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

- 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

High resilience of carbon transport in long-term drought-stressed mature Norway spruce trees within 2 weeks after drought release

Under ongoing global climate change, drought periods are predicted to increase in frequency and intensity in the future. Under these circumstances, it is crucial for tree\u27s survival to recover their restricted functionalities quickly after drought release. To elucidate the recovery of carbon (C) transport rates in c. 70-year-old Norway spruce (Picea abies [L.] KARST.) after 5 years of recurrent summer droughts, we conducted a continuous whole-tree $^{13}$C labeling experiment in parallel with watering. We determined the arrival time of current photoassimilates in major C sinks by tracing the $^{13}$C label in stem and soil CO$_{2}$ efflux, and tips of living fine roots. In the first week after watering, aboveground C transport rates (CTR) from crown to trunk base were still 50% lower in previously drought-stressed trees (0.16 ± 0.01 m h$^{-1}$) compared to controls (0.30 ± 0.06 m h$^{-1}$). Conversely, CTR below ground, that is, from the trunk base to soil CO$_{2}$ efflux were already similar between treatments (c. 0.03 m h$^{-1}$). Two weeks after watering, aboveground C transport of previously drought-stressed trees recovered to the level of the controls. Furthermore, regrowth of water-absorbing fine roots upon watering was supported by faster incorporation of $^{13}$C label in previously drought-stressed (within 12 ± 10 h upon arrival at trunk base) compared to control trees (73 ± 10 h). Thus, the whole-tree C transport system from the crown to soil CO$_{2}$ efflux fully recovered within 2 weeks after drought release, and hence showed high resilience to recurrent summer droughts in mature Norway spruce forests. This high resilience of the C transport system is an important prerequisite for the recovery of other tree functionalities and productivity

Dynamics of initial carbon allocation after drought release in mature Norway spruce—Increased belowground allocation of current photoassimilates covers only half of the carbon used for fine‐root growth

After drought events, tree recovery depends on sufficient carbon (C) allocation to the sink organs. The present study aimed to elucidate dynamics of tree-level C sink activity and allocation of recent photoassimilates (C$_{new}$) and stored C in c. 70-year-old Norway spruce (Picea abies) trees during a 4-week period after drought release. We conducted a continuous, whole-tree $^{13}$C labeling in parallel with controlled watering after 5 years of experimental summer drought. The fate of C$_{new}$ to growth and CO$_{2}$ efflux was tracked along branches, stems, coarse- and fine roots, ectomycorrhizae and root exudates to soil CO$_{2}$ efflux after drought release. Compared with control trees, drought recovering trees showed an overall 6% lower C sink activity and 19% less allocation of C$_{new}$ to aboveground sinks, indicating a low priority for aboveground sinks during recovery. In contrast, fine-root growth in recovering trees was seven times greater than that of controls. However, only half of the C used for new fine-root growth was comprised of C$_{new}$ while the other half was supplied by stored C. For drought recovery of mature spruce trees, in addition to C$_{new}$, stored C appears to be critical for the regeneration of the fine-root system and the associated water uptake capacity

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

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, i.e. 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 ~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 threefold. Water-limited P. abies released two-thirds of its exudate-C into the surface soil, whereas it was only one-third in droughted F. sylvatica. 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

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

Please cite as: Benjamin D Hafner, Melanie Brunn, Marie J Zwetsloot, Fabian Weikl, Karin Pritsch, Kyohsuke Hikino, Nadine K Ruehr, Emma J Sayer, Taryn L Bauerle. (2022) Data from: Carbon allocation to root exudates is maintained in mature temperate tree species under drought. [dataset] Cornell University eCommons Repository. https://doi.org/10.7298/6r80-8a15Data in support of the following research: 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, i.e. 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 L. and Picea abies (L.) Karst. 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 ~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 threefold. Water-limited P. abies released two-thirds of its exudate-C into the surface soil, whereas it was only one-third in droughted F. sylvatica. 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.German Research Foundation (DFG), grant numbers: PR555/2-1, PR292/22-1, GR 1881/5-1, MA1763/7-1, MA1763/10-1 Bavarian State Ministry for Nutrition, Agriculture and Forestry and Bavarian State Ministry for Environment and Consumer Protection, grant number: W047/Kroof II