Dynamic change of forest ecological carrying rate in Yunnan province

Based on the concept of green development, with Yunnan Province as the research object and forest ecological carrying rate as the target layer, two sub-systems of forest ecological carrying capacity and environmental pollution pressure were constructed. The TOPSIS method was used to calculate the forest ecological carrying capacity and environmental pollution pressure index of Yunnan Province from 2011 to 2020, so as to obtain the forest ecological carrying rate. The results showed that: (1) the forest ecological carrying capacity index in Yunnan Province increased first, then decreased and then increased in the past 10 years. (2) The environmental pollution pressure index of Yunnan Province showed an overall rising trend in the past 10 years. (3) The forest ecological carrying rate in Yunnan Province is still in overload state. Therefore, to improve the level of forest ecological carrying capacity in Yunnan Province, it is necessary to formulate relevant policies from two aspects: forest ecological carrying capacity and environmental pollution.


Introduction
As the largest ecosystem on land, forest can effectively regulate climate and precipitation [1][2], and provide numerous ecological services for human beings [3], such as water, entertainment and habitat, etc.At the same time, it can purify and absorb air pollution to a certain extent, and provide a constant power for human social and economic development.
In the 1990s, excessive logging and neglect of planting caused serious damage to ecological environment [4].Nowadays, we are in the age of science and technology, which brings convenience but also deterioration of ecological environment [5].Economic development will inevitably accelerate the urbanization process, which brings pressure on forest resources and the global environment [6][7], and also causes the disharmony between man and land.Land use pressure has a huge impact on forest resources and forest coverage [8], as well as on forest landscape change [9], which has begun to improve in recent years.Land ecological security has also changed from insecurity to security [10].Therefore, how to deal with the relationship between urban construction and ecological environment has become the key [11].Human life quality and the natural environment are intimately intertwined.In order to ensure human life and sustainable development, it is important to pay attention to bordering areas' pollution emissions in addition to local pollution sources [12].Forests are also under pressure from various aspects when maintaining their own renewal, such as natural disasters such as forest diseases and pests, forest fires [13][14][15], overgrazing [16], and air pollution [17].
Ecological carrying capacity is often studied together with ecological footprint, which is also called ecological occupation, ecological footprint and ecological trace [18][19][20].Parker and Burgess first proposed the concept of ecological carrying capacity.They held the opinion that ecological carrying capacity referred to the most individuals of a certain species that can coexist in a given ecosystem [21].Forest ecological carrying capacity is closely related to forest ecological security.Forest ecological carrying capacity can be studied from the aspects related to ecological security.For example, North America [22], the United Kingdom [23] and New Zealand [24] measure forest ecological security from the perspective of forest ecological health.Entropy weight TOPSIS method is mostly used to measure and analyse the dynamic trend of forest ecological carrying rate [25][26].
At present, researches on ecological carrying capacity mainly focus on Marine ecological carrying capacity, water ecological carrying capacity and land ecological carrying capacity, and most of them focus on areas with high economic development level.There are few reports on forest carrying capacity in western China.Forest is an ecosystem component with the highest biomass in the world, and species diversity can be used to evaluate the sustainable development of forest [27].The carrying capacity of forest resources is directly proportional to natural conditions [28], which will directly affect the survival of human beings and the global ecosystem [29].Yunnan Province has the largest number of plant species in China, and is known as the kingdom of plants.Plant types such as tropical, subtropical, temperate and cold temperate zones are distributed in Yunnan Province.There are many ancient, derived and exotic plant species and groups, which are highly representative in areas with rich forest resources in the world.Therefore, this paper takes Yunnan Province as the research object, uses entropy weight TOPSIS model to study its forest ecological carrying rate, analyzes the forest carrying state of Yunnan Province, and provides reference for the world environmental protection and sustainable development.

