Disentangling the effects of tree species and microclimate on heterotrophic and autotrophic soil respiration in a Mediterranean ecotone forest

Understanding how forest ecosystems influence the response of soil respiration (R-S) to climate drivers is essential to accurately predict soil carbon dioxide (CO2) fluxes in a changing environment. This is particularly crucial in areas of contact between coniferous and broadleaved forests in the Mediterranean region which are already experiencing a warmer and drier climate. We present a case study in a Mediterranean ecotone forest where R-S components-heterotrophic (R-H) and autotrophic (R-A)- were monitored beneath pure Scots pine (Pinus sylvestris L.) and Pyrenean oak (Quercus pyrenaica Willd.) stands using the root-trenching method. We used generalized linear models to predict the soil CO2 efflux based on interactions between soil water conent (SWC) and soil temperature. Regardless of the tree species, we found that the strong and inverse water availability and thermal seasonality of the Mediterranean climate intensely constrained soil microbial activity. The incorporation of the soil temperature-moisture covariation thus greatly improved the quality of the models compared to approaches that consider soil temperature alone. We also identified species-specific responses influencing both the total amount of R-A and its sensitivity to environmental variables. Root respiration in the Scots pine stand showed greater vulnerability to the decline in SWC throughout the summer than in the oak stand. The R-A in the Pyrenean oak stand was mainly limited by low soil temperatures in winter, indicating low maintenance rates during vegetative dormancy. Mean annual R-H rates were highest in the Scots pine stand, probably driven by the larger litterfall rates and soil carbon (C) stocks; however, the apparent turnover rate of soil organic C in the oak stand was almost twice as fast as in the pine stand. While our observations are limited to a case study, our work shows that both soil moisture and forest composition can significantly control the temperature dependence of R-S components under a Mediterranean climate.