Tree diameter distributions in natural and managed old Pinus sylvestris-dominated forests

The characteristics and variability of tree diameter distributions were studied in natural and managed old Pinus sylvestris L.-dominated forests in the middle boreal vegetation zone of eastern Fennoscandia. The managed forest stands in the study were located in Häme in southwestern Finland, and the natural stands were located in the Vienansalo wilderness area in northwestern Russia. In natural stands, the mean tree density (tree height > 1.3 m) was significantly higher compared to managed stands, but the two stand categories did not differ in their total basal areas. The shape of the pooled diameter distribution was descending in natural stands and bimodal in managed stands. Within the two stand categories, there was considerable stand-to-stand variation in the shapes of diameter distributions. In general, the range in diameters was larger in natural stands compared to managed ones. This was because large living trees (diameter > 40 cm) were common in natural stands, while in managed stands they were almost totally lacking. Natural stands where the last forest fire occurred 50-60 years ago differed clearly from stands with a longer time lapse since the last fire, particularly in having a higher density of small trees. Diameter distributions were characterized with indices, such as Shannon and Siipilehto, and the Gini coefficient. Taken separately, these indices poorly described and segregated diameter distributions, but with several indices it was possible to characterize the differences between diameter distributions. Both natural and managed stands had heterogeneous diameter distributions, but managed stands lacked some structural features, such as large trees and trees with structural diversity characteristics, which are likely to reduce their ability to sustain high biodiversity. Our results demonstrate that high variability of tree sizes and stand structures is an important diversity feature of naturally dynamic Pinus-dominated forests in Fennoscandia.