Invasion of broad-leaf tree species into a larch plantation: seasonal light environment, photosynthesis and nitrogen allocation

We have studied how tree seedlings with differing leaf phenological traits change their nitrogen allocation in seasonally changing light environments. Specifically, we have investigated seasonal changes in maximum leaf photosynthetic rates in situ, and changes in nitrogen, chlorophyll and ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) content. We have also estimated nitrogen allocation characteristics from leaf nitrogen to photosynthetic proteins, light-harvesting protein and other proteins in four successional tree species growing in a larch plantation. The gap phase species, Magnolia hyporeuca Siebold et Zucc., displays higher photosynthetic rates to compensate for the short leafy period and allocates more nitrogen to Rubisco. In contrast, tree species that grow at the forest floor, Prunus ssiori F. Schmidt and Carpinus cordata Blume, use the leafless period of the canopy trees to change their allocation of nitrogen from Rubisco to chlorophyll in line with canopy closure. Quercus mongolica Fischer ex Ledeb. var. crispula (Blume) Ohashi is a mid-successional tree species, and maintains a high photosynthetic rate in September because it allocates more nitrogen to Rubisco. We conclude that phenological differences in tree seedlings are clearly reflected in the nitrogen allocation pattern at gaps and at the forest floor in larch plantations.