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Abstract A comprehensive survey and analysis of alien invasive plants at the scenic spot Chengbi Lake was conducted by combining route investigation with interviews and quadrat sampling. There were 21 families, 33 genera and 42 species in the scenic spot Chengbi Lake. Among them, herbal plants accounted for 69%. The Compositae and Amaranthaceae families were the most frequent, represented by 18 species in total; and 36 species of these alien invasive plants originated from America, accounting for 85.7% of the total. Fifty-seven percent of the species were introduced as ornamental plants. Seven of them were severely harmful, including Wedelia trilobata, Eupatorium odoratum, Lantana camara, Mirabilis jalapa, Solanum erianthum, Ipomoea cairica and Alternanthera philoxeroides. Strategies to increase defensive awareness, reinforce ecological protection, regulate species introduction and improve supervision mechanisms are discussed.
Key words The scenic spot Chengbi Lake; Alien invasive plants; Survey; Analysis
Alien invasive plants refer to plants that migrate and form populations in natural or semi-natural ecosystems or habitats due to intentional or unintentional human activities and thus threaten the ecological environment where they are introduced to or some species in the environment［1］. Biological invasion, especially the invasion of exotic plants, has caused extremely serious ecological consequences and huge economic losses on regional and global scales［2］. The investigation and prevention of alien invasive plants has become an important task in todays ecological research, especially in plant protection and ecosystem restoration［3］. The Chengbi Lake Tourism Scenic Area is an important original ecological park, and the The non-renewable animal and plant resources contained in it are an important natural cultural heritage of mankind, and are irreplaceable in ecological environment protection and sustainable development of human society. However, with the development of the tourism industry in Youjiang District and the intensification of human activities, the natural ecosystem of the Chengbi Lake Scenic Area has been damaged to a certain extent and is facing a serious threat from invasive plants. Therefore, it is necessary to investigate and analyze the distribution of alien invasive plants in the Chengbi Lake Scenic Area, and the implementation of effective control measures is very necessary. However, relevant research results have not yet been found. In this study, the species, life forms, origins, invasion routes, distribution characteristics and damage degree of alien invasive plants in the scenic spot Chengbi Lake were mainly analyzed on the basis of field investigation and reference materials, and their future intrusion trend and prevention and control measures were preliminarily discussed, aiming to provide a scientific basis for further scientific research and ecological protection. Materials and Methods
General situation of the research area
Baise City is located in the west of Guangxi Zhuang Autonomous Region, at the junction of the three provinces of Yunnan, Guizhou and Guangxi, and the upper reaches of the Youjiang River, with a total area of 36 300 km2［4］. The specific geographical coordinates are 106°07′-106°56′ E and 23°33′-24°18′N. The Chengbi Lake Scenic Area is located 7 km northeast of Baise City, Guangxi Zhuang Autonomous Region. It was listed in the first batch of provincial-level scenic spots by the Guangxi government in 1988. In recent years, it has also been rated as an important national water conservancy scenic spot［5］. The scenic area is dominated by natural landscapes and is known because of "green mountain, elegant water, beautiful island, fresh fish". Chengbi Lake has a water surface area of 39 km2 and a total storage capacity of 1.154 billion m3. Chengbi Lake has a large water source forest protection area with a total area of 770 km2. It has a subtropical monsoon climate, with an annual average temperature maintained at around 22.2 ℃. The summer is rainy, often hot and humid, and the winter and spring are relatively dry. There is very abundant sunshine throughout the year［5］. In addition, the terrain of Chengbihu Lake area tends to be flat, and is surrounded by hills. The shape of the landform is diverse, and the geographical advantages are also very obvious. Chengbi Lake has a forest coverage rate of 70%, is rich in wetland resources, and maintains good biodiversity［5］.
Research methods
This study investigated invasive alien plants that pose a direct or potential threat to the ecosystem or landscape of the scenic area［6］. By consulting relevant literature through books, the Internet, etc.［7-9］, the distribution, hazard and prevention of invasive alien plants were fully understood, and invasive plants in the Chengbi Lake Scenic Area were initially screened and sorted out, so as to facilitate the preparation of field investigations.
Route surveys were conducted along roads or areas with frequent human activities, the types of alien invasive plants and their associated plants distributed along the line were recorded, so as to grasp the distribution of alien invasive plants in the Chengbi Lake Scenic Area as a whole. In addition, in order to better analyze the invasion routes and invasion trends of invasive alien plants in the scenic spot, route surveys were conducted on the surrounding areas of the scenic spot. On the basis of the route surveys, a representative distribution area in the scenic area is selected to conduct the sample survey. The specific methods included the vertical sample survey method and the horizontal representative habitat quadrat survey method. In various habitats at different altitudes, 2 m × 2 m quadrats were set up to investigate the geographical location, habitat characteristics, and plant distribution, plant height, coverage, phenophase, ecological location, etc. Meanwhile, various plants were photographed, and the list of alien invasive plants was determined by consulting references and querying specimens.
