar, CO 2 molar fraction in this case, within the control volume underneath the EC measurement level. It is given by the difference in the quasi-instantaneous profiles of concentration at the beginning and end of the EC averaging period, divided by the averaging period. The approaches used to estimate SC largely vary, from measurements based on a single sampling point usually located at the EC measurement height, to measurements based on profile sampling. Generally a single profile is used, although multiple profiles can be positioned within the control volume. Measurement accuracy reasonably increases with the spatial sampling intensity, however limited resources often prevent more elaborated measurement systems. In this study we use the experimental dataset collected during the ADVEX campaign in which turbulent and non-turbulent fluxes were measured in three forest sites by the simultaneous use of five towers/profiles. Our main objectives are to evaluate both the uncertainty of SC that derives from an insufficient sampling of CO 2 variability, and its impact on concurrent NEE estimates.Results show that different measurement methods may produce substantially different SC flux estimates which in some cases involve a significant underestimation of the actual SC at a half-hourly time scales. A proper measuring system, that uses a single vertical profile of which the CO 2 sampled at 3 points (the two closest to the ground and the one at the lower fringe of the canopy layer) is averaged with CO 2 sampled at a certain distance and at the same height, improves the horizontal representativeness and reduces this (proportional) bias to 2–10% in such ecosystems. While the effect of this error is minor on long term NEE estimates, it can produce significant uncertainty on half-hourly NEE fluxes. [ABSTRACT FROM AUTHOR]

• Spatial survey of leaf inclination for most common broadleaf tree species in Europe. • Leaf angle distribution might be considered a species-specific trait for most cases. • Spherical approximation not a valid assumption for European broadleaf tree species. • Leaf angle distribution for 50 common broadleaf tree species in Europe also reported. • We strongly encourage the collection of actual in situ measurements where possible. Leaf angle distribution influences forest canopy processes like radiation balance, photosynthesis, and evapotranspiration. Indeed, a strong sensitivity to variability in the leaf angle distribution is reported for many models. Difficulties in conducting leaf angle distribution measurements limit data availability and explanations of its species-specific phenology and variation across environmental gradients. This leads to the situation that leaf angle distribution is often the most poorly constrained model parameter. Here, we report a spatial survey of leaf angle distributions and their seasonal and vertical changes for five most abundant forest broadleaf tree species in Europe according to the spatially representative Level I ICP Forests monitoring network. Although we found evidence that leaf angle distribution might be considered a species-specific trait for all studied species except Betula pendula , we advocate the use of leaf angle distributions obtained from actual leaf inclination measurements whenever possible. Data on leaf angle distributions for 50 widespread forest broadleaf tree species in Europe is also reported. [ABSTRACT FROM AUTHOR]

Abstract: Eddy-covariance measurements are performed at several hundred sites all over the world on a long-term basis. The gathered data are used to characterise ecosystem exchanges of trace gases, water and energy and to validate or constrain process-based models. There is an increasing demand on standardised and comprehensive quality flagging and uncertainty quantification of these fluxes. In this paper, we review established quality assessment procedures and present a comprehensive newly composed strategy emphasising tests on high-frequency raw data, expanding existing tests on statistics, fluxes and corrections, plus quantification of errors. Moreover, representativity of fluxes is checked by footprint analysis. This strategy is applied within the recently launched TERENO network of ecosystem observatories, and its robustness is demonstrated for data acquired with different measurement set-ups. Four test data sets from TERENO and one data set from CarboEurope-IP were subjected to this quality assessment. The presented strategy is compared with established quality assessment schemes, and it is demonstrated that unrealistic fluxes are now efficiently excluded while retaining the largest possible amount of high quality data. Additionally, the algorithms applied provide comprehensive, reproducible, qualitative and quantitative uncertainty estimates for users of eddy-covariance flux data. [Copyright &y& Elsevier]

Abstract: Extensive field measurements have been performed at three CarboEurope-Integrated Project forest sites with different topography (Renon/Ritten, Italian Alps, Italy; Wetzstein, Thuringia, Germany; Norunda, Uppland, Sweden) to evaluate the relevant terms of the carbon balance by measuring CO2 concentrations [CO2] and the wind field in a 3D multi-tower cube setup. The same experimental setup (geometry and instrumentation) and the same methodology were applied to all the three experiments. It is shown that all sites are affected by advection in different ways and strengths. Everywhere, vertical advection (F VA) occurred only at night. During the day, F VA disappeared because of turbulent mixing, leading to a uniform vertical profile of [CO2]. Mean F VA was nearly zero at the hilly site (Wetzstein) and at the flat site (Norunda). However, large, momentary positive or negative contributions occurred at the flat site, whereas vertical non-turbulent fluxes were generally very small at the hilly site. At the slope site (Renon), F VA was always positive at night because of the permanently negative mean vertical wind component resulting from downslope winds. Horizontal advection also occurred mainly at night. It was positive at the slope site and negative at the flat site in the mean diurnal course. The size of the averaged non-turbulent advective fluxes was of the same order of magnitude as the turbulent flux measured by eddy-covariance technique, but the scatter was very high. This implies that it is not advisable to use directly measured quantities of the non-turbulent advective fluxes for the estimation of net ecosystem exchange (NEE) on e.g. an hourly basis. However, situations with and without advection were closely related to local or synoptic meteorological conditions. Thus, it is possible to separate advection affected NEE estimates from fluxes which are representative of the source term. However, the development of a robust correction scheme for advection requires a more detailed site-specific analysis of single events for the identification of the relevant processes. This paper presents mean characteristics of the advective CO2 fluxes in a first site-to-site comparison and evaluates the main problems for future research. [Copyright &y& Elsevier]