The transition from tropical to subtropical (warm temperate) evergreen
forests is more clearly apparent in East Asia, from Nepal to the western
Pacific coast, than elsewhere in the tropics. We review the nature of
this transition and hypothesize the physical, ultimately climatic,
factors that may maintain it, with a special focus on how the increasing
instability and warming of climates will affect these forests. A
primary climatic mediator of the transition is proposed, thereby
offering a testable hypothesis for the climate–forest transition
relationship. What is known of this transition is summarized in context
of the primary climatic mediators of elevational zonation of forest
formations in equatorial Asia to the tree line, in the Himalaya at the
India-Indo-Burma northern tropical margin, and as both elevational and
latitudinal zonation in southern China. Consequent secondary edaphic and
other physical changes are described for the Himalaya, and hypothesized
for southern China. The forest ecotones are seen to be primarily
defined by tree floristic change, on which account changes in structure
and physiognomy are determined. The montane tropical-subtropical
transition in the Himalaya is narrow and observed to correlate with an
as yet ill-defined frost line. A distinct tropical-subtropical
transition forest is recognized in the southwest China mountains. There
is a total change in canopy species at the Himalayan ecotone, but
subcanopy tropical species persist along an elevational decline of c.
400 m. The latitudinal transition in South China is analogous, but here
the tropical subcanopy component extends north over ten degrees
latitude, albeit in decline. The tropical-subtropical transition is
uniquely clear in East Asia because here alone a tropical wet summer-dry
winter monsoon extends to 35° north latitude, encompassing the
subtropical evergreen forest, whereas subtropical evergreen forests
elsewhere exist under drier temperate summer climate regimes.