Study area
Yunnan Province is very rich in forest resources, with the forest coverage rate reaching 55.04%, the total forest area exceeding 210,000 km 2 , the forest stock volume approaching 2 billion m 3 , and the area ratio of national nature reserves reaching 3.86%.At present, the number of forest villages in Yunnan Province has reached 235, and the number of provincial forest villages has exceeded 1,000.The national target of forest greening rate in rural areas is 30%, and Yunnan Province has reached 45%, far exceeding this target.The number of forest fires in Yunnan Province during the 13 th Five-Year Plan period decreased by 71% when compared to the 12 th Five-Year Plan period, and the total area of forest affected by the 13th Five-Year Plan period decreased by 68%.It can be seen that Yunnan Province is rich in forest resources, but there are still many problems to be solved in forest ecological protection and environmental pollution control in Yunnan Province.

Index construction and data source
Nature is the basic condition for human survival and development.The forest ecological carrying rate in Yunnan Province can be divided into two subsystems of forest ecological carrying capacity and environmental pollution pressure.The forest ecological carrying capacity system includes resource index, damage rate index and location index, and the environmental pollution pressure system includes environmental pollution pressure index, as shown in Table 1 for details.
The forest coverage rate and forest stock per unit area can represent the quantity and quality of forests in Yunnan Province.The proportion of national nature reserves can reflect people's willingness to protect nature in Yunnan Province.The proportion of natural forest refers to the ratio of the area of natural forest to the total forest area in Yunnan province.The higher the proportion of natural forest, the stronger the forest ecological capacity of Yunnan Province.The proportion of afforestation area is the proportion of afforestation area and total forest area in Yunnan Province.The higher the value of this index, the stronger the intention of Yunnan province to improve the richness of forest resources.
The area affected by forest fire represents the extent of forest fire in Yunnan Province.The incidence rate of forest pests represents the damage degree of forest system in Yunnan Province.Where average annual precipitation is high, the diversity of forest species is higher, and where precipitation is low, the diversity of forest species is lower.Average temperature is also one of the important locations representing forest productivity and species diversity.The average annual sunshine duration can affect the growth of vegetation and is one of the important locations of forest ecosystem.The discharge of waste water, the production of general industrial solid waste, the discharge of SO2, and the discharge of particulate matter in the environmental pollution pressure index will exert pressure on the forest ecosystem of Yunnan Province, and also represent the negative impact of human activities on the forest ecosystem of Yunnan Province.

Estimation method of forest ecological carrying rate in Yunnan Province
TOPSIS model is a positive ideal solution and a negative ideal solution to define decision-making problems.The entropy weight TOPSIS model combined with the entropy weight method and TOPSIS model is applied to the evaluation of forest ecological carrying rate in Yunnan Province, which can reflect the weight and dynamic evolution trend of each index of forest ecological carrying rate in Yunnan Province.

Forest ecological carrying capacity and environmental pollution pressure index measurement method.
The first part is the calculation of entropy weight.The first step is to take the subsystem of forest ecological carrying capacity as an example.There are different units of indicators, so it needs to carry out dimensionless processing.
T ij is the standardized value of the J th index of forest ecological carrying rate in Yunnan Province in the year i.When the index value is positively correlated with the forest ecological carrying capacity of Yunnan Province, formula (1) is used; when it is negatively correlated, formula (2) is used.In the second step, formula (3) is used to calculate the entropy of the index.Third, formula ( 4) is used to calculate the entropy weight of the index.W j is the entropy weight of the J th index.
The second part is the establishment of TOPSIS model.In the first step, the entropy weight of formula ( 5) is used to establish the normalized matrix   .The second step is to determine the positive ideal solution vector and negative ideal solution vector of the forest ecological carrying capacity index in Yunnan province, as shown in formula ( 6) and (7).The third step is to calculate the Euclidean distance between the evaluation index of forest ecological carrying capacity in Yunnan province and the ideal solution vector, as shown in formula (8) and (9).In the fourth step, the evaluation index of forest ecological carrying capacity in Yunnan Province was calculated by formula (10). (8) The measurement method of environmental pollution pressure index in Yunnan Province is consistent with the forest ecological carrying capacity index algorithm, which will not be repeated here.

