Assessment of the heartwood contribution to carbon accumulation in Pinus sylvestris L. trees under different forest site conditions

Background: The heartwood (HW) proportion in the trunk of mature trees is an important characteristic not only for wood quality but also for assessing the role of forests in carbon sequestration. We have for the first time studied the proportion of HW in the trunk and the distribution of carbon and extractives in sapwood (SW) and HW of 70-80 year old Pinus sylvestris L. trees under different growing conditions in the pine forests of North-West Russia. Method: We have examined the influence of conditions and tree position in stand (dominant, intermediate and suppressed trees) in the ecological series: blueberry pine forest (Blu) - lingonberry pine forest (Lin) - lichen pine forest (Lic). We have analyzed the influence of climate conditions in the biogeographical series of Lin: the middle taiga subzone - the northern taiga subzone - the transition area of the northern taiga subzone and tundra. Results: We found that the carbon concentration in HW was 1.6%-3.4% higher than in SW, and the difference depended on growing conditions. Carbon concentration in HW increased with a decrease in stand productivity (Blu-Lin-Lic). In medium-productive stands, the carbon concentration in SW was higher in intermediate and supressed trees compared to dominant trees. In the series from south to north, carbon concentration in HW increased by up to 2%, while in SW, it rose by 2.7%-3.8%. Conclusions: Our results once again emphasized the need for an empirical assessment of the accurate carbon content in aboveground wood biomass, including various forest growing conditions, to better understand the role of boreal forests in carbon storage.