Review on savanna vegetation in the dry hot river valleys of southwestern China

The deep valleys in southwestern China experience a dry, hot climate and support savanna-like vegetation, largely shaped by the “foehn” effect. This vegetation is primarily dominated by grasses, interspersed with scattered shrubs and trees, resembling the physiognomy of savannas in other parts of the world. The savanna-like ecosystem hosts a rich diversity of species adapted to the dry, hot valley habitat, including many local endemic plants. In terms of life forms, phanerophytes dominate, accounting for 48.0% of the total species, while hemicryptophytes account for 23.3%. Regarding leaf characteristics, microphyllous and nanophyllous plants represent 42.5% and 20.6% of the total, respectively; among tree species, 39.3% have compound leaves, reflecting adaptations to arid habitats. The flora in the hot, dry valleys of southwest China are generally dominated by tropical elements, but are influenced by regional geological history and geography. Floristic similarity between the Jinshajiang and Yuanjiang river valleys is high, with 73.8% and 53.8% at the genus and species levels, respectively, representing the greatest similarity among the hot, dry valleys. The origin and evolution of this flora are closely related to major geological events in Yunnan since the Tertiary, including uplift of the Himalayas. Some species exhibit floristic affinities with the savannas of India and Africa, supporting the hypothesis that the Indian plate separated from Africa, collided with Asia, and transported African plants to the continent. The high generic similarity between the Jinshajiang and Yuanjiang river valleys supports the river capture hypothesis. In contrast, the savanna-like vegetation in the Nujiang River valley is dominated by tropical floristic elements, which is likely related to the northward push of the Indian plate into the Asian plate and the subsequent northward shift of northern Myanmar. These geological events help explain the floristic composition, characteristics, and distribution patterns of the savanna-like vegetation in the dry, hot valleys of southwestern China.