Measurement and evaluation standards of forest ecological carrying rate.
The forest ecological carrying capacity and environmental pollution pressure evaluation index of Yunnan Province obtained from Formula (10) can further obtain the forest ecological carrying rate of Yunnan Province.According to previous studies [25], the specific calculation method is forest ecological carrying rate (I) = (forest ecological carrying capacity index) (S) / (Environmental pollution pressure index) (P).The evaluation standards of forest ecological carrying rate in Yunnan Province is shown in

Empirical analysis
According to the results calculated in the second part, the trend of forest ecological carrying capacity and environmental pollution pressure index in Yunnan Province was analyzed, and the factors affecting the forest ecological carrying rate in Yunnan Province were analyzed.

Trend analysis of forest ecological carrying capacity and environmental pollution pressure index in Yunnan Province
According to Figure 1, it can be seen that the forest ecological carrying capacity index of Yunnan Province increased during 2011-2013, decreased during 2014-2017, and increased again during 2018-2020.The index of forest ecological carrying capacity in Yunnan Province reached its peak in 2013 and its lowest in 2017.The main reason is that in 2013, the incidence of forest pests in Yunnan Province was the lowest in 10 years, only 0.21%.In 2017, the average annual sunshine duration in Yunnan Province was the lowest in 10 years, only 1,930 hours.Compared with the previous year, the decline in 2015 was the largest, with a decline of 16.36%.The main reason was that in 2014, the wastewater discharge in Yunnan Province increased by 157,892,900 tons compared with the previous year.
According to Figure 2, it can be seen that the environmental pollution pressure index of Yunnan Province fluctuated and increased during 2011-2020, and fluctuated within the range of 0.36-0.38 during 2011-2015.The environmental pollution pressure index in Yunnan Province increased significantly from 2016 to 2017, among which, the environmental pollution pressure index in 2016 and 2017 increased by 15.37% and 30.93% compared with the previous year, respectively.The main reason is that in 2016, the waste water discharge in Yunnan Province increased by 4.47% compared with the previous year.In 2017, the waste water discharge and the production of general industrial solid waste in Yunnan Province increased by 2.22% and 4.60% respectively compared with the previous year.From 2018 to 2019, the environmental pollution pressure index in Yunnan Province decreased slightly, mainly because the SO 2 emissions in 2018 and 2019 in Yunnan Province decreased by 35.65% and 4.69% compared with the previous year, respectively.The environmental pollution pressure index in Yunnan Province increased significantly in 2020 compared with the previous year, mainly because the discharge of wastewater increased by 2.08% compared with the previous year.In the past 10 years, the environmental pollution pressure index in Yunnan Province has increased by 41.09%.Therefore, it is necessary to pay attention to the discharge of waste water, gas and solid waste to reduce the environmental pollution pressure index in Yunnan Province.

Analysis of forest ecological carrying rate in Yunnan Province
According to the forest ecological carrying capacity and environmental pollution pressure index of Yunnan Province from 2011 to 2020, the forest ecological carrying rate of Yunnan Province can be obtained by formula (11).According to Figure 3, it can be seen that the forest ecological carrying rate of Yunnan Province increased during 2011-2013, decreased during 2014-2017, increased during 2018-2019, and decreased in 2020.In general, the forest ecological carrying rate in Yunnan Province experienced surplus, balance and overload state from 2011 to 2017.From 2018 to 2019, the forest ecological carrying rate in Yunnan Province changed from overload to equilibrium state, and there was an overload trend in 2020.Therefore, we can improve the forest ecological carrying capacity in Yunnan Province by increasing the afforestation area, enriching the forest structure in Yunnan Province, increasing the proportion of national nature reserves and reducing the forest damage rate.At the same time, for the discharge of waste water, waste gas and solid waste, the fiscal expenditure and the number of treatment facilities can be increased to reduce the environmental pollution pressure in Yunnan Province.The forest ecology of Yunnan Province has changed from surplus state to overload state, which indicates that the relationship between economic development and forest ecology has not been properly handled in the economic development, resulting in the imbalance between economic development and forest ecology.In recent years, the forest ecological carrying rate of Yunnan Province has been rising, which is because Yunnan Province has practiced the "two mountains" theory in recent years and has been fighting to be the vanguard of ecological civilization construction.While growing the economy, we should be mindful of protecting the ecological environment and stick to low-carbon and green development.

