Structural and functional differences in the belowground compartment of healthy and declining beech trees

Functional studies of tree decline have mainly focused on process inside the plant showing possible alterations of carbon transport, storage and hydraulic functions. However, the processes occurring at the plant-soil interface have been seldom investigated. Our objective is to examine carbon functional alterations in the belowground compartment in the case of a long term beech decline. Soil and nutrient content profiles were characterized under five healthy and five declining trees. Seasonal root growth and rhizodeposition were characterized using, ingrowth cores, and microbial biomass combined with soluble carbon organic content, respectively. Podzolisation associated with soil acidification and deficiencies in Ca, Mg and Mn were observed in the soil under declining trees, but not under healthy trees. Spring fine root growth was higher in declining trees than in healthy trees but there were only minor differences concerning rhizodeposition proxies. In our study, we showed that the tree health status is associated to a marked heterogeneity of soil characteristics. Podzolisation close to declining trees leads to a local mineral deficiency which probably stimulates their fine root growth but without altering their flux of carbon exudates.