Litter decomposition rates in relation to litter stocks in boreal coniferous forests along climatic and soil fertility gradients

The contribution of trees, understorey dwarf shrubs, and mosses in the soil organic matter accumulation in boreal forest soils is still poorly understood. To determine the relationship between litter decomposition and accumulation, we conducted litter decomposition trials and quantified the litter components accumulated on the forest floor in south and north boreal forests at two site types, sub-xeric and mesic forests. Litter used in the litter decomposition trial and the accumulated litter were also chemically characterized. The decomposition rates of bilberry, needle and the moss litter analyzed as accumulated dry weight and C loss did not differ along the climatic and fertility gradients. While the chemical quality of the tree, dwarf shrub, and moss litter differed only slightly by location in the north or the south, there were major differences in the accumulated litter stocks on the forest floor. Litter produced by the trees constituted the most important litter C stock in the south boreal forests, whereas the litter produced by mosses in the north formed as large or an even larger stock than the litter produced by trees. Also the quantity of accumulated dwarf shrub litter was higher in the north than south. According to our results, the differences in the quality and quantity of soil OM stocks between the south and north boreal forests is largely a result of quantitative difference in the litter input by trees vs. mosses and dwarf shrubs, rather than differences in the litter quality or decomposition rates between the north and the south.

Research highlights▶ Decomposition rates did not differ along boreal climatic and fertility gradients. ▶ Accumulated litter stocks varied substantially between north and south boreal forests. ▶ Tree litter constituted the most important litter carbon stock in south boreal forests. ▶ In north boreal forests moss litter carbon stock equaled or exceeded tree litter stock.