Carbon balance of a boreal bog during a year with an exceptionally dry summer

Northern peatlands are important terrestrial carbon stores, and they show l arge spatial and temporal variation in the atmospheric exchange of CO2 and CH4. Thus, annual carbon balance must be studied in detail in order to pred ict the climatic responses of these ecosystems. Closed-chamber methods were used to study CO2 and CH4 in hollow, Sphagnum angustifolium lawn, S. fuscu m lawn, and hummock microsites within an ombrotrophic S, fuscum bog. Microm eteorological tower measurements were used as a reference for the CH4 efflu x from the bog. Low precipitation during May-August in 1994 (84 mm below th e long-term average for the same period) and a warm July-August period caus ed the water table to drop by more than 15 cm below the peat surface in the hollows and to 48 cm below the surface in high hummocks. Increased annual total respiration exceeded gross production and resulted in a net C loss of 4-157 g/m(2) in the different microsites. Drought probably caused irrevers ible desiccation in some lawns of S. angustifolium and S. balticum and in S . fuscum in the hummocks, while S. balticum growing in hollows retained its moisture and even increased its photosynthetic capacity during the July-Au gust period. Seasonal (12 May-4 October) CH4 emissions ranged from 2 g CH4- C/m(2) in drier S. fuscum hummocks and lawns to 7 and 14 g/m(2) in wetter S . angustifolium-S. balticum lawns and hollows, respectively. Aerodynamic gr adient measurements at the tower showed slightly higher CH4 flux rates than the average estimates for the whole bog obtained by closed-chamber methods . Winter C efflux comprised 30 g CO2-C/m(2) and 1 g CH4-C/m(2) out of a tot al loss of 90 g C/m(2) on average in the bog, and there was an estimated an nual loss of 7 g C/m2 by leaching. This study shows how delicately the bore al bog's C balance in different microsites depends on climatic variations, especially the distribution of precipitation. It also confirms that severe C losses can occur in boreal bogs during extended summer droughts, even in years with annual temperatures close to the long-term average and with prec ipitation clearly greater than the long-term average.