Geographical Patterns of Floristic Transition of Seed Plant in North-south Range-gorge Region in Western Yunnan and the Biogeographical Significance

Floristic transition is one of the important keys to understanding the origin,emigration of flora and predicting vegetation distribution.The spatial distribution of floristic transition was little known until now,especially in Yunnan,China.We examined spatial patterns of floristic transition in north-south range-gorge region in Western Yunnan and the biogeographical significance.Also,in this paper,the relationships between floristic transition and latitude,altitude,climatic index(mean annual temperature)were probed.Based on flora information from field plots and literatures,the correlations between floristic transition and altitude,latitude and climatic indices were examined,and floristic transition was predicted by GIS.The proportions of temperate elements increased from north to south or with the increase of altitude,while the proportions of tropical elements decreased.With the increase of mean annual temperature,the proportions of temperate elements decreased,while that of tropical elements increased.Floristic transition increased,then decreased against energy gradients,which was consistent to their ecological characters.When mean annual temperature was around 12.4℃,there was floristic equilibrium point.When mean annual temperature was below 2.9℃,temperate elements fully took the flora,and the corresponding vegetation was temperate needle forests.When mean annual temperature was above 21.8℃,tropical elements fully took the flora,and the corresponding vegetation was rain forests.Strong transition was found in mountains of Ailao and Wuliang with altitude from 1 500～2 500 m.Weak transition was found in area with altitude below 1 500 m,or above 3 000 m.