Net Carbon Exchange of three Boreal Forests during a Drought

The boreal region forms a nearly continuous circumpolar belt extending between 50° and 70° N in North America, Europe, and Asia. Interactions between the atmosphere and the vast boreal forest biome, and the impact of climate change on its carbon cycle is, therefore, of great importance to understanding the global carbon cycle.

The present study analyses eddy correlation flux measurements of CO₂ and climate data obtained at three sites of Fluxnet Canada, formerly part of the Boreal Ecosystem-Atmosphere Study (BOREAS) and the Boreal Ecosystem Research and Monitoring Sites Program (BERMS). The sites, located in northern Saskatchewan are in extensive mature stands of aspen, black spruce, and jack pine, and less than 100 km apart. The proximity of these sites provides a unique opportunity to compare the responses of different ecosystems to comparable synoptic-scale meteorological forcings and interannual climate variability. Four recent consecutive years (2000 - 2003) of measurements from the above sites were analysed. The time series starts with a year with temperature and precipitation well within the long-term average. The subsequent years were affected by a drought occurring in western Canada. In addition, an early (warm) spring occurred in 2001 and a late (cold) spring in 2002.

These forcings significantly affected the annual carbon budget of these forests. During the first drought year, low soil water content significantly decreased ecosystem respiration at the aspen site, probably due to reduced heterotrophic respiration in the upper soil layers. In the second and third year of the drought, the trees appeared to suffer from water stress and thus showed reduced gross ecosystem photosynthesis (GEP). The conifer sites, on the other hand, showed only very limited reaction to the drought. The early spring resulted in higher GEP at the deciduous aspen site, but did not affect spring GEP at the conifer sites. The late spring, however, affected all three sites. It will be shown that these responses are important in assessing under what conditions southern boreal forests are likely to switch from being a sink to a source of atmospheric carbon dioxide.