Snowmelt Infiltration Through Partially Frozen Soil in Finnish Lapland

Abstract

Snowmelt timing is critical for tree growth in high latitudes, but threshold conditions with respect to root-zone soil water availability in spring is not well understood. In spring 2008 we measured snowpack thickness, apparent snow water (ASW), air and soil temperature, as well as soil water content (SWC) in Mustavaara fell (67˚59’N, 24˚06’E), Finnish Lapland. Three monitoring stations were established on a fell (mountain shaped by Pleistocene glaciations) gradient: forest at 394 m.; forest line at 447 m and tree line at 480 m a.s.l. Forest hosts stands Norway spruce (Picea abies) and are underlain by Haplic Podzol developed on glacial tills. Before the onset of the snowmelt (on 16 th of April) the snowpack was at the thickest, 104 cm, in the forest line as compared to those of 85 cm in forest, and in 45 cm in treeline. Soil temperature (20-cm-depth) remained below 0˚C until the 1 st of June. Due to air temperature rise notably above 0˚C on 27 th of April, the onset of snowmelt occurred throughout the elevation gradient. At all stations (treeline, forest line, and forest) the effect of snowmelt was seen as the rise of ASW (29 th of April) two days later and the rise of SWC (30 th of April) three days later, hence suggesting that snowmelt water infiltrate rather unimpeded through the soil. The maximum SWC was observed to be rather simultaneous with snow disappearance in the treeline, whereas in the forest and forest line snow disappeared almost a month later than maximum SWC. We contend that snowmelt infiltration through partially frozen soil significantly contributes to ground water reserves and soil water availability, rather than soil temperature, is pivotal for the start of height increment of trees in northern boreal conditions.