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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 (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.

Drought is a severe natural risk that increases drying-rewetting frequencies of soils. Yet, it remains largely unknown how forest ecosystems respond to dry-wet cycles, hampering our ability to evaluate the overall sink and source functionality for this large carbon pool. Recent investigations suggest that the release of soluble carbon via root exudation increases under drought, influencing soil carbon stabilization and mineralization. However, an integration of root exudation into the carbon allocation dynamics of drought stressed trees is missing. We hypothesized that roots in dry soil layers have a higher exudation rate than roots in more moist layers across different soil depths. Further, we tested if higher exudation rates under drought are attenuated by reduced root abundance in dry soils and if the fraction of root exudation from total carbon allocation increases with decreasing photosynthesis rates under drought. At the KROOF experimental site in southern Germany, where mature beech (Fagus sylvatica L.) and spruce (Picea abies (L.) Karst.) trees were exposed to artificial drought stress for five consecutive growing seasons, we show that at the root level root exudation rate increases in drier soils. Especially roots in the upper soil profile and roots of spruce trees increased root exudation under drought. When scaled to whole tree level, we did not find differences in total exudation between drought stressed and control trees, indicating sustained root exudation at the tree level under drought. As photosynthesis rates and therefore total carbon assimilation was substantially reduced under drought (by 50 % in beech and almost 70 % in spruce), the fraction of root exudation from total assimilation slightly increased for drought stressed trees. Our results demonstrate that stimulation of root exudation rates with drought exists in natural temperate forest ecosystems but might be mitigated by reduced fine root abundance under drought. Nevertheless, increased exudation per root surface area will have localized impacts on rhizosphere microbial composition and activity especially in the topsoil exposed to more extreme dry-wet cycles. Finally, also the exudate composition can help to determine how priming of soil organic matter relates to belowground carbon allocation dynamics and to disclose processes of complementary species interaction and should be emphasised in future studies.

This contribution presents the result of a free-air 13C labeling experiment on mature Norway spruce (P. abies [L.] KARST.) upon watering after five years of recurrent summer drought in southern Germany, focusing on whole tree allocation processes. Mature spruce trees had been exposed to recurrent summer drought from 2014 to 2018 through complete exclusion of precipitation throughfall from spring to late fall (i.e., March to November). In early summer 2019, the drought stressed spruce trees were watered to investigate their recovery processes. In parallel with the watering, we conducted a whole-tree 13C labeling in canopies and traced the signal in various C sinks, i.e. stem phloem and CO2 efflux, tree rings at different heights, coarse roots, fine root tips, mycorrhiza, root exudates, and soil CO2 efflux.We hypothesize that drought stressed spruce preferentially allocates newly assimilated C to belowground sinks upon drought release. Conversely to our expectations, allocation to belowground C sinks was not stimulated in drought stressed compared to control spruce. Likewise, the relative amount of recently fixed C allocated to aboveground sinks did not differ between treatments. Our findings suggest that the belowground C sinks are not of higher priority for the allocation of newly assimilated C upon watering after long-term drought. The observed allocation pattern is discussed taking total above- and belowground biomass as well as C source/sink relations into account.