Soil hydrological properties and conditions, site preparation, and the long-term performance of planted Scots pine (<em>Pinus sylvestris</em> L.) on upland forest sites in Finnish Lapland

Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) forests, which differ from each other both ecologically and economically, predominate in Finnish Lapland. The need to study the effect of both soil factors and site preparation on the performance of planted Scots pine has increased due to the problems encountered in reforestation, especially on mesic and moist, formerly spruce-dominated sites. This thesis examines the soil hydrological properties and conditions, and related physical properties, on 10 pineand 10 sprucedominated upland forest sites in Finnish Lapland. The long-term effects of site preparation on soil factors are also studied. Finally, the effects of both site preparation and reforestation methods, as well as soil hydrological factors, on the long-term performance of planted Scots pine are summarized. In general, the soil properties were comparable to those reported earlier for till soils in Finnish Lapland and Fennoscandia. The results showed that pine and spruce sites in Lapland have significantly different soil physical properties. Under field capacity or wetter soil moisture conditions, planted pines presumably suffer from excessive soil water and poor soil aeration on most of the originally spruce sites, but not on the pine sites. The studies also suggested that the changes in soil physical properties and organic matter content due to site preparation may affect the soil water regime and, as a result, the prerequisites for forest growth for more than two decades after site preparation. The air-filled porosity at field capacity (–10 kPa) and in situ in the ploughed ridges were significantly higher, and the bulk density and in situ soil water content lower than in the untreated intermediate areas. There was high variation in the survival and mean height of the planted pines. The study suggested that on formerly spruce-dominated sites, pine survival is the lowest on sites that dry out slowly after rainfall events, and that height growth is the fastest on soils that reach favourable aeration conditions for root growth rapidly after saturation, and/or where the average air-filled porosity near field capacity is large enough for good root growth. Survival, but not mean height, could be enhanced by employing intensive site preparation methods like ploughing instead of lighter site preparation methods on spruce sites. From the point of view of survival, there seems to be a relatively broad assortment of site preparation methods suitable for coarser-textured pine sites. Site preparation methods affecting the nutrient status of the soil, such as ploughing and especially prescribed burning, seem to enhance the height growth of Scots pine over several decades after reforestation on formerly pine-dominated sites. The use of soil water content in situ as the sole criterion for sites suitable for pine reforestation was tested and found to be a relatively uncertain parameter. However, the thesis identified new potential soil variables, such as the water retention curve parameters α and n, affecting either the long-term survival or height growth of planted Scots pine. The use of these variables as criteria for sites suitable for pine should be tested using other data in the future.