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

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Abstract

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.

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Brunn, M., Hafner, B. D., Zwetsloot, M. J., Weikl, F., Pritsch, K., Hikino, K., … Bauerle, T. L. (2022). Carbon allocation to root exudates is maintained in mature temperate tree species under drought. New Phytologist, 235(3), 965–977. https://doi.org/10.1111/nph.18157

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