Research Objects, Scope, Contents and Methods

Woody plant dominance includes two ways of division as follows:

+ Division of forest dominance Sarukhan (1978) used the IVI index, which was proposed by Curtis and McIntosh (1950) and is called the Importance Value Index (IV). Tree species with an IVI > 5% are ecologically significant species. Take 1-5 dominant species or groups of tree species with the largest IV (ranked from largest to smallest) with a total IV accounting for 50% or more to name the forest plant dominance. This index is well applied to the study of multi-layered forests and large-diameter trees.

+ Division of communities and dominance according to Thai Van Trung (1970) [17]: Take the composition of N% tree species or dominant genus to divide the indicators: Community: There is an absolute dominance of individuals of 1-2 species (or genera) accounting for over 90% of the number of individuals of the species in the community layer (layer A) on the surveyed area unit. Dominance: the number of individuals of less than 10 species (or genera) accounts for 40-50% or more of the total number of individuals of the community (layer A) on the surveyed area unit (individuals of each species account for 4-5%). This division method is applied to forests with small diameters, on a narrow area.

According to Le Quoc Huy (2005), the Importance Value Index (IVI) was applied by Curtis & Mclntosh (1950); Phillips (1959); Mishra (1968) to represent the structure, correlation and dominance order between species in a plant population. The IVI index represents better and more comprehensively the relative properties of the ecosystem compared to the absolute single values of density, frequency, dominance, etc. The IVI index of each species is calculated using one of the following two formulas:

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1. IVI = RD + RF + RC (Rastogi, 1999 and Sharma, 2003).

2. IVI = RD + RF + RBA (Mishra, 1968).

In which: RD is relative density, RF is relative frequency of occurrence, RC is relative canopy cover and RBA is the total relative cross-sectional area of each stem.

species. The IVI index of a species reaches a maximum value of 300 when the research site has only that tree species.

1.2.3.2. Research on diversity of woody plants

The biodiversity of a typical ecosystem or a certain territory is expressed in different categories. First of all, it is the diversity of taxa (phylum, class, family, genus, species, etc.); then it is the diversity in the structure of the ecosystem, the mutual relationship between the ecosystems and communities that create a sustainable ecological balance, existing naturally.

When studying vegetation, some authors have conducted quantitative analysis of biodiversity indicators. Specifically, when studying biodiversity of Can Gio forest - Ho Chi Minh City, Vien Ngoc Nam (2008) used the following indicators: Margalef richness index - d; Piejoue evenness index - J'; Simpson dominance index - D; Shannon diversity index - H' and Caswell index (V) ... for evaluation.

These indices have provided a large amount of information on species composition, rare species, common species and adaptive capacity. Quantification in this way is extremely important for current forest protection work.

Assessment of biodiversity of Huong Son special-use forest plants. In the study area, 28 randomly distributed standard plots were investigated, each plot had an area of 500m2 . The following information was measured: species, number of plant species for trees, shrubs and herbs; number of individuals of each species; diameter of each individual; canopy cover of the total number of individuals calculated separately for each species in each standard plot (Hoang Viet Anh et al., 2008).

According to Vien Ngoc Nam et al. (2014), to assess species biodiversity between regions, use the following indicators:

+ Alpha biodiversity : Related to information on species composition and quantity of a specific research area or site, such as a standard plot of 20m x 50m (quadrat).

+ Beta biodiversity: describes the difference in species composition between two adjacent research sites along a transect; the beta index is low when the species composition of the two research sites is highly similar and vice versa. This value reaches its maximum when there is no common species between the two research sites (similarity is zero).

+ Gamma biodiversity: is defined as the level of encountering a complementary species when changing geographically in different areas of a habitat type. This diversity indicates the difference in species composition and biodiversity indicators of 2 large habitats/habitats that are far away/close to each other.

Thus, forest biodiversity includes not only trees but also the myriad species of plants, animals and microorganisms living in forested areas and their associated genetic diversity.

General comments on the overview of research issues

Research on forest structure of forest types is a very complicated work. In our country, research on forest structure has only been focused on since the 20th century. During the French colonial period, research on forest structure was mainly done by French foresters.

The above mentioned works are the basis and orientation for forestry research in Vietnam later. Since 1960, our country has had many research works on mixed forest structure, leaving an extremely valuable source of data for readers in forestry production and forest management and use. Typical authors such as: Dong Si Hien (1974), Thai Van Trung (1978), Nguyen Van Truong (1983) ...

Along with the development of information technology, many authors have continued to study more deeply the correlation law, model the experimental distributions to find the most suitable equation for each type of forest and have quantified relatively accurately some growth indicators of the research forest. At the same time, the authors have studied and proposed a system of division

Forest type aims to fully understand and perceive the nature of a forest type in an area or territory in order to optimize forest formation conditions by silvicultural techniques (forest planting, nurturing, forest use, restoration, protection, conservation, etc.) to improve the efficiency and quality of the forest.

In the field of forestry, the most important are the research works of Thai Van Trung (1963, 1973) and Nguyen Van Truong (1983).

Based on Loschau's forest status classification, in 1984 the Ministry of Forestry (now the Ministry of Agriculture and Rural Development) improved, expanded and issued a classification system (abbreviated as QPN 6-84) for widespread application nationwide and has been used to this day.

With the increasing demand for forest business, the subdivision of forest types needs to be precise. The task of the topic is to, in addition to determining the qualitative, conduct more in-depth research on forest structure such as: species structure, species diversity, forest adaptability through studying the distribution rules and regeneration situation under the forest canopy... From there, discover and provide optimal qualitative and quantitative data on growth indicators as a basis for proposing improvements and building effective solutions for forest management, protection, business and conservation.

