Future climate warming is expected to enhance plant growth in temperate ecosystems and to increase carbon sequestration. But although severe regional heatwaves may become more frequent in a changing climate their impact on terrestrial carbon cycling is unclear. Here we report measurements of ecosystem carbon dioxide fluxes, remotely sensed radiation absorbed by plants, and country-level crop yields taken during the European heatwave in 2003.We use a terrestrial biosphere simulation model to assess continental-scale changes in primary productivity during 2003, and their consequences for the net carbon balance. We estimate a 30 per cent reduction in gross primary productivity over Europe, which resulted in a strong anomalous net source of carbon dioxide (0.5 Pg Cyr21) to the atmosphere and reversed the effect of four years of net ecosystem carbon sequestration. Our results suggest that productivity reduction in eastern and western Europe can be explained by rainfall deficit and extreme summer heat, respectively. We also find that ecosystem respiration decreased together with gross primary productivity, rather than accelerating with the temperature rise. Model results, corroborated by historical records of crop yields, suggest that such a reduction in Europe's primary productivity is unprecedented during the last century. An increase in future drought events could turn temperate ecosystems into carbon sources, contributing to positive carbon-climate feedbacks already anticipated in the tropics and at high latitudes.

Europe-wide reduction in primary productivity caused by the heat and drought in 2003

Investigating soil moisture-climate interactions in a changing climate: A review

FLUXNET is a global network of micrometeorological flux measurement sites that measure the exchanges of carbon dioxide, water vapor, and energy between the biosphere and atmosphere. At present over 140 sites are operating on a long-term and continuous basis. Vegetation under study includes temperate conifer and broadleaved (deciduous and evergreen) forests, tropical and boreal forests, crops, grasslands, chaparral, wetlands, and tundra. Sites exist on five continents and their latitudinal distribution ranges from 70°N to 30°S. FLUXNET has several primary functions. First, it provides infrastructure for compiling, archiving, and distributing carbon, water, and energy flux measurement, and meteorological, plant, and soil data to the science community. (Data and site information are available online at the FLUXNET Web site, http://www-eosdis.ornl.gov/FLUXNET/.) Second, the project supports calibration and flux intercomparison activities. This activity ensures that data from the regional networks are intercomparable. And third, FLUXNET supports the synthesis, discussion, and communication of ideas and data by supporting project scientists, workshops, and visiting scientists. The overarching goal is to provide information for validating computations of net primary productivity, evaporation, and energy absorption that are being generated by sensors mounted on the NASA Terra satellite. Data being compiled by FLUXNET are being used to quantify and compare magnitudes and dynamics of annual ecosystem carbon and water balances, to quantify the response of stand-scale carbon dioxide and water vapor flux densities to controlling biotic and abiotic factors, and to validate a hierarchy of soil–plant–atmosphere trace gas exchange models. Findings so far include 1) net CO 2 exchange of temperate broadleaved forests increases by about 5.7 g C m −2 day −1 for each additional day that the growing season is extended; 2) the sensitivity of net ecosystem CO 2 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of CO 2 flux density exhibits peaks at timescales of days, weeks, and years, and a spectral gap exists at the month timescale; 4) the optimal temperature of net CO 2 exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor flux densities.

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FLUXNET: A New Tool to Study the Temporal and Spatial Variability of Ecosystem-Scale Carbon Dioxide, Water Vapor, and Energy Flux Densities

This paper discusses the advantages and disadvantages of the different methods that separate net ecosystem exchange (NEE) into its major components, gross ecosystem carbon uptake (GEP) and ecosystem respiration (Reco). In particular, we analyse the effect of the extrapolation of night-time values of ecosystem respiration into the daytime; this is usually done with a temperature response function that is derived from long-term data sets. For this analysis, we used 16 one-year-long data sets of carbon dioxide exchange measurements from European and US-American eddy covariance networks. These sites span from the boreal to Mediterranean climates, and include deciduous and evergreen forest, scrubland and crop ecosystems. We show that the temperature sensitivity of Reco, derived from long-term (annual) data sets, does not reflect the short-term temperature sensitivity that is effective when extrapolating from night- to daytime. Specifically, in summer active ecosystems the long

https://www.bgc-jena.mpg.de/bgi/uploads/Publ/Publications/Reichstein_etal_2005.pdf

On the Separation of Net Ecosystem Exchange into Assimilation and Ecosystem Respiration: Review and Improved Algorithm

. Recently reported trends of carbon dioxide uptakes pose the question if trends may results of the digitalization of gas analysers and sonic anemometers used in the 1990s. Simulating a 12-bit digitalization and the instrument error reported for the R2 and R3 sonic anemometers elsewhere, the influence of these deficits in comparison to the 16-bit digitalization were quantified. Both issues have an effect only on trace gas fluxes of small magnitude, mainly for the carbon rather than for the water vapour fluxes. The influence on the annual net ecosystem exchange is negligible.

