A team of scientists of the CEA (DSM and DSV), CNRS, Smithsonian Institute, and Peruvian and Bolivian research stations, recently published [1] a monthly follow-up over several years of the leaf-area index (LAI) in three stations distributed on a climatic gradient of South America, in French Guiana, Peru and Bolivia.

As expected, the method confirms that the seasonal variation is minimum close to the climatic equator (Guyana), and increases away from this line (Peru and especially Bolivia). It brings however, a surprising new and well marked result in the three studied sites: the leaf cover is maximum at the beginning of the dry season, and minimum during the rainy season. This result can be understood with the argument that the surface devoted to photosynthesis tends to be as high as possible when the solar radiation is maximum, but goes against the usual intuitions of the ecologists.

This finding has been just corroborated by a publication estimating the leaf area index by indirect methods [2] based on satellite data covering a large part of the Amazonian basin.

The initial publication presents direct, ground-based measurements of the leaf area index. Whoever knows the tropical forest will appreciate the quantity of work and the quality of international collaborations needed for a monthly follow-up in natural sites over several years and in three Amazonian countries … Ground data were taken by using a method and an instrument developed in a DSM-DSV collaboration with CNRS [3].
The leaf area index being a good indicator of the water and carbon exchanges with the atmosphere, climate models - in particular those developed at the CEA by the LSCE - will have to take this result into account.