Long-term water and carbon dioxide fluxes from a boreal forest. Methods and applications

Direct long-term measurements of carbon dioxide and water vapour exchange are needed over forested ecosystems to determine their net budgets of carbon and water and to parameterize, improve, and test biogeochemical, ecological and hydrological models. Responding to this need, an eddy-correlation system was designed with special emphasis on accuracy, long-term stability, minimum maintenance requirements, and winter-time applicability. The system includes commercially available and specially constructed sensors, thermostatic housing and lightning protection, data acquisition and evaluation software and remote data access. It has successively been improved with respect to errors such as frequency response or calibration stability. Since May 1994, three such systems have been running continuously above a coniferous forest, resulting in a time series of fluxes that covers 98% of the whole period. Source-area analysis of the flux data proved to be important for relating total evaporation to evaporation components measured at tree level. It is even more important for estimating the net carbon budget, since the significant night-time fluxes measured at one point above the forest generally represent different source areas than the day-time fluxes, and may even not be representative of the site. Interannual variations were considerable during the period of observation. While the Norunda forest was a net source of carbon with respect to the atmosphere during the whole period, the source strength was significantly reduced during 1996, probably owing to differences in climate. Evaporation significantly decreased during 1996, relative to 1994 and 1995, apparently due to persistent vegetation damage caused by the extraordinarily long and cold winter of 1995/1996. 128 refs, 28 figs, 7 tabs