Ecosystem carbon stocks and their temporal resilience in a semi-natural beech-dominated forest

Abstract

Forest management, including setting aside non-intervention forests, is currently debated as a measure to efficiently mitigate climate change. Temperate, old-growth forests are rare and the long-term effect of setting aside forest reserves, e.g. for habitat conservation, on forest carbon stocks and carbon sequestration remains uncertain. In this study, we assessed the five principal forest carbon pools in a semi-natural, temperate forest in Denmark and evaluated the changes in biomass and dead wood carbon stocks over 20 years. We also benchmarked the current carbon stocks against those in managed beech-dominated forests in the same region. In the most recent inventory, carbon stocks totalled 395 Mg ha−1 of which 47% was in above-ground live biomass, 11% in below-ground biomass, 9% in dead wood, 2% in the forest floor, and 31% in the top 75 cm of the mineral soil. Compared to similar, but managed forests in the same region, carbon stocks in above- and below-ground biomass as well as in dead wood were notably larger. However, analyses of above- and below-ground biomass and dead wood carbon stocks showed remarkable stability during the past 20 years, despite major disturbances in terms of windthrows and various pathogens. Despite the overall stability of carbon stocks we observed some significant spatio-temporal changes. Notably, the observed dynamics illustrate a general retreat of Q. robur and increasing dominance of F. sylvatica under a small-scale disturbance regime. The forest floor and soil carbon stocks averaged 131 ± 4 Mg ha−1 and were very similar to those of managed beech-dominated forests, suggesting little potential for soil carbon sequestration from setting forests aside as unmanaged. Our study has implications for selection of adequate forest management strategies to efficiently mitigate climate change as it confirms the large and persistent carbon stocks in old-growth forests compared to managed forests, but offers no evidence of continued carbon sequestration in old-growth forests.