Bodo Wichura

Micrometeorological measurements during the total solar eclipse of August 11, 1999

Micrometeorological measurements of radiation, atmospheric and soil parameters, and turbulent energy and momentum e uxes, ozone and carbon dioxide e uxes have been conducted over a maize e eld at FreisingWeihenstephan in Southern Germany during the total solar eclipse on August 11, 1999. For the period 30 minutes before and after the totality the weather conditions at the location where the micrometeorological measurements were made was satisfactory. Several connections between the irradiation and other meteorological parameters over a maize e eld have been found. The time response between irradiation and the long-wave upward radiation was only a few minutes, whereas almost all parameters caused by the turbulent transport had a time shift of up to 30 minutes. A period of nearly 30 minutes with reduced turbulence regime after the totality was found. Using a wavelet transformation for the time series, a change of time scales from longer to shorter ones was observed before the totality, and after the turbulence increased in the short time scales. The investigation of the residuum of the closure of the energy balance showed that with a time shift for the latent heat e ux (unlike the net radiation) after the totality, a better energy budget closure was obtained.

Carbon budget of a spruce forest ecosystem

The investigation of carbon fluxes is of immense interest in ecosystem and climate research. Forest ecosystems may be a sink for anthropogenic carbon, if the assimilation is larger than the respiration. Alternatively, increasing temperatures due to climate change (IPCC 2001) may be a reason for increasing respiratory fluxes. While low-altitude spruce sites in Germany are significant carbon sinks (e.g. Bernhofer et al. 2003), sites above 600 m a.s.l. are only small sinks or may change their character by climate change. Therefore the Weidenbrunnen site in the Lehstenbach catchment was selected as a EUROFLUX site (Valentini et al. 2000) and was also used in the following CARBOEUROFLUX program for systematic investigations with respect to the data quality of turbulent fluxes. Overviews of the European carbon program and of the worldwide FLUXNET program are respectively given by Valentini (2003) and Baldocchi et al. (2001). All relevant references are also provided herein. Furthermore, the site was used for process studies to separate assimilation and respiration fluxes, and to study the exchange conditions between the forest and the atmosphere (Wichura et al., this Vol.). All of these studies were part of the ecosystem research of the Lehstenbach catchment, the main research area of the Bayreuth Institute of Terrestrial Ecosystem Research (BITOK). The main results for the carbon dioxide flux measurements since 1997 are discussed in this chapter.

Structure of Carbon Dioxide Exchange Processes Above a Spruce Forest

Several micrometeorological techniques,’such as the flux-gradient method or the eddy covariance technique, offer the potential to measure net fluxes of water vapor, CO2 and other trace gases exchanged between ecosystems and the atmosphere (e.g., Baldocchi and Meyers 1998). Subsequent data analyses allow the calculation of net ecosystem CO2 exchange. These net fluxes, however, reflect the balance between different component fluxes. In the case of CO2, two opposing fluxes contribute to this net flux: CO2 uptake during photosynthesis and CO2 release during respiration from above- and belowground organisms. Distinguishing among these components is critical to obtain insights into the processes underlying ecosystem responses to climate forcing (Buchmann 2002). This is because environmental parameters, such as temperature and soil moisture, differentially affect biological activities (e.g., Baldocchi et al. 2001).

The BAYSOFI Campaign – Measurements carried out during the total solar eclipse of August 11, 1999

The total solar eclipse of August 11, 1999 provided a unique opportunity to observe the input of fast day-night and night-day transitions, under high solar elevation around noon, on the earth-atmosphere-biosphere system. Within the interdisciplinary e eld campaign BAYSOFI, measurements of radiation, boundary layer micrometeorology and photochemistry, photosynthesis and transpiration were carried out at Freising-Weihenstephan and several locations nearby focusing on short-term effects of the eclipse. Although the overall grosswetterlage on August 11 was not favourable for viewing the eclipse, with clouds covering most of central Europe, observational conditions at Weihenstephan were fair due to a large hole in the cloud layer which appeared just half hour before totality lasting for more than one hour. Thus signie cant effects of the eclipse on radiation, photolysis rates, OH, the temperature, wind, turbulence structure and stratie cation, ozone and CO 2 e uxes, photosynthesis, transpiration and sap e ow of trees could be observed which are reported and discussed in the following sequence of papers.

Long-Term Carbon and Water Vapour Fluxes

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.