Stump decomposition rates after clear-felling with and without prescribed burning in southern and northern boreal forests in Finland

Abstract

The high importance of stumps for biodiversity, carbon and nutrient cycling requires narrowing the uncertainties in their decomposition rate assessment. We estimated the stump decomposition rates of the main European boreal tree species along a chronosequence of 0-, 5-, 10-, 20-, 30-, 40- and 50-year-old clear-felled sites with and without prescribed burning in southern and northern Finland. Estimation was done based on the bulk density and mass loss of above- and below ground wood and bark. The factors explaining the variation in mass loss during decomposition in decreasing order of importance were: stump part > tree species > vegetation zone > stump size > prescribed burning. In northern sites, stump turn-over time ( t 95) was 153, 104 and 100 years for Scots pine ( Pinus sylvestris), Norway spruce ( Picea abies) and birch ( Betula sp.), respectively. The decomposition rates of above- and below ground wood did not differ from each other. In southern sites, where only the above-ground parts of stumps were studied, the turn-over time was 61, 56 and 43 years for pine, spruce and birch, respectively. In southern sites, the wood and bark of all tree species decomposed faster compared to those in northern sites, except for charred pine wood. Burning slowed down the decomposition of un-fragmented parts of pine and spruce bark in all sites and pine wood in southern sites (mean bark cover 32%). It also facilitated the decomposition of birch wood in northern sites (mean bark cover 78%) and increased bark fragmentation. Decomposition of birch bark in northern sites and birch wood in southern sites was slower for larger diameter stumps. Pine and spruce bark as well as birch bark in southern sites showed the opposite tendency since the size effect was dominated by the effects of fragmentation and burning. The bark of pine and spruce decomposed faster than the wood, whereas birch bark decomposed more slowly than birch wood. Bark fragmentation accelerated mass loss in all cases except for birch stumps in northern sites. Decomposition factors are interrelated and their relative importance should be taken into account when up-scaling the species-specific decomposition rate constants. Refinements to decomposition rate estimates could aid in management scenarios to maximize carbon and nutrient storage and ensure the availability of stumps for maintaining biodiversity in intensively managed forests.