Growth of first-order branches in Betula platyphylla saplings as related to the age, position, size, angle, and light availability of branches

The growth of first-order branches was measured in 25 saplings of Betula platyphylla Sukatchev var. japonica Hara growing under various light conditions in northern Hokkaido, northern Japan, and the effect of age, tip height, length, and angle of branches and light availability at the branch tips on branch growth were examined. Branch growth was evaluated as the total biomass growth of current-year long shoots within a first-order branch (TBG), the number of long shoots currently produced by the first-order branch (CSN), and the mean biomass of current-year long shoots within the first-order branch (MSG). In general, TBG, CSN, and MSG were negatively dependent on branch age and positively dependent on height, length, angle (from the horizontal), and light availability of the branch. The relationships between each of TBG, CSN, and MSG and the independent variables were individual specific. The dependency of TBG, CSN, and MSG on the light availability at the branch tips was affected by the maximum light availability at the individual level, suggesting that branch growth is affected not only by branch-level resources but also by conditions at the individual level. Based on these results, two concepts for understanding branch growth, branch autonomy and correlative inhibition, were discussed, and prediction models for TBG, CSN, and MSG were presented.