Keena trees across the plateau began dying simultaneously in the 1980s. Researchers suspect acid deposition from distant pollution, making this an important ecosystem for long-range atmospheric contamination.
The Silent Sickness
In the late 1970s and early 1980s, a strange affliction began to spread through the montane cloud forests of Horton Plains National Park. Trees, particularly the main species Calophyllum walkeri—known locally as Keena—started dying from the top down. This phenomenon, called forest die-back, involves the progressive death of twigs and branches, leading to a skeletal appearance and eventually the collapse of the entire tree. Surveys revealed a severe problem, with some affected areas showing a nearly 50% loss of vegetation.
The die-back affects at least 22 plant species within the park, but Calophyllum walkeri is the most visibly and severely impacted. This evergreen tree, which can grow up to 7 meters tall, lives in the high-elevation forests, typically found between 1,220 and 2,400 meters. The ecosystem of Horton Plains is a unique montane grassland and cloud forest, sitting at an elevation of 2,100–2,300 meters (6,900–7,500 ft). This high-altitude environment, constantly bathed in mist and fog, makes the ecosystem particularly sensitive to atmospheric changes. The simultaneous decline across a wide area pointed not to a localized pest or disease, but to a more pervasive environmental stressor.
A Distant Cause
The leading hypothesis for the die-back is acid deposition, a consequence of long-range air pollution. Industrial emissions, primarily sulfur dioxide (SO2) and nitrogen oxides (NOx), can travel hundreds of kilometers in the atmosphere before falling back to Earth as acid rain, fog, or dust. Researchers suspect that pollutants from India and other parts of Southeast Asia are being transported to the central highlands of Sri Lanka.
Acid deposition does not typically kill trees directly. Instead, it weakens them through several mechanisms. It can damage the protective waxy coating on leaves, making them unable to perform photosynthesis efficiently. It also leaches essential mineral nutrients like calcium and magnesium from the soil, while simultaneously releasing toxic heavy metals like aluminum. This combination starves the trees of nutrients and poisons their root systems. The constant moisture in a cloud forest can concentrate these acidic pollutants, bathing the foliage in a harmful solution that strips away nutrients.
While other factors like drought, frost, and even bark damage from the local Sambar deer population have been considered, the widespread and species-specific nature of the die-back strongly implicates airborne chemical agents. The forest at Horton Plains has become an unintentional monitor, showing the far-reaching effects of industrial pollution on a remote and otherwise pristine environment.
💡Fun Facts
Horton Plains is part of the Central Highlands of Sri Lanka, a UNESCO World Heritage Site.
The main affected tree, *Calophyllum walkeri*, is endemic to Sri Lanka, meaning it is found nowhere else on Earth.
The plains form the headwaters for three of Sri Lanka's largest rivers: the Mahaweli, Kelani, and Walawe.
Stone tools dating back to the prehistoric Balangoda culture have been discovered in the park.
The park is open daily from 6:00 AM to 6:00 PM. Ticket counters close at 4:00 PM. It is best to start hikes early to avoid mist that often obscures views after 9:00 AM.
Admission
Entrance fees vary for foreign and local visitors and are tied to the US dollar exchange rate. Expect to pay around $55-$75 USD for an adult foreign tourist, including vehicle fees.
Accessibility
The main 9.5 km circular trail is well-marked but covers uneven terrain. Visitors drive to a central car park and proceed on foot. The area is remote with no direct public transport to the park entrance.
Horton Plains Cloud Forest Die-BackCentral Province, Sri Lanka