Chapter 2. OBJECTS, SCOPE, CONTENT AND METHODS OF RESEARCH

2.1. Research subjects, time and location

* Research subjects:

The research subjects are secondary forest states under the management authority of Van Ban Forestry One Member Co., Ltd., Lao Cai province.

* Research period: From February 2018 to June 2019.

* Research location: Liem Phu commune and Duong Quy commune, Van Ban district, Lao Cai province.

2.2. Research limitations

Research on secondary forest status (IIa) under the management authority of Van Ban Forestry One Member Co., Ltd., Lao Cai province.

2.3. Research content and methods

2.3.1. Research content

To achieve the research objectives, the topic conducts the following contents:

(1) Study some characteristics of the structure of the tree layer;

(2) Study of layer structure characteristics;

(3) Research on the evaluation of regenerated tree structure;

(4) Proposing silvicultural solutions, promoting regeneration and forest protection and management.

2.3.2. Research method

2.3.2.1. Method of inheriting basic documents

Inheriting documents and survey data on natural conditions, socio-economic conditions of the research area, and reference documents related to the topic of domestic and foreign authors.

2.3.2.2. Field investigation method

To collect data, we conducted a survey by route and ÔTC as follows:

* Survey lines: Determined in 2 directions parallel and perpendicular to the contour lines. The distance between the 2 lines is 50 - 100 m depending on the terrain. Along both sides of the survey line, arrange TTC and slab plots to collect data.

* Standard plots: To collect vegetation data, we apply the method of establishing 2000m2 OTC ( 40x50m) for all states, each state establishes 3 OTCs at the foot, slope, and top positions (so the total number of OTCs needed is 6 plots). To collect data on regenerated trees in the OTC, a system of slab plots with dimensions (5x5m) is established. The slab plots are arranged on diagonals, perpendiculars, and edges of the OTC. The total area of the slab plots must reach at least 1/3 of the OTC area. In addition, along both sides of the survey line, slab plots are placed to collect additional data.

- Data collection:

On the survey route, statistics on species composition, land use history, and vegetation characteristics are collected. Along both sides of the route, OTCs and plots are arranged to collect data.

+ In the 2000m2 OTC, determine the terrain location, exposure direction, slope, and land use history. Collect data on vegetation: measure height, trunk diameter (at a height of 1.3m), canopy diameter for trees with diameter D  6cm, determine canopy cover, and density of fresh vegetation.

Height measurement: Trees under 4.0m in height are measured directly with a pole with divisions of 0.1m. Trees over 4.0m in height are measured with a SUNNTO 627124 ruler adjusted according to the direct height measurement method.

Diameter measurement: Measure all trees with D 1.3  6 cm. Measure at breast height (D 1.3m ), measure directly with a caliper (add together the two directions DT-NB, divide by 2 to get the average value) with an accuracy of 0.10cm. Or measure the circumference with a tape measure and then deduce the diameter: D 1.3 = C 1.3 /π . (where C 1.3 is the circumference at 1.3 m; π = 3.14)

Canopy diameter: Measure according to the canopy projection on the horizontal plane in the two directions East-West and South-North, then calculate the average value.

Determining canopy cover (Canopy cover is the proportion of land area covered by vegetation): Survey by point network method, the survey method is conducted as follows: On each OTC, establish parallel lines at equal distances. On each of these lines, survey 100 points, survey canopy cover points are scored as follows:

If the survey point is inside the canopy, we give a score of 1.0. If the survey point is on the edge of the canopy, we give a score of 0.5. If the survey point is outside the canopy, we give a score of 0.0.

After investigating 100 points in the OTC, we calculate the canopy cover according to the formula: TC% = ∑number of points/100; In which: TC% is the canopy cover of the OTC.

The survey results are recorded in the form (Form 1):

+ Count quantity, determine species composition, measure height, evaluate quality and origin of regenerated trees.

The results of the investigation are recorded in the investigation form.

+ Survey of fresh vegetation: In a plot (5x5m). Survey of shrubs, survey of fresh vegetation (ground cover vegetation) according to the following criteria: main species name, average height, coverage, growth status. Survey results are recorded in the survey form.

+ Soil profile investigation: on each OTC, dig a soil profile in the middle of the OTC, the investigation results are recorded in the form (Form 04).

2.3.3. Processing data on tree strata composition

To evaluate the ecological compositional structure of woody plant communities, we use the Importance Value Index (IVI), calculated according to the formula:

In there:

IVIi (%) 

WhoMove RFi

(2.1)

 IVI i is the importance index (composition ratio) of the ith species.

 Ai is the relative abundance of the ith species:

Who (%) 

N i

N i

i  1

x 100

(2.1.1)

In which: Ni is the number of individuals of the ith species; s is the number of species in the population

 D i is the relative dominance of the i-th species:

Di (%) 

Price x 100

 Gi

i  1

(2.1.2)

In which: G i is the cross-sectional area of the ith species; s is the number of species in the community.

G ( cm 2 )   x

D i

(2.1.3)

i 2 

i  1 

With: D i is the diameter 1.3 m (D 1.3) of the i-th tree; s is the number of species in the community

 RF i is the relative frequency of occurrence of the ith species:

RF (%)  F i

x 100

F i

i  1

(2.1.4)

In which: F i is the frequency of occurrence of the ith species; s is the number of species in the community.

FNumber of samples with the i type appearing x 100

Total number of research samples (2.1.5)

Accordingly, tree species with IVI index ≥ 5% are truly ecologically significant in the forest stand. According to Thai Van Trung (1978), in a forest stand, the group of tree species that accounts for more than 50% of the total number of individuals in the upper tree layer is considered the dominant species group.

Composition of regenerated trees

Determine the average number of trees by species based on the formula:

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