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

A whole-tree growth model was developed to simulate the relationships betwe en gross production and tree growth of Aucuba japonica, an evergreen broadl eaved shrub. The model is based on the allometry of plant organs and incorp orates the processes of flowering, branching, and allocation of biomass amo ng different organs. Growth of model plants was simulated under different v alues of annual gross production per leaf area (LGP) for 15 years. The tree form was mainly determined by the critical shoot size for initiating flowe ring (SF), but the average increase of new shoot mass (INM) for two success ive years had marked effects on the growth pattern of A. japonica in additi on to SF. Under small LGP conditions, plants with larger SF did not fewer o r postponed the initiation of flowering. With increasing LGP, plants with l arger SF had a greater advantage because of their high efficiency for verti cal growth. A smaller SF was observed in A. japonica var. borealis, which i s distributed in heavy snowfall areas, and resulted in precocious flowering with a higher reproductive rate. The small plant size and shoot size of va r. borealis may be the result of selection for preventing the delay of the year in which reproduction starts under smaller LGP conditions. On the othe r 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.