Organic carbon stock losses by disturbance: Comparing broadleaved pioneer and late-successional conifer forests in Mongolia’s boreal forest

Abstract

The boreal forest biome is prominent for its high organic carbon pools, the stability of which is important for climate change mitigation. Natural and anthropogenic disturbance is converting boreal forests from late-successional conifer stands to broadleaved pioneer forests. The present paper analyzes how the degradation of Larix sibirica forest to mixed Betula platyphylla-L. sibirica stands or finally to monospecific B. platyphylla forest affects organic carbon stock density in Mongolia’s boreal forests, which are representative of large forest areas at the southern fringe of this biome in Inner Asia. Conversion of larch forest to mixed forest with 60% larch and 40% birch resulted in the reduction of carbon stock density by 7%, whereas the complete replacement of larch by birch reduced ecosystem carbon stock density by 19%. These reductions only referred to biomass stocks, whereas the soil organic carbon stock remained stable indicating that losses during and after the disturbance were offset by carbon sequestration in the highly productive initial stages of pioneer forests. The magnitude of carbon stock degradation by conversion into broadleaved pioneer forest suggests that the existing estimate of 1.5–1.7 Pg C for the total organic carbon pool in Mongolia’s boreal forest is not substantially affected by the present level of forest conversion resulting in a share of 6.4% of birch in Mongolia’s total forest area, but could become more relevant with progressive degradation of late-successional forests.