By Henry Fountain, The New York Times, April 23, 2014
Years of drier conditions in the Congo River basin in central Africa appear to be affecting trees in the region’s vast rain forests, scientists reported on Wednesday.
Writing in the journal Nature, the researchers said the capacity of the trees to photosynthesize had declined. If this trend continues, they suggested, a long-term result could be changes in the structure and composition of the region’s forests, the largest expanse of rain forest in the world after the Amazon. Those potential changes — which could eventually mean a shift from a classic rain forest with a closed canopy of trees to a more open, savanna-like environment — could affect the region’s biodiversity and its capacity to fix and store carbon dioxide from the atmosphere.
The lead researcher, Liming Zhou of the University at Albany, cautioned that the analysis had used only data from remote-sensing satellites, including one that uses forest greenness as an indirect measure of photosynthetic capacity. He and others said that field studies were needed to confirm any changes that are occurring — to determine, for instance, whether trees are actually dying from the drought, which the satellite data did not show.
“This is just a very first step,” Dr. Zhou said.
Many models of climate change project that periods of drought will increase in the tropics, home to most of the world’s rain forests. Average rainfall in the Congo basin has been declining for several decades, and while this may not be a consequence of climate change — one recent study suggests that it is part of a natural cycle — studying the impact will help scientists understand what may be in store as the planet warms.
“This is the type of signal you’d expect if a region is experiencing a directed shift in climate,” said Jeffrey Q. Chambers, a geographer at the University of California, Berkeley, and an author of a commentary on the research, also published in Nature. “What needs to happen now is continued observation to better understand whether, in fact, this is a climate-related trend.”
“Satellite data can only tell you so much,” Dr. Chambers said. “You really need to get into the field and see what’s happening.”
Most studies of the impact of reduced rainfall on rain forests have taken place in the Amazon. Conditions in that region have been different from those in the Congo basin — the Amazon experienced two short but intense droughts in the past decade, rather than a more gradual, long-term drought — and the effects have been studied on the ground as well as from orbit. Very few field studies have been conducted in the Congo basin.
Although rain forests stretch into West Africa, cloud cover and pollution affect the quality of satellite data from that region. So Dr. Zhou and his colleagues focused on the Congo basin, using data from several sensor systems, including one that produces images of the entire planet every few days at various wavelengths of light, visible and otherwise.
Use of this kind of satellite imagery is a subject of much debate because the position of the sun can produce shadows in the images that can affect the analysis. But Dr. Chambers said the researchers appeared to have minimized that problem by looking at images collected during the same period from year to year.
They found that the forests had become less green over the last decade. That correlates with changes in the health of the vegetation, including a decline in the amount of the green pigment chlorophyll, which plants use for photosynthesis. Data from other satellite sensors, including ones that showed changes in water content in the vegetation and in the land, were consistent with the finding on greenness, as were the data on rainfall.
“Trees are not as vigorous without enough water,” Dr. Zhou said, and over time, drought will favor deciduous trees rather than broad-leaved evergreens, which account for most of the trees in a tropical rain forest.
The period of the study was too short to see such large-scale changes. “But that’s what we’re worried about for the future, in the context of global warming,” Dr. Zhou said.