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Technical Note: Possible errors in flux measurements due to limited digitalization

Sites for flux towers such as organized in the FLUXNET network (e.g. Baldocchi et al., 2001) are often chosen according to their ecological importance and micrometeorological aspects play only a minor role in many cases. As a result, sites are often located in complex terrain and are characterized by heterogeneous land cover with short fetches over the target land use type, i.e. conditions that compromise the collection of high quality meteorological data sets. The application of the eddy-covariance technique, which is commonly used for flux determination (e.g. Aubinet et al., 2000; Baldocchi et al., 2000), requires therefore a site dependent quality control to allow for a robust interpretation of the flux measurements (Foken et al., 2004). A key component in the data quality protocol are footprint models that determine the spatial context of a measurement by defining a transfer function between sources or sinks of the signal and the sensor position. The derived source area is crucial for the interpretation of micrometeorological data sets, e.g. by determining the fetch requirements under changing atmospheric stability regimes or by assessing the influence of distorting terrain elements on the measurements. A powerful tool to model source areas over forest are Lagrangian Stochastic (hereafter referred to as LS) footprint models (e.g. Baldocchi, 1997; Rannik et al., 2000; Kljun et al., 2002; Rannik et al., 2003), because this technique allows the consideration of effects of canopy flow on the measured fluxes, and a more realistic treatment of diffusion. As drivers, Lagrangian stochastic models use characteristics of prevailing turbulence to calculate trajectories of individual air parcels, such as the profiles of the mean wind speed u, the wind fluctuations (σu; σv; σw), or the dissipation rate of turbulent kinetic energy e. However, as only few generally valid theories are known for the flow in the canopy space (e.g. Lee, 1998; Finnigan, 2000), for within canopy flow these parameters often have to be approximated with crude generalizations and certain ad hoc assumptions (Schmid, 2002). This study aims at testing the sensitivity of a Lagrangian Stochastic footprint model to the input parameters describing the turbulent flow field, with a focus on the within canopy flow processes. A highquality long-term dataset of turbulence measurements within and above a tall spruce canopy was used to extract detailed turbulence statistics as input parameters for the footprint model. From these measurements, representative profiles of the input parameters required for the footprint modeling are derived by application of different filters to the original data set. We deployed spectral analysis using a wavelet analysis tool (Thomas and Foken, 2005) to extract and detect single coherent structures along the vertical profile. Based on this analysis, typical exchange regimes are determined to characterize the degree of coupling between the canopy space, the atmosphere and the ground surface. The resulting description of the turbulent flow field varies in both the spatial and temporal context, as statistics are derived specifically for each exchange regimes and corresponding wind direction. Two additional simpler methods to describe the canopy flow regime will be applied for means of comparison. First, the parameterization of the flow statistics presented by Rannik et al. (2003) will be used, which have been tested thoroughly for Lagrangian Stochastic footprint modeling. This dataset was derived by measurements at the Hyytiala site in Finland, for a forest architecture that has characteristics significantly different from those at our testing site. Second, a model by Massman and Weil (1999) to parameterize the profiles of the flow statistics based on profiles of the leaf area index will be employed. The impact of the application of these different descriptions of canopy flow will be tested by comparing size and position of the source areas computed by the footprint model, as well as by the determined composition of land cover types within the source area and their correlation to the measured eddy-covariance fluxes.

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On the sensitivity of Lagrangian Stochastic footprint modeling to within canopy flow statistics derived from Wavelet analysis

Microclimate was investigated within a heterogeneous spruce forest in Northern Bavaria, Germany, at the Waldstein–Weidenbrunnen site, especially during the EGER project in 2007, 2008 and 2011. Besides standard tower measurements, two innovative measuring techniques were used to investigate horizontal and vertical gradients. A particular focus was paid to advection within the homogeneous part and its effect on NEE, as well as gradients near a forest edge, measured by a mobile measuring system.

Forest Climate in Vertical and Horizontal Scales

Interactive comment on "Towards a more harmonized processing of eddy covariance CO₂ fluxes : algorithms and uncertainty estimation" by D. Papale et al.

This study aims to demonstrate the applicability of acoustic remote sensing to observing coherent structures above tall vegetated canopies and thus to get deeper insight into the dynamics of coherent structures. Therefore a Sodar-Rass system was operated during two field campaigns obtaining time series with a resolution of 0.4 Hz. Coherent structures were successfully observed in the vertical wind speed data. Comparisons with simultaneously operated sonic anemometers yielded a remarkable agreement in the wavelet variance spectra. The characteristic event durations of coherent structures were found to range from approx. 20 s to several minutes with smaller structures missing with increasing height. Coherent structures were found to exist mainly in the roughness sublayer above tall vegetation. Both small-scale and large-scale events were found to exist at the same time and possibly interacting with each other.

/pdf/detection-of-coherent-structures-in-high-frequent-time-2xgsnv3a6m.pdf

Thomas Foken

Papers

Technical Note: Possible errors in flux measurements due to limited digitalization

On the sensitivity of Lagrangian Stochastic footprint modeling to within canopy flow statistics derived from Wavelet analysis

Forest Climate in Vertical and Horizontal Scales

Interactive comment on "Towards a more harmonized processing of eddy covariance CO₂ fluxes : algorithms and uncertainty estimation" by D. Papale et al.

Detection of coherent structures in high frequent time series of a monostatic Sodar-Rass system