Beech decline reshapes fine root traits, microbial composition and soil carbon–nutrient cycling

Abstract Tree decline increasingly influences forest functioning world‐wide; however, below‐ground processes influencing tree health remain poorly understood. This study evaluates the changes in root–microbe interactions of beech trees ( Fagus sylvatica ) in response to tree decline. Over 2 years, we investigated fine root dynamics, functional traits, microbial community composition, and enzymatic activity in the rhizosphere and bulk soil of declining and healthy beech trees at their southern range limit. Declining trees exhibited reduced fine root biomass, higher turnover rates and shifts towards longer and thinner fine roots. These changes altered the diversity and composition of fungal and bacterial communities, decreasing the relative abundance of ectomycorrhizal fungi while promoting the proliferation of saprophytic fungi. The competitive and antagonistic interactions among these functional guilds, along with increased root mortality, likely contributed to the heightened enzyme activity observed in the rhizosphere of declining trees. Synthesis . This study highlights the critical role of rhizosphere interactions in nutrient cycling and tree health. We found that tree decline triggers cascading effects that influence fine root, microbial communities and soil enzyme activities in the rhizosphere. By focusing on these critical below‐ground processes, our research enhances the understanding of beech tree decline and underscores the need to consider these below‐ground dynamics in future studies to further understand overall forest decline.