Below-ground carbon (C) allocation studies in boreal forests are scarce and have high levels of uncertainty in ecological and modelling studies. The uncertainty of fine root turnover and the heterogeneity of fine root distribution are the main barriers to quantifying the below-ground C allocation. Unravelling the below-ground C litter inputs of boreal forests, including fine roots and ectomycorrhizal (EcM) mycelia, could provide fundamental information for quantifying biogeochemical cycles. This thesis evaluated the below- and above-ground litter C inputs along a site type gradient of Scots pine (Pinus sylvestris) sites in southern Finland, and a distinct silver birch (Betula pendula) site in northern Finland. Furthermore, the Scots pine pioneer/fibrous root growth phenology was observed and compared with the modelled growth of the above-ground organs (predicted by the dynamic CASSIA model) in southern Finland in 2018, when there was an unusual summer drought. Fine root turnover was observed by minirhizotrons (MR) and the root growth phenology was observed by flat-bed scanners, both of which direct methods are known to provide reliable results in root research.
Based on the daily root growth monitoring experiments, we found that the timing of intensive root growth lagged behind the growth of above-ground organs (shoots, secondary xylem, buds, and needles). Interestingly, we found a clear root growth increase while the needle growth decreased, which may have been caused by a shift of non-structural carbohydrates (NSC) from above-ground to below-ground. The low temperature and summer droughts may have constrained the fibrous root growth, but not influenced the pioneer root growth, which indicates that pioneer roots could be more tolerant to severe climate variations.
Increasing nutrient availability could clearly increase the above-ground C allocation but not the below-ground allocation. Our study sites CT, VT, MT were named after Cajander’s Finnish site type theory in the order of increasing nutrient availability. Our study found that the nutrient-poor site CT tends to have significantly higher fine root longevity and biomass than the relatively nutrient-rich sites VT and MT. Fine roots could allocate more biomass below the ground and survive longer in nutrient-poor conditions. The distal tips of tree roots reflect the forest foraging ability, as shown by the fact that EcM root tips per basal area and fine root biomass per basal area both increased gradually from nutrient-rich to nutrient-poor sites and from low to high latitudes. Overall, we found that below-ground litter accounts for 21-58% of total litter inputs in boreal forests. This finding indicates that the C allocation pattern could be a specific effect of species and latitudes. The Scots pine in the southern sites allocated up to one third of total litter inputs below the ground but the northern silver birch allocated over half of total litter inputs below the ground.
In conclusion, we suggest that the growth phenology and litter inputs of below- and above-ground organs should always be observed and quantified together. Understory species contributed significantly to litter C inputs which should not be neglected in boreal forests.
Moreover, future studies should be focused on the shifting of below- and above-ground C allocation response to extreme climate and also on the need to include EcM mycelia and root exudates in the accounting of below-ground litter pools.