The study determines the differences in the chemistry of small headwater streams located in areas with different plant communities in Finnish Lapland. Water samples from 76 catchments with four different plant communities were collected and analyzed. The statistical analyses of water temperature, water specific electric conductivity and the concentration of main ions, revealed the greatest variation in ion concentrations related to rock weathering: Ca2+, Mg2+, HCO3– in the studied catchments. Enrichment of water with these ions occurred in catchments representing forest communities with thick soils, especially in the boreal forest community. This suggests that future changes in the northern treeline may affect the surface water chemistry depending on the direction of changes. In case of the expansion of the treeline to the north due to climate warming, an increase in water mineralization and a shift of hydrochemical facies towards a Ca-Mg-HCO3-type can be expected. On the other hand, the regression of the northern treeline could be expected to result in a reduction of nutrient pools and leaching them into streams. Eventually, it could result in the depletion of soils and the dominance of atmosphere-derived ions in waters, and the hydrochemical facies will shift towards Na-Cl-type.