Results and Analysis
According to the comprehensive results of the route surveys, sample surveys and field surveys, and with reference to relevant literatures and graph comparison［10-13］, it was finally determined that there were about 42 major alien invasive plant species in the Chengbi Lake Scenic Area, belonging to 33 genera of 21 families (Table 1). According to the analysis of Table 1, in the set sample plots, alien invasive plants were distributed in areas at different altitudes in the vertical direction. Among the various alien invasive plant, Eupatorium odoratum, Lantana camara, Ipomoea cairica, Bidens pilosa, Conyza canadensis and Euphorbia hirta had different degrees of distribution between the altitudes of 133 and 422 m surveyed, while Wedelia chinensis, Mirabilis jalapa, Solanum erianthum and Oxalis corymbosa were mainly distributed in the center of the tourist attraction at a lower altitude of 157 to 192 m.
According to the horizontal survey of various habitat representatives in the tourist attraction, the distribution of invasive plants including L. camara, E.odoratum, C. canadensis, B. pilosa and E. hirta was also observed in artificial turf, wasteland at forest edges, residential areas and lakeside. It should be especially pointed out that a large number of S. erianthum and M. jalapa in the scenic reservoir dam. In the residential areas of the scenic spot, due to the cultivation and introduction by local residents, there were many invasive plants that can be used as medicines, fruits and vegetables, such as Talinum paniculatum, Cassia tora, C. occidentalis, Mimosa pudica, Cleome gynandra, etc. At present, although these plants are scattered and lightly harmful, they must be protected against further escape and spread in the scenic area.
In addition, the types of associated plants in the quadrats surveyed were relatively simple, and they were at a disadvantage both in quantity and in plant height. The coverage of invasive alien plants averaged about 77%, and some even as high as 100%. For example, in the artificial turf, along roadsides at the foot of hills and in nurseries under the forest, W. chinensis spread widely with a coverage of 100%, and it is highly harmful. The specific distribution of alien invasive plants in the Chengbi Lake Scenic Area is especially typical in the area around the Bashou scenic spot. Analysis of invasion pathways of alien invasive plants
The main invasion pathways of alien invasive plants can be divided into intentional introduction, unintentional introduction and natural invasion［14］. The invasion of alien invasive plants in China is mainly intentional introduction. However, in the scenic spots such as the Chengbi Lake Tourism Scenic Area, the unintentional introduction through human activities such as tourists will also have a great impact on the invasion of alien plants.
Intentional introduction refers to the conscious and purposeful introduction of a species by human beings into its ecological environment outside its distribution and diffusion potential［14-15］. The correct and appropriate introduction will increase the diversity of local ecosystems to a certain extent, enrich biological resources, increase social benefits, and promote development. However, blind introduction without risk assessment will pose an irreversible threat to the local ecological environment, and will also affect the development of local tourism. Once alien invasive plants escape and spread, the impact is even greater. At present, alien invasive plants that have caused direct or potential threats in the Chengbi Lake Scenic Area are mainly introduced as ornamental, garden or turf plants, such as W. trilobata, M. jalapa, I. cairica and L. camara.
Unintentional introduction refers to the intrusion of a species into another ecological environment with the import and export of vehicles, agricultural products and goods or by attaching to the fur or clothing in the unconscious of human beings［16］. As far as tourist attractions are concerned, the unintentional introduction of invasive alien plants is mainly caused by unintentional introductions by tourists from all over the world, such as S. torvum, E. hirta and B. pilosa.
Natural intrusion mainly refers to the propagation of plant seeds and other propagules along with airflow, water flow, birds and beasts to areas beyond their distribution and diffusion potential［16］, such as E. odoratum, E. adenophorum, Crassocephalum crepidioides and C. canadensis. In addition, naturally introduced alien invasive plants have serious harms once they form a population advantage.
