Patterning of subtropical/warm-temperate evergreen broad-leaved forests in East Asian mountains with special reference to shoot phenology

Abstract

The distribution pattern of subtropical/warm-temperate evergreen broad-leaved forest along altitudinal and latitudinal gradients in east Asia was examined with special reference to tree shoot phenology of the component evergreen trees. There were two prncipal evergreen tree groups, laurofagaceous notophyll trees and symploco-myrsinaceous microphyll trees. The lauro-fagaceous trees constitute the lower montane forest zone in tropical mountains and occupy the canopy layer of northern limit forest of the evergreen broad-leaved trees in mid-latitudes while the symploco-myrsinaceous trees constitute the upper montane forest zone in tropical mountains and occupy the understory of northern limit forest. In tropical forests, evergreen trees manifest diverged shoot growth behavior such as continuous, intermittent, and manifold, and leaf phenology such as evergreen, leaf exchanging, and deciduous. In the northern limit of evergreen forest, however, the tree behavior more or less converged into intermittent, single flush type of evergreen leaves with vernal leaf abscission. The evergreen leaves had relatively long leaf life span of more than 1.5 yr though within the northern limit areas leaf life span was changed into shorter one with the increase of latitudes. The two evergreen tree groups having characteristic shoot phenological features were the essential groups of canopy and understory guilds of the northern limit forest: the lauro-fagaceous trees having a short shoot growing duration with many scaled, protected buds in the canopy, and the symploco-myrsinaceous trees having a relatively long shoot growing duration with a few-scaled or cataphyllary or naked buds in the understory. Moreover, they shared different habitat conditions within the same climax area of the subtropical/warm-temperate forest zone, mainly along topographical, hydro-edaphological, or disturbance gradients. Throughout the broad northern limit areas of evergreen broad-leaved forests, canopy (lauro-fagaceous trees) and understory (symploco-myrsinaceous trees) guilds are consistently composed of the same genera or families having specific leaf size and shoot phenological features, but in each genus or family, the number of constituent species gradually decreased with the increase of latitudes.