LONG-TERM MEASUREMENTS OF BOREAL FOREST CARBON BALANCE REVEAL LARGE TEMPERATURE SENSITIVITY

We present results from two years' net ecosystem flux measurements abo ve a boreal forest in central Sweden. Fluxes were measured with an edd y correlation system based on a sonic anemometer and a closed path CO2 and H2O gas analyser. The measurements show that the forest acted as a source during this period, and that the annual balance is highly sen sitive to changes in temperature. The accumulated nux of carbon dioxid e during the full two-year period was in the range 480-1600 g CO2 m(-2 ). The broad range is caused by uncertainty regarding assessment of th e night-time fluxes. Although annual mean temperature remained close t o normal, the results are partly explained by higher than normal respi ration, due to abnormal temperature distribution and reduced soil mois ture during one growing season. The finding that a closed forest can b e a source of carbon over such a long period as two years contrasts sh arply with the common belief that forests are always carbon sinks.