## 4 Discussion
### Relationship between the Above- and Belowground Phenology of deciduous Trees  
In 2019, some links between the root phenology and senescence could be observed. For example, a peak in the new root count of the beech trees in the KS and the PB could be observed during or just before the moment with the highest probability on the phenological transition date marking the re-organization phase of senescence, respectively. In the oak trees, the trend in the new root count peaked concurrently with the first peak in the phenological transition data marking the re-organization phase of senescence. However, similar dynamics for beech and oak were not observed in 2020. On the other hand, the fine-roots of birch did show growth during the entire period of senescence in both years. We can observe potential links between the below- and aboveground phenology. Because of the lack of seasonality in the observed fine-root phenology, the coupling between the below- and aboveground phenology was, however, not consistent across years for beech and oak. Studies on the formation, differentiation, and maturation of xylem (i.e., xylogenesis) on the same species also suggested that the relationships between the xylogenesis and the timing of the bud burst or leaf senescence are unclear, or missing [144; 145; 146; 147]. In the trend of the cumulative root surface of the beech trees in the KS and PB, one might observe two moments of increase just after the bud burst in 2019 and 2020. The observation that the bud burst in beech occurs later or concurrently with the onset of the wood formation is known from the literature and can be associated with the wood anatomy of beech (i.e., semi-diffuse porous wood). Similar to beech, the bud burst in birch (diffuse porous wood) occurs later or concurrently with the onset of wood formation. The wood formation in oak (ring porous wood), on the other hand, starts before the bud burst [148; 149; 150]. However, the observed moments of increase in the cumulative root surface of the beech trees might also sign towards an effect of the re-allocation of resources from different plant organs to the fine-root growth [151; 152; 153; 154; 4, 35]. In the literature, internal competition for carbohydrates among plant organs is usually mentioned as a potential explanation for the presence of one- (usually between June and September) or two-yearly peaks in the fine-root phenology of deciduous trees [155; 156; 157; 158; 142; 159; 160; 4, 18]. However, our results mainly suggest that internal processes are less important for fine-root phenology than external drivers. In fact, the lack of parallel observations between the belowground and aboveground phenology may suggest that the phenology of the former is mainly driven by carbohydrate reserves, colonization through mycorrhizae, or meteorological variables (e.g., soil moisture or temperature) that are different from the drivers of the latter (e.g., air temperature, vapor pressure deficit, etc.) [40; 161]. There may be two aspects for which the link between the below- and aboveground phenology may be obvious: the species life strategy (i.e., successional stage) and drought conditions. Relationships between the above- and belowground phenology might depend on the species' life strategy in this study. For example, as a pioneer species, birch has an early bud burst and early onset of its re-organization phase of senescence. Likewise, birch has fine-roots with a shorter median lifespan than those in beech and oak. Perhaps the life strategy of birch may, through a different timing in the internal allocation of carbohydrates, explain why its fine-root phenology is different from those of beech and oak. However, phylogenetic relationships might also be important here as both beech and oak are members of the Fagaceae, but Birch is a member of the Betulaceae. Furthermore, it is known that the root-to-shoot ratio tends to increase under seasonal drought stress, because of a decrease in the fine-root biomass [11; 162; 163; 164; 165; 166; 167].