Composition of families of alien invasive plant
According to the situation of field investigation and systematic research with reference to relevant literatures, 42 major alien invasive species were found in the Chengbi Lake Scenic Area, belonging to 33 genera of 21 families (Table 1), among which Compositae, Amaranthaceae and Solanaceae are major ones. Among the 42 alien invasive species, there were 14 species of Compositae, 4 species of Polygonaceae, and 3 species of Solanaceae, a total of 21 species, accounted for 50% of the invasive species in the Chengbi Lake Scenic Area. In addition, there are two species of Mimosaceae, Caesalpiniaceae, Euphorbiaceae and Convolvulaceae. There was one species in Oxalidaceae, Cleomaceae, Portulacaceae, Crassulaceae, Chenopodiaceae, Verbenaceae, Nyctaginaceae, Gramineae, Cactaceae, Cyperaceae, Basellaceae, Myrtaceae and Malvaceae. Composition of origins of invasive alien plants
Among the original invasive plants in the Chengbi Lake Scenic Area, there were 36 species originating from the Americas, accounting for 85.7% of the invasive plants in the Chengbi Lake Scenic Area, which was the main origin. The second one was Africa, where 5 species originated, accounting for 11.9%. There was only one from Asia, accounting for 2.4%, specifically from India. However, in the Chengbi Lake Scenic Area, alien invasive plants originating from Europe and Oceania have not yet been discovered.
Composition of life forms of alien invasive plants
The life forms of invasive plants in the Chengbi Lake Scenic Area were divided into vines, herbs, shrubs, sub-shrubs and small trees. Among them, there were 29 species of herbal species, accounting for 69.0% of the invasive plants in the scenic spot, followed by shrubs, 9 species, accounting for 21.4%, 2 species of vines, accounting for 4.8%, and one sub-shrub and small tree, both accounting for 2.4%.
Hazard analysis of alien invasive plants
The method for dividing the harm degree of invasive alien species divided the harm of current invasive plants in the Chengbi Lake Scenic Area into serious, moderate and mild based on the distribution of the current invasive plants in the field survey and the direct or potential threat to the ecological environment and economy of the Chengbi Lake Scenic Area［17］. The harm of alien invasive plants to the Chengbi Lake Scenic Area mainly included two aspects, namely, the destruction to the ecological environment of the scenic spot and the destruction to the scenic features and landscape construction of the scenic spot, which will fundamentally threaten human development and social economy［18-21］.
Alien invasive plants can destroy the ecological balance and endanger the ecological environment
Invasive alien species are highly invasive and diffusive due to strong adaptability, strong stress resistance, rapid growth and difficulty of eradication, and the lack of natural enemies that can contain them. Once invasive alien plants invade, they will rapidly multiply seedlings in large quantities, competing with the surrounding native plants for sunlight, water and fertilizer while secreting allelopathic substances, and they will inhibit the growth of other plants around their individuals by suppressing and repelling native species. As a result, others plants cannot grow or reproduce normally, or even die, so that they can easily form dominant populations, gradually occupy the ecological position of native species, and then form a single superior plant community, which causes fundamental changes in the species composition and structure of the ecosystem, affecting biodiversity. And ultimately, the ecosystem is subject to irreparable damage. Invasive alien plants affect the construction of scenic spots and hinder social and economic development
On the one hand, the invasion of scenic spots by invasive plants will affect the construction of scenic spots and the image of scenic spots, thus reducing the benefits of scenic spots. On the other hand, the prevention of invasive alien plants requires a lot of manpower and material resources, and ultimately will affect the income of the scenic spots. Scenic spots are an important pillar of local tourism industry. The income of scenic spots is partly related to the development of social economy.
Conclusions and Discussion
Analysis on the future invasion trend of alien invasive plants in the Chengbi Lake Scenic Area
On the basis of a comprehensive analysis of the results of the field survey, combined with the situation in the surrounding areas of the scenic spot, the future invasion trend of alien invasive plants in the Chengbi Lake Scenic Area was predicted.
On the one hand, there is already distribution in the Chengbi Lake Scenic Area, especially the severely endangered species, such as W. chinensis, E. odoratum and M. jalapa, which have a tendency to spread further. The main diffusion direction is the ecological environment with weak human management and few original species, especially the wasteland created by human intervention. On the other hand, although the Chengbi Lake Scenic Area has not yet been invaded, there are invasive alien species, especially invasive plants, such as Ageratum houstonianum, P. nil, Tithonia diversifolia, Eichhornia crassipes, etc., distributed at different levels around it or in Baise and even throughout the country, which all may be introduced with the intention of managers or unintentionally introduced by visitors into the scenic area.
Problems in the prevention and control of invasive alien plants
According to relevant information and surveys, there are still many problems in the prevention and control of invasive plants in the Chengbi Lake Scenic Area［22-26］.
Insufficient understanding of invasive plants by scenic spot staff and tourists
Analysis shows that artificial unintentional introduction is an important way for invasive alien plants to invade scenic spots. On the one hand, because local people or foreign tourists lack the knowledge of invasive plants, they cannot judge whether a certain plant is an invasive plant or not, and do not know what kind of harm will be caused by the introduction. This allows visitors to unintentionally introduce alien invasive plants into the scenic area. On the other hand, managers in the scenic area lack the knowledge of alien invasive plants, resulting in insufficient prevention and control of invasive plants. Limited investment in the scenic spot
To a certain extent, the infrastructure and management level of the Chengbi Lake Scenic Area has yet to be further improved. The passenger traffic in the scenic spot has a certain seasonality, and especially in the off-season of tourism, the lack of management of managers in the scenic spot will also provide opportunities for the invasion of alien invasive plants. Once invade and spread, invasive plants are difficult to eradicate. In addition, a lot of manpower and financial resources are spent in the control process, and the investment in these two aspects is still very limited.
