Tree species mixtures - a common feature of southern Swedish forests

Summary The proportions of forests in southern Sweden with the most common single or mixed species compositions were determined using the data provided by the Swedish National Forest Inventory. Forests including a second tree species with a basal area of at least 10 per cent, in addition to the most abundant species, were defined as mixed forests. The most common compositions were spruce, pine/spruce, pine and spruce/birch. Overall, mixed forest was more common than single-species forest, and approximately two-thirds of both spruce-dominated and pine-dominated forests included a sufficient proportion of other tree species to be classified as mixed forests, according to the above definition. Differences in composition related to age, site and ownership classes were analysed. Hypotheses that pine/birch mixtures only occur in early-successional stages and that spruce/birch mixtures occur mainly on wet sites were rejected. In fact, 90 per cent of spruce/birch mixtures were found on mesic and moist-mesic sites. The hypothesis that pine/ spruce mixtures are more frequent in private forest was also rejected. In conclusion, the study found high proportions of mixed forest across age and site classes and provides background information for assessing the potential to refine silvicultural methods applicable, e.g., to young spruce/birch or very old pine/spruce stands.