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.

/pdf/fluxnet-a-new-tool-to-study-the-temporal-and-spatial-2izxj9ikwz.pdf

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

In this study we analyse eddy-covariance flux measurements of carbon dioxide and water vapour from 18 years at Waldstein–Weidenbrunnen (DE-Bay), a Norway spruce forest site in the Fichtelgebirge, Germany. Standard flux partitioning algorithms have been applied for separation of net ecosystem exchange NEE into gross primary production GPP and ecosystem respiration Reco, as well as gap-filling. The site has always been a carbon sink, and annual net uptake ( − NEE) shows a positive trend with values around 40 g C m−2 a−1 for 1997–1999 up to 615 ± 79 g C m−2 a−1 for 2011–2014. This is related to a strong increase in GPP, while Reco is slightly enhanced. Evapotranspiration increases coherently with NEE, while atmospheric demand, that is, potential evaporation, shows inter-annual variability, but no trend. Comparisons with studies from other warm-temperate coniferous forests show that our NEE estimates are at the upper range of the distribution, but still realistic. Also evapotranspiration estimates, evaluated in the Budyko framework, are in a similar range but with a large inter-annual variability. We identified instrumental problems and variability from different flux partitioning algorithms as a large source of uncertainty, but with only minor influence on the trends found. Warming and rising CO2-concentrations are consistent with the observed trend, but cannot be disentangled from site-specific changes such as the recovery from “Waldsterben” after liming and an increase in heterogeneity after a wind-throw, which likely plays the most important role in the observed dynamics. As such transitions from an “ideal” to a disturbed or heterogeneous site are likely more-often the case at FLUXNET stations built 10–20 years ago, a systematic bias in regional studies can only be avoided by taking each single site history into account.

Long-Term Carbon and Water Vapour Fluxes

This chapter describes the challenges and the history of micrometeorology. For sake of comprehensiveness, it also provides an overview of essential definitions that the reader might consider becoming familiar with before delving deeper into the present volume. Furthermore, this chapter provides the historical perspective of the evolution of a rapidly maturing field right up to the development of recent footprint tools used in research as in applications. It should be apparent to all that such an overview can only scratch the surface while some of the details will be described in the following chapters. It goes without saying that this overview is tinted by the experiences of the authors.

History and Definition

Measuring the isotopic composition of trace gas flu xes can provide additional information on ecosystem gas exchange, when ecosystem processes, like assimilation, discri minate against heavier isotopes. In the case of CO 2 exchange, different mass-balances for bulk CO 2 and its CO2 or CO O isotopes can be used to separate respiration from photosynthetic assimilation. Up to now, detectors for direct isotope mea surements in the field lack the precision needed fo r fast eddy covariance (EC) flux measurements. The collection of updraft an d downdraft whole-air samples using the relaxed edd y accumulation technique (REA) allows simultaneously determining tr ace gas concentrations and isotope ratios by high p recision laboratory analysis. At the same time whole-air REA relaxes sev eral of the technical problems related to REA sampli ng on traps. In tests using air from a tank the complete whole-a ir REA sampling system and its foil balloon bag rese voirs showed no signs of contamination after cleaning. The standard eviations of δC and δO isotope ratios were only slightly higher than the precision specified for the laboratory analysis procedure. First experiment results showed that is otopic differences (updrafts−downdrafts) were large enough to yield signa l to noise ratios greater than five when applying h yperbolic deadbands during REA sampling (HREA). The performance of the in strument and the HREA sampling method are investigat ed by simulation of the sampling process for bulk CO 2, which serves as proxy scalar. Measurements by whole-air HREA in combination with h igh precision isotope analysis can quantify the iso fluxes of CO2 and COO. Furthermore, additional information is collected on the scalar correlation of bulk CO 2 and its stable isotopes, which represents the relatively short timescale of updrafts and downdrafts in the turbulent exchange a bove an ecosystem. This information is essential to check the scalar s imilarity assumptions made in the HREA and EC/flask m ethod for the quantification of isofluxes.

/pdf/whole-air-relaxed-eddy-accumulation-for-the-measurement-of-45tcrskmt3.pdf

Whole-air relaxed eddy accumulation for the measurement of isotope and trace-gas fluxes

The chapter gives a brief overview of the measuring sites of the Bayreuth Center of Ecology and Environmental Research at the Waldstein mountain, Germany, northeast Bavaria, with a special focus on the Waldstein-Weidenbrunnen site (FLUXNET site DE-Bay) as well as the Pflanzgarten and Kohlerloh sites. The climate of the area, the structure of the forest stands with plant area index profiles, and the soil properties are described. There is a special focus on the tower installations, the permanent program, and the special experimental campaigns. Pictures of all sites and all relevant data are given, while the instrumentation is listed in Appendix A. Due to the storm event “Kyrill” in January 2007, the southern part of the forest was destroyed. A trace gas flux footprint analysis is presented for conditions after a large wind throw.

Thomas Foken

Papers

Long-Term Carbon and Water Vapour Fluxes

History and Definition

Whole-air relaxed eddy accumulation for the measurement of isotope and trace-gas fluxes

Description of the Waldstein Measuring Site

Eddy sampling methods, a comparison using simulation results