Forest ecosystems represent a major type ofland use in Germanyand in Europe. They provide a number of functions, or ecosystem services, beneficial to humans, namely biomass production, regulation of the water- and energy cyde, C and N sequestration, erosion control, recreation, and they act as habitat for numerous species. The stability of forest ecosystems in Europe as influenced by the deposition of air pollutants has been a matter of debate for more than 20 years. Besides atmospheric deposition, other environmental conditions affecting forest ecosystems, such as temperature, CO content of the atmosphere 2 and precipitation, have significantly changed in the past and continue to change in the future. Quantifying and predicting the effects of these changes on ecosystem functioning are achallenge to ecosystem research and also a requirement to establish sustainable use of forest ecosystems in the future. This book summarizes results of long-term, interdisciplinary ecosystem research conducted in two forested catchments and coordinated at the Bayreuth Institute of Terrestrial Ecosystem Research (BITÖK), University of Bayreuth, Germany. It does not aim to summarize all the research of BITÖ K in the past decade, which would go far beyond the studies in these two catch­ ments. Instead, we concentrate here on the long-term developments in the biogeochemistry of carbon and mineral elements and on the water cyde, at both the plot and the catchment scale.