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…