Drought may take a toll on Congo Rainforest, NASA Satellites Show

A new analysis of NASA satellite data shows that Africa's Congo rainforest, the second-largest tropical rainforest in the world, has undergone a large-scale decline in greenness over the past decade.The study, lead by Liming Zhou of University at Albany, State University of New York, shows that between 2000 and 2012, the decline affected an increasing amount of forest area and intensified. The research, published April 23 in Nature, is one of the most comprehensive observational studies to explore the effects of long-term drought on Congolese rainforests using several independent satellite sensors.Scientists use the satellite-derived "greenness" of forest regions as one indicator of a forest's health. While this study looks specifically at the impact of a persistent drought in the Congo region since 2000, researchers say that a continued drying trend might alter the composition and structure of the Congo rainforest, affecting its biodiversity and carbon storage.

"It's important to understand these changes because most climate models predict that tropical forests may be under stress due to increasing severe water shortages in a warmer and drier 21^st century climate," Zhou said.

Previous research used satellite-based measurements of vegetation greenness to investigate changes in the Amazon rainforest, notably the effects of severe short-term droughts in 2005 and 2010. Until now, little attention has been paid to African rainforests, where ground measurements are even sparser than in the Amazon and where droughts are less severe but last longer.To clarify the impact of long-term drought on the Congo rainforest, Zhou and colleagues set out to see if they could detect a trend in a satellite measure of vegetation greenness called the Enhanced Vegetation Index. This measure is developed from data produced by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA's Terra satellite. The scientists focused their analysis on intact, forested regions in the Congo basin during the months of April, May and June each year - the first of the area's two peak rainy and growing seasons each year.
The study found a gradually decreasing trend in Congo rainforest greenness, sometimes referred to as "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's in the form of rainfall, water stored in the ground, water in near-surface soils, or water within the vegetation.