Abstract:

Background & Aims: Paleobotanical studies show that the basic patterns of modern flora and vegetation in Yunnan originated in the Paleogene. These findings have greatly changed our previous understanding of the origin and evolution of modern vegetation and flora in Yunnan. In this article, the formation and evolution of the modern flora and vegetation of Yunnan will be described by linking geological events to palaeobotanical findings. The proposed formation and evolution of the flora and vegetation of Yunnan are detailed by combining paleobotanical results with the history of geological events since the Cenozoic period and the formation of southwestern monsoon climate.

Review Results: Flora and vegetation of Yunnan have a more ancient history than previously believed. Their formation and evolution have been clearly influenced by the geological events that have occurred since the Cenozoic period. The geographical patterns of Yunnan flora reveal that it had tropical origins and tropical-subtropical Tertiary flora of East Asia in the early stages but was influenced mainly by tropical Asian flora and north temperate flora in the later stages. The uplift of the Himalayan-Qinghai-Tibet Plateau and the accompanying geological events directly affected the evolution of the flora in Yunnan. The development and strengthening of the southwest monsoon climate were the main factors for the formation of vegetation, and specifically tropical rain forest vegetation, in Yunnan. The tropical flora and vegetation (except flora and savanna-like vegetation in hot dry valleys) displayed an increasing bias distribution pattern from northwest to southeast in Yunnan, which could be directly affected by the clockwise rotation and southeast displacement of the Simao-Lanping geological plate and the northward displacement of the plate in North Myanmar. The differentiation of the flora of the subtropical evergreen broad-leaved forests in Yunnan was closely related to the Himalayan uplift in the late Miocene. The formation and strengthening of the southwest monsoon climate by further uplift of Himalayas further promoted the divergence of the subtropical evergreen broad-leaved forests. The evolution of savanna vegetation in the hot dry valleys of Yunnan was believed to be closely related to the geological events such as deep valley formations and river captures caused by the Himalayan uplift, and some representative plant species in the savannas showed a likeness with savannas in India and Africa. The discontinuous remnant tropical deciduous forests (monsoon forest) in Yunnan were believed to be closely associated with the tropical deciduous forests in mainland Southeast Asia during a more seasonal or arid period in or before the Pliocene to Pleistocene caused by a stronger monsoon. The sclerophyllous evergreen broad-leaved forest dominated by Quercus species (Fagaceae) was unique, Tethys-associated remnant vegetation evolved from the former tropical-subtropical evergreen broad-leaved forests in Yunnan. The present tropical rain forest was proposed to be the most recent vegetation type in Yunnan following the formation and strengthening of the southwest monsoon climate.

Conclusion: Our research indicates that since the Oligocene, the evolution of flora and vegetation in Yunnan has been affected by the uplift of the Himalayas and the various accompanying geological events. The development and strengthening of southwest monsoon climate is the main factors for the formation of vegetation in Yunnan, especially for the occurrence of tropical rain forest.

Key words: flora and vegetation, geological events, southwest monsoon climate, origin and evolution, Yunnan