Soil and water nutrients in stem-only and whole-tree harvest treatments in restored boreal peatlands

In boreal peatlands felling and tree harvest are commonly carried out as part of peatland restoration. Stem-only harvest is the principal harvest method and it leaves the live crown material (felling residue) containing most tree nutrients at the site. Whole-tree harvest, where felling residue is removed, is not favored due to higher transport costs, although it might better promote the recovery of nutrient-poor peatlands towards pristine conditions. We investigated whether initial differences in N mineralization and decomposition rates observed between tree harvest methods continued out to 6 years after restoration and whether the spatial variation in water table (WT) level and water nutrient concentrations parallels with the observed pattern in mineralization and decomposition rates. The study was done at 15 peatland sites in Natura 2000 protection areas in Finland during 2007-2013. Concentration of ammonium in soil water was higher in the stem-only harvest treatment compared to that of the whole-tree harvest treatment, whereas the previously observed differences in net N mineralization and decomposition rates had leveled out by the sixth year after restoration. The spatial variation created by the ditch network still affected the hydrology and peatland functions so that the nutrient concentrations were higher near ditches than in other locations, implying potential risk for nutrient leaching. Based on this study, there is no reason to prefer either harvest method over the other in nutrient-poor drained peatlands with low tree volumes, which constitute the majority of available peatland restoration area in Finland.