This paper considers problems associated with ecological processes in floodplain forests in Leipzig, central Germany, caused by the biological production of the forest stand. With methods of the RP-HPLC (High Peformance Liquid Chromatography) humic substances in the canopy precipitation, stem flow and soil solution at one investigation plot in a typical hardwood floodplain forest stand at the Weisse Elster river system has been investigated. It could be shown that the origin and contents of the soil humic acids depend on the input of organic acids produced in the forest stand, most of all in the stems. The paper should demonstrate the utilisation of one very special method to quantify and to qualify organic acids ans their transport in a floodplain forest ecosystem.