Effects of Forest Practices on Water Resources Recharge in the Boreal Climate

The aim of this study was to evaluate effects of forestry practices on water resources in the middle-boreal zone of central Finland. The long-term research was performed at Kivesvaara experimental plots (64∘28′ N, 27∘33′ E) and Oijusluoma small catchments (65∘50′ N, 29∘0′ E), with a detailed snow observation at Paljakka (65∘26′ N, 26∘26′ E). Forestry practices included clear-cut, shelterwood and seed cutting with different techniques of soil preparation and reforestation. The clear-cut (particularly by the total biomass harvest including stump removal) has increased the water yield (a positive feedback). But, on the other hand, it has caused a drop in retention and lag-time, and an increase of peak-flows, as well as increasing mobility of nitrogen and phosphorus. Thus, the clear-cut of mature spruce stands (Picea abies) increased the groundwater recharge by 63 mm (i.e., 24%), and the total biomass harvest of one-half of the experimental catchment increased the water yield by 44 mm (i.e., 15%) during three years after the harvest. A significant increase in contents of nitrogen persisted in groundwaters more than 25 years after the clear-cut, and 10 years after the natural regeneration (shelterwood or seed cutting). During the monitored period of 28 years, the total output of nitrogen from subsequent forest plantations ranged from 12 to 21 tons per hectare (i.e., exceeding 5–9 times the output from control mature spruce stands). By natural regeneration, the ten-year mean load of nitrogen exceeded 2.5–5 times the load at control plots. After the total biomass harvest (a whole tree and stump removal), the mean annual outflow of nitrogen and phosphorus was 5.4 and 0.11 k g h a −1 (exceeding 2.5 and 1.8 times the pre-treatment stream-flow). These results confirmed the hypothesis of a significant impact of forest practices on water resource control in the boreal forest biome.