Forest Structure and Fine Root Biomass Influence Soil CO2 Efflux in Temperate Forests under Drought

Soil respiration is rarely studied at the landscape scale where forest and soil properties can be important drivers. We performed forest and soil inventories in 150 temperate forest sites in three German landscapes and measured in situ soil CO2 efflux with the soda-lime method in early summer 2018 and 2019. Both years were affected by naturally occurring summer droughts. Our aim was to investigate the impact of forest structural and compositional properties, soil properties and climate on soil CO2 efflux at the landscape. Forest properties explained a large portion of soil CO2 efflux variance (i.e., 14% in 2018 and 20% in 2019), which was comparable or larger than the portion explained by soil properties (i.e., 15% in 2018 and 6% in 2019), and much larger than that of climate. Using Structural Equation Modeling, we found that forest structural properties, i.e., tree density and basal area, were negatively linked to soil CO2 efflux, while forest composition, i.e., conifer share and tree species richness, was not important. Forest structure effects on soil CO2 efflux were either direct or mediated by fine root biomass under dry summer conditions. Summer soil CO2 efflux was positively linked to fine root biomass but not related to total soil organic carbon stocks or climate. Forest structural properties influence soil CO2 efflux under drought events and should be considered when predicting soil respiration at the landscape scale.