Tree mortality in the dynamics and management of uneven-aged Norway spruce stands in southern Finland

This study focused on tree mortality in spruce-dominated stands managed using the single-tree selection method in southern Finland. Together with regeneration and tree growth, mortality is one of the basic elements of the stand structure and dynamics in selection stands. The study was based on data acquired from a set of 20 permanent experimental plots monitored with repeated measurements for 20 years. The average mortality in the number of stems (N) was 4.45 trees ha(-1)a(-1), in basal area (G) 0.07 m(2) ha(-1)a(-1), and in stemwood volume (V) 0.56 m(3)ha(-1)a(-1). In relative terms it was 0.50% ofN, 0.30% ofGand 0.27% ofV, respectively. Wind and snow were the most common causes of mortality, while deaths by biotic causes (mammals, insects, pathogens) were extremely rare. Some 6-10% of the total loss in the number of stems and volume was attributable to the loss or removal of trees that sustained serious damage in harvesting. Most of the mortality occurred in the smallest diameter classes of up to 20 cm. Such a high mortality among small trees can have an adverse influence on the sustainability of selection structures if not successfully checked in harvesting and management.