Carbon dioxide efflux density from the floor of a central Siberian pine forest

Total and forest floor carbon dioxide flux densities (F(CO₂)) and environmental variables were measured for 18 consecutive midsummer days during July 1996 in a 215-year-old stand of Pinus sylvestris L. trees located 40 km southwest of the village of Zotino in central Siberia, Russia (61°N, 89°E, 160 m asl). Forest floor F(CO₂) was regulated by surface soil water content, related to the limited storage capacity of the sandy soil equivalent to only 4 mm water per 100 mm depth of soil. Following 12 mm rainfall, forest floor FCO₂ increased by 52% to a maximum value of 4.1 μmol m^-2 s^-1. However, the rate had returned to the general lower level by the next day in response to rapid drying of the surface soil. There was little correspondence between forest floor F(CO₂) and the distributions of root and soil carbon or soil temperature. However, for soil samples returned to the laboratory, sieved to remove roots and re-watered, microbial respiration rate was positively and exponentially related to temperature. Measurements of forest floor F(CO₂) by eddy covariance were in good agreement with the chamber data during the daytime when the atmosphere was regularly mixed by turbulence. Micrometeorological flux measurements at the forest floor and above the trees showed how, on average, 77% of the carbon sequestered by tree canopy photosynthesis was lost to the atmosphere by root and soil microbial respiration during the observation period. On a daily basis, the boreal forest was generally a modest net sink (-75 mmol m^-2 per day), but also a small carbon source on hot and dry days.