Vegetation succession and biomass production after peat ash and PK-fertilization on the cutaway peatland of Näsmyran in Hälsingland, Sweden

Sammanfattning: By 2010, the total area of cutaway peatlands ready for after use in Sweden is expected to be 3 000 – 5 000 ha. Afforestation of terminated peat cuttings is one of the most favoured after use options and can be suitable for commercial timber production if drainage and nutrition status are taken care of. This study was conducted on a long-term afforestation experiment on Näsmyran in Hälsingland, Sweden. In 1991, five different tree species were planted on 40 plots with four different fertilizer treatments. The aim of this study was to assess the total biomass production and the vegetation development as a response to the fertilization. After 13 vegetation periods all plots were dominated by natural regeneration of downy birch, silver birch and pine from the surrounding forests. Still, on the fertilized plots the planted trees were thriving. The total stem density after 13 years was 69 900 stems/hectare on the peat ash treatment (corresponding to 80 kg P and 104 kg K), 43 500 stems/ha on P40K80 (40 kg P and 80 kg K/ha), 39 300 stems/ha on P80K120, 21 200 stems/ha on P0K0, and 18 800 stems/ha on Control (no fertilizer; no planting). The highest total biomass production, 96 tonnes per hectare (13 yrs) was recorded for the treatment P80K120, while P40K80 and ASH had a total production of 64 and 55 t/ha, respectively. The P0K0 and the Control produced only 7 and 5 t/ha, respectively. While the highest PK-fertilizer dose resulted in the highest total tree biomass production, peat ash fertilization resulted in the highest number of stems/ha. The most productive species, in terms of single tree volume and biomass, was lodgepole pine. The areal production of Scots pine was, however, higher than for lodgepole pine on P40K80 and ASH, due to the contribution from natural regeneration for Scots pine. Comparing the two birch species, downy birch shows higher stem densities than silver birch, while the opposite is true regarding production (both single tree production and areal production). Considering the natural regeneration, planting of Scots pine may not be necessary when the cutaway peatland is fertilized with PK-fertilizer or peat/wood ash, provided that natural seed sources are found within seeding distance (approximately 200 m). Without fertilization even after 13 vegetation periods, there was still no appropriate vegetation establishment found on the unfertilized plots. There is no doubt that establishment and development of the trees on Näsmyran are the results of fertilization.