Coarse woody debris and stand characteristics in mature managed and old-growth boreal mesic forests in southern Finland

Abstract

Stand structure was studied on thirty 1-ha plots in mature managed and old-growth forest stands dominated by Norway spruce ( Picea abies L. Karst.) in southern Finland. The forests were classified into three categories (10 plots in each category) according to the age of the dominant spruces and signs of previous cutting: (1) mature managed (95–118 years), (2) overmature managed (124–145 years) and (3) old-growth (129–198 years). The average volume of the living trees was 299 m 3/ha in mature, 331 m 3/ha in overmature and 396 m 3/ha in old-growth stands. The clearest difference in the living stand characteristics between the categories was in the numbers of large pines ( Pinus sylvestris L.) and deciduous trees, which were both ca. 10 times more frequent in old-growth than in mature stands, and intermediate in overmature stands. The average volume of coarse woody debris (CWD) with a minimum diameter of 5 cm was 14 m 3/ha in mature (range 2–28 m 3/ha), 22 m 3/ha in overmature (7–38 m 3/ha) and 111 m 3/ha (70–184 m 3/ha) in old-growth stands. Stand age and basal area of the cut stumps together explained over 70% of the variation in the volume of CWD. In old-growth forests, ca. 70% of the volume of CWD consisted of logs, 23% of intact dead standing trees and 7% of broken snags. Most of the volume of CWD belonged to the 20–29 and 30–39 cm diameter classes, but the mean proportion of large trunks ≥40 cm was also considerable, 24%. In managed forests man-made CWD, consisting of cut stumps, bolts and logging residues, accounted for an average of 25% of the total volume of CWD. In many cases most of the CWD volume belonged to the small diameter classes, 5–9 and 10–19 cm, and the mean proportion of large trunks was only 8%. We suggest that, considering the low proportion of both old-growth and overmature forests in southern Finland and the long time required for old-growth characteristics to develop, the most efficient short-term management strategy to increase structural diversity and old-growth attributes in managed forests would be to apply harvesting methods retaining the old-growth characteristics (i.e. large living trees, snags and logs) that already exist in overmature and mature stands.