The Study On Origion And Evolution Of Karst Tiankeng Flora In Dashiwei, Guangxi

A total number of 1987 samples have been collected in the study to investigate the flora of Dashiwei Tiankeng, Guangxi.784 species, belongs to 144 familives and 239 genera are identificated, among these,137 pteridophytes,7 gymonspers, and 640 angiosperms. Accounting the floristic composition and geographical distribution elements, we research the natural characteristic of the Tiankeng flora. The cluster analysis has been studied according to the community’s important value (Ⅳ) of species in vary Tiankeng which is in the different geological development period. The results help reveal the origin and evolution of the Tiankeng flora.According to floristic and community ecological data, the origin and evolution of Tiankeng flora can be divided to 3 stages:the origin stage of the Tiankeng flora (referred to as the first stage), such as Baidong, Meiguidating, Banyuedong, Maoqidong, Mafengdong Tiankeng flora, the competition stage of the flora (the second stage), such as Shenmu, Sujia, Luojia Tiankeng flora, and the stable stage (the third stage), such as Dacao, Chuandong, Dengjiatuo Tiankeng flora.(1) The origin stage of the Tiankeng flora (OS, the First stage)In the early development of the Tiankeng, the pithead is smaller than the bottom, the sunlight is dimer and more air humidity than the surface ground. There are 110 vascular species belong to 53 families and 83 genera. But with the diameter gradually increase of the pithead, the sunlight enhanced and the temperature raised gradually. With the factors improvement for plant living, the soil moisture and humus increased. The evolution of the Tiankeng flora can be further divided into 3 sub-stages:in the first sub-stage (OS1) the inhabitat is the best dim and only 8 bryophytes species live in early Tiankeng development. In the second sub-stage (OS2) the sunlight slightly enhanced with the Tiankeng pithead increasing, some shade fern plants, such as Ctenitis membranifolia、Dryopteris scottii、 Selaginella delicatula settled in Tiankeng. In the third sub-stage (OS3) the sunlight is further enhanced, some shade shrub species invade in the flora, such as Tetrapanax papyrifer^ Aucuba chinensis、 Schefflera bodinieri. The first stage of flora is beginning at bryophytes settle, ended at the plant adaptation to the higher temperature and moisture(HT-M) environment than that of the ground.In the geographical distribution analysis, the temperate genus proportion is much higher than the other Tiankeng flora stages, about 50% in all of genera, but the important value of these genera and species is very low. However the important value of species which adapt to HT-M environment is high in the third sub-stage, showing trend of Tiankeng flora evolution is toward to adaption to HT-M environment.(2) The competition stage of the Tiankeng flora (CS, the Second stage)With the more sunlight increase, the more humidity and soil thickness in the competition stage of flora. There are 296 species belonging to 110 families in this stage, and more shade tolerant tree species live in the plant community. The tropical distribution genera accounted for 70%,10% higher than that of the surface ground flora, and the IV of species adapt to HT-M environment higher than the third evolution stage and the surface ground flora, which reflecting the HT-M environment in this stage. Some deciduous plants adapting to cold and drought environment invade this stage, although their IV is not very big. The dynamic evolution of the second stage Tiankeng flora is caused by the competition between the plant adapt to aquosity and aridity. The evolution of second stage of flora started at Manglietia aromatica, Miliusa sinensis, Machilus oreophila adapt to the HT-M environment, and ended the constructive species of surface ground plant community adapt to the arid environment.(3) The stable stage of the Tiankeng flora (SS, the Third stage)There are adequate sunlight In the developed Tiankeng with drought bottom and the flat terrain.483 species live in these Tiankeng flora. The community is mainly composed of heliophilous species。With Tiankeng ageing, cliff constantly caving and the bottom raising, decreasing the ability to block the cold wind, the underground river blocking, the air changed to be more dry and cold in winter than that of the second stage. Maintained by tropical genera, their species are no longer adapting to this habitat and gradually eliminated in the end. The third stage begin at the constructive species of surface ground plant community adapting to the dry environment, and tend to be ecological convergance of the surface ground flora in the end.(4) Being located between Sino-Japan 14 (SJ), and Sino-Himalayan flora,14 (SH), the Tiankeng flora is built by these two flora species, but the proportion of the 14 (SH) genus is higher than the 14 (SJ) of all the Tiankeng flora. It is obviously been effected by Sino -Himalayan flora more than by Sino-Japan flora.(5) In this study reported a new record genus Cotinus and seven species new record in Guangxi, they are Cotinus coggygria var. glaucophylla; Lilium nepalense var. ochraceum; Lysimachia lancifolia; Sinocenecio bodinieri; Pilea unciformis; Anoectochilus gengmanensis; Androsace runcinata.