Fine root longevity and carbon input into soil from below- and aboveground litter in climatically contrasting forests

•Determines fine root turnover of Norway spruce and understorey in two climatically contrasting forests in Finland.•Shows that spruce fine root live longer at the northern site than at the southern site.•Calculates the ratio of below- and aboveground litter production in these and four additional sites.•Relates the ratio of below- and aboveground litter production to site fertility.•Shows that nutrient-poor sites produce relatively more litter belowground than aboveground. The major part of carbon (C) flow into forest soil consists of continually renewed fine roots and aboveground litterfall. We studied the belowground C input from the fine root litter of trees and understorey vegetation in relation to their aboveground litterfall in two Norway spruce (Picea abies L.) stands located in northern and southern Finland. The production of fine roots was estimated by using turnover and biomass data from minirhizotrons and soil cores. The foliage litter production of trees was estimated from litter traps, and that of the understorey vegetation from its annual growth and coverage. Finally, we augmented the data with four spruce plots in Sweden in order to study the above- and belowground litter ratios along latitudinal and soil fertility gradients. The fine root biomass of spruce trees per stand basal area was almost double in the northern site compared to the southern site. Furthermore, spruce fine roots in the north persisted significantly longer (97±2weeks) than spruce roots in the south (89±2weeks) or understorey fine roots at both sites. The annual production of tree foliage litter was higher in the southern stand, but the total amount of litter (including trees and understorey, above- and belowground) was similar at both sites, as was the ratio between the above- and belowground litter production. The role of understorey vegetation was greater in the northern site where it was responsible for 23% and 33% of below- and aboveground litter production, respectively, compared to 11% and 15% in the south. Thus, both below- and aboveground understorey C input is substantial and should be taken into account in ecosystem C cycle models. The regression between the aboveground:belowground litter production-ratio and the C:N-ratio of the organic layer (combined data from Finland and Sweden), showed that the share of belowground litter production increased when site fertility decreased. This shift in the litter production pattern from above- to belowground in the least f