LAUSR

Abstract Information on pathways and rates of decomposition of coarse woody debris (CWD) from one of the dominant boreal tree species – Siberian larch (Larix sibirica Ledeb.) – is scarce. We examined presence and distribution of internal decay in its living trees and decomposition rates in an 88-year chronosequence of logs in an old-growth mesic mixed European boreal forest. Fifty-two percent of the trees started decaying prior to death. Butt or stem rot was caused by white- and brown-rot biotrophic fungi in almost equal proportions. Those trees lost on average 29% of their initial wood mass. The mean decomposition rate of wood as related to initial conditions was 0.007 yr−1 for wood with initial biotrophic decay and 0.010 yr−1 for wood with saprotrophic decay. Bark lost its mass and volume faster – at a rate of 0.056 yr−1. The rate of carbon (C) loss for the whole stem was 0.024 yr−1 in wood affected by saprotrophic fungi and 0.007 yr−1 when the tree was affected by biotrophic fungi. When calculating C losses due to decomposition, separating bark and wood with different C concentrations and decomposition patterns as well as the rate of biotrophic and saprotrophic decay enables us to refine estimates of the role of Siberian larch CWD in C cycling. Further research is still needed to resolve uncertainties around the role of fungal communities and the origins and decomposition rates of butt rots and stem rots in living trees of Siberian larch, as well as the distribution patterns of these fungi and decomposition processes in individual trees and at the ecosystem level.