Growth and Nutrition of Scots Pine on Drained and Fertilized Purple Moor Grass Fens in Central Finland

While fens dominated by Purple Moor Grass (Molinia caerulea (L.) Moench) have generally been considered unsuitable for forest drainage due to their low content of mineral nutrients, the fact that they also are rich in nitrogen has been acknowledged to offer silvicultural possibilities. This study examined the nutrient status and growth of Scots pine stands (with downy birch as a secondary species) on two drainage and fertilization experiments. Both of the sites were nitrogen-rich and originally scarcely wooded or treeless fens. Nokkapuronsuo (64 degrees 34' N, 28 degrees 23' E) was drained in the 1950s and fertilized in 1962, 1980 and 2002. Hanhiselka (64 degrees 53' N, 26 degrees 04' E) was drained in 1967 and fertilized in 1980. Disintegrated phosphate, potassium chloride, PK-fertilizer for peatlands and wood ash were used to fertilize the sites. The data included monitoring the foliar nutrient concentrations and post-fertilization stand volume growth during the 27-44 years from fertilizer application. The needle analyses of Scots pine revealed a severe deficiency of phosphorus (P) and potassium (K) in the unfertilized control trees. Fertilization treatments of P (36-133 kg ha(-1)) and K (66-361 kg ha(-1)) improved the respective nutrient concentrations above the deficiency limits. The effect of single P addition was more pronounced and prolonged than that of K addition. In general, the more K included in the single or repeated fertilization treatment, the greater was the growth of trees during the study period. At both sites, the stand growth of unfertilized trees remained low (less than 0.5 m(3) ha(-1) a(-1)). Fertilization increased both stand density and the volume growth of the single trees. On the fertilized plots tree growth remained at considerably higher levels than the control throughout the whole study period. At Nokkapuronsuo the highest yields varied between 3 and 4 m(3) ha(-1) a(-1) during the 44 years and at Hanhiselka between 5 and 6 m(3) ha(-1) a(-1) during the 27 years. In conclusion, refertilizing during the stand rotation, once with phosphorus and twice with potassium, is needed to ensure the continued good nutrient status of trees on Molinia fens.