Associations between catchment characteristics (altitude, slope, proportion of peatland, site fertility, total stem volume, and its distribution among the main tree species), climatic drivers (temperature sum, latitude, precipitation and runoff) and atmospheric deposition and concentrations and export of C, N, P and Fe in 21 unmanaged boreal catchments (0.07 to 14 km 2 ) were examined in data sets spanning 3 to 32 years. In the entire data base including catchments from southernmost Finland to the Arctic Circle, N and P concentrations were associated with climatic drivers and deposition, while a catchment characteristic, peatland percentage, refl ecting topography and site fertility was the best predictor for TOC (total organic carbon) and total Fe concentrations and export. In a subgroup of 18 catchments located in similar climatic conditions and low N deposition in eastern Finland (<600 kg N km -2 yr -1 ), peatland percentage accounted for 52-75 % of the export of TOC, TN (total nitrogen), TON (total organic nitrogen), NH 4 -N and total Fe, while slope was the best predictor of TP (total phosphorus) export (r 2 = 0.56, p < 0.001). Furthermore, among these 18 catchments TOC export accounted for as much as 95 % of the variance in TON export, and 61-73 % of the variance in NH 4 -N, total Fe and TP export. Total stem volume of the tree stands and their distribution between tree species and vegetation types were not important predictors for concentrations/export. The peatland percentage of the 21 catchments ranged widely, from 0 to 88 %, with an average value of 34 % as in entire Finland. Consequently, among the 21 catchments there was also a large variability both in average TOC and pH, 1.2-30 mg l -1 and 4.2-7.1, respectively. The average annual TOC export was 6,200 kg km -2 yr -1 , signifi cantly lower values were recorded for the export of TN, TP and total Fe (130, 5 and 280 kg km -2 yr -1 , respectively). N and P export was mainly in an organic form. The TIN (total inorganic N) proportion averaged 13 % and elevated NO 3 -N export occurred only in fertile, high pH sites in southernmost Finland, where stream C/N ratio was lowest. On average, 97 % of the TOC, 94 % of the TN and 79 % of the TP was in a dissolved form.

A two dimensional model, FEMMA, to describe water and nitrogen (N) fluxes within and from a forested first-order catchment (Kangasvaara in Eastern Finland) was constructed by linking the most significant processes affecting the fluxes of water, ammonium, nitrate and dissolved organic nitrogen along a hillslope from the water divide to the stream. The hillslope represents the average flowpath of water in the catchment and the model was used to estimate the N fluxes for a catchment in eastern Finland before and after clear-cutting. The simulated results were in reasonable agreement with the nitrate, dissolved organic N and dissolved total N measurements from the study catchment and with other results in the literature. According to the simulations, the major sinks of N after clear-cutting were immobilisation by soil microbes, uptake by ground vegetation and sorption to soil. These sinks increased downslope from the clear-cut area, indicating the importance of an uncut buffer zone between the stream and the clear-cut area in reducing N exports. The buffer zone retained 76% of the N flux coming from the clear-cut area. Nitrification was a key process in controlling the N export after clear-cutting and N increases were mainly as nitrate. Most of the annual N export took place during the spring flood, when uptake of N by plants was minimal.

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Estimates of increased nutrient export caused by forest clear-cuttings are mostly based on long-term paired catchment studies, where the treatment effect is calculated using pre-treatment dataset regression and post-treatment records of nutrient loads. In these studies uncertainty in the regression between the pre-treatment loads from the control and from the treatment catchments is typically neglected, even though it affects determination of the magnitude and duration of the treatment effect. This uncertainty is described in terms of variance in regression coefficients and residuals. The aim was to study how uncertainty in a pre-treatment dataset is propagated to estimates of the treatment effects using two sets of paired catchment data in eastern Finland, where the total phosphorus loads following forest clear-cutting and site preparation were investigated. The results showed that neglecting the uncertainty in the pre-treatment data did not change the conclusions when the treatment effect was strong enough. However, when the treatment effect was small, neglecting the uncertainty in the pre-treatment dataset easily leads to over-interpretation of the results. The uncertainty in a pre-treatment dataset should be taken into account in the paired catchment studies in order to avoid bias in the estimation of management effects on the loads.