Non-standardized introduction assessment
In order to further beautify the scenic environment, the scenic spot will introduce plants from other regions and even abroad, and there is a lack of risk assessment in the introduction process. When the ecological impact is not fully considered, it is not favorable to the prevention and control of invasive plants.
Insufficient scientific research and monitoring management
The investigation, prevention and control of alien invasive plants is a basic but highly professional work. As the systematic distribution of invasive plants in the Chengbi Lake Scenic Area has not yet been investigated, identified and analyzed, and the long-term and effective monitoring of invasive plants widely distributed in the scenic area has not been carried out, resulting in the lack of effective management of the scenic spot.
Lack of management regulations
At present, there are few relevant regulations on the scenic spots in China, and there are no relevant regulations on the management of invasive plants. Therefore, the Chengbi Lake Scenic Area lacks a legal basis for managing alien invasive plants, and there is a certain lack of management strength.
Methods and measures for prevention and control of alien invasive plants in the Chengbi Lake Scenic Area
In view of the existing problems in the prevention and control of invasive plants in the Chengbi Lake Scenic Area, combined with relevant information［16］and local favorable resources, methods and measures for preventing and controlling invasive plants in the scenic area are proposed below.
Popularizing the knowledge of alien invasive plants
First of all, the management department of the Chengbi Lake Scenic Area can conduct professional training on alien invasive plants for the staff in the scenic area, especially the management personnel, to popularize relevant knowledge. Meanwhile, we must strengthen publicity through various channels, so that visitors can have a deep understanding of the hazards and distribution of invasive plants, and actively participate in the control. Specifically, relevant information on invasive plants that needs to be controlled in the Chengbi Lake Scenic Area can be incorporated into the travel guide, or relevant promotional materials can be handed out. In addition, propaganda signs can be set in areas where the invasive plants are widely distributed in the scenic area. Strengthening the protection of the original ecological environment
Previous studies at community scales have shown that native species have a certain degree of "natural" resistance to invasive alien species, and that in areas with rich native species diversity, alien species have low ability to invade and spread［27］. Therefore, effective protection of the original ecological environment in the scenic area is the fundamental way to prevent and control the invasion and spread of alien invasive plants. All relevant administrative departments of the scenic spot should take effective measures to increase the protection of the original vegetation community and its habitat, and maintain the unaffected natural ecosystem as much as possible.
Conducting scientific research, determining the direction of prevention
In the absence of basic research on alien invasive plants, the management department of the Chengbi Lake Scenic Area can cooperate with local research institutes or universities to study the species composition, quantity, specific distribution and hazards of alien invasive plants in the Chengbi Lake Scenic Area. Meanwhile, according to the analysis results, main defense species and regions can be established, and the prevention and protection work in the areas with weak ecological environment and the surrounding areas of tourist routes should be strengthened.
Introducing alien invasive plants safely, effectively managing for long term
The existing alien invasive plants such as W. chinensis in the Chengbi Lake Scenic Area were originally introduced as landscape plants, but they eventually escaped and spread due to lack of management, causing harm. Therefore, when introducing landscape plants, we should consider and strictly refer to relevant laws and regulations to ensure the safety of introduction. It is necessary to conduct risk assessment, initially determine that there is a certain feasibility, and carry out eco-efficiency trials before introduction, and introduction can be conducted when it is determined that it will not threaten ecological safety. After the introduction, management should be strengthened, and attention should be paid to long-term and effective ecological monitoring.
Finding invasive plants early, removing them timely
The invasion process of alien invasive plants is generally divided into four periods: introduction and escape period, population establishment period, stagnation period and diffusion period［28］. Once invasive alien plants enter the diffusion period, they will grow and spread very rapidly, making it difficult to eradicate. In the Chengbi Lake Scenic Area, it is necessary to strengthen the classification management of species, and to detect alien invasive plants or plants suspected of invasive alien species very early. Once an alien invasive species is found, it is necessary to identify it in time, and to ascertain its source, invasion route and hazard as much as possible. On this basis, appropriate methods and measures should be adopted to completely remove it. Meanwhile, we must do a good job of isolation of the surrounding environment and strengthen the protection of similar ecological environment. Conclusions
Baise City is one of the 12 key red tourist areas in the country. The Chengbi Lake Scenic Area is an important pillar of Baise tourism. The investigation and analysis found that there are many invasive plants in the Chengbi Lake Scenic Area, which are harmful, and there is a trend of further spread. It is necessary to increase prevention and control, and carry out effective control from different aspects to ensure the ecological safety and economic development of Baise City and realize the harmonious development of man and nature.
