A mountain forest where trees are climbing uphill. The tree line has shifted 200 meters higher since the 1960s as temperatures warm. A forest on the move.
An Uphill Migration
In the mountains of Laos, an entire forest is migrating. The Phou Khao Khouay evergreen forest is moving uphill, a direct response to a warming climate. The treeline—the highest elevation where trees can grow—has advanced approximately 200 meters upslope since the 1960s. This shift is a clear, measurable indicator of long-term temperature increases in the region.
Treelines are sensitive ecological boundaries, or ecotones, limited primarily by temperature. Globally, a warming climate allows trees to colonize higher, previously colder altitudes. This phenomenon, known as thermophilization, is happening in mountain ranges worldwide, but the scale of the shift in Phou Khao Khouay is particularly distinct. The forest's elevation ranges from 200 meters to 1,761 meters, providing a steep gradient for this migration to occur. The mean annual temperature is 26.6°C, but subtle, long-term increases are enough to push the boundaries of tree survival higher up the slopes. The dominant forest types undergoing this change include evergreen forests and stands of coniferous trees like Pinus latteri and Fokienia hodginsii.
A Vertical Laboratory
The 1,920-square-kilometer Phou Khao Khouay National Park is a vertical laboratory for studying climate change impacts. Established in 1993, the park's varied topography of sandstone cliffs, river gorges, and extensive flat uplands creates diverse microclimates. The upward shift of tree species has cascading effects on the entire ecosystem. As the forest composition changes, the habitats for countless other species are altered.
The park has considerable biodiversity, including Asian elephants, white-cheeked gibbons, and the near-extinct green peafowl. The movement of the forest means that animals dependent on specific vegetation for food and shelter must either adapt or follow the shifting habitat. This process can create competition with species already established at higher elevations and disrupt established food webs. The park’s underlying geology is a sedimentary complex of schist, clay, and sandstone, which influences soil composition and water retention, further complicating how different plant species respond to warming. Researchers monitor these changes to understand how resilient—or vulnerable—such ecosystems are to continued climate shifts.
💡Fun Facts
The name Phou Khao Khouay translates to "Mountain of the Water Buffalo Horn," a name derived from local legends and the shape of the mountain range.
The park is a watershed for three major rivers—the Nam Mang, Nam Leuk, and Nam Gnong—which all flow into the Mekong River.
Botanical surveys between 2015 and 2019 identified 14 species of flowering plants in the park that were previously unrecorded in Laos.
The park hosts several ethnic groups, including Hmong, Khmu, and Yao, with three villages located entirely inside the park's boundaries.
The visitor center is open daily from 8:00 AM to 5:00 PM; the park itself is generally accessible 24/7.
Admission
An entrance fee is typically required. Costs for guided tours can range from $30 to $100 USD.
Accessibility
The terrain is mountainous and rugged. Access often requires a 4x4 vehicle, especially on the steep and rocky dirt roads leading to the plateau. Trails can be slippery during the wet season.