Significance Climate change increases the frequency of drought events and leads to higher variability in precipitation. Drought impairs rhizosphere (root and the root-associated microbiome) functioning in trees and leads to a reduced assimilate supply belowground. It remains unclear if rhizosphere and thus whole-tree functioning can quickly recover after drought release. We show that rhizosphere metabolic activity in previously drought-exposed 100-y-old Scots pine increased in response to subtle soil moisture increases (induced by light rainfall). As a consequence of this activity change, the belowground allocation of new assimilates was immediately stimulated. Even light rainfall events can lead to a fast recovery of rhizosphere functioning and the increased C and energy demand is instantly met by altered whole-tree assimilate allocation.