Dynamics of net ecosystem CO2 exchange and heterotrophic soil respiration following clearfelling in a drained peatland forest

摘要

In this study we show for the first time with gas exchange measurements, how ecosystem C dynamics evolve after clearfelling in a peatland forest. Clearfelling decreases peat decomposition rate through two mechanisms: aerated peat volume decreases when water-table rises, and increased surface dryness (increased irradiation) limits decomposition of litter. However, since trees are removed and ground vegetation suffers greatly after clearfelling, photosynthesis is low and the site is losing C after clearfelling. New tree stand is needed to shift this direction. Northern forested peatlands contain a large pool of terrestrial carbon (C). When drained for forestry, these peatlands may become sources of C or remain C sinks, depending on site nutrient level, climate, and average water level depth. However, previous studies do not consider the impact of other silvicultural practices on the C dynamics in drained peatlands. Clearfellings cause disturbance in ecosystem C dynamics and might lead to losses of C to the atmosphere. The initial development of C dynamics following clearfelling in a drained peatland forest in Finland was investigated in a 4-year study which included monitoring of ground vegetation development and chamber based measurements of net ecosystem exchange (NEE), ecosystem respiration (R ECO) and heterotrophic peat soil respiration (R PEAT). NEE measurements showed that the site was mainly a net CO2 source (May-October mean efflux of 1990gCO2 ma degree 2 seasona degree 1) to the atmosphere during the three measurement seasons after clearfelling. This was caused by decrease in photosynthesizing biomass and by high R ECO (May-October mean efflux of 2540gCO2 ma degree 2 seasona degree 1). The recovery of ground vegetation biomass was rather fast. However, during the 3 years after clearfelling its capability to fix carbon could not compensate the high R ECO, which was mainly caused by excessive amounts of easily decomposable fresh organic matter (dying tree roots, logging residue and dying ground vegetation). Clearfelling slightly decreased the decomposition rates of peat (R PEAT) (May-October mean efflux of 1130gCO2 ma degree 2 seasona degree 1) i.e., the oldest and largest C storage of the site, through two mechanisms: R PEAT in deeper layers was restricted by higher water table level and in surface layers by excessive dryness, which lowered the temperature response of R PEAT. We conclude that clearfelling strongly affects ecosystem C dynamics but does not affect the peat C storage. The appearance of new tree stand is needed before the ecosystem could act as a C sink again.