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 architecturewas studied in evergreen broad-leaved trees of subtropical/warm-temperaterain forests of southern and central Japan. Winter buds werecategorized into three types based on external morphology anddevelopmental processes: naked, hypsophyllary and scaled buds.Each shoot tip with intermittent growth was covered with a smallnumber 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 nakedbuds, immature leaves resumed their growth and developed tomature leaves the following spring; thus these trees had nospecial organs to cover shoot tips during winter. In trees withhypsophyllary buds, some hypsophylls covering the shoot tipsthrough the year were shed without further growth when new shootsstarted to grow in the spring. In trees with scaled buds, newlygrowing shoots had hypsophyllary buds at their tips in spring.After the completion of stem elongation, the buds were replacedby scaled buds (often covered with more than 30 scales) in summer.These scaled buds grew during autumn and winter until a newflush of growth the following spring. The three bud types correspondedto forest stratification in the northern-limit forest: the nakedbuds of Rubiaceae and Myrsinaceae in the ground layer; the hypsophyllarybuds of various families (e.g. Symplocaceae, Myrsinaceae) inthe understorey; and the scaled buds of Fagaceae and Lauraceaein the forest canopy. The position and activity of buds on abranch were reflected in the architectural patterns of the treesin different layers of the forest. The scaled-bud trees hadwell-protected, abundant axillary buds and are probably suitedto survive in the forest canopy (with frequent disturbances),whereas the single terminal bud of hypsophyllary-bud trees cansurvive in the less disturbed, resource-limited understoreyof the forest.Copyright 1998 Annals of Botany Company Bud structural type; bud formation; bud growth; shoot elongation; shoot-growth cycle; branching architecture; forest stratification.