Lowered water table level decreases boreal mire NECB - a question of increased decomposition or decreased photosynthesis?

The fundamental prerequisite for development and maintenance of mire ecosystems is a positive water balance maintaining a water table level close to the soil surface. One potential effect of climate change at higher latitudes is decreased positive water balance, i.e. increased evapotranspiration and/or decreased precipitation during the growing season leading to a lowered water table level. A lowered water table level is well known to reduce both the net ecosystem exchange (NEE) and most likely also the Net Ecosystem Balance (NECB). Most commonly a reduced NEE is interpreted as resulting from increased respiration. Therefore, a water table draw down is often viewed as a severe threat to the large long-term carbon stores occurring in high latitude peatlands. We used eddy covariance derived data on NEE from a high latitude mire in Northern Sweden during a year with severe drought during the growing season to separate the effects between photosynthesis and ecosystem respiration. The long term annual average NEE at the site is ~59 g C m-2 yr-1 which during the year with summer drought was reduced to 17 g C m-2 yr-1 resulting in a NECB not different from zero. Detailed analyses of the diurnal variation in NEE as well as ordinary NEE-partitioning into gross photosynthesis and respiration respectively revealed a drastic decrease in daytime CO2 uptake while the nighttime CO2 emission hardly was affected at all. Thus, for this widespread type of mire the most significant direct effect of severe droughts is reduced photosynthesis rather then increased respiration.