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.