Chronic canopy decline alters root exudation patterns and rhizosphere chemistry in Fagus sylvatica

Abstract Aims Root exudates mediate carbon (C) and nutrient exchange between plants and soil microorganisms, yet their responses to chronic tree decline remain poorly understood. We investigated how canopy dieback affects the quantity and chemistry of root exudates in Fagus sylvatica L. trees growing at the southern limit of their distribution, where recurrent drought and warming drive forest decline. Methods We combined continuous microclimate monitoring, fine root trait analyses, in situ exudate collection, and untargeted metabolomics to compare declining and healthy trees. Metabolite and functional profile of exudates were characterized through hierarchical molecular classification and stoichiometric and thermodynamic trait characteristics. Results Declining trees released nearly twice as much C through root exudation as healthy individuals. Exudation rates increased with soil moisture in declining trees only, whereas soil temperature showed a similar negative relationship with exudation rates in both healthy and declining trees. Metabolomic analyses revealed distinct chemical fingerprints: exudates from declining trees were enriched in nitrogen and energetically poor compounds, reflected in higher nominal oxidation state of carbon and N:C ratios but lower Gibbs free energy. Conclusions Chronic canopy decline alters both the rate and composition of root exudation, shifting rhizosphere chemistry toward low-energy, nitrogen-enriched substrates that may stimulate microbial respiration and nutrient turnover but reduce soil C stabilization. These findings demonstrate that decline-driven changes in tree C allocation extend belowground, reshaping root–microbe interactions and potentially weakening the long-term C balance of drought-prone forests.