### The Fine-Root Lifespan, Turnover Rate, and Phenology of Temperate Deciduous Trees  
The fine-root lifespan and turnover rate affect the timely uptake of resources, the competition among species, and the carbon fluxes in the soil [19]. It is therefore of paramount importance that we understand the coupling between the fine-root lifespan and the turnover rate (and the fine-root phenology). For example, due to the difference in carbohydrate reserve content, the lifespan and the turnover rate of the fine-roots may differ per season of fine-root birth. Fine-roots born in spring may thus live shorter lives than those born later [8; 32]. The literature reports a wide variation in the fine-root phenological patterns among species and years. Observed patterns in the fine-root phenology throughout one year are linked to the timely availability of resources and may thus include: a constant fine-root production, a variable fine-root production with one peak, or a variable fine-root production with multiple peaks (usually two) [19; 33; 34; 35]. Deciduous trees in temperate regions usually have a variable fine-root production with values that peak in June in the Northern Hemisphere, remaining relatively constant throughout summer and decreasing later on [1; 36]. However, drought and heat stress may cease the fine-root production [1; 37; 38]. Relatively little information is available on the phenology of fine-roots in deciduous trees [39]. Therefore, it is no surprise that even less attention is dedicated to the coupling between the below- and aboveground phenological processes in deciduous trees. In fact, most information on this coupling is related to the differences in resource acquisition determined by leaf or root traits, rather than by differences in the actual phenology [40; 41]. Whenever inferences are made between the below- and aboveground phenology in deciduous trees, these are almost exclusively drawn between phenological patterns in the fine-roots and the bud burst, while usually neglecting the leaf senescence period. In addition, these inferences between above- and belowground phenology are usually drawn using data of relatively low resolution (i.e., approximately once a month) [15; 19; 42]. Recent technologies do allow observations of the belowground phenology with a higher resolution (i.e., up to once a day) but involve significant costs [43]. While knowing the fine-root lifespan, turnover, and phenology are important for climate modeling (especially for belowground processes and the parametrization of soil carbon fluxes) and timber production (due to the storage of carbohydrates in the roots), less is known about fine-roots dynamics than aboveground dynamics [19; 44; 45; 46; 47; 23; 48]. Three reasons may account for this knowledge gap. First, there are considerable difficulties in estimating the fine-root lifespan, turnover rate, and phenology, and none of the current measurement methods account for all these difficulties, which include: measuring a process of continuous growth and mortality that is not clearly defined, a relatively inaccessible study system, labor-intensive measurements, and soil disturbance. Second, as opposed to estimations of the leaf lifespan, our estimations of the fine-root lifespan, turnover rate, and phenology vary significantly among and within species and among and within years with very few trends [8; 49]. In fact, estimations of the root lifespan can vary by more than fivefold among species, while estimations of the leaf lifespan are usually similar among species [50]. At first sight, the drivers controlling the fine-root lifespan, turnover, and phenology are unclear and seemingly unconnected to aboveground tree traits with few exceptions (e.g., the positive correlation between the fine-root diameter and wood production or fine-root biomass and aboveground biomass) [23; 29; 51].