Allocation of nitrogen within the crown during leaf expansion in Quercus serrata saplings.

Early season leaf growth requires a large amount of nitrogen, and the amount of N provided for new leaf development has been well tested. Although shoot position within the crown strongly influences leaf properties, little is known about absorbed and remobilized nitrogen allocation in the tree crown. Thus, we investigated differences in the allocation of recently absorbed nitrogen in the tree crown. To quantify nitrogen allocation, we conducted 15N tracer experiments using potted saplings of the temperate deciduous oak (Quercus serrata Thunb. ex. Murray). Allocation of 15N within the crown varied significantly: the top leaves received more remobilized nitrogen than did the lateral leaves, suggesting that remobilized nitrogen is predominantly allocated to the top shoots, which are important for height growth. On the other hand, the proportion of currently-absorbed nitrogen to total nitrogen in the lateral leaves was more than twice that in the top leaves. We also detected the input and the output of nitrogen in the top leaves after the completion of leaf expansion, indicating that significant nitrogen cycling occurs even after full leaf expansion.