A new study has debunked a widely-held belief in the scientific community that global warming would cause the tree line -- the mountain zone after which trees stop growing -- to advance upslope.

The initial findings of the research, which is being conducted under the National Mission for Himalayan Studies implemented by the ministry of environment, forest & climate change, suggest that with increase in global temperatures, tree lines were not shifting to higher elevations. Researchers said that one reason could be that warmer temperatures were leading to drier conditions which negatively affected seed survival.

Researchers from various institutes, including GB Pant National Institute of Himalayan Environment and Sustainable Development (GBPIHED), Wildlife Institute of India, Birbal Sahni Institute of Palaeobotany in Lucknow, Kumaun University, University of Kashmir and Nainital-based NGO Central Himalayan Environment Association (CHEA), started the study in March 2016. The study is being conducted in three sites: Kashmir (Sinthan Top) in western Himalayas, Uttarakhand (Tungnath) in central Himalayas and Sikkim (Duksan Top) in eastern Himalayas.

Scientists are studying how tree lines respond to climate change since an advancing tree line would have implications for biodiversity of the ecotone (area of transition of vegetation between two plant communities). The expansion of tree line could impact rare species and plant communities in this region.

Study coordinator and former scientist from Forest Research Institute S P Singh said, “Many scientists have predicted that the tree line will expand with global warming. But we did not see evidence of it in our study.”

Singh said that an increase in temperature leads to drier conditions which negatively impacts seed survival. He added that global warming was a complex phenomena and its effect on tree lines would take time to be clearly established.

A few other studies have also found that response of tree lines to observed climate warming has been mixed. A study conducted in 2009 by Melanie Harsch from Lincoln University in New Zealand analyzed a global dataset of 166 sites for which tree line dynamics had been recorded since 1900 AD. It found that 47% of the tree lines did not show any elevational shift while 52% advanced to higher elevations. Another study published in December 2016 in the journal Global Change Biology found that two subalpine tree species of western North America were not moving to higher elevations as temperatures rise. The study found that warming decreased the young trees’ odds of survival.

Researchers in the present study also said that tree line, timber line and ecotone should be defined based on vegetation and not on geographical parameters. This is because several species had developed unique survival strategies to tackle climate change, scientists said.

R S Rawal, a scientist at GBPIHED, said, “Climate change is inducing species to come up with strange survival strategies. Tall trees in Himalayas were able to germinate with the help of a shrub species, Juniperus, which formed a thicket over seeds of these trees to protect them from snow, rain and air.”

The study also found that contrary to scientific belief, temperature plunged 0.5 degree Celsius with 100m rise in altitude. It was earlier believed that a decrease of 0.6 degree Celsius was witnessed with 100m rise in altitude.

Source: June 28, 2017, The Times of India