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Editor: Yingzhi GUANG Proofreader: Xinxiu ZHU
Key words The scenic spot Chengbi Lake; Alien invasive plants; Survey; Analysis
Alien invasive plants refer to plants that migrate and form populations in natural or semi-natural ecosystems or habitats due to intentional or unintentional human activities and thus threaten the ecological environment where they are introduced to or some species in the environment［1］. Biological invasion, especially the invasion of exotic plants, has caused extremely serious ecological consequences and huge economic losses on regional and global scales［2］. The investigation and prevention of alien invasive plants has become an important task in todays ecological research, especially in plant protection and ecosystem restoration［3］. The Chengbi Lake Tourism Scenic Area is an important original ecological park, and the The non-renewable animal and plant resources contained in it are an important natural cultural heritage of mankind, and are irreplaceable in ecological environment protection and sustainable development of human society. However, with the development of the tourism industry in Youjiang District and the intensification of human activities, the natural ecosystem of the Chengbi Lake Scenic Area has been damaged to a certain extent and is facing a serious threat from invasive plants. Therefore, it is necessary to investigate and analyze the distribution of alien invasive plants in the Chengbi Lake Scenic Area, and the implementation of effective control measures is very necessary. However, relevant research results have not yet been found. In this study, the species, life forms, origins, invasion routes, distribution characteristics and damage degree of alien invasive plants in the scenic spot Chengbi Lake were mainly analyzed on the basis of field investigation and reference materials, and their future intrusion trend and prevention and control measures were preliminarily discussed, aiming to provide a scientific basis for further scientific research and ecological protection. Materials and Methods
General situation of the research area
Baise City is located in the west of Guangxi Zhuang Autonomous Region, at the junction of the three provinces of Yunnan, Guizhou and Guangxi, and the upper reaches of the Youjiang River, with a total area of 36 300 km2［4］. The specific geographical coordinates are 106°07′-106°56′ E and 23°33′-24°18′N. The Chengbi Lake Scenic Area is located 7 km northeast of Baise City, Guangxi Zhuang Autonomous Region. It was listed in the first batch of provincial-level scenic spots by the Guangxi government in 1988. In recent years, it has also been rated as an important national water conservancy scenic spot［5］. The scenic area is dominated by natural landscapes and is known because of "green mountain, elegant water, beautiful island, fresh fish". Chengbi Lake has a water surface area of 39 km2 and a total storage capacity of 1.154 billion m3. Chengbi Lake has a large water source forest protection area with a total area of 770 km2. It has a subtropical monsoon climate, with an annual average temperature maintained at around 22.2 ℃. The summer is rainy, often hot and humid, and the winter and spring are relatively dry. There is very abundant sunshine throughout the year［5］. In addition, the terrain of Chengbihu Lake area tends to be flat, and is surrounded by hills. The shape of the landform is diverse, and the geographical advantages are also very obvious. Chengbi Lake has a forest coverage rate of 70%, is rich in wetland resources, and maintains good biodiversity［5］.
Research methods
This study investigated invasive alien plants that pose a direct or potential threat to the ecosystem or landscape of the scenic area［6］. By consulting relevant literature through books, the Internet, etc.［7-9］, the distribution, hazard and prevention of invasive alien plants were fully understood, and invasive plants in the Chengbi Lake Scenic Area were initially screened and sorted out, so as to facilitate the preparation of field investigations.
Route surveys were conducted along roads or areas with frequent human activities, the types of alien invasive plants and their associated plants distributed along the line were recorded, so as to grasp the distribution of alien invasive plants in the Chengbi Lake Scenic Area as a whole. In addition, in order to better analyze the invasion routes and invasion trends of invasive alien plants in the scenic spot, route surveys were conducted on the surrounding areas of the scenic spot. On the basis of the route surveys, a representative distribution area in the scenic area is selected to conduct the sample survey. The specific methods included the vertical sample survey method and the horizontal representative habitat quadrat survey method. In various habitats at different altitudes, 2 m × 2 m quadrats were set up to investigate the geographical location, habitat characteristics, and plant distribution, plant height, coverage, phenophase, ecological location, etc. Meanwhile, various plants were photographed, and the list of alien invasive plants was determined by consulting references and querying specimens.
