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Satellites Show Drought May Put Stress on Congo Rain Forest
June 2, 2014

The following is based on an article originally published April 23 on the NASA website.

A new analysis of NASA satellite data shows that Africa’s Congo rain forest has undergone a large-scale decline in greenness over the past decade.

The study, led by Liming Zhou, shows that between 2000 and 2012, the decline affected an increasing amount of forest area. The research, published April 23 in the magazine Nature, is one of the most comprehensive observational studies to explore the effects of long-term drought on the Congo rain forest using independent satellite sensors.

“Most climate models predict tropical forests may be under stress due to increasing severe water shortages in a warmer and drier 21st-century climate,” Zhou said.

Scientists use satellite images showing the vegetation “greenness” of forests as one indicator of the area’s health. The measure of greenness is developed from data produced by a resolution imaging instrument on NASA’s Terra satellite. 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.

The study found a gradually decreasing trend in Congo rain forest greenness, sometimes referred to as “browning.” The browning of the forest canopy is consistent with observed decreases in the amount of water available to plants, whether in the form of rainfall, groundwater, water in near-surface soils or water within the vegetation. Scientists say that a continued drying trend could alter the composition of the Congo rain forest, affecting its biodiversity and carbon storage.

Climate factors known to affect vegetation growth were in line with the observed browning. Land surface temperatures, for example, were observed to increase over most of the study area. Decreased cloudiness allowed more solar radiation to reach the plants, an action that typically promotes photosynthesis. But in this case, it likely posed an extra stress on the plants from the depletion of soil moisture.

“Forests of the Congo Basin are known to be resilient to moderate climate change because they have been exposed to dry conditions. 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,” said researcher Sassan Saatchi.

How the changes affect individual plant species remains to be seen. One possibility is that drier conditions may favor deciduous trees at the expense of evergreen trees.

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

Photo credit: NASA