Study shows forests have been browning for more than 20 years

Nov 23, 2013 11:25 GMT  ·  By

An international collaboration of scientists argues in a new study that the world's forests have been undergoing a browning process for more than 20 years, and say that this process leads to a significant decrease in overall photosynthetic activity. 

Photosynthesis is one of the most important natural processes occurring anywhere on Earth. Through it, plants convert carbon dioxide from the atmosphere into oxygen, in the presence of light, water and nutrients from the ground. Without it, we would not be able to breath our planet's air.

In the new paper, published in a recent issue of the journal Global Change Biology, experts reveal that high-elevation tropical forests across five continents are loosing more and more foliage each year, in response to global warming and climate change.

This conclusion is based on a formula called Normalized Difference Vegetation Index (NDVI), which calculates how green or brown a certain area is. Data for these comparisons are collected by satellites that measure how much light is reflected form particular regions on the ground.

As seen from orbit, a certain sector of forests can have either lush foliage – which makes it appear greener – or less foliage, which makes it appear browner. As a trend, since the 1990s, tropical forests are turning browner every year, suggesting an extreme reaction of changing temperature patterns.

For this study, researchers analyzed NDVI data on tropical forests at altitudes between 1,000 and 6,000 meters (3,000 to 20,000 feet), collected between 1982 to 2006, Mongabay reports. A total of 47 regions in Africa, the Americas, South Asia and Southeast Asia were covered by the research.

Overall, some 50,000 square kilometers (20,000 square miles) of forests were included in this study. Throughout these areas, a greening trend was replaced by a browning trend around the mid-1990s. The shift occurred simultaneously worldwide.

“The browning was with respect to the maximum greenness attained in each year – so it was a decline in the maximum photosynthetically active leaf biomass attained in the entire year,” explains researcher Jagdish Krishnaswamy, from the Ashoka Trust for Research in Ecology and the Environment.

“Something was affecting the ability of tropical mountain vegetation to sustain the same canopy biomass in the early 1990s. We think it was temperature related moisture stress, or loss of moisture regimes such as mist due to warming,” concludes the expert, who was also the lead author of the new study.