The extra growth may have wider impacts, particularly on the flow of water into the rivers that flow down from the Himalayas, says Karen Anderson of the University of Exeter, UK. “If the ecology is changing, that will have impacts on the hydrology. Nobody’s considered that before.”
Anderson’s team used data from NASA’s Landsat satellites to study high-altitude vegetation in the Himalayas between 1993 and 2018. They focused on the subnival zone: the highest of the regions above the treeline that has seasonal, but not permanent, snow cover. Subnival plants are small and include grasses, shrubs and mosses.
The researchers found that the Himalayan subnival zone is vast, with an area at least 5 times that covered by permanent ice and snow. However, because it is difficult to reach, it hasn’t been studied much. “There’s hardly any ecological data from this region at all,” says Anderson.
By tracking which regions showed up green in satellite images, the team found that subnival vegetation has increased since 1993. The biggest increase occurred between 5000 and 5500 metres above sea level.
It isn’t clear precisely why the plants are colonising higher ground. “My initial guess would be that the primary driver is temperature limitation being removed,” says Anderson. The Himalayas are warming particularly rapidly, due to humanity’s greenhouse gas emissions, so elevations that were once too cold for plants may now be tolerable. Modelling studies have suggested that plants will move uphill for this reason. Anderson says the extra carbon dioxide in the air could also be helping the plants grow.
Most studies of the impact of warming on the Himalayas have focused on the melting of glaciers, which could play havoc with the water supplies of neighbouring countries such as India by causing floods and droughts. Anderson says it is vital to find out how the new plants will affect the flow of water. They may absorb more heat than bare ground, leading to more warming and thus faster melting of snow – or they could lock up moisture, helping to smooth out the flow of water.
It is unclear how much higher plants could go as the climate warms. Already, some hardy plants have been found growing 6159m above sea level. However, the highest mountain peaks are still very cold, covered in snow and ice and they lack soil. “Whether or not plants could grow on the peak of Everest, I think that’s probably unlikely within our lifetime or even within 100 years,” says Anderson.
Journal reference: Global Change Biology , DOI: 10.1111/gcb.14919