Differences in surface–subsurface temperature regimes and soil moisture between vegetation types across a boreal-alpine ecotone

The boreal-alpine ecotone is distinguished by abrupt vegetation shifts along short altitudinal gradients, with climate warming driving treeline shifts upwards and northwards. In this study, we examine the hypothesis that distinct vegetation types present within this ecotone exhibit significant differences in surface–subsurface temperature regimes, as well as in soil moisture levels. This was done by measuring subsurface, surface and above-surface temperatures and soil moisture along a boreal-alpine ecotone during the growing season at Dagalifjell, southern Norway, for four different vegetation types: 1) pine-dominated coniferous forest, 2) mountain birch-dominated forest line, 3) shrubland and 4) lichen heath. Furthermore, the one-dimensional version of the Spatial Forest Hydrology (SpaFHy) model was employed to explore the potential of simulating soil moisture dynamics for typical boreal-alpine vegetation using openly available remote sensing data. The results revealed a decline in subsurface temperatures along the elevation-dependent vegetation gradient from coniferous forest (7.47 ± 0.66 °C) to lichen heath (5.56 ± 0.68 °C), with statistically significant differences ( p < 0.05) across all vegetation types. The highest recorded soil moisture values were observed in shrubland (24.75 ± 13.55 VWC%, volumetric water content) and in the forest line (23.97 ± 6.86 VWC%), whereas the lowest values were found in the coniferous forest (15.22 ± 11.63 VWC%). Notably, significant differences in soil moisture were detected among the four vegetation types, except for the shrubland and forest line. The SpaFHy model successfully replicated the general trends in soil moisture; however, it fell short in accurately capturing daily fluctuations and variations within individual plots. This indicates that further development is needed for better representation of fine-scale soil moisture variations typically observed in boreal-alpine environments. Our findings highlight distinct patterns in soil thermal and moisture regimes across different vegetation types, underscoring the need to understand these interactions when assessing potential vegetation shifts in ecotonal environments under climate change.