Soil water drives distribution of northern boreal conifers Picea abies and Pinus sylvestris

•The SWC > 0.24 m3m−3 is the edaphic constraint for P. sylvestris.•P. abies is occupying wet-mesic tills derived from mafic rocks.•P. abies is absent on tills derived from felsic granulite.•Climate-driven factors cannot explain the divergence in the conifer forest lines. Considering the climate change the role of edaphic drivers for the distribution of cold-hardy conifers, Norway spruce (Picea abies (L.) Karst.), and Scots pine (Pinus sylvestris L.) is crucial. Even though both species reach latitude 70oN, P. abies forest line is located at exceptionally lower latitudes in Fennoscandia compared to P. sylvestris tree line. Hence the divergence in mafic (basic) and felsic (acidic) lithologies and thereby physicochemical properties of parent glacial tills may play a critical role in tree species distributions. The objective of this study was to explore soil water content (SWC) and nutrient regimes in Finnish Lapland by transecting the main lithological units; mafic Central Lapland Greenstone Belt (CLGB) in the south and felsic Lapland Granulite Belt (LGB) in the north. Eight one ha in size plots were sampled at 10 m intervals for soil chemistry and texture and measured for SWC, gamma-ray, electrical conductivity, temperature and matric potential. In binary decision tree (DT) analysis, soil physics explained the growth sites of P. abies and P. sylvestris with 97% accuracy, the most important variable being SWC. The constraint for P. sylvestris was SWC > 0.24 m3m−3. Based to the soil chemistry DT, 95.4% of the P. abies and P. sylvestris growth sites were correctly classified. The most important variables were Cr, Ti, Mn and P. Distribution of conifers follows lithology, such that P. abies occupies mesic-wet tills derived from ultramafic and mafic rocks of the CLGB, whereas P. sylvestris thrives only on dry and coarse-textured, often granitoid tills. We argue that dry and nutrient-poor tills of felsic garnet gneisses of the LGB constitute a dispersal barrier for P. abies. The above tree species specific constraints provide guidance for future dispersal models.