LONG-TERM EFFECT OF PLOUGHING ON SOIL HYDROLOGY IN NORTHERN FINLAND

Abstract

Intensive site preparation methods such as ploughing and mounding have been widely used on reforestation areas in northern Finland due to the high soil moisture and low temperature. Especially on sites formerly occupied by Norway spruce the prolonged saturation of top mineral soil after snowmelt and during rainy seasons may frequently threaten the success of reforestation. The long-term effect of site preparation on soil volumetric water content (SWC) was studied in ploughed ridges and the untreated intermediate areas of prescribed burning, patch scarification, disk trenching and ploughing plots in eight reforestation experiments two decades later. SWC in the mineral topsoil (0–15 cm) was measured eight times during two summers on 96 plots using the TDR method and 600 probes installed vertically under humus layer. In addition, the differences in the soil physical properties between the ploughed ridges and intermediate areas were investigated. The results showed no significant lateral effects of ploughed ditches (spacing 5 m) on the top mineral soil hydrology of the intermediate areas two decades after site preparation. SWC in the intermediate areas of ploughed plots was similar to that of the plots treated with the lighter site preparation methods. The soil water content at field capacity (-10 kPa) and the topographic wetness index showed significant effects on SWC. The soil bulk density and measured SWC were significantly lower, and the total porosity, air-filled porosity at field capacity and measured air-filled porosity higher in the ploughed ridges than in the adjacent intermediate areas. The results suggest that ploughing has a long-term favorable effect on the soil aeration in ploughed ridges. This may be crucial during the rainy growing seasons and explain the lower seedling mortality found on the regeneration areas treated with ploughing compared with light site preparation methods.