A view of the entire African rainforest area
(green) transitions into a view of the Congo region (mostly brown) that is the
subject of new research published in Nature. The study area represents intact
areas in the Congo rainforest where satellite data are high
quality.
Image Credit:
NASA Goddard's Scientific Visualization
Studio
A new analysis of NASA satellite data shows 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, led by Liming Zhou of University at Albany, State University of
New York, shows between 2000 and 2012 the decline affected an increasing amount
of forest area and intensified. The research, published Wednesday in Nature, is
one of the most comprehensive observational studies to explore the effects of
long-term drought on the Congo rainforest using several independent satellite
sensors.
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"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," Zhou said.
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.
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 whether 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 is in the form of
rainfall, water stored in the ground, water in near-surface soils, or water
within the vegetation.
In the Congo rainforest, a browning trend (brown)
dominates smaller areas that show a greening trend (green) during April, May and
June each year from 2000 to 2012.
Image Credit:
NASA Goddard's Scientific Visualization
Studio
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.
"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 (JPL)
in Pasadena, Calif.
Climate factors known to affect vegetation growth were also 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, which typically promotes photosynthesis, but in
this case it likely posed an extra stress on the plants from the resulting
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 in the past few hundred
years," Saatchi said. "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 area remains to be
seen. For example, drier conditions may favor deciduous trees at the expense of
evergreen trees.
"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 other authors for this research include Yuhong Tian at I.M. Systems
Group, Inc. at the Center for Satellite Applications and Research, the science
arm of National Oceanic and Atmospheric Administration's Satellite and
Information Service; Ranga Myneni at Boston University; 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, Md.; and Conghe Song and Taehee Hwang at University of North
Carolina at Chapel Hill.
NASA monitors Earth's vital signs from land, air and space with a fleet of
satellites and ambitious airborne and ground-based observation campaigns. NASA
develops new ways to observe and study Earth's interconnected natural systems
with long-term data records and computer analysis tools to better see how our
planet is changing. The agency shares this unique knowledge with the global
community and works with institutions in the United States and around the world
that contribute to understanding and protecting our home planet.
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Guillermo Gonzalo Sánchez Achutegui
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