Initial effects of post-harvest ditch cleaning on greenhouse gas fluxes in a hemiboreal peatland forest

Ditch cleaning (DC) is a well-established forestry practice across Fennoscandia to lower water table levels (WTL) and thereby facilitate the establishment of tree seedlings following clear-cutting. However, the implications from these activities for ecosystem-atmosphere greenhouse gas (GHG) exchanges are poorly understood at present. We assessed the initial DC effects on the GHG fluxes in a forest clear-cut on a drained fertile peatland in hemiboreal Sweden, by comparing chamber measurements of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes from soil and ditches in DC and uncleaned (UC) areas over the first two post-harvest years. We also evaluated spatial effects by comparing fluxes at 4 m and 40 m from ditches. We found that 2 years after DC, mean (+/- standard error) WTL of-65 +/- 2 cm was significantly lower in the DC area compared to-56 +/- 2 cm in the UC area. We further observed lower gross primary production and ecosystem respiration in the first year after DC which coincided with delayed development of herbaceous ground vegetation. We also found higher CH4 uptake but no difference in N2O fluxes after DC. Greater CH4 uptake occurred at 4 m compared to 40 m away from both cleaned and uncleaned ditches. Model extrapolation suggests that total annual GHG emissions in the second year were reduced from 49.4 +/- 17.0 t-CO2-eq-ha(-1) -year(-1) in the UC area to 27.8 +/- 10.3 t-CO2-eq-ha(-1) -year(-1) in the DC area. A flux partitioning approach suggested that this was likely caused by decreased heterotrophic respiration, possibly because of enhanced soil dryness following DC during the dry meteorological conditions. CH(4 )and N2O fluxes from clear-cut areas contributed < 2 % to the total (soil, ditches) GHG budget. Similarly the area -weighted contributions by CO2 and CH4 emissions from both cleaned and uncleaned ditches were < 2 %. Thus, our study highlights that DC may considerably alter the post-harvest GHG fluxes of drained peatland forests. However, long-term observations under various site conditions and forest rotation stages are warranted to better understand DC effects on the forest GHG balance.