Carbon allocation to roots of suppressed Norway spruce increases immediately after selection harvest

Selection harvesting in fertile, drained peatlands is an alternative for even-aged forestry, where clearcutting takes place at the end of each rotation period. Avoiding clear-cuts has been promoted due to reducing negative externalities, like nutrient loading to waterways and significant greenhouse gas emissions after harvesting. Our aim was to understand the responses of suppressed Norway spruce trees to selection harvesting. We analysed biomass accumulation and priorities of carbon allocation to stems, proximal roots and distal roots. We quantified carbon isotope composition (δ13C) and cross-sectional growth of proximal (supporting) roots and distal (water and nutrient transportation) roots. The study took place in a drained, fertile boreal peatland site in southern Finland, where a selection harvest was conducted in 2016, while an area as a control was left untouched. Our results show that suppressed Norway spruce trees increased their cross-sectional growth of proximal roots and distal roots twice as much as that of the stem during the first five years after the selection harvest. The timing and magnitude of carbon allocation to proximal roots and distal roots were immediate and equal, underlining the fact that trees were investing in mechanical support and nutrient and water uptake. These results show that the climate benefits of selection harvest were not delayed. Instead of immediate growth of stems, we found immediate growth of roots.