Discussion on the Origin of Mid-Montane Wet Evergreen Broad-Leaved Forest in Yunnan

Mid-montane wet evergreen broad-leaved forest is widely distributed in the upper montane areas of Yunnan between 1800~3400 m. This forest has the same ecological and physiognomic characteristics as the evergreen broad-leaved forest of the subtropical lowlands; however, although it has a tropical geographical affinity in its flora, it occurs in warm-temperate to temperate climates. Using the data of a well-studied mid-montane wet evergreen broad-leaved forest in the Ailao Mountains as an example, the apparent disharmony between the forest and the climate where it occurs is discussed. The forest is comprised of 821 seed plant species, belonging to 386 genera in 110 families. In regards to the geographical elements of the flora, tropical families are major contributors, suggesting ancient tropical origin. The tropical distributions make up 47.75% of total genera and 33.45% of total species, strongly suggesting tropical affinity of the flora. The present temperate climate, where the montane wet evergreen broad-leaved forest occurs, is similar to the climate in the eastern regions of China with deciduous broad-leaved forests, in which tropical genera are only minor contributors to the flora. Obviously, the high proportion of tropical elements and the evergreen broad-leaved physiognomy of the forest on the upper montane of the Ailao Mountains exhibit apparent dissonance with the local climate. This might be explained by the rapid uplift of the Ailao Mountains with the Himalayas during the Quaternary period, with the indigenous flora not conspicuously changing although the habitat became a temperate one. Thus, the former evergreen broad-leaved forest found in the tropical/subtropical lowlands has continued in the high montane habitat with the uplift of the terrain.