Vegetation Zonation and Conifer Dominance Along Latitudinal and Altitudinal Gradients in Humid Regions of the Western Pacific

Abstract

In humid regions of the western Pacific, conifers and deciduous broadleaf trees dominate in colder climate while evergreen broadleaf trees dominate in warmer climate. There are two geographically discontinuous forest zones of conifer-dominated forests, i.e., boreal zone and temperate-to-tropical zone. Between them, there is a deciduous broadleaf forest zone with warm summer and cold winter (Kira’s warmth index, WI from 45 to 85 °C, and coldness index, CI −15 °C). Boreal conifer forests are distributed at high latitudes or altitudes in Japan (“subarctic or subalpine forest”). Temperate conifer forests occur in a narrow belt on the Pacific Ocean side in Japan and are more extensive and distributed to warmer climate at high altitudes in Taiwan and in temperate zone of New Zealand, where they are called “conifer-broadleaf” forests. These temperate conifer (or mixed) forests grade into tropical conifer forests at high altitudes in Southeast Asia and New Guinea. Thus, these forests may be collectively called “temperate-to-tropical conifer-mixed forests” in contrast to boreal conifer forest. It appears that relatively cool summer (or year-round low temperature on tropical mountains) limits the growth of evergreen broadleaf trees, while mild winter (or the absence of winter on tropical mountains) gives the competitive advantage to conifers over deciduous broadleaf trees, leading to conifer dominance in the temperate-to-tropical conifer-mixed forests.