In what appears to be another grim outcome of climate change, a study has found that forests in eastern Himalayas are gradually ‘browning’, with trees withering and foliage declining even during productive seasons. Similar changes were noted in tropical mountain forests across the world.

Among the 47 protected areas across five biodiversity hotspots selected for the study, were Kangchendzonga national park in Sikkim and Namdapha national park in Arunachal Pradesh. It used satellite images from 1982 to 2006, which revealed a common trend: there was mild greening till the mid 1990s and then came a sudden and steady reversal which is making these forests appear drier and brown.

This may mean that the trees in these forests are not able to transpire at the optimum level and their photosynthesis activity has reduced due to temperature rise.

“One would imagine that the mountains would become more green with the rise in temperature, but it is not so,” said Jagdish Krishnaswamy, one of the authors and a scientist at Ashoka Trust for Research in Ecology and Environment ( ATREE). “There is a temperature induced moisture stress which is causing the trees to wither. There is less foliage even during the most productive time of the year in almost all the five regions we have studied,” The study also points to a complete loss of certain moisture regimes in these forests.

“The globally consistent browning trends that we observe indicate that such phenomenon is probably more widespread in tropical mountains, with implications for primary production and species diversity at all levels,” the study concludes.

Researchers used normalized difference vegetation index (NDVI), which monitors live green areas from remote sensing data, from 1982 to 2006 and focused it on select high altitude (1,000 to 5,000 metres above mean sea level) protected areas in the tropical belt. During this period the rise in temperature and trends in precipitation was different for different regions. Despite that, scientists spotted a similar browning trend in all the areas covered.

“In the Himalayas we see a temperature rise of about 1.5 degree C between 1982 and 2006. But it’s not the same for other continents that were studied,” explained Krishnaswamy. Except for South America, all the other four regions studied experienced a steady rise in temperature during the period but precipitation trends were inconsistent.

Says Robert John, faculty at the Indian Institute of Science Education and Research and co-author of the study, “It’s interesting that all these five regions are distinct climatically. We found that the browning trend is statistically significant. It’s real. The trend may not have started at the same time in all the regions though.”

Scientists from The Center for International Forestry Research (CIFOR) in Indonesia have also co-authored this study that has been accepted for publishing on the Global Change Biology journal.

Among others, the study has covered Mt Kilimanjaro and Rwenzori in Africa; Huascaran and Medidi in South America; Jigme Dorji and Jigme Singye national park in Bhutan in south Asia along with the two other forests in India and Khakaborazi national park in Myanmar; Lorentz national park in Indonesia, southeast Asia.