Study of biological diversity has long been the concern of the ecology. Plant species diversity and species richness is the comprehensive reflection of the uniform distribution, the difference can not only reflect the characteristics of plant communities in composition, structure, characteristics of spatial and time characteristics, but also reflect the plant community [1] [2] . Forest is one of the most important ecological systems of land types, the species diversity and spatial pattern has long been one of the hotspots in the research, forest vegetation as an important part of forest ecosystem, plays an important role in maintaining the function of forest ecosystems, forest ecosystems to maintain their stability [3] [4] [5] .

Cibagou nature reserve is located in the southeast of Tibet, and was typical Alpine Canyon terrain. Because of its unique geographical location and well- preserved primary forest ecosystem characteristics, natural academic research has been attracted attention of researchers in different research field for long- time, but limited by traffic infrastructure, only few researches has been conducted over the past few decades, such aslarge animal species research [6] [7] [8] , insect classification research [9] , study on the Characteristics of Actinomycetes [10] . The investigation of plant resources was only carried out the part of plant species investigation, and the plant species community diversity research has not been conducted and analyzed. In this study, we investigated the vegetation under different elevation gradients in the Cibagou National Nature Reserve, and analyzed species diversity by the commonly analysis method. The aim was to provide some references for the comprehensive management of vegetation, the protection and development of biodiversity in Cibagou Nature Reserve.

Community as an important part of biodiversity, species diversity is the very important content in community ecology; altitude is one of the main plant community species composition control mountains natural gradient, elevation gradient change will lead to the change of factors such as temperature, precipitation, light conditions, thus affecting species distribution. Species diversity and the relationship between the altitude gradient has no unified conclusion, species diversity gradually lower approximation with altitude in the change of latitude gradient model has been proved in many mountains [13] [14] . The species diversity of zonal plant communities in western Yunnan was studied and results showed that the density of species decreased with the increasing of altitude gradient [15] , Zhang [16] also has a consistent conclusion based on Luya Mountain plant community diversity in Shanxi province. However, there are many studies have shown that the temperature and humidity of the elevation in the range has most species survival conditions, thus presents the species richness increased with altitude showed a trend of unimodal curve change [17] [18] , namely lower after rising first. Grytnes [19] found that high species richness values appeared at an altitude of 1500 - 2500 min the Himalayas or Nepal. In the study of plant community pattern in Haba Snow Mountain, Yunnan, the peak of herbaceous plant abundance appeared at an altitude of 3000 m - 3500 m [20] . The study of the Altai Mountains in Xinjiang also showed that the abundance and diversity of herbaceous plants showed a single peak Curve increases [21] . The reason may be that the study did not include a complete elevation gradient, that is, the peak of the species diversity was not studied in the study of altitudinal gradient [22] . Su [23] have shown that the Tibetan Sejila Mountain herbal plant richness was a double peak curve; about 3500 m above sea level and an altitude of 4500 m are high.

The results showed that the species richness of understory plants in Cibagou Nature Reserve increased first and then decreased with the increase of altitude gradient, showing a trend of single peak curve and peak appeared between the attitude 2327 m - 2670 m, Shannon-wiener diversity index peak appeared at an altitude of 2327 m. The diversity of plant species in different forest plants is relatively large, but the variation of plant diversity varies with altitude and is generally low. Possible reason was the primeval forest nature reserves is relatively complete, tall trees and high shrubs level obviously, lead to light in a certain extent become limiting factors of the growth of undergrowth, at an altitude of 1749 m is most obvious, the undergrowth only 30% of the total coverage, with altitude gradient rise, different forest types of forest thinning, forest light penetration enhancement, undergrowth coverage increased significantly, species richness and diversity were slightly rising trend, and with the higher altitude, temperature may become a limiting factor of plant growth, thereby limiting the part of the plant’s growth.

This study was financially supported by follow fund: Collection and utilization of medicinal flower resources in Tibet; Research on Tibet resources plant information system based on Android (2015XZ01G28); Fund of Tibet science and Technology Department (2016ZR-NQ-09); Humanities and social science projects in Universities in Tibet (sk2015-36).

Wang, W., Xing, Z., Li, W.B. and Yang, X.L. (2017) Study on Diversity of Undergrowth Plant Community in Cibagou Nature Reserve. American Journal of Plant Sciences, 8, 2149-2158. https://doi.org/10.4236/ajps.2017.89144