Long-term responses of soil and forest floor respiration to increasing temperature in a mixed deciduous forest

Forests play a crucial role in the carbon biogeochemical cycle. A significant amount of carbon is lost every year through soil respiration (SR). SR is known to respond exponentially to soil temperature (ST), but it is still unclear how SR responded to the long-term increases in temperature. Hence, we currently cannot predict if global warming would result in an overall increase or decrease of CO₂ release from forest soils. 
In this context, our study aims to understand how SR has changed over time in a mixed deciduous forest in Switzerland. The study site is located in the Lägeren (CH-LAE) forest at 689 m a.sl., which is mainly composed by European beech trees (Fagus sylvatica) and Norway spruce (Picea abies). SR was measured with a closed chamber through survey campaigns in 2006, 2007, 2021 and 2022. In addition, continuous measurements of forest floor Net Ecosystem Exchange (NEE_ff) with a below-canopy Eddy covariance system have been running since 2014. We then partitioned these fluxes to obtain forest floor respiration (R_ff). A random forest analysis was performed to investigate SR and R_ff drivers; SR responses to ST were analysed with the Lloyd-Taylor (1994) equation.  
The aims of this research are (1) to compare the magnitude of SR and R_ff, (2) to evaluate the drivers of SR and R_ff, and finally (3) to investigate the change of ST, SR and R_ff over time. We expect that ST is the main driver, and that the magnitude of SR and R_ff is comparable. Moreover, we hypothesize a long-term acclimation of SR and R_ff to the increasing air and soil temperatures recorded at the study site. We indeed found ST driving SR and R_ff, except for drought conditions when soil moisture becomes a limiting factor. We also observed that the sensitivity of SR to ST has increased over time, suggesting higher CO₂ fluxes from the forest soil with increasing temperature due to climate change.