Glacial tills in Lapland vary in texture and hydraulic characteristics. This is because tills are local and the amount and quality of the fine-grained matrix is a result of the type of source rock. Scots pine (Pinus Silvestis L) has been favoured on artificial regeneration for the last forty years. Several cases of seedling diebacks have been occurred on the fine-grained till soils, which formerly have been Norway spruce (Picea abies Karst.) dominated forests. It is suggested that high soil moisture is a critical factor for the survival of pine seedlings. Since summer is short, a crucial question is the duration of the spring saturation of till soil after snowmelt.

Time domain reflectometry (TDR) was used to monitor soil moisture variations on sites with failed Scots pine regeneration. The sites were established on tills derived from fine-grained schists. Dielelectric properties of tills recorded simultaneously with air and soil temperature and precipitation from September 1994 through December 1995.

In 1995 daily mean temperatures rised above 0^oC on mid-May resulting snowmelt by the end of May. However, dielectric coefficient in the surface layers (A0 5cm, A 10cm and BC 30cm) rised from E_till < 6 (partially frozen) to E_till = 13 as early as mid-April. This was true in tills derived from black schist and chlorite amphibole schist. The dielectric change was due to soil temperature rise from -0.5^oC to near 0^oC on mid-April suggesting thermal heat flux from the parent till. On mid-May dielectric coefficient rised dramatically in fine-grained schist tills due to snowmelt. Saturation (E_till > 30) reached in a week and it lasted until third week of June. Soil moisture in fine-grained tills maintained E_till > 13 (equivalent water content, theta > 20 cm³cm^-3) until October. Rainfall effects were clearly visible in schist tills, too. In contrast, snowmelt and rainfalls did not have significant effect on dielectric coefficient of sandy till with E_till < 13 from April through October.

The fine-grained schist tills in Lapland are suggested to be too high in soil moisture for Scots pine. The spring saturation due to low hydraulic conductivity of till lasts the first three weeks of June, the most important time for growth.

Raimo Sutinen
Geological Survey of Finland
P.O.Box 77
SF-96101 Rovaniemi, Finland
Phone: +358-60-3297 303
Fax : +358-60-3297 289