<p>The translocation and allocation of carbon (C) and nitrogen (N) within tree crowns play critical roles in enhancing resource-utilization efficiency, tree growth, and development. However, understanding of these processes, which are essential for determining effective crowns and guiding proper pruning practices, remains incomplete. Here, we conducted in situ isotope tracing experiments on six-year-old <i>Betula alnoides</i> trees using two stable isotopes (¹³C and ¹⁵N) to investigate the patterns of C and N translocation and allocation across different crown layers in the growth season. The relative abundances of foliar ¹³C and ¹⁵N in each crown layer were monitored over a 28-day chase period (at 0, 1, 3, 5, 7, 14, and 28 days post<Emphasis Type="Underline">-</Emphasis>labelling). Translocation of newly assimilated C among crown layers was limited in the growth season, supporting the theory of branch autonomy for C. While newly root-absorbed N was preferentially allocated to the upper crown layers, leaf-absorbed N was primarily translocated between adjacent layers, with the lower layers acting as the primary nutrient storage sites. These distinct utilization patterns, namely C autonomy within each crown layer and N interdependence via adjacent-layers translocation and lower-layer storage during the growth season, provide novel insights into the functional roles of crown layers and offer a scientific basis for optimizing artificial pruning regimes to promote the efficient cultivation of large, knot-free timber in forest management.</p>