Bud Structure and Shoot Architecture of Canopy and Understorey Evergreen Broad-leaved Trees at their Northern Limit in East Asia

The morphology of winter buds, shoot growth and branching architecture was studied in evergreen broad-leaved trees of subtropical/warm-temperate rain forests of southern and central Japan. Winter buds were categorized into three types based on external morphology and developmental processes: naked, hypsophyllary and scaled buds. Each shoot tip with intermittent growth was covered with a small number of immature leaves or hypsophylls when growth ceased. Hypsophylls protect the apical meristem during its resting period, hence we termed them hypsophyllary buds. In trees with naked buds, immature leaves resumed their growth and developed to mature leaves the following spring; thus these trees had no special organs to cover shoot tips during winter. In trees with hypsophyllary buds, some hypsophylls covering the shoot tips through the year were shed without further growth when new shoots started to grow in the spring. In trees with scaled buds, newly growing shoots had hypsophyllary buds at their tips in spring. After the completion of stem elongation, the buds were replaced by scaled buds (often covered with more than 30 scales) in summer. These scaled buds grew during autumn and winter until a new flush of growth the following spring. The three bud types corresponded to forest stratification in the northern-limit forest: the naked buds of Rubiaceae and Myrsinaceae in the ground layer; the hypsophyllary buds of various families (e.g. Symplocaceae, Myrsinaceae) in the understorey; and the scaled buds of Fagaceae and Lauraceae in the forest canopy. The position and activity of buds on a branch were reflected in the architectural patterns of the trees in different layers of the forest. The scaled-bud trees had well-protected, abundant axillary buds and are probably suited to survive in the forest canopy (with frequent disturbances), whereas the single terminal bud of hypsophyllary-bud trees can survive in the less disturbed, resource-limited understorey of the forest.