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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.

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's 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.