Dielectric constant time stability of glacial till at a clear-cut site

Temporal stability of spatial patterns (time stability) of field measured soil water content has been attributed to soil texture and topographic curvature in agricultural soils, but limited information is available on time stability of soil water content of forest soils composed on non-stratified glacial tills. We studied time stability of soil water regimes by measuring soil dielectric (ε) properties of fine-grained glacial till (0–15 cm) at 1-m-spacing along a 110-m-long transect with TDR twice a week through one growing season, from the 31st of May to the 1st of October 2001, in Finnish Lapland. The formerly Norway spruce (Picea abies L. Karst.) covered site was clear-cut, subsequently burnt over and mechanically prepared with a disk drencher, and then regenerated to Scots pine (Pinus sylvestris L.). Ten years after silvicultural treatments the measurements of soil ε, analyzed with Spearman's rank correlation coefficients, showed significant time stability (rs = 0.83–0.93; P < 0.01) of soil water content for the whole season. Precipitation in July (122 mm) was twice as high as the average (1982–1999), but a persistent pattern of the soil ε occurred during both the drying phase which lasted until the end of June and the wetting phase which took until the end in September. Soil texture and topographic curvature were found to be secondary to coarse fragment content in contributing to spatial persistence of the soil ε. The minute recovery of vegetation after forestry practices also demonstrated that evapotranspiration and root water uptake played a negligible role in the soil water dynamics. Our results imply that the time stability of the spatial pattern of soil water regimes in glacial tills largely accounts for the composition of tree species in northern boreal forests and should be taken into consideration when artificial regeneration is contemplated in the harsh climate of Lapland.