The world's richest known diversity of tree species is in the upper Amazon region of Peru, where as many as 300 species can be counted in a typical hectare (about 2 1/2 acres), a botanist has reported in the January issue of the Proceedings of the National Academy of Sciences.

Until now, botanists had thought Southeast Asian forests were the most diverse, but those forests have no more than 200 tree species per hectare. Central African forests peak at about 120 species per hectare. Comparable figures for most of the United States range from one species per hectare to a dozen or so.

The Peruvian findings were made by marking off sample hectares and identifying every tree measuring at least 2 1/2 inches in diameter at breast height.

"In the first 50 trees sampled at Yanamono {Peru}, only two species were repeated," Alwyn H. Gentry wrote in his report. Gentry, who does much field work in Peru, is based at the Missouri Botanical Garden in St. Louis, a major center for research on endangered Amazon forests.

The same Peruvian region may be the world's richest in all forms of life. Nearby are the documented record-holders for maximum species diversity among butterflies, amphibians, reptiles, birds and mammals.