Twenty years of heat and water balance climatology at the Hartheim pine forest, Germany

The Hartheim pine forest is an ecosystem in the dry region of the southern upper Rhine valley in Germany. A project “Monitoring the Hartheim pine forest” was initiated in 1973 and is continuing. One of its objectives is to provide long-term measurements of the net radiation and its components, to calculate the energy fluxes and other hydrological parameters. Sufficient studies on the climatology of the ‘primitive’ elements of climate exist in central Europe. But long-term series of measurements of the physical processes controlling these elements above different characteristic surface types are missing. The study of heat balance monographies (e.g. Budyko, 1974; Miller, 1981; Kessler, 1985; Hantel, 1989; Baumgartner, 1990) illustrates this deficit drastically. An aspect of our investigations is to study the long-term behaviour of the above-mentioned complex physical variables in relation to weather and climate. Furthermore, we examine whether it is possible to distinguish between the changes, for example, of the ratio of net radiation to global radiation caused by the dynamics of climate and the changes caused by alterations of the ecosystem. Natural and human influences on the pine stand at Hartheim led to an 8-m growth in height during the monitoring period. We present some examples showing that the dynamics of the weather hide other effects within the frame of 20 years of measurements. On the other hand, evident changes in interception and transpiration are due to a changing forest ecosystem.