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.
A Biologically Unexplored Region
Much of the Phongsaly montane forest lies within the Phou Den Din National Protected Area, a 2,220 square kilometer reserve that borders Vietnam. Due to its remoteness and difficult access, it is one of the least surveyed protected areas in Laos. The terrain is rugged, with elevations inside the park reaching over 2,000 meters. These forests are predominantly dry evergreen and semi-evergreen, with large tracts of old-growth forest remaining intact.
The forests support a significant diversity of animal life, though comprehensive surveys are sparse. They provide a habitat for the critically endangered Northern white-cheeked gibbon (Nomascus leucogenys), with Laos holding the largest remaining populations of the species. Other recorded mammals include Asiatic black bears, sun bears, gaurs, and possibly tigers. The constant moisture and cloud cover also create ideal conditions for a variety of orchids, ferns, and medicinal plants, many of which may be new to science.