50 YEARS OF CHANGE IN A SWEDISH BOREAL OLD-GROWTH PICEA-ABIES FOREST

Changes in the tree layer (> 1.3 m) and sapling layer (< 1.3 m, including seedlings) of a Swedish boreal old-growth Picea abies (Norway spruce) forest from the 1930s to the 1980s were studied in permanent plots. The plots were established in 1938-1939 and re-analysed in 1983-1988. Regeneration, mortality, turnover rate in the tree layer and amount of decomposing logs as well as the time required for complete decomposition of logs were investigated using the detailed data from the 1930s. Ca. 25 % of the trees present during the first analysis were no longer alive. This mortality was balanced by recruitment from the sapling layer. The rate of mortality suggests a turnover time for the tree layer of ca. 200 yr. The number of spruces in the sapling layer has increased by ca. 85 %, hypothetically in response to an increase in amount of decomposing wood that can serve as nurse logs and stumps. The mean time for total decomposition was calculated as ca. 200 yr. Spruce regeneration on logs does not occur until the log is at least ca. 50 yr old. The survival pattern in the sapling layer suggests a high mortality rate at the seedling stage (less-than-or-equal-to 1 yr) and a low mortality rate at the sapling stage. In conclusion, it is suggested that the amount of coarse woody debris available for regeneration, the occurrence of seedlings, and seedling mortality constitute concurrent factors through which climatic fluctuations, in a long-term perspective, direct stand recruitment and density. As a consequence, these boreal forests will be kept in a dynamical equilibrium.