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Africa's Green Heart Turning Brown

Africa’s Congo rainforest, the second-largest tropical rainforest in the world, has lost its much greenness over the past decade, a new analysis of satellite data shows.

The study demonstrates that a persistent drought in the Congo region since 2000 has affected the greenness of an increasing amount of forest area and that the browning trend has intensified over the 13 years of the study.

Scientists use the satellite-derived measurements of forest “greenness” as one indicator of a forest’s health.

Published Wednesday in the journal “Nature,” the research uses several independent satellite sensors to measure the effects of long-term drought on the Congo rainforest.

The research team was led by Dr. Liming Zhou, associate professor of atmospheric and environmental sciences with University at Albany, State University of New York.

“It’s important to understand these changes because most climate models predict tropical forests may be under stress due to increasing severe water shortages in a warmer and drier 21st century climate,” said Zhou, who led a team of scientists from Australia, China, France and the United States.

Straddling the equator and covering 700,000 square miles (1,800,000 km2) in six countries, the Congo rainforest contains a quarter of the world’s remaining tropical forest.

This rainforest is inhabited by some 450 species of mammals, including forest elephants, okapi, bonobos, and all three subspecies of gorillas – the mountain gorilla, lowland gorilla, and the eastern lowland gorilla.

There are 300 reptile species, 200 amphibians, over 1,000 species of birds and more than 11,000 different plant species.

Zhou and his team say if the drying trend continues it might alter the composition and structure of the Congo rainforest, affecting its biodiversity and ability to absorb the greenhouse gas carbon dioxide.

Theirs is the most comprehensive study of the effects of long-term drought on Congolese rainforest, although similar research has investigated greenness changes in the Amazon rainforest after the severe short-term droughts in 2005 and 2010.

“Previous research used the satellite-measured vegetation greenness to investigate the effects of two severe short-term droughts in the Amazon rainforest, while little attention has been paid to African rainforest due the lack of ground measurements,” observed Ranga Myneni at Boston University, a co-author of the study.

Zhou and his team analyzed intact, forested regions in the Congo basin each year during the months of April, May and June – the first of the area’s two peak rainy and growing seasons.

The study found a gradually decreasing trend in Congo rainforest greenness, or “browning,” suggesting a slow adjustment to the long-term drying trend.
This is in contrast to the more immediate response seen in the Amazon, such as large-scale tree mortality, brought about by more episodic drought events.

The browning of the forest canopy is consistent with observed decreases in the amount of water available to plants, whether that is in the form of rainfall, water stored in the ground, water in near-surface soils, or water within the vegetation.

These changes in available water were detected in part with NASA satellites including the NASA/JAXA Tropical Rainfall Measuring Mission, NASA’s Quick Scatterometer (QuikScat), and NASA’s Gravity Recovery and Climate Experiment, a joint mission with the German Aerospace Center.

The scientists also used the Enhanced Vegetation Index developed from data produced by the Moderate Resolution Imaging Spectroradiometer, MODIS, instrument on NASA’s Terra satellite.

“Combining measurements from different sensors has given us more confidence in the results of the MODIS data and provided us with insights into the environmental and physiological mechanisms of the browning observed by the MODIS data,” said co-author Sassan Saatchi of NASA’s Jet Propulsion Laboratory in Pasadena, California.

The scientists observed climate factors known to affect vegetation growth that resulted in browning. Land surface temperatures increased over most of the study area. Fewer clouds allowed more solar radiation to reach the plants. More sunshine typically promotes photosynthesis, but in this case it likely posed an extra stress on the already drying plants and trees.

Saatchi said, “Forests of the Congo basin are known to be resilient to moderate climate change because they have been exposed to dry conditions in the past few hundred years. However, the recent climate anomalies as a result of climate change and warming of the Atlantic Ocean have created severe droughts in the tropics, causing major impacts on forests.”

How the changes affect individual plant species in the Congo forests remains to be seen. Drier conditions may favor deciduous trees at the expense of evergreen trees, the scientists say.

“Our assessment is a step toward an improved understanding of how African rainforests respond to increasing drought,” Zhou said. “We need to consider the complex range of processes affecting different tropical rainforest species before we can fully assess the future resilience of tropical forests.”

“The study is likely the most comprehensive observational study thus far exploring the effects of long-term drought on Congolese rainforests, involving analyses of several independent remote sensing products (optical, thermal infrared, microwave and gravity) over the Congo,” said co-author Yuhong Tian with I.M. Systems Group at NOAA’s National Environmental Satellite, Data, and Information Service.

The other authors for this research include:
Philippe Ciais at Laboratoire des Sciences du Climat et de l’Environnement, Gif sur Yvette Cedex, France
Yi Y. Liu at University of New South Wales, Australia
Shilong Piao at Peking University, China; Haishan Chen at Nanjing University of Information Science and Technology, China
Eric Vermote of NASA’s Goddard Space Flight Center, Greenbelt, Maryland
Conghe Song and Taehee Hwang at University of North Carolina at Chapel Hill


Article by: Environmental News Service




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