Dynamics of net ecosystem CO 2 exchange and heterotrophic soil respiration following clearfelling in a drained peatland forest

Abstract

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 CO 2 source (May–October mean efflux of 1990 g CO 2 m −2 season −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 2540 g CO 2 m −2 season −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 1130 g CO 2 m −2 season −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.