Higher carbon storage in primary than secondary boreal forests in Sweden.

Boreal forests provide considerable global land carbon storage and uptake, but they are being rapidly transformed to managed secondary forests, with poorly quantified implications for ecosystem carbon storage. Here we present data from extensive mapping and field inventories of carbon storage in primary forests in Sweden and use multiple methods to show that primary forests store ~72% (70 to 74% across methods) more carbon than managed secondary forests in vegetation, deadwood, soils, and harvested wood products combined. Soils constitute both the largest carbon store and the largest difference between these forest types. The total carbon storage difference between primary and managed secondary forests is 2.7 to 8.0 times larger than previous estimates. Our results challenge estimated past and future contributions of boreal forest management to atmospheric carbon dioxide concentrations. Editor's summary: Global demand for wood products is increasing, as are calls for protecting tropical forests from deforestation. In boreal regions, where much of the forest has not been previously logged, demand has led to clear-cutting of such old, "primary" forest. Pascual et al. showed the potential impacts of clearing primary boreal forests by comparing the carbon storage of primary and secondary (previously logged) forests in Sweden. Including vegetation, dead wood, wood products, and soils, primary forests stored over 70% more carbon than secondary forests, a difference several times greater than previous estimates. These results reveal previously underappreciated value in protecting primary boreal forests. —Bianca Lopez 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. Illustration of methods and data used to estimate C storage differences between primary and managed secondary forests.: The map shows primary forests in Sweden that have either been inventoried for this study or contain national forest inventory samples. Middle panels illustrate how C storage in the ecosystems and in harvested wood was estimated. The right-hand panel illustrates the resulting C storage. [ABSTRACT FROM AUTHOR]