Carbon allocation to root exudates is maintained in mature temperate tree species under drought.

Summary: Carbon (C) exuded via roots is proposed to increase under drought and facilitate important ecosystem functions. However, it is unknown how exudate quantities relate to the total C budget of a drought‐stressed tree, that is, how much of net‐C assimilation is allocated to exudation at the tree level.We calculated the proportion of daily C assimilation allocated to root exudation during early summer by collecting root exudates from mature Fagus sylvatica and Picea abies exposed to experimental drought, and combining above‐ and belowground C fluxes with leaf, stem and fine‐root surface area.Exudation from individual roots increased exponentially with decreasing soil moisture, with the highest increase at the wilting point. Despite c. 50% reduced C assimilation under drought, exudation from fine‐root systems was maintained and trees exuded 1.0% (F. sylvatica) to 2.5% (P. abies) of net C into the rhizosphere, increasing the proportion of C allocation to exudates two‐ to three‐fold. Water‐limited P. abies released two‐thirds of its exudate C into the surface soil, whereas in droughted F. sylvatica it was only one‐third.Across the entire root system, droughted trees maintained exudation similar to controls, suggesting drought‐imposed belowground C investment, which could be beneficial for ecosystem resilience. [ABSTRACT FROM AUTHOR]