Amount and Fall Rates of Snags in the Primeval Middle Boreal Spruce Forests of the Vepssky Forest Reserve

In view of global climate change and biodiversity decline, long-term research in primeval forests escaped from timber harvesting is especially important. We aimed at analysing the dynamics of number and volume of standing dead trees (snags), as well as the patterns of tree mortality and the longevity of snags in primeval middle-boreal spruce stands. The study is based on the database of tree stand inventories performed on the eleven permanent sample plots (SPs) for 25–46 years. The volume of snags during the period from 1971 to 2019 varied from 0.4 to 164 m3 ha–1, with an average annual turnover rate (the period from the tree death to snag fall) from 0.07 to 32.7 m3 ha–1 year–1. In several biogeocoenoses, the peaks in tree mortality through decline after windthrows were observed, resulting in the stock of coarse woody debris being comparable to the stock of a forest stand. The rate and the mode of tree mortality, and the species distribution of snags, depend on the soil moisture regime, tree species composition and age structure of the forest stand. Before death, some trees were weakened for a period up to 46 years. The time of tree weakening before death (the period from the moment when mechanical damage, traces of insect activity, diseases, etc. were noted in the tree, until the moment when a tree died) did not have a significant effect on the duration of snag longevity. The duration of snag longevity varies from 2 to 30 or more years, depending on interrelated factors such as tree species and size, moisture conditions, tree species composition of forest stand, and the successional status of the biogeocoenosis.