Why is Aucuba japonica smaller in heavy snowfall areas ? A growth simulation of evergreen broad-leaved shrubs based on shoot allometry, critical shoot sizes for flowering and photosynthetic production

A whole-tree growth model was developed to simulate the relationships between gross production and tree growth of Aucuba japonica, an evergreen broad-leaved shrub. The model is based on the allometry of plant organs and incorporates the processes of flowering, branching, and allocation of biomass among different organs. Growth of model plants was simulated under different values of annual gross production per leaf area (LGP) for 15 years. The tree form was mainly determined by the critical shoot size for initiating flowering (SF), but the average increase of new shoot mass (INM) for two successive years had marked effects on the growth pattern of A. japonica in addition to SF. Under small LGP conditions, plants with larger SF did not flower or postponed the initiation of flowering. With increasing LGP, plants with larger SF had a greater advantage because of their high efficiency for vertical growth. A smaller SF was observed in A. japonica var. borealis, which is distributed in heavy snowfall areas, and resulted in precocious flowering with a higher reproductive rate. The small plant size and shoot size of var. borealis may be the result of selection for preventing the delay of the year in which reproduction starts under smaller LGP conditions. On the other hand, the large SF of A. japonica var. japonica, which is distributed in light snowfall areas, suppresses flowering under small LGP conditions, and flowering occurs only after achieving a certain amount of vertical growth.