Climate change is altering Himalayan tree landscape — drought-resistant fir trees are replacing birch

Climate change is altering the tree line landscape of the central Himalayas, according to new research. Birch trees, a deciduous broadleaved species that dominates the region, are being replaced by fir, a slow-growing evergreen conifer. This shift is likely to alter the ecological function of Himalayan ecosystems, the paper has warned.

Fir trees (Abies spectabilis) are typically found at a lower altitude compared to the Himalayan birch (Betula utilis), but the research noted a shift in the pattern. While both species are climbing upslope on the highlands, fir is climbing faster.

Warming climate is likely to blame, said the study recently published in the journal Nature Plants. The conifers are likely benefiting from a warmer climate while birch trees are struggling to grow due to stress from increased temperatures and lack of moisture, the scientists said.

Both species are common in many valleys, forming tree lines — which mark the boundary of a habitat where trees can thrive, but cannot grow beyond. In some areas, the treelines are a mix of conifers and broadleaves. In tree lines primarily composed of birch, fir trees followed within several metres, the study noted.

However, fir and birch tree lines are shifting upslope at different rates. Over the past 200 years, fir has climbed at an average of 11 centimetres (cm) per year, while birch has moved at a slower pace of 6 cm annually. The scientists estimate that fir will outcompete birch at mixed tree lines, eventually becoming the dominant tree line species in the Himalayas.

The central Himalayas, home to some of the largest and most continuous stretches of alpine tree lines globally, are experiencing rapid climatic changes. The alpine tree line is the altitude beyond which trees are unable to grow.

The Himalayan mountain ecosystem is one of the most rapidly warming regions in the world, making it a critical area to study the impacts of climate change, the authors of the paper highlighted.

The research was carried out in Nepal’s Sagarmatha National Park, a UNESCO World Natural Heritage site that also includes Mount Everest and the Annapurna Conservation Area (Manang district), a trans-Himalayan region. To analyse tree growth, scientists extracted pencil-thin cores from tree trunks to study annual growth rings.

Climate data indicates that the region has experienced warming and drying conditions in recent decades. The Annapurna valley, situated in a rain shadow zone, receives less precipitation than neighbouring areas and has seen rising temperatures alongside declining rainfall.

“The valley receives less precipitation than nearby areas due to the rain shadow effect of the Annapurna massif and has experienced increasing temperatures and declining precipitation in recent decades,” the authors noted.

Approximately 80 per cent of the region’s rainfall occurs during the Indian summer monsoon, also known as the Southwest Monsoon. The study sites were carefully chosen to minimise anthropogenic disturbances and avoid landslide or avalanche-prone areas.

The research projected potential tree line shifts under various global warming scenarios by 2100 — including temperature increases of 2 degrees Celsius, 3.6°C and 4-5°C — to assess impacts on temperature, humidity and precipitation.

Scientists found that fir trees have been growing more in recent decades, likely because they benefit from warmer temperatures. In contrast, birch trees, which need more moisture, are growing less due to heat-related dryness. 

Other factors, like fewer birch seedlings and interactions with other plants, are also causing this decline. Fir and birch compete for space, sunlight, water and nutrients, making it even harder for birch to grow.

Researchers expect fir trees to become more common in the coming decades because they adapt better to warmer conditions. Unlike birch, which struggles to reproduce in higher temperatures, fir can handle the heat more easily.

Further, thick snow cover can make it harder for birch seedlings to grow, but warmer spring weather may speed up snowmelt and improve soil moisture in early spring.

“Birch survival is highly dependent on the supply of water from melting snow during the early growing season. Rapid warming and frequent pre-monsoon drought may thus increase soil moisture deficits with potential negative impacts on birch establishment,” the study observed.

Climate change-driven disturbances such as early snowmelt, snow fungi, avalanches, landslides, insect outbreaks, hotter droughts and fires will further hinder the recruitment of less resilient species like birch.

This change is likely to alter how the ecosystem functions.

The shift of dominant plant functional types from deciduous to evergreen may also alter ecological functions such as the rate of litter decomposition, carbon and nitrogen cycling, water-use efficiency, interactions with animals and fungi and energy fluxes, with further impacts on the composition of alpine plant communities,” the paper said.