Results and Analysis
According to the comprehensive results of the route surveys, sample surveys and field surveys, and with reference to relevant literatures and graph comparison［10-13］, it was finally determined that there were about 42 major alien invasive plant species in the Chengbi Lake Scenic Area, belonging to 33 genera of 21 families (Table 1). According to the analysis of Table 1, in the set sample plots, alien invasive plants were distributed in areas at different altitudes in the vertical direction. Among the various alien invasive plant, Eupatorium odoratum, Lantana camara, Ipomoea cairica, Bidens pilosa, Conyza canadensis and Euphorbia hirta had different degrees of distribution between the altitudes of 133 and 422 m surveyed, while Wedelia chinensis, Mirabilis jalapa, Solanum erianthum and Oxalis corymbosa were mainly distributed in the center of the tourist attraction at a lower altitude of 157 to 192 m.
According to the horizontal survey of various habitat representatives in the tourist attraction, the distribution of invasive plants including L. camara, E.odoratum, C. canadensis, B. pilosa and E. hirta was also observed in artificial turf, wasteland at forest edges, residential areas and lakeside. It should be especially pointed out that a large number of S. erianthum and M. jalapa in the scenic reservoir dam. In the residential areas of the scenic spot, due to the cultivation and introduction by local residents, there were many invasive plants that can be used as medicines, fruits and vegetables, such as Talinum paniculatum, Cassia tora, C. occidentalis, Mimosa pudica, Cleome gynandra, etc. At present, although these plants are scattered and lightly harmful, they must be protected against further escape and spread in the scenic area.
In addition, the types of associated plants in the quadrats surveyed were relatively simple, and they were at a disadvantage both in quantity and in plant height. The coverage of invasive alien plants averaged about 77%, and some even as high as 100%. For example, in the artificial turf, along roadsides at the foot of hills and in nurseries under the forest, W. chinensis spread widely with a coverage of 100%, and it is highly harmful. The specific distribution of alien invasive plants in the Chengbi Lake Scenic Area is especially typical in the area around the Bashou scenic spot. Analysis of invasion pathways of alien invasive plants
The main invasion pathways of alien invasive plants can be divided into intentional introduction, unintentional introduction and natural invasion［14］. The invasion of alien invasive plants in China is mainly intentional introduction. However, in the scenic spots such as the Chengbi Lake Tourism Scenic Area, the unintentional introduction through human activities such as tourists will also have a great impact on the invasion of alien plants.
Intentional introduction refers to the conscious and purposeful introduction of a species by human beings into its ecological environment outside its distribution and diffusion potential［14-15］. The correct and appropriate introduction will increase the diversity of local ecosystems to a certain extent, enrich biological resources, increase social benefits, and promote development. However, blind introduction without risk assessment will pose an irreversible threat to the local ecological environment, and will also affect the development of local tourism. Once alien invasive plants escape and spread, the impact is even greater. At present, alien invasive plants that have caused direct or potential threats in the Chengbi Lake Scenic Area are mainly introduced as ornamental, garden or turf plants, such as W. trilobata, M. jalapa, I. cairica and L. camara.
Unintentional introduction refers to the intrusion of a species into another ecological environment with the import and export of vehicles, agricultural products and goods or by attaching to the fur or clothing in the unconscious of human beings［16］. As far as tourist attractions are concerned, the unintentional introduction of invasive alien plants is mainly caused by unintentional introductions by tourists from all over the world, such as S. torvum, E. hirta and B. pilosa.
Natural intrusion mainly refers to the propagation of plant seeds and other propagules along with airflow, water flow, birds and beasts to areas beyond their distribution and diffusion potential［16］, such as E. odoratum, E. adenophorum, Crassocephalum crepidioides and C. canadensis. In addition, naturally introduced alien invasive plants have serious harms once they form a population advantage.
Composition of families of alien invasive plant
According to the situation of field investigation and systematic research with reference to relevant literatures, 42 major alien invasive species were found in the Chengbi Lake Scenic Area, belonging to 33 genera of 21 families (Table 1), among which Compositae, Amaranthaceae and Solanaceae are major ones. Among the 42 alien invasive species, there were 14 species of Compositae, 4 species of Polygonaceae, and 3 species of Solanaceae, a total of 21 species, accounted for 50% of the invasive species in the Chengbi Lake Scenic Area. In addition, there are two species of Mimosaceae, Caesalpiniaceae, Euphorbiaceae and Convolvulaceae. There was one species in Oxalidaceae, Cleomaceae, Portulacaceae, Crassulaceae, Chenopodiaceae, Verbenaceae, Nyctaginaceae, Gramineae, Cactaceae, Cyperaceae, Basellaceae, Myrtaceae and Malvaceae. Composition of origins of invasive alien plants
Among the original invasive plants in the Chengbi Lake Scenic Area, there were 36 species originating from the Americas, accounting for 85.7% of the invasive plants in the Chengbi Lake Scenic Area, which was the main origin. The second one was Africa, where 5 species originated, accounting for 11.9%. There was only one from Asia, accounting for 2.4%, specifically from India. However, in the Chengbi Lake Scenic Area, alien invasive plants originating from Europe and Oceania have not yet been discovered.
