3. Results
3.1. Biomass Production
There were no statistically significant differences in biomass production during the ten-year period between the mulch and control treatments, although the mean biomasses of both pine and birch tended to be slightly higher on the mulched plots (Figure 1).
3.2. Metal Concentrations
3.2.1. Foliage
The Cu and Ni concentrations of pine needles tended to increase with age but no statistically significant differences could be found between the treatments (Table 1). In case of birch the Cu and Ni concentrations of the leaves were slightly higher in the mulch treatment, but the differences were not statistically significant (Table 1).
3.2.2. Bark
The highest Cu and Ni concentrations of all the sampled compartments were those of pine bark from the control plots (Figure 2). The t-test showed that the between- treatment difference was statistically significant for the Cu concentrations (t = 4.039; p = 0.016).
The mean Cu concentration of the birch bark was slightly higher on the mulch plots, but the difference was not statistically significant. The corresponding Ni con-centrations were about the same for both treatments (Fi-gure 2).