Size-dependent variation in leaf functional traits and nitrogen allocation trade-offs in Robinia pseudoacacia and Cornus controversa

Tree species vary in how they invest resources to different functions throughout their life histories, and investigating the detailed patterns of ontogenetic changes in key functional traits will aid in predicting forest dynamics and ecosystem processes. In this context, we investigated size-dependent changes in key leaf functional traits and nitrogen (N) allocation trade-offs in black locust (Robinia pseudoacacia L., an N-fixing pioneer species) and giant dogwood (Cornus controversa Hemsl., a mid-successional species), which have different life-history strategies, especially in their light use. We found that the leaf mass per area and leaf carbon concentrations increased linearly with tree size (diameter at breast height, DBH), whereas leaf N concentrations decreased nonlinearly, with U- and hump-shaped patterns in black locust and giant dogwood, respectively. We also discovered large differences in N allocation between the two species. The fraction of leaf N invested in cell walls was much higher in black locust than in giant dogwood, while the opposite was true for the light harvesting N fraction. Furthermore, these fractions were related to DBH to varying degrees: the cell wall N fraction increased with DBH for both species, whereas the light harvesting N fraction of giant dogwood decreased nonlinearly and that of black locust remained constant. Instead, black locust reduced the fraction of leaf N invested in other N pools, resulting in a smaller fraction compared to that of giant dogwood. On the other hand, both species had similar fraction of leaf N invested in ribulose-1,5-bisphosphate carboxylase/oxygenase across tree size. This study indicated that both species increased leaf mechanical toughness through characteristic changes in N allocation trade-offs over the lifetimes of the trees.