Analysis on the obstacle factors of forest ecological carrying rate in Yunnan Province
The obstacle degree of the indicator layer can be obtained using formula (11), and after that, the obstacle degree of the state layer can be determined.The calculated results show that there has been a significant impact on the forest ecological carrying rate in Yunnan Province in recent years from both the forest resources and environmental pollution.This is indicated by the high obstacle degree of the state layer between the forest resource index and the environmental pollution pressure index.The obstruction degree of the indication layer is sorted concurrently, as seen in Table 3.As can be seen from Table 3, the ratio of natural forest area always occupied the first place in 2011 and 2012.In 2013 and 2014, the incidence of forest pests began to appear in the main obstacle factors, indicating that the use of high-tech means to reduce the incidence of pests is also very important in the process of improving the forest ecological carrying rate.From 2014 to 2016, the barrier degree of annual average precipitation and annual average sunshine duration began to increase.Annual average precipitation has an important impact on the distribution of species, and species are more abundant in places with sufficient precipitation, and vice versa.The average annual sunshine duration has an important impact on the growth of vegetation.The longer the average annual sunshine duration is, the faster the growth of vegetation will be, thus affecting the richness of vegetation resources.Yunnan Province's forest ecology has been unable to handle the emission of exhaust gas and other harmful substances in the last three years, and the forest has been in a state of overload.From 2018 to 2020, Yunnan Province had the highest obstacle degree of SO2 emission.The amount of stock per unit area and the rate of forest coverage have also started to become obstacles.It has been discovered that in the last three years, neither the stock nor the rate of forest coverage have increased in Yunnan Province.The best way to develop the forest resources in Yunnan Province is to expand the area that is being afforested and enrich the existing forest resources.

Discussions
(1) The richness of the forest's resources, the extent of damage caused by pests and fires, and the conditions of the forest's location are the main concerns of the subsystem of the forest's ecological carrying capacity.The system is a direct reflection of the forest ecological carrying rate relative to the forest resources.The following measures can improve the forest ecological carrying capacity index value in Yunnan Province.First, keep up with China's crucial "returning cropland to forest" policy, which has been in place since 2002.It aids in enhancing Yunnan Province's forest supply and coverage rate.Therefore, this policy should be continued by the government.Second, the natural forest contains the greatest variety of biological species.The Yunnan Province's forest ecological carrying capacity index is enhanced by the growth of the natural forest, resulting in a green and healthy condition for the forest ecosystem.Therefore, the natural forest should be included in the ecological protection red line.Thirdly, forest damage rate is a major obstacle to the improvement of forest ecological carrying capacity, especially forest fires and the incidence of forest pests.Therefore, it is necessary to strengthen the public awareness of forest fire prevention, and use high technology to carry out field fire monitoring.For pests, financial support could be increased to encourage the development of biotechnology and to monitor and control them in real time.
(2) The generation and release of waste water, waste gas, and solid waste in Yunnan Province are the main concerns of the environmental pollution pressure subsystem.The value is used to represent the pressure of environmental pollution on forests in Yunnan Province.Generally speaking, pressure increases with value.In Yunnan Province, the relationship between environmental pollution pressure and forest ecological carrying rate is negative, so appropriate steps should be done to lower the value of environmental pollution pressure.First, we should guide the transformation of heavy polluting enterprises to green and low energy consumption.Penalties will be imposed on illegal discharges, thereby reducing the discharge of industrial waste water, gas and solid waste, and ultimately achieving high-quality development.Second, guide residents to achieve energy conservation and emission reduction, so as to reduce domestic wastewater discharge.
(3) Forest ecological carrying rate is the ratio between forest ecological carrying capacity index and environmental pollution pressure index, and its ratio can tell whether the forest carrying state is overloaded, balanced and surplus, and its evolution trend, which is generally 1 as the limit.Few research have previously examined ecological carrying rate instead of previous studies' major focus on ecological carrying capacity.It is possible to evaluate the ecological status of forests more objectively by including the pressure from environmental contamination in the evaluation index system.However, there are still many shortcomings.First, when establishing the environmental pollution pressure index, few indicators are selected, which cannot fully reflect the environmental pollution pressure.Second, in addition to environmental pollution pressure, forest ecology will be subject to other pressures, and other pressures can be considered in the future.Thirdly, the differences among the 16 cities and states in Yunnan Province cannot be seen by taking the province as the research unit.In the future, the city and state as the research unit can be used to study the spatial distribution among the cities and states.
(4) Previous studies have used a "general stress index" to describe the level of energy consumption, and the higher the energy consumption, the corresponding increase in waste emissions, so the reduction of this index will also help improve the ecological carrying capacity of forests [30][31].General pressure index can also represent the degree of environmental pollution.When applied to the field of forest ecology, the higher the value is, the greater the environmental pollution pressure of the forest will be.Industry and other energy secret-oriented industries can be guided by the government to adjust the industrial structure and transition to low energy consumption, so as to reduce the pressure of environmental pollution and improve the ecological carrying capacity of forests.

