A Phytogeographic Frontier
The montane forests of Laos's northernmost Phongsaly Province represent a distinct biological dividing line. This region is the meeting point for two of the world's major floristic regions: the Indochinese Floristic Region of tropical Asia to the south, and the temperate Sino-Himalayan floristic region to the north. The forest exists at high altitudes, with the provincial capital itself sitting at 1,400 meters, making it the highest in Laos. At elevations around 1,800 meters, the climate cools sufficiently to support a temperate ecosystem that is out of place in the surrounding tropical lowlands.
This unique, high-elevation environment allows for the growth of montane oak forests. Tree species from the family Fagaceae, which includes oaks (Quercus) and chestnuts (Castanopsis), are characteristic of these forests. Species such as Quercus griffithii can be found here, forming open conifer-hardwood forests. These forests are essentially southern extensions of temperate ecosystems, clinging to the cool, misty mountaintops at the very edge of their viable range. The annual rainfall in these mountains can be as high as 2,500 mm, supporting an evergreen environment.