Composition of life forms of alien invasive plants
The life forms of invasive plants in the Chengbi Lake Scenic Area were divided into vines, herbs, shrubs, sub-shrubs and small trees. Among them, there were 29 species of herbal species, accounting for 69.0% of the invasive plants in the scenic spot, followed by shrubs, 9 species, accounting for 21.4%, 2 species of vines, accounting for 4.8%, and one sub-shrub and small tree, both accounting for 2.4%.
Hazard analysis of alien invasive plants
The method for dividing the harm degree of invasive alien species divided the harm of current invasive plants in the Chengbi Lake Scenic Area into serious, moderate and mild based on the distribution of the current invasive plants in the field survey and the direct or potential threat to the ecological environment and economy of the Chengbi Lake Scenic Area［17］. The harm of alien invasive plants to the Chengbi Lake Scenic Area mainly included two aspects, namely, the destruction to the ecological environment of the scenic spot and the destruction to the scenic features and landscape construction of the scenic spot, which will fundamentally threaten human development and social economy［18-21］.
Alien invasive plants can destroy the ecological balance and endanger the ecological environment
Invasive alien species are highly invasive and diffusive due to strong adaptability, strong stress resistance, rapid growth and difficulty of eradication, and the lack of natural enemies that can contain them. Once invasive alien plants invade, they will rapidly multiply seedlings in large quantities, competing with the surrounding native plants for sunlight, water and fertilizer while secreting allelopathic substances, and they will inhibit the growth of other plants around their individuals by suppressing and repelling native species. As a result, others plants cannot grow or reproduce normally, or even die, so that they can easily form dominant populations, gradually occupy the ecological position of native species, and then form a single superior plant community, which causes fundamental changes in the species composition and structure of the ecosystem, affecting biodiversity. And ultimately, the ecosystem is subject to irreparable damage. Invasive alien plants affect the construction of scenic spots and hinder social and economic development
On the one hand, the invasion of scenic spots by invasive plants will affect the construction of scenic spots and the image of scenic spots, thus reducing the benefits of scenic spots. On the other hand, the prevention of invasive alien plants requires a lot of manpower and material resources, and ultimately will affect the income of the scenic spots. Scenic spots are an important pillar of local tourism industry. The income of scenic spots is partly related to the development of social economy.
Conclusions and Discussion
Analysis on the future invasion trend of alien invasive plants in the Chengbi Lake Scenic Area
On the basis of a comprehensive analysis of the results of the field survey, combined with the situation in the surrounding areas of the scenic spot, the future invasion trend of alien invasive plants in the Chengbi Lake Scenic Area was predicted.
On the one hand, there is already distribution in the Chengbi Lake Scenic Area, especially the severely endangered species, such as W. chinensis, E. odoratum and M. jalapa, which have a tendency to spread further. The main diffusion direction is the ecological environment with weak human management and few original species, especially the wasteland created by human intervention. On the other hand, although the Chengbi Lake Scenic Area has not yet been invaded, there are invasive alien species, especially invasive plants, such as Ageratum houstonianum, P. nil, Tithonia diversifolia, Eichhornia crassipes, etc., distributed at different levels around it or in Baise and even throughout the country, which all may be introduced with the intention of managers or unintentionally introduced by visitors into the scenic area.
Problems in the prevention and control of invasive alien plants
According to relevant information and surveys, there are still many problems in the prevention and control of invasive plants in the Chengbi Lake Scenic Area［22-26］.
Insufficient understanding of invasive plants by scenic spot staff and tourists
Analysis shows that artificial unintentional introduction is an important way for invasive alien plants to invade scenic spots. On the one hand, because local people or foreign tourists lack the knowledge of invasive plants, they cannot judge whether a certain plant is an invasive plant or not, and do not know what kind of harm will be caused by the introduction. This allows visitors to unintentionally introduce alien invasive plants into the scenic area. On the other hand, managers in the scenic area lack the knowledge of alien invasive plants, resulting in insufficient prevention and control of invasive plants. Limited investment in the scenic spot
To a certain extent, the infrastructure and management level of the Chengbi Lake Scenic Area has yet to be further improved. The passenger traffic in the scenic spot has a certain seasonality, and especially in the off-season of tourism, the lack of management of managers in the scenic spot will also provide opportunities for the invasion of alien invasive plants. Once invade and spread, invasive plants are difficult to eradicate. In addition, a lot of manpower and financial resources are spent in the control process, and the investment in these two aspects is still very limited.
