Rising temperatures pushing treelines up

Kathmandu, Nov. 29: Rising temperatures in the Himalayas are causing significant changes in forests, with fir trees quickly replacing birch trees at alpine treelines. A treeline is the altitude above which trees do not grow. 

According to research, fir trees (tall and evergreen trees commonly found in colder regions and mountainous areas) have been spreading and moving uphill faster (about 11 cm per year) compared to birch trees (about 6 cm per year) over the past 200 years.

The shift, driven by climate warming, could reshape forest ecosystems and affect the services they provide, like water conservation and climate regulation, said a research ‘Accelerated succession in Himalayan alpine treelines under climatic warming’ published on "Nature Plants" recently.

Researchers, including Dr. Shalik Ram Sigdel, Xiangyu Zheng, Flurin Babst, J. Julio Camarero, Shan Gao, Xiaoxia Li, Xiaoming Lu, Jayram Pandey, Binod Dawadi, Jian Sun, Haifeng Zhu, Tao Wang, Eryuan Liang and Josep Peñuelas.

Their research focused on tracking ecosystem processes along environmental gradients by analyzing plant functional traits.

Dr. Sigdel, the lead author of the research, explained that the study focuses on understanding how alpine treelines and shrub lines respond to climate change across different spatial scales, with a particular emphasis on the Himalayas. Using dendroecological tools (methods and techniques used to study tree rings), the research examined forest dynamics in the central Himalayas, he said.

The findings reveal that late-successional fir trees, which thrive in warmer and shadier conditions, are moving upslope more rapidly than pioneer birch trees. 

Fir trees are increasingly dominating many treeline areas, outcompeting birch due to their greater adaptability to changing climatic conditions. This shift highlights the accelerating impact of climate change on forest ecosystems in the region.

“This change is a clear sign of how climate warming is speeding up forest succession, a natural process where one group of species replaces another over time. Experts believe this could lead to lasting impacts on the structure and function of Himalayan forests, changing how they support wildlife, store water, and help fight climate change,” Dr. Sigdel said.

Forest succession and climate change

Researchers analysed the dynamics of mixed treelines, comprising pioneer birch trees and late-successional fir trees. By studying the age structure of tree populations, spatial patterns, and recruitment processes, the team reconstructed decades of treeline shifts and simulated future scenarios using advanced treeline models.

Johanna Toivonen, a researcher from the School of Forest Sciences at the University of Eastern Finland, highlighted the implications saying, “Fir, being more drought-resistant, is likely to replace birch as the dominant species at treelines in the Himalayas. This transition, driven by climate warming, could have profound effects on ecosystem functioning and services, including water conservation and climate regulation.”

This shift in species dominance is not merely a local phenomenon but a harbinger of broader ecological changes. Late-successional species like fir can significantly alter the structure and function of forests. For instance, a transition from deciduous forests (dominated by birch) to coniferous forests (dominated by fir) could affect soil health, water retention, and biodiversity.

The study also underscores the importance of treelines as ecological indicators. Treelines are highly sensitive to climatic changes, making them ideal for observing how ecosystems respond to warming temperatures, said Dr. Sudeep Thakuri, a Climate Scientist.

Dr. Thakuri explained that rising temperatures are influencing treelines, causing vegetation to shift upward. Various studies, including those conducted in Taplejung, Barun Valley, Sagarmatha region, Manang, Shey Phoksundo, and Rara, indicate that Salla (pine) trees and Betula (birch) trees are moving to higher elevations compared to their previous ranges due to increasing temperatures.

“From various studies, we can confirm that changing climatic conditions are altering vegetation patterns. Areas that once had bushes are now seeing the growth of trees like pines and birch,” he added.

Another similar study by Narayan Gaire of Tribhuvan University, Ze-Xin Fan of Key Lab of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Parveen K. Chhetri of California State University, Dominguez Hills and Santosh K. Shah of Birbal Sahni Institute of Palaeosciences also found out that the country’s Himalaya is experiencing rising temperatures, erratic rainfall and more frequent droughts, which significantly impact alpine treelines. Treelines, which range in elevation from 3400 to 4500 metres above sea level, are vital indicators of environmental changes in high-altitude regions. 

The study also showed that treeline shifts vary by site and species, with an average upward movement of 0.46 metres per year, though rates range from 0 to 2.6 metres per year depending on local conditions. Factors such as winter and summer temperatures and spring precipitation play a key role in these dynamics.

Interestingly, treeline responses are not limited to upward shifts, some areas display varied patterns due to complex interactions between biotic and abiotic factors. The study emphasises the need for more comprehensive research across multiple species, locations, and environmental factors to better understand these changes, according to the study.

Scientists recommend more long-term studies to understand these changes better and to prepare for the effects on local communities and ecosystems. The findings highlight the urgent need to address climate change and its impact on sensitive mountain regions.