Leaf expansion period is important for plant nitrogen acquisition in temperate broad-leaved species; previous study showed an enzyme, nitrate reductase, activity had a distinct peak in the middle of leaf expansion period followed by a low profile kept throughout the growing period. In boreal forests, however, growing period is shorter than in temperate area, and it is likely that the boreal species have different seasonal patterns of nutrient acquisition from the temperate species. In this study, we focused on the nitrate assimilation in boreal plant leaves, and described seasonal patterns of the nitrate assimilation by boreal species in comparison with temperate species.

We measured leaf area and weight of Alnus tenuifolia, Betula papyrifera and Populus tremuloides in a boreal forest in interior Alaska, US, and nitrate reductase activity (NRA) was simultaneously measured as an index of nitrate assimilation. The measurement started at the beginning of leaf expansion, and continued throughout the leaf expansion period.

Leaf areas reached to the maximum in about three weeks. Peaks of NRA were found in the leaf expansion period in all three species, and the leaf expansion stage with the peak NRA differed among species. However, NRA per leaf calculated from NRA per weight and leaf weight generally increased throughout the leaf expansion period.