Conclusions
(1) The forest ecological carrying capacity index of Yunnan Province increased firstly, then decreased and then increased during 2011-2013.The index of forest ecological carrying capacity in Yunnan Province was the highest (1.53) in 2013 and the lowest (0.65) in 2017.This was related to the incidence of forest pests and the average annual sunshine duration.Yunnan Province had the lowest incidence of pests in 2013 and the lowest average annual sunshine duration in 2017.
(2) The environmental pollution index of Yunnan Province showed an overall upward trend in the past 10 years, indicating that the forest status of Yunnan Province was increasingly affected by environmental pollution in recent years, which was related to the annual increase in the total amount of wastewater discharge.But the decline in two of the past three years indicates that Yunnan has begun to control the production of waste water, gas and solid waste, realizing that economic development should be of high quality and not at the expense of the ecological environment.
(3) The forest ecological carrying rate of Yunnan Province is still in the overload state, but in recent years, the forest ecological carrying rate has begun to rise, indicating that the forest state of Yunnan Province is developing towards a good trend, but it still needs to make great efforts to restore to the surplus state before 2014.
(4) In places where forest resources are not very rich, increasing afforestation area can effectively improve the amount of forest resources and is beneficial to improving forest ecological carrying capacity [32].In areas with poor forest quality, the protection of national nature reserves and natural forests should be strengthened.In areas under heavy environmental pollution pressure, the government should develop in the direction of high quality, reduce pollutant emissions, protect urban forests, and ultimately improve the ecological carrying capacity of forests [33][34].

Figure 1 .
Figure 1.Forest Ecological carrying capacity index of Yunnan Province from 2011 to 2020.

Figure 2 .
Figure 2. Environmental pollution pressure index of Yunnan Province from 2011 to 2020.

Figure 3 .
Figure 3. Trend analysis of forest ecological carrying rate index in Yunnan Province from 2011 to 2020.

Table 1 . Construction of forest ecological carrying rate index in Yunnan Province.
Among them, the resource index data came from China Statistical Yearbook (2012-2021), the victim rate data from China Forestry Statistical Yearbook (2011-2020), and the location index from Yunnan Statistical Yearbook (2012-2021).Data of environmental pollution indicators were obtained from China Environmental Statistical Yearbook (2012-2021).

Table 2 .
Evaluation standards of forest ecological carrying rate.Analysis method of barrier factors of forest ecological carrying rate In order to analyse the factors affecting the forest ecological carrying rate in Yunnan province, the obstacle degree model was introduced to diagnose the obstacle factors.For details, see Formula (11), where the value of Q ij represents the impact degree of each index on the forest ecological carrying rate in Yunnan Province.

Table 3 .
Main obstacle factors of forest ecological carrying rate in Yunnan Province.