Impacts of Drought-Induced Tree Mortality on Soil Microbial Communities: Insights from a Scots Pine Mesocosm Experiment

Increased tree mortality attributed to water limitation is documented for various tree species at different sites globally. Nevertheless, our understanding of tree mortality effects on soil microbial communities remains scarce. Therefore, we conducted a mesocosm experiment with young Scots pine saplings and natural forest soil to follow changes in soil microbial communities during a tree mortality event. Scots pine saplings were completely deprived of water until they died. Shifts in soil microbial communities during this event were assessed by metabarcoding in parallel with measuring plant vitality and physicochemical soil properties. Drought history influenced tree mortality, and a high individuality between trees was observed. Tree mortality was accompanied by reduced stomatal conductance, discoloring of needles, increased defoliation, and shrinkage of the stem diameter. Soil NO3- concentrations increased after tree death through diminished plant uptake and potentially increased microbial nitrification. Microbial abundance and community composition were affected by tree death and drought legacy. Copiotrophic bacterial taxa decreased during tree mortality while oligotrophic increased, probably slowing soil C turnover. Fungal saprotrophs decreased, while symbiotrophs increased in abundance, indicating possible competition between facultative saprotrophic ECM and free-living saprotrophs in the early phase after tree death. Overall, drought-induced tree mortality promoted changes in soil bacterial and fungal communities, potentially affecting soil processes in forest ecosystems.