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   . 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    .
2. Materials and Methods
2.1. Natural Geography Characters
The Cibagou Nature Reserve is located in the southeastern part of the Qinghai- Tibet Plateau in the southeastern part of the Tibet Autonomous Region. It is the intersection of the Himalayas and the Hengduan Mountains. The geographical coordinates are North latitude 28˚34' - 29˚07', East latitude 96˚52' - 97˚10', attitude 2500 m. Cibagou Nature Reserve area annual precipitation of more than 1000 mm, the average annual temperature was range 10˚C - 20˚C, the average annual humidity was range 60% to 70%, frost-free period was more than 200 d. Cibagou Nature Reserve is 76 km long from north to south and 33 km from east to west. The total area is about 1 × 105 hm2. The forest area of 5 × 104 hm2, forest coverage rate of up to 54.6%, the total forest volume was about 1.6 × 108 m3. According to the forest (vegetation) type classification system can be divided into mountain subtropical evergreen broad-leaved forest, mountain deciduous broad- leaved forest, temperate hardwood evergreen oak forest, mountain temperate pine forest, mountain Berlin, subalpine deciduous coniferous forest, subalpine Dark coniferous forest, shrub, meadow.
2.2. Research Methods
2.2.1. Sample Selection Settings, and Forest Plant Community Survey
The experiment was carried out in the Cibagou Nature Reserve in July 2016, walked about 30 km along the cibagou gully. The vicinity of the Ciba Ditch Ecological Protection Station is the end of the survey. In the case of obvious division of forest vegetation, select the appropriate sample to investigate, set two 10 m × 10 m quadrats, respectively, in the sample four corners and the center set 2 m × 2 m small sample (a total of 10) for the investigation of forest Under vegetation. The height, coverage, quantity and other information of understory plant vegetation were determined according to the types of plant species, and the important values of different plant species were calculated. The calculation formula was as follows  :
2.2.2. Plant Diversity Calculation Method
The plant community was systematically analyzed by species richness, species diversity and species uniformity.
Margalef Richness Index:
Menhinick Richness Index:
Shannon-wiener diversity index:
Simpion diversity index:
Pielou evenness index:
Sheldon Evenness Index:
where S is the number of species in the sample, N is the number of all species in the sample, and Pi is the important value of species i.
3.1. Analysis of Plant Community Composition and Important Value under Different Altitudinal Gradients
The difference of plant composition in understory reflects the habitat condition of forest land  . According to the investigation results, we can divide the Cibagou forest types were divided into Castanopsis forest, mixed forest, Acer green broad-leaved forest, oak forest, poplar and birch mixed forest, Quercusaquifolioide forest with 6 species. Can be seen from Table 1, at different altitudes in the forest, understory plant species have obvious differences, at an altitude of 1749 m Castanopsis forest, understory plant coverage was lowest, only 30% species numbers were only 3, the highest importance value was 0.518 for Lepisorusthunbergianus, followed by Campylotropis macrocarpa and Cymbidium hookerianum, important values were 0.255 and 0.172 respectively; at an altitude of 2327 m oak forest, understory plant total coverage was reached 95%, species number was 14, the highest importance value for the Impatiens linghziensis and Galium aparine Linn. var. echinospermum.
At an altitude of 1960 m mixed forest, plant community coverage was 80% and plots in the number of species was 13, significant higher values of the
Table 1. The important values of plant species in different forest stands at different altitudes in Cibagou.
Pteracanthus alatus, Impatiens cristata, Aster albescens, important values were 0.226, 0.208, 0.102; at an altitude of 2092 m Acer green broad-leaved forest, plant total the coverage was only 40% and species number was 9, the important value of Pteris cretica L. var. nervosa and Tetrastigma serrulatum, were 0.279 and 0.116; the elevation of 2670 m poplar and birch mixed forest, plant total coverage is 80%, the number of species was 10, higher important value for Fargesia setosa, Oxalis acetosella L. ssp. leucolepis, Polygonum nepalense, important values were 0.218, 0.209, 0.181; at an altitude of 2880 m in Quercus aquifolioides forest, plant total coverage is 90%, higher important value for Fragaria nubicola, Pteridium aquilinum (L.) Kuhnvar. latiusculum, Panax pseudoginseng Wall. var. notoginseng, Impatiens linghziensis, important values were 0.332, 0.199, 0.140, 0.107.
3.2. Species Diversity of Plant Species in Different Elevations of the Cibagou Nature Reserve
Using Menhinick richness index and Margalef abundance index to analysis, the evaluation of richness from Figure 1, you can see, at an altitude of 1749 m to 2880 m range, species richness are low, the overall showed a trend of rise before they are lower; The Shannon-wiener index and Simpson index of different elevation gradient undergrowth species diversity were analyzed (see Figure 2), the results show no obvious difference change with altitude gradient, only at an altitude of 2327 m diversity is relative taller; The Pielou evenness and Sheldon index of different elevation gradient plant species evenness were analyzed (see Figure 3), the results show kindness and ditch undergrowth evenness first decreases after rising trend on the whole. Overall, at an altitude of 2327 m and an altitude of 2670 m in plant species richness is relatively high, at an altitude of 2327 m in species diversity is relatively high, at an altitude of 2670 m species evenness, lowest possible reasons for the actual survey, at an altitude of 2327 m and 2670 m number appears more rare species.
Figure 1. Plant richness in different altitude gradients.
Figure 2. Diversity of plant species at different altitudes.
Figure 3. Plant species uniformity at different altitudes.
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   . 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  , Zhang  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   , namely lower after rising first. Grytnes  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  . The study of the Altai Mountains in Xinjiang also showed that the abundance and diversity of herbaceous plants showed a single peak Curve increases  . 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  . Su  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).
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