Distribution of Vascular Plants along Altitudinal Gradients in Hoang Lien National Park, Vietnam

The present study was conducted to examine the distributional characteristics of floral communities along the altitudinal gradients in Hoang Lien National Park (HLNP), located in Lao Cai province, Vietnam. We recorded the relatively abundant flora system with 3252 species (including 361 endemic species and 237 endangered species), belonging to 1126 genera, 230 families and 6 different phyla. Methodology of sampling, specimen collection and identification, statistical analysis are simultaneously used for investigating the complex changes of composition and richness of plant assemblages. The study results indicated the divisions in quantity and composition, especially differentiation of endemic and rare species in accordance with altitudinal gradients.

According to Bussmann & Sharon (2006), the vegetation cover has changes in structure, species composition, dominant species group, and density of plant communities along elevation belts. Evaluating the relative importance of the factors that might determine elevational richness patterns remains challenging (Krömer, Acebey, Kluge, & Kessler, 2013). Among the factors of geometric constraints, area and climate, altitude is one of the most important factors affecting species richness patterns in mountain ecosystems, because it has played an essential role in driving drastic changes in temperature, water availability as well as overall area (Beals, 1969;Bruun et al., 2006;Lee, Chun, Song, & Cho, 2012). The research results show that the correlation between species richness, plant communities with elevation, and changes of environmental factors by altitude is the cause of the diversity of habitats, abundance of plant communities and formation of vegetation belts.
Flora diversity in HLNP (Lao Cai province) is recognized class A-the highest biodiversity value of Vietnam by the Global Environment Fund. HLNP is a typical high mountain area; the only place has three elevations in Vietnam (Lap, 1976). There have been a large number of studies evaluating climate differentiation as well as ecological surveys and plant distribution investigations according to elevation belts (Gottfried et al., 1999;Kem & Dilger, 1994;Ly, Thu, Tue, Tan, & Quang, 1996;Pham, 1999Pham, -2001Rahbek, 1997;Thin & Harder, 1996;Tsyganov, Milbau, & Beyens, 2013;Vo, 1970;Vu & Nguyen, 2005Zhao, Li, & Chai, 2018). However, these previous studies have only been limited in collecting specimens, analyzing and classifying but are not systematic. Besides, many of the previous studies also researched in the specific survey areas as well as each altitude but did not mention the trend of the vegetation communities in HLNP.
The purpose of the present study was to provide specific information on plant distribution of characteristics of flora along altitudinal gradients in HLNP in Vietnam as a whole. The study results also aimed to provide basic materials for rational use of natural resources, economic development and protection of the ecological environment in and around the study area.

Experimental area
The study was conducted in HLNP, Lao Cai Province (22˚07' -22˚23'N, 103˚00' -104˚00'E) ( Figure 1) from June, 2016 to December, 2018. The main types of terrain include high mountains, valleys, hillsides could be divided into two main forms: upper montane and lower montane terrain. HLNP is composed of rocks such as granite, amphilolite, filit, limestone, which is most commonly granite, in humid tropical and subtropical conditions. The annual average precipitation is relatively high, with 80% of rainfall from May to October. The annual average air temperature is from 15˚C to 24˚C, with the highest temperature of  of 20 × 30 m and were considered to represent the general characteristics of each altitude. Each plot was sampled exhaustively and the area was kept constant, thus controlling both sampling effort and area in our measure of species richness per plot (Lomolino, 2001;Rahbek, 1997 (Brummitt, 1992). Endemic plants identification was referred to "Vietnamese flora" (Thin et al., 2000(Thin et al., -2010, while rare plants followed to "List of Vietnamese plants" (Thin et al., 2001(Thin et al., -2005 and "Authors of Plant Names" (Brummitt & Powell, 1992).
Preliminary description of types of flora cover was built by naming and describing types of flora patterns in HLNP based on UNESCO's vegetation classification framework (1973) which was applied to Vietnam by Phan Ke Loc (Loc, 1985). In order to achieve accurate analysis of species composition and identify dominant species in the vegetation cover structure, all of species in the standard quarters must be sampled.

Data analysis
The data obtained from the experiments described above were analyzed with SPSS Statistics with version 25 with analysis of descriptive statistics.
Evaluate the change of vegetation cover along elevation belts: in terms of changes in quantity and species composition, changes in vegetation status, distribution of specific species of elevation belts, correlations between belts, we calculated Sorensen's formula (1911( , recorded by Thin, 2004 to compare the relationship between the belts by Primer version 6 software. where: S is the Sorenson index (value from 0 to 1); a is the number of species of community A; b is the number of species of the community B and c is the number of common species of two communities (A and B). The closer the value is to S, the closer the relationship of the two communities is. On the contrary, S has a value of nearly 0, indicating that the two communities do not have a close relationship.

Distribution of major plants Rare plants
Hoang Lien National Park is also considered to be the center of rare plants such as Fokienia hodginsii, Taxus chinensis, Coptii spp., Berberis spp., Mahonia japonica, Aristolochia spp., Asarum spp., Panax spp. The (Table 2).

Discussion
With the aim of collecting the specific data of vegetation diversity in and around Hoang Lien National Park, investigations were conducted and documented positive results.
Compared to previous researches, we confirmed the additional number of 1228 species, 350 genera, 30 families comparing to 2024 species, 771 genera, 200 families of vascular plants recorded according to Thin N. N., & Thoi N. T., 1999; as well as contributed a new number of 820 species, 223 genera, 21 families comparing to 2432 species, 898 genera and 209 families following to Tri, N. Q., 2009(Tri, 2009).
The survey of plant distribution along altitude gradient showed that the number of plant communities decreased when the altitude increased, except for the region between the belt of below 700 m and 700 -1700 m ( Figure 2). Almost previous studies have documented that the higher the height a.s.l, the lower the quantity and richness (Bai, Sang, & Axmacher, 2011;Wang, Long, Wang, Ding, & Wang, 2007), while there is also the report getting acceptance of the increase of species quantity in the middle belt and the trend of richness has yet to be exactly demonstrated (Yang et al., 2014).
Sorensen index indicated the level of relationship between 2 plant communities. In HLNP, Sorensen index is changed from 0.45 to 0.69 (Table 3)     to the altitude of 700 -1700 m, which could be reasoned of favorable climate factors, because environmental factors has directly attached to flora richness (Tsyganov et al., 2013;Zhao et al., 2018). This belt is also less affected by human activities than the height of below 700 m, leading to the greater abundance. With the higher altitudes, topology and slope as well as strict climate caused the limit of vegetation diversity. The concentration distribution of vascular plants as well as rare species, endemic species at elevations below 1700 m, especially from 700 -1700 m shows the importance of this belt with protecting plant diversity, preserving plant resources in Hoang Lien Son area (Lao Cai province) and conserving biodiversity in Vietnam in general.

Conclusion
This study found the decrease of flora diversity with the upper elevations, except for the middle belt of 700 -1700 m with the richest quantity in HLNP. Besides, the Sorensen reflected the close relationship of flora richness among altitudes.
There are five altitudinal gradients of typical plants including the elevations of below 700 m, 700 -1700 m, 1700 -2200 m, 2200 -2800 m, and above 2800 m.
Each plant elevation has different characteristics of distribution, composition and number of taxon. Effective solutions must be therefore taken to protect the sustainable ecological environment. An enormous change in the composition of plant species, endemic species, and rare species by elevation belts requires a policy and strategy to conserve plant diversity, preserve typical ecosystems, and protect rare species suitable for each altitude.