Plant Diversity ›› 2020, Vol. 42 ›› Issue (04): 255-280.DOI: 10.1016/j.pld.2020.04.001

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The tropical-subtropical evergreen forest transition in East Asia: An exploration

Peter Ashtona, Hua Zhub   

  1. a Arnold Arboretum, Harvard University, Royal Botanic Gardens, Kew, UK;
    b Center for Integrative Biology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Yunnan, PR China
  • Received:2019-12-20 Revised:2020-04-06 Online:2020-08-25 Published:2020-10-14
  • Contact: Peter Ashton, Hua Zhu
  • Supported by:
    This project was funded by The National Natural Science Foundation of China (41471051, 31970223). Thuy Nguyen has greatly assisted with her knowledge of the ecology and flora of the northern Vietnamese mountains, advising and translating relevant literature; Fang Wen shared his experience of forests and landscapes in the far south-east of Guangxi autonomous regions; ShinIchiro Aiba provided critical information of the ecology of Okinawa forests; Liping Li provided invaluable assistance with translation, and references; while Peter Grubb and Eben Goodale commented on a draft, and the latter sought the advice of colleagues in China on our behalf. And a particular word of thanks to Peter Grubb, who provided us with extensive notes of his own on southern Chinese flora and forests, accompanied by invaluable comments from the late Len Webb and Tim Whitmore. And last, my (P.S.A.) very special thanks to Raymond Porter, who introduced discipline, precision and elegance to my unruly English presentation.

Abstract: The transition from tropical to subtropical (warm temperate) evergreen forests is more clearly apparent in East Asia, from Nepal to the western Pacific coast, than elsewhere in the tropics. We review the nature of this transition and hypothesize the physical, ultimatelyclimatic, factors that may maintain it, with a special focus on how the increasing instability and warming of climates will affect these forests. A primary climatic mediator of the transition is proposed, thereby offering a testable hypothesis for the climate-forest transition relationship. What is known of this transition is summarized in context of the primary climatic mediators of elevational zonation of forest formations in equatorial Asia to the tree line, in the Himalaya at the India-Indo-Burma northern tropical margin, and as both elevational and latitudinal zonation in southern China. Consequent secondary edaphic and other physical changes are described for the Himalaya, and hypothesized for southern China. The forest ecotones are seen to be primarily defined by tree floristic change, on which account changes in structure and physiognomy are determined. The montane tropical-subtropical transition in the Himalaya is narrow and observed to correlate with an as yet ill-defined frost line. A distinct tropical-subtropical transition forest is recognized in the southwest China mountains. There is a total change in canopy species at the Himalayan ecotone, but subcanopy tropical species persist along an elevational decline of c. 400 m. The latitudinal transition in South China is analogous, but here the tropical subcanopy component extends north over ten degrees latitude, albeit in decline. The tropical-subtropical transition is uniquely clear in East Asia because here alone a tropical wet summer-dry winter monsoon extends to 35° north latitude, encompassing the subtropical evergreen forest, whereas subtropical evergreen forests elsewhere exist under drier temperate summer climate regimes.

Key words: Tropical, Subtropical, Forest transition, Ecotone, East Asia