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The long-term impacts of current forest management methods on surface water quality in Fennoscandia are largely unexplored. We studied the long-term effects of clear-cutting and site preparation on runoff and the export of total nitrogen (total N), total organic nitrogen (TON), ammonium (NH 4 -N), nitrate (NO 3 -N), total phosphorus (total P), phosphate (PO 4 -P), total organic carbon, and suspended solids (SS) in three paired-catchments in Eastern Finland. Clear-cutting and soil preparation were carried out on 34 % (C34), 11 % (C11), and 8 % (C8) of the area of the treated catchments and wide buffer zones were left along the streams. Clear-cutting and soil preparation increased annual runoff and total N, TON, NO 3 -N, PO 4 -P, and SS loads, except for SS, only in C34. Runoff increased by 16 % and the annual exports of total N, TON, NO 3 -N, and PO 4 -P by 18, 12, 270, and 12 %, respectively, during the 14-year period after clear-cutting. SS export increased by 291 % in C34, 134 % in C11, and 16 % in C8 during the 14, 6, and 11-year periods after clear-cutting. In the C11 catchment, NO 3 -N export decreased by 12 %. The results indicate that while current forest management practices can increase the export of N, P and SS from boreal catchments for many years ([10 years), the increases are only significant when the area of clear cutting exceeds 30 % of catchment area.

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The effects of wood ash fertilization on soil chemical properties were studied in three young Scots pine (Pinus sylvestris L.) plantations and a Norway spruce (Picea abies (L.) Karst.) plantation with different site fertility in southern Finland. The dose of 3 t ha -1 of loose wood ash was applied to 4 replicate plots (25 × 25 m) at each experiment. Humus layer and mineral soil samples were taken before the treatment and 7 and 16 years after wood ash application. Results showed that neutralization as well as fertilization effects of wood ash on forest soil were of long duration. An ash-induced pH increase of 0.6-1.0 pH units and exchangeable acidity (EA) decrease of 58-83% were detected in the humus layer 16 years after wood ash application. The decrease in acidity was most pronounced on the Calluna site with initially the lowest pH and highest EA. In the mineral soil the increase in pH was observed later than in the humus layer. After 16 years, the mineral soil pH was increased (0.2-0.3 pH units) on the Vaccinium and Myrtillus sites. A corresponding and in most cases a signifi cant increase in the extractable Ca and Mg concentrations was detected in both the humus layer and in the mineral soil. Wood ash signifi cantly increased the effective cation exchange capacity (CEC e ) and base saturation (BS) but decreased the concentration of exchangeable Al in both soil layers on all the sites. No response of N availability to wood ash application could be found.

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Rapid growth of ground vegetation following clear-cutting is important to site productivity because vegetation retains nutrients in the ecosystem and can decrease nutrient leaching prior to stand re-establishment. Aboveground biomass, nutrient contents (N, P, K and Ca) and species composition of ground vegetation were determined 1 year before and for 7 years after clear-cutting of a mixed forest dominated by Norway spruce [Picea abies (L.) H. Karst.] in eastern Finland. The biomass of the feather mosses [Pleurozium schreberi Brid. and Hylocomium splendens (Hedw.) B. S.& G.] and the dwarf shrubs (Vaccinium myrtillus L. and V. vitisidaea L.), which had dominated the ground vegetation in the mature forest, significantly decreased after clearcutting. However, with the exception of H. splendens, these species had recovered within 3-5 years. The biomass of Deschampsia flexuosa (L.) Trin. considerably increased soon after clear-cutting, and Epilobium angustifolium L. appeared 3-5 years after cutting. These species contributed to the retention of nutrients not simply because of their biomass but also because of higher nutrient concentrations in their tissues. Total biomass and nutrient contents of the ground vegetation exceeded those of the pre-cutting levels. The proportion of ground vegetation biomass and nutrient contents represented by mosses decreased after cutting, while V. myrtillus, although reduced after cutting, remained a marked nutrient sink. The results suggest that H. splendens is the most sensitive species to cutting, but the biomass of P. schreberi, V. myrtillus and V. vitis-idaea return to initial levels soon after clear-cutting as do the nutrient contents of ground vegetation.

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