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

Editorial: Surface fluxes over land in complex terrain

. Recently reported trends of carbon dioxide uptake pose the
question of whether trends may be the result of the limited digitalization of gas
analysers and sonic anemometers used in the 1990s. Modifying a 12 bit
digitalization and the instrument error reported for the Gill R2 and R3 sonic
anemometers found elsewhere, the influence of these deficits in comparison to
the now commonly used 16 bit digitalization were quantified. Both issues have
an effect only on trace gas fluxes of small magnitude, mainly for the carbon
dioxide rather than for the water vapour fluxes. The influence on the annual
net ecosystem exchange is negligible, because other errors resulting from
gap filling routines, for example, are much larger.

/pdf/possible-errors-in-flux-measurements-due-to-limited-2vnoufdvgo.pdf

Possible errors in flux measurements due to limited digitalization

Within micrometeorology the term modeling is not uniquely defined. It refers to various methods covering a range of complexity extending from simple regressions up to complicated numerical models . In applied meteorology (agro meteorology and hydro meteorology) simple analytical models are very common. Modeling of evaporation is particularly important but sophisticated numerical methods are not yet widely used in this research area. The following chapter describes different types of models and their limitations beginning with simple analytical methods up to numerical models of near-surface energy and matter transport. The application of models in heterogeneous terrain receives special attention and related flux averaging approaches are addressed in a separate subchapter.

Modeling of Energy and Matter Exchange

Zusammenfassung Für das östliche Oberfranken (Referenzzeitreihe) konnte ein linearer Trend der Jahresmittelwerte der Lufttemperaturen von 1.3 K in 40 Jahren (1961 bis 2000) festgestellt werden. Für die Wintermonate Dezember bis Februar beträgt der Temperaturtrend für den gleichen Zeitraum 2.1 K. Im Jahresverlauf zeigt im Winter v. a. der Monat Dezember eine erhebliche Temperaturzunahme (2.4 K). Im Frühling weisen die Monate März und Mai hochsignifikante positive Trends auf, wohingegen im Sommer nur im August eine erhebliche Erwärmung von 2.3 K zu beobachten ist. Im Herbst hingegen läst sich eine eindeutige Trendentwicklung nicht nachweisen. Kaum Veränderungen zeigen die Jahressummen der Niederschläge der Referenzzeitreihe für das östliche Oberfranken. Im Jahresverlauf jedoch läst sich eine erhebliche Veränderung des Niederschlagsregimes nachweisen. Auffällig sind die hochsignifikanten Niederschlagsabnahmen im Frühling (April & Mai: –16 mm pro Jahrzehnt), die hochsignifikante Zunahme im Juli und die deutliche Reduzierung im August und der starke Trend zu einer höheren Niederschlagssumme im Herbst. Auf dieser Basis und unter Zugrundelegung globaler Klimaszenarien wurden die in den nächsten 50 Jahren zu erwartenden Folgen auf den Tourismus (Bedeutungsverlust Wintersport), das Kurwesen (Verschiebung Bioklima), die Forstund Landwirtschaft (Änderung Wuchsbedingungen Wald) abgeschätzt und kurz skizziert.

Klimawandel in Oberfranken

The eddy-covariance method represents the only direct way to measure the turbulent fluxes of momentum, temperature, trace gases, and particles between the land surface and the atmosphere. It is a direct measurement of the net carbon-dioxide budget and dry deposition. For that purpose, it is widely used in networks of long-term ecosystem observatories around the world and is the centerpiece of intensive field campaigns investigating biosphere–atmosphere exchange processes. The instrumentation typically consists of a 3-D sonic anemometer/thermometer and one or more additional gas analyzers that are able to measure the high-frequency fluctuations of the scalar to be transported. These instruments are mounted on a meteorological mast to sample the turbulent field under the assumption that eddies are carried along with the mean wind. Further prerequisites of the method are horizontal homogeneity, steady-state conditions and well-developed turbulence. For successful application of the method, a series of quality tests and flux corrections is required, which will be presented together with commonly used instrumentation and postprocessing software. Moreover, we will provide a historical overview and provide guidelines for site selection and setup and the necessary maintenance procedures.

Thomas Foken

Papers

Editorial: Surface fluxes over land in complex terrain

Possible errors in flux measurements due to limited digitalization

Modeling of Energy and Matter Exchange

Klimawandel in Oberfranken

Eddy-Covariance Measurements