Annual carbon balance in a hemiboreal silver birch and Norway spruce mixed stand chronosequence

Mixed forests are expected to be highly productive carbon (C) sequestering ecosystems. The study reports the effect of stand age on net ecosystem production (NEP) in a hemiboreal mixed forest chronosequence composed of silver birch and Norway spruce. The biometric C budgeting method was applied in seven chronosequence mixed stands aged between 20 and 70 years. Average NEP across the studied development classes ranged from 3.8 to 5.3 tC ha−1 yr−1. The highest value of NEP, 7 tC ha−1 yr−1, was estimated for a 30-year-old middle-aged stand. The main driver shaping the annual NEP of the stands along the successional gradient was the net primary production (NPP) of trees, while the contribution of the ground vegetation was minor. Annual cumulative soil heterotrophic respiration (Rh) varied between 4.1 and 5.5 tC ha−1, irrespective of stand age. Birches produced the major share (61%) of the annual aboveground litter input to the soil while the belowground C input through fine root production was largely formed of spruces (63%). The studied birch-spruce mixed stands on fertile soils acted as strong C sinks. Still compared to the literature data for corresponding pure stands, no overyielding effect of mixed stands was revealed.