The transition from tropical to subtropical(warm temperate)evergreen forests is more clearly apparent in East Asia,fromNepal to the western Pacific coast,than elsewhere in the tropics.We review the nature of this tran...The transition from tropical to subtropical(warm temperate)evergreen forests is more clearly apparent in East Asia,fromNepal to the western Pacific coast,than elsewhere in the tropics.We review the nature of this transition and hypothesize the physical,ultimately climatic,factors thatmay 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 climateeforest 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 narrowand observed to correlate with an as yet ill-defined frost line.A distinct tropical-subtropical transition forest is recognized in the southwest Chinamountains.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 35north latitude,encompassing the subtropical evergreen forest,whereas subtropical evergreen forests elsewhere exist under drier temperate summer climate regimes.展开更多
In the southern mountain ranges of Yunnan province,China,deep valleys of several large rivers create rain shadows with hot dry summers,and are locally designated tropical;towards the north,notably in the Lancang(Upper...In the southern mountain ranges of Yunnan province,China,deep valleys of several large rivers create rain shadows with hot dry summers,and are locally designated tropical;towards the north,notably in the Lancang(Upper Mekong)valley,these regions may experience frost during winter.The woody forest canopy of these valleys is predominantly deciduous,with evergreen elements in the north,where the canopy is open and the forest savanna-like.However,we here present tall forest with a closed deciduous canopy and semi-evergreen subcanopy observed in hot dry valleys of these rivers and their tributaries in the tropical south.The structure and physiognomy of these forests resemble the tall(moist)deciduous forest formation widespread in South Asia and Indo-Burma.Furthermore,these forests are largely composed of tropical elements at both the generic(80%)and the species level(>70%),indicating that these forests are indeed tropical.We originally hypothesized that these isolated forests represent refugia of a pre-Holocene extension of tall(moist)deciduous forest formation of South Asia and Indo-Burma.The sample plot we established to test this hypothesis confirmed that these forests share the structure and physiognomy of the tall(moist)deciduous forest formation;however,the plots also showed that these forests lack the characteristic and dominant species of the formation's Indo-Burmese range.The tree flora,in particular,indicates that both deciduous and evergreen elements are instead mostly derived from the adjacent tropical semi-evergreen forests of tropical southern China;yet they also include an important endemic element,which implies that these forests have survived as refuges possibly since the Pliocene.The exceptional representation of evergreen elements in these forests indicates that they have rarely been subject to hot fires or domestic cattle browsing,adding to the unique nature of the forests and further justifying their strict conservation.展开更多
基金The National Natural Science Foundation of China(41471051,31970223)。
文摘The transition from tropical to subtropical(warm temperate)evergreen forests is more clearly apparent in East Asia,fromNepal to the western Pacific coast,than elsewhere in the tropics.We review the nature of this transition and hypothesize the physical,ultimately climatic,factors thatmay 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 climateeforest 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 narrowand observed to correlate with an as yet ill-defined frost line.A distinct tropical-subtropical transition forest is recognized in the southwest Chinamountains.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 35north latitude,encompassing the subtropical evergreen forest,whereas subtropical evergreen forests elsewhere exist under drier temperate summer climate regimes.
基金funded by The National Natural Science Foundation of China(41471051,31970223)the major project of Yunnan Science and Technology Department-Yunnan University joint fund(2018FY001(-002))“Yunnan Vegegraphy Research”.Fig.1 was made by Yang Jianbo from GIS Lab in Xishuangbanna Tropical Botanical Garden,Chinese Academy of Sciences.
文摘In the southern mountain ranges of Yunnan province,China,deep valleys of several large rivers create rain shadows with hot dry summers,and are locally designated tropical;towards the north,notably in the Lancang(Upper Mekong)valley,these regions may experience frost during winter.The woody forest canopy of these valleys is predominantly deciduous,with evergreen elements in the north,where the canopy is open and the forest savanna-like.However,we here present tall forest with a closed deciduous canopy and semi-evergreen subcanopy observed in hot dry valleys of these rivers and their tributaries in the tropical south.The structure and physiognomy of these forests resemble the tall(moist)deciduous forest formation widespread in South Asia and Indo-Burma.Furthermore,these forests are largely composed of tropical elements at both the generic(80%)and the species level(>70%),indicating that these forests are indeed tropical.We originally hypothesized that these isolated forests represent refugia of a pre-Holocene extension of tall(moist)deciduous forest formation of South Asia and Indo-Burma.The sample plot we established to test this hypothesis confirmed that these forests share the structure and physiognomy of the tall(moist)deciduous forest formation;however,the plots also showed that these forests lack the characteristic and dominant species of the formation's Indo-Burmese range.The tree flora,in particular,indicates that both deciduous and evergreen elements are instead mostly derived from the adjacent tropical semi-evergreen forests of tropical southern China;yet they also include an important endemic element,which implies that these forests have survived as refuges possibly since the Pliocene.The exceptional representation of evergreen elements in these forests indicates that they have rarely been subject to hot fires or domestic cattle browsing,adding to the unique nature of the forests and further justifying their strict conservation.
