Fertilization to increase and sustain tree growth in coniferous stands in Sweden

Intensified forest management practices, together with the deposition of air pollutants, have considerable potential implications for silviculture. These factors influence N availability in soil and also cause soil acidification, involving reductions in the levels of plant-available base cations. The aim of the work described in this thesis was to evaluate the need for compensatory additions of other nutrients apart from N, from a forest yield perspective, after whole-tree harvesting and N fertilization, focusing on Swedish conditions but including also some studies from Finland and Norway. All studies were undertaken in field experiments on mineral soils, representing a wide range of site conditions. Stem growth and nutrient concentrations in needles were assessed in closedcanopy Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) stands. Whole-tree harvesting in thinnings reduced stem growth (by 5-10% on average) for at least 10 years. These growth reductions were counteracted fully by adding inorganic Nfertilizer and, thus, there was no need for additions of other nutrients. The stem growth responses to additions of wood ash without N were small and variable, and not statistically significant at any of the studied experimental sites. Experiments at a large set of experimental sites previously subjected to repeated N fertilizations showed that the addition of nutrients other than N generally had negligible effects on the short-term (4-15 years) growth response. Furthermore, it was concluded that N fertilization, with the addition of B, generally did not cause any serious long-term (20-30 years) nutrient deficiencies in trees, even after substantial N additions