Shifting Himalayan Tree landscape and its impact on Ecosystems

In the breathtaking expanse of the majestic Himalayan mountains, it’s not solely seasoned climbers who engage in a fervent race to reach the peaks. Trees, too, are embroiled in a dramatic contest for survival and dominance amidst these towering heights. In numerous locations, birch, which stands as the predominant tree species of these high-altitude highlands, is gradually ceding ground to fir, a slow-growing evergreen conifer. This significant shift, as revealed by a recent study published last month in the distinguished journal Nature Plants, could potentially reshape habitats for various other species and fundamentally alter the intricate dynamics of the entire Himalayan ecosystem. The study’s findings emphasize that it is “necessary to take a community view” when assessing how a changing climate is impacting these complex ecosystems, remarks Pieter De Frenne, an esteemed ecologist at Ghent University who was not directly involved in the research.

The central Himalayas proudly boast one of the world’s most extensive and continuous tree lines, a stunning natural border that showcases the resilience of these flora. Over the expansive time frame of 2 million to 5 million years, birch trees (Betula utilis) established their presence on newly exposed rocky terrain, which was left behind by the relentless advance and retreat of glaciers. Subsequently, the Himalayan fir (Abies spectabilis) began to spread into this varied landscape. Over the course of time, this natural evolution has resulted in a diverse mosaic of forests where either birch or fir trees dominate the scene. Currently, the Himalayas are experiencing episodes of warming and drought at an accelerated pace compared to the global average. The question of how its tree lines—those defining markers that signify the highest altitude at which trees can thrive—are responding to these swift and dramatic changes has long captivated botanist Shalik Ram Sigdel. In the course of his extensive work, he frequently encountered tree lines composed of a vibrant mix of both birches and firs. “I became really curious how one species was responding compared to the other,” he reflects thoughtfully.

The team focused on key forests near Mount Everest and in the western Annapurna Conservation Area, choosing stands with minimal human disturbance and similar climatic conditions. At each site, they selected plots roughly twice the size of a typical basketball court, containing between 400 and 700 trees.

The rugged terrain and unpredictable weather in the Himalayas made fieldwork challenging. “There were times when a clear day suddenly turned snowy, and we could barely find our way back to the hotel using GPS,” he recalls.

For each tree, they drilled into trunks near the ground and extracted pencil-thin cores. The ages of older trees were determined by counting annual growth rings. For younger trees, which don’t yet have visible growth rings, they counted tiny marks left by annual growth, known as bud scars.

With those data, Sigdel and colleagues could track the spread of each species along the tree line, identify the number of newly growing trees, and analyze their locations and growth patterns. The team found that fir trees have been reproducing more in recent decades, whereas birch trees had their peak reproduction between the 1920s and 1970s, with lower rates in recent years. In addition, fir trees are shifting uphill at about 11 centimeters per year—roughly the length of a pen—compared with birch trees, which move at about 6 centimeters per year, about the width of a smartphone.

The researchers predicted how tree lines might shift by 2100 under three warming scenarios: low warming (up to 2°C), moderate warming (up to 3.6°C), and high warming (4°–5°C) compared with preindustrial levels. By examining tree growth under these different temperature, precipitation, and humidity conditions, they found that fir trees would thrive in warmer, drier conditions, whereas birch would struggle to keep up. With continued warming under most warming scenarios, they predict, fir will continue to surge upward and claim higher territories, and the birch tree line is likely to stay put or even slip downhill.

Experts note that change could be concerning for the people and animals that call these mountains home. “Expanding tree lands and shrublands are shrinking snow leopard habitats, which prefers open landscapes for hunting,” says Parveen Chhetri, an environmental scientist at California State University, Dominguez Hills who was not involved in the study. And forests encroaching into grazing lands also puts them at odds with ranchers, he notes. This is already happening in Langtang, a central Himalayan valley, where “locals have even begun cutting tree lines to reclaim grazing land for yaks and sheep,” he adds.

Ultimately, Chhetri says, these kinds of studies can give locals and forest managers the tools to forecast how these tree lines will morph and shift in response to different climate dynamics and so they can plan for the future.