## 5 Conclusions  
We summarize here the answers to our three research questions:  
(I) How do the below- and aboveground phenology of deciduous trees relate to each other?  
We observed no clear and repeating yearly patterns in the belowground phenology of beech, oak, or birch. Consequently, no consistent coupling between the below- and aboveground phenology of these species could be observed. In 2019, we observed peaks in the new root count concurrently with the phenological transition date marking the (re-organization) phase of senescence in beech and oak. Our data, which were asymmetrically distributed with strong effects from outliers, suggest that the life strategy of a species influences both the below- and aboveground phenology and the fine-root lifespan and turnover rate values.  
(II) Are there differences in the fine-root lifespan, turnover rate, and phenology among beech, oak, and birch?  
The fine-root phenology in beech and oak (both late-successful species and members of the Fagaceae) showed more similarities among each other than in comparison with birch (pioneer species and member of the Betulaceae). Likewise, our results also showed significant differences in the fine-root lifespan and the turnover rate of beech, oak, and birch. More specifically, beech and oak had the longest fine-root lifespan and the lowest turnover rate, respectively.  
(III) Are there differences in the fine-root lifespan and the turnover rate among roots with different diameters or among those born in different seasons?  
Fine-roots with a larger diameter did not always have a greater fine-root lifespan than fine-roots with a smaller diameter. Fine-roots born in spring also did not always live shorter than fine-roots that were born later. Significant differences, and a wide variance in the estimations, were especially found in the fine-root lifespan and the turnover rate whenever the fine-root data were divided by the root diameter class or the season of root birth.  
The yearly pattern in the phenology of the fine-roots was unclear with a wide variance in the cumulative root surface and the new root count. It is likely that the lack of clear and repeating phenological trends, and the wide variance in the fine-root lifespan and the root turnover rate estimations, might have been due to the extreme heat and drought stress observed during the period of 2019-2020.  
Conceptualization, methodology, and funding acquisition, M.C.; investigation, H.X.N., C.F., T.G., P.W. and B.M.; formal analysis, B.M.; validation, P.S. and I.O.; visualization, B.M.; writing--original draft, B.M.; writing--review & editing, A.P., H.X.N., P.S., P.W., LO., M.C. and B.M. All authors have read and agreed to the published version of the manuscript.  
B.M. and M.C. have been funded by a DOCPRO4 fellowship and the ERC Starting Grant LEAF-FALL (714916) by the University of Antwerp and the European Research Council, respectively. I.O. and P.S. (in part) were supported by Estonian Research Council grant PRG916.  
Data Availability StatementData will be made available upon acceptance when required.  
**Acknowledgments:** Thanks are due to [PERSON] (PLECO), the Flemish Agency for Forest and Nature (ANB), the Belgian Armed Forces, and the Municipality of Brasschaat.  
**Conflicts of Interest:** The authors declare no conflict of interest.