Growth of genetically improved stands of Norway spruce, Scots pine and loblolly pine

Genetically improved material of Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.) and loblolly pine (Pinus taeda L.) is commonly used for forest regeneration in Sweden and the south-east of the USA. The potential genetic gain in terms of volume growth is, at present, in the range 10-20 % for Norway spruce and Scots pine and similar or even greater for loblolly pine, and will increase in the future. As a basis for optimizing management, it is necessary to have accurate growth and yield forecasts that take genetic improvement into consideration. The objective of this research was to analyze and model the effects of tree improvement on tree growth and tree slenderness (height-diameter ratio). Results from two field trials of Scots pine and loblolly pine showed that genetics and spacing affected both tree growth and tree slenderness, indicating that growth models need to consider genetics as well as competition to predict the development of various genetic entries correctly (Paper I, II). Paper III described the development of a height growth model for unimproved material and this was subsequently evaluated for various genetic entries of Norway spruce and Scots pine. The analysis indicated that the model predicted the height development relatively well for genetically improved Norway spruce. However, for Scots pine, the model needed to be modified. By incorporating a genetic component, the prediction errors were significantly reduced for Scots pine. The phenotypic plus-tree selection conducted during the 1970s and 1980s was found to have little impact on the height-diameter ratios for Norway spruce and Scots pine in Sweden (Paper IV). Small differences in height-diameter relationships were also found for loblolly pine seedling entries in the south-eastern USA (Paper II). However, the moderate heritability of height-diameter ratio for Norway spruce and Scots pine indicates that breeding can modify such ratios (Paper IV). Selecting for diameter only would result in less slender stems of both species, while selecting for height only would result in less slender Norway spruce trees and more slender Scots pine trees