The effect of drainage on fine-root biomass, production, and turnover in hemiboreal old-growth forests on organic soils

Information on the capacity of organic soils to capture and store carbon in old-growth forests in the hemiboreal forest zone is scarce and fragmented. However, fine-root data can provide valuable insights into soil carbon fluxes. Thus, the aim of the current study was to provide estimates of the fine-root biomass (FRB), fine-root production (FRP), and fine-root turnover (FRT) rate by tree species and other functional groups in old-growth (stand age 131-179 years) forests on mesotrophic organic soils dominated by Scots pine (Pinus sylvestris L.), with and without the effects of forest drainage. The sequential soil coring method was used to estimate the FRB and FRP. The total FRB was significantly higher in the undrained sites (6.8 +/- 0.3 t ha-1) than in the drained sites (3.97 +/- 0.1 t ha-1). The FRB of Scots pine in the undrained forest was significantly higher (1.7 +/- 0.1 t ha-1) than in the drained forest (0.5 +/- 0.1 t ha-1), supporting an extensive foraging strategy. The significantly higher mean FRB of Norway spruce (Picea abies [L.] Karst.) (1.4 +/- 0.1 t ha-1) in the drained sites than the undrained sites (0.7 +/- 0.2 t ha-1) can be explained by there being a higher proportion of spruce in the stand compositions, thus a higher standing volume of this species and an increased FRB. The FRB of dwarf shrubs (2.43 +/- 0.2 t ha-1) formed the largest part of the total FRB in the undrained sites and the second largest (1.16 +/- 0.1 t ha-1), following Norway spruce, in the drained sites. The total FRP was similar between the undrained (2.05 +/- 0.31 t ha-1 year-1) and drained (1.82 +/- 0.26 t ha-1 year-1) stands. However, considerable variability in the FRP was observed between different sites of the same forest site type. The FRT rate of Scots pine was twice as high in the drained sites than the undrained sites, suggesting faster nutrient and carbon input into the drained soil compared with the undrained soil. Estimates of FRB, FRP, and FRT rates for different functional groups can be used in carbon-cycle modeling and in further calculations to estimate the carbon budget (balance) in forests on organic soils.