Anders Ahlström

INTRODUCTION
Boreal forests are crucial for mitigating global climate change by capturing and storing large amounts of atmospheric carbon dioxide. However, previously unmanaged primary boreal forests are being rapidly transformed to managed secondary forests to meet the rising global demand for materials and energy. Understanding the effects of boreal primary to managed secondary forest transformations on carbon (C) storage is critical for constraining the global C budget and for evaluating the potential of northern forests in climate change mitigation strategies, which include considerations of increased biomass use for energy and materials.
RATIONALE
Previous attempts to quantify the impacts of boreal primary to managed secondary forest transformations have been hampered by limited C storage observations. The high spatial heterogeneity in environmental characteristics and historical land use in boreal areas adds to the uncertainty and calls for regional studies. We combined Sweden’s national forest inventory data with extensive field inventories of live trees, deadwood, and soils to estimate C storage in primary and managed secondary forests in Sweden. We used multiple methods to estimate the C storage difference between primary and managed secondary forests. We added estimates of missing data on deadwood C stored in stubs and roots left on site after harvest and C stored in harvested wood products for managed secondary forests, which allowed us to compare the land C storage in primary and managed forest systems.
RESULTS
We found that primary forests store ~72% more C than managed secondary forests in vegetation, deadwood, soils, and harvested wood products combined (land C storage). Among the compartments, soils constitute both the largest C store and the largest difference between primary and managed secondary forests. Our estimated difference in land C storage between primary and managed secondary forests in Sweden is 2.7 to 8.0 times larger than reported in global state-of-the-art data-driven studies and bookkeeping models informing the Global Carbon Project.
CONCLUSION
Although the high-intensity forest management common in Sweden implies that our results may not be directly transferable to the broader boreal biome, they suggest that previous estimates have substantially underestimated the C cost of transforming primary to managed secondary boreal forests. Our findings suggest that preservation and conservation of European boreal forests may be more effective as a climate change mitigation strategy than previously thought.