A method for calculating nutrient loads from forestry: Principles and national applications in Finland

The influence o f forests on the hydrology and chemical characteristics o f rivers. lakcs and groundwater i s of paramount importance. Forests help prevent hydrological extremes, such as floods, erosion and eutrophication of surface waters. However, forestry practices, such as cutting, drainage. fertilization and scarification, also affect the hydrology. erosion and leaching of nutrients from catchments (KE:->TTA\1IES 1998). The forestry practices adopted, the proportional size of the treatcd area, and the natural characteristics at the local level will al! affect the extent of the impact of forestry on water bodies. Separating man-induced loading (point and nonpoint) from natural leaching is necessary to monitor progress in implementing current environmental policies. This task is especially complicated in the case of forestry practices where loads are !ow and temporary and the si tes affected change from year to ycar. The main problems arc adapting results from small, thoroughly studied catchmcnts to the riverbasin scale. Vcrifying the accuracy of calculations made of forestry loadings together with unknown natural !eaching is also uncertain. A third and very practical problem i s the laek o f relevant da ta an d statistics on forestry practices. although satcllite images can help considerably (LEPISTb et al. 200 l). The ai m o f this study was to devclop methods for calculating the potential, long-term phosphorus (P) and nitrogen (N) loads from forestry at national, river-basin and coastal-region scales and to cxamine different scenarios. "Potential" here refers to the loads entering waters as a result o f forestry practices without any calculations of the loss in rivers and lakes.