Effects of drought and disturbance on the CO2 and CH4 fluxes in a mixed forest and spruce monoculture

Spruce monocultures have been intensively planted across a wide area of Europe to increase timber production and meet the demand from society. However, evidence suggests that species monocultures may not be as resilient to drought spells and heat waves compared to mixtures of two or more species. The advantage of mixed forests over monocultures is particularly evident when the mixed species occupy different niches, reducing inter-specific competition and enabling better growth and increased carbon sequestration. However, it remains unclear how drought events and heat waves affect carbon sequestration in the soil and how this differs between mixed forests and species monocultures. In this study, we conducted two years of intensive monitoring of soil CO2 and CH4 fluxes, measured soil microbial diversity, and assessed long-term (tree ring) and seasonal tree growth to quantify carbon sequestration in a mixed forest and a spruce monoculture. Results showed that severe drought in 2022 significantly reduced the growth of Norway spruce stand and its' forest floor and soil CO2 fluxes but at lesser intensity impacted C fluxes of European beech and silver fir stand. The bark beetle outbreak in 2023 caused rapid tree infestation and die-back only in the spruce stand (followed by salvage clear-cut harvesting) which subsequently increased soil CO2 emissions via a sudden increase in litter input from dead trees, soil temperature and water content from reduction of shade and evapotranspiration.