Human land use influences important properties of terrestrial ecosystems, such as energy flow, standing crop, and biomass turnover. Human interference with ecological energy flows may be studied by calculating the human appropriation of NPP (HANPP), defined as the difference between the net primary production (NPP) of potential vegetation and the actual NPP remaining in ecosystems after harvest. Comparing the standing crops of the potential vegetation and actually prevailing vegetation, we demonstrate the human impact on the amount of carbon stored in living vegetation. We discuss these concepts using empirical results for aboveground vegetation in Austria calculated from statistical data and from land use and land cover models derived from remote-sensing data. According to our calculations the human appropriation of aboveground NPP in Austria today amounts to ~50%. The aboveground standing crop (biomass stock) of the vegetation prevailing in Austria today is ~64% lower than that of the potential vegetation. ¿ 2001 American Geophysical Union