Publisher Summary The topographic heterogeneity of the Mekong Basin produces highly variable mosaics of seasonal and total rainfall. Mean annual rainfall within the Mekong Basin varies from as little as 600 mm to 5000 mm or more in portions of the upper Annamite Range. Total rainfall generally shows a strong correlation with the seasonality of rainfall, with areas of lower rainfall exhibiting a longer dry season. The upper watershed of the Mekong River drainage in northwestern Yunnan, Tibet, and Qinghai Provinces of China provides a transition from tropical and subtropical forests of Southeast Asia to temperate forests to the north. Lower valleys below 1000 m elevation with relatively mesic conditions characteristically support semievergreen forest. The canopy exhibits a mixed dominance of both evergreen and deciduous broad-leaved species, with deciduous tree species attaining their major importance on drier valley slopes. A large area of the upper Mekong drainage at intermediate elevations is characterized by a dissected cover of subtropical conifer forest with Pinus yunnanensis occurring over a broad range but commonly at 1500–3000 m, and Pinus armandii at 2500–3000.