Non-standardized introduction assessment
In order to further beautify the scenic environment, the scenic spot will introduce plants from other regions and even abroad, and there is a lack of risk assessment in the introduction process. When the ecological impact is not fully considered, it is not favorable to the prevention and control of invasive plants.
Insufficient scientific research and monitoring management
The investigation, prevention and control of alien invasive plants is a basic but highly professional work. As the systematic distribution of invasive plants in the Chengbi Lake Scenic Area has not yet been investigated, identified and analyzed, and the long-term and effective monitoring of invasive plants widely distributed in the scenic area has not been carried out, resulting in the lack of effective management of the scenic spot.
Lack of management regulations
At present, there are few relevant regulations on the scenic spots in China, and there are no relevant regulations on the management of invasive plants. Therefore, the Chengbi Lake Scenic Area lacks a legal basis for managing alien invasive plants, and there is a certain lack of management strength.
Methods and measures for prevention and control of alien invasive plants in the Chengbi Lake Scenic Area
In view of the existing problems in the prevention and control of invasive plants in the Chengbi Lake Scenic Area, combined with relevant information［16］and local favorable resources, methods and measures for preventing and controlling invasive plants in the scenic area are proposed below.
Popularizing the knowledge of alien invasive plants
First of all, the management department of the Chengbi Lake Scenic Area can conduct professional training on alien invasive plants for the staff in the scenic area, especially the management personnel, to popularize relevant knowledge. Meanwhile, we must strengthen publicity through various channels, so that visitors can have a deep understanding of the hazards and distribution of invasive plants, and actively participate in the control. Specifically, relevant information on invasive plants that needs to be controlled in the Chengbi Lake Scenic Area can be incorporated into the travel guide, or relevant promotional materials can be handed out. In addition, propaganda signs can be set in areas where the invasive plants are widely distributed in the scenic area. Strengthening the protection of the original ecological environment
Previous studies at community scales have shown that native species have a certain degree of "natural" resistance to invasive alien species, and that in areas with rich native species diversity, alien species have low ability to invade and spread［27］. Therefore, effective protection of the original ecological environment in the scenic area is the fundamental way to prevent and control the invasion and spread of alien invasive plants. All relevant administrative departments of the scenic spot should take effective measures to increase the protection of the original vegetation community and its habitat, and maintain the unaffected natural ecosystem as much as possible.
Conducting scientific research, determining the direction of prevention
In the absence of basic research on alien invasive plants, the management department of the Chengbi Lake Scenic Area can cooperate with local research institutes or universities to study the species composition, quantity, specific distribution and hazards of alien invasive plants in the Chengbi Lake Scenic Area. Meanwhile, according to the analysis results, main defense species and regions can be established, and the prevention and protection work in the areas with weak ecological environment and the surrounding areas of tourist routes should be strengthened.
Introducing alien invasive plants safely, effectively managing for long term
The existing alien invasive plants such as W. chinensis in the Chengbi Lake Scenic Area were originally introduced as landscape plants, but they eventually escaped and spread due to lack of management, causing harm. Therefore, when introducing landscape plants, we should consider and strictly refer to relevant laws and regulations to ensure the safety of introduction. It is necessary to conduct risk assessment, initially determine that there is a certain feasibility, and carry out eco-efficiency trials before introduction, and introduction can be conducted when it is determined that it will not threaten ecological safety. After the introduction, management should be strengthened, and attention should be paid to long-term and effective ecological monitoring.
Finding invasive plants early, removing them timely
The invasion process of alien invasive plants is generally divided into four periods: introduction and escape period, population establishment period, stagnation period and diffusion period［28］. Once invasive alien plants enter the diffusion period, they will grow and spread very rapidly, making it difficult to eradicate. In the Chengbi Lake Scenic Area, it is necessary to strengthen the classification management of species, and to detect alien invasive plants or plants suspected of invasive alien species very early. Once an alien invasive species is found, it is necessary to identify it in time, and to ascertain its source, invasion route and hazard as much as possible. On this basis, appropriate methods and measures should be adopted to completely remove it. Meanwhile, we must do a good job of isolation of the surrounding environment and strengthen the protection of similar ecological environment. Conclusions
Baise City is one of the 12 key red tourist areas in the country. The Chengbi Lake Scenic Area is an important pillar of Baise tourism. The investigation and analysis found that there are many invasive plants in the Chengbi Lake Scenic Area, which are harmful, and there is a trend of further spread. It is necessary to increase prevention and control, and carry out effective control from different aspects to ensure the ecological safety and economic development of Baise City and realize the harmonious development of man and nature.
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