Soil-Driven Timberline of Spruce (Picea abies) in Tanaelv Belt–Lapland Granulite Transition, Finland

Abstract

Climate-driven northern limits of cold-hardy conifers, Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.), reach latitude 70°N in Fennoscandia. Spruce timberline is located at significantly lower latitudes than pine timberline, but factors contributing to this exceptional pattern have remained obscure. We assessed soil-water and soil-nutrient regimes across the spruce timberline in Finnish Lapland through the use of terrestrial electromagnetic (EM) measurements. These included soil dielectric permittivity (ε), which is positively correlated to soil-water content (θv); gamma radiation (γ), which is negatively correlated to soil-water content (θv); and electrical conductivity (σa), which is a measure of solute content of a soil. The soil EM data were acquired at 15 sites carrying mature (older than 150 yr) stands dominated by either spruce or pine; the sites fall along a 50-km-long transect that crosses the spruce timberline. The correlation and artificial neural network (ANN) analyses revealed that the soil-moisture and solute content were significantly higher in glacial tills of Norway spruce stands compared to those of stands formed by Scots pine. The stands dominated by spruce were associated with mesic-wet and nutrient-rich tills derived from mafic amphibole and hornblende gneisses of the Tanaelv Belt. Dry and nutrient-poor till derived from felsic garnet gneisses of the Lapland Granulite appeared to constitute an edaphic dispersal barrier for Norway spruce.