Characteristics of Riparian Rare Plant Communities on the Shennongjia Mountains,Central China

Riparian communities at middle altitude of the Shennongjia Mountains,with Euptelea pleiospermum and Cercidiphyllum japonicum as principal part,harbor many Tertiary-relic,rare and/or endemic species,some of which are mainly or exclusively confined to riparian habitats.In order to study the characteristics of the rare plant communities,we investigated 48 plots along riparian zones of the four main rivers(Yandu River,Xiangxi River,Nan River and Du River) on the Shennongjia Mountains.Species composition,areal-types,life-form spectrum,leaf characteristic and community vertical structure were analyzed.A total of 749 species were recorded,belonging to 412 genera and 144 families.Of these,there were 256 monotypic genera and 72 monotypic families.Analysis of the areal-types of spermatophyte indicated that tropical and subtropical elements were equivalent with temperate elements at family level,but tempe-rate elements were dominant at genus level.As to endemic elements,6 families and 50 genera were ende-mic to East Asia and 2 families and 27 genera were endemic to China.Physiognomy of these communities was characterized by phanerophytes and hemicryptophytes with mesophyllic and microphyllic herbaceous single leaves.Stratification of tree layer,shrub layer and herb layer was distinct.Tree layer was further subdivided into two sub-layers:upper and lower tree layers.From upper tree layer to shrub layer,the coverage increased gradually.Richness of endemic and/or relic plants indicates the old-growth and relictual characteristics of the riparian rare plant communities.All the features of species composition,areal-types,life-form spectrum and leaf characteristic reveal that the characteristic of the rare communities is characterized as a transition from evergreen broad-leaved forest to deciduous broad-leaved forest.The vertical structure makes it possible to provide favorable light for several riparian rare plants during their whole life history.Specific position,peculiar topography and well water/thermal condition make subtropical mountain riparian zones to be an important area for rare plants conservation.