Occurrence and Distribution of Alternanthera philoxeroides in the Youjiang River Valley

Guikang JIA Zhiying ZHANG

Abstract The occurrence and distribution of Alternanthera philoxeroides in different habitats of four districts and counties in the Youjiang River Valley were investigated from September to November in 2018. The results show that A. philoxeroides certainly has occurred in watercourses, ridges, ditches, roadsides and green belts in those four districts and counties of The Youjiang valley. According to the sampling investigation, ridges have the highest frequency of occurrence, followed by ditches and roadsides. The A. philoxeroides living in ditches grows better than other four habitats, and the plants are stronger and vigorous. In the geographical division, the distribution areas of A. philoxeroides in Tianyang District and Tiandong County are the largest. Youjiang District and Pingguo County have a small amount of A. philoxeroides distributed along farmland, green belts and village houses, which has the characteristics of high local population density and strong aggregation, but the areas are smaller than other counties.

Key words The Youjiang valley; Alternanthera philoxeroides; Occurrence frequency; Distribution

Received: July 23, 2019Accepted: November 2, 2019

Supported by National Natural Science Foundation of China (31660171); The Basic Ability Improving Project of Young and Middleaged Teachers in Guangxi (KY2016YB421); 2013 Backbone Teacher Project of Baise University (210203009).

Guikang JIA (1977-), male, P. R. China, associate professor, master, devoted to research about Invasive ecology and plant biodiversity.

*Corresponding author. Email: jiaguikang@163.com.

Alternanthera philoxeroides is a plant in Alternanthera of Amaranthaceae, native to southern Paraguay in South America and the Rio de la Plata basin in northeastern Argentina, as well as wetlands in Paraguay and Parana［1］. During the Second World War, A. philoxeroides was introduced into China and planted in the suburbs of Shanghai and the northern plains of Zhejiang as a horse feed. It was later used as a pig feed and planted in various parts of the South, making it available in the southern provinces in a short period of time. Studies have shown that A. philoxeroides can cause the losses of rice, corn and vegetable yields of 45%, 19% and 20%, respectively［2-3］.

The surface of the stems of A. philoxeroides is usually graygreen, sometimes with purplered. The leaves are opposite and shorthaired on both sides. Its capitulum is solitary in leaf axils. The flowers have five tepals, white or pink, and the flowering period is from May to November. The flowers are generally not strong, and the plant depends mainly on stem bud reproduction［4-6］. Since the 1980s, due to the lack of scientific understanding and effective control measures for A. philoxeroides, the natural sprawl area has expanded rapidly in different regions of the southern provinces, which is very serious to the local ecological environment［7］, and the phenomena of enclosing river channels and invading farmland have appeared in many provinces and are becoming more frequent and serious. In the process of invasion of A. philoxeroides, the terrestrial type shows stronger spreading ability and survivability, and the aquatic type has even more serious harm to the river environment. The Youjiang River Valley has also suffered from the invasion of A. philoxeroides in recent years and has become one of the most serious disaster areas. The invasion of A. philoxeroides in the valley area affects local crop production［8］, threatening the survival of native herbaceous plants and causing ecological imbalances. The investigation on the frequency and distribution of A. philoxeroides in different habitats and different regions in the Youjiang River Valley has important reference value for the relevant departments to take preventive and control measures.

General Situation of Research Area

The Youjiang River Valley is located in the hinterland of the western mountainous area of Guangxi. The river valley is surrounded by a large area of hilly mountains at its two sides. The middle is a narrow valley plain. This area has a south subtropical monsoon climate with an annual average temperature at 21.0-22.1 ℃ and an annual average precipitation of 1 100-1 200 mm. The climate has the characteristics of rain and heat in the same season, warm winter and dry spring, and gets warm quickly in spring［9］. The Youjiang River Valley is the center of the "Nanning-Guizhou-Kunming Economic Belt" and the most convenient sea passageway in the southwestern part of China in the strategy of developing the western region. It has abundant natural resources and its regional superiority has been highlighted in recent years. Its scope includes two district and two counties (Youjiang District, Tianyang District, Tiandong and Pingguo County) of Baxian City, and it is about 130 km long in the eastwest direction and about 20 km wide in the northsouth direction. The Youjiang River Valley is a famous grain producing region in Guangxi and an important fruit and vegetable export region in the transportation of vegetables and fruits from the southern areas to the northern cities.

Research Methods

From September to November 2018, a field investigation was conducted on the valley plain of the Youjiang River Valley and representative villages and towns in the mountainous areas on both sides. The investigation mainly adopted the onsite investigation method, the route investigation method and the quadrat investigation method［10］, and directed at the frequency and distribution of A. philoxeroides in the Youjiang River Valley and the growth of A. philoxeroides in various habitats. The investigated habitats include roadsides, lakes and reservoirs, riversides, ditches, ridges, and urban greening areas. The quadrats of the investigation fully covered various villages and towns in the Youjiang River Valley, forming a network investigation scope covering all areas of the Youjiang River Valley with National Highway 323 and 324 as the main routes and the provincial, county and villagelevel roads as auxiliary routes.

For the systematicness and scientificity of the investigation, the routes were set as "two lines and two branches". Two lines: With the National Highway 323 and 324 as the main lines, the occurrence and distribution of A. philoxeroides in the villages and towns along the way were investigated; and two branches: two kinds of branch lines along the National Highway 323 and 324, i.e., county roads and village roads, were selected by a certain distance for the investigation, such as Provincial Road 206, County Road X736, XB50, Pingguo County 313 line. Onsite investigation and random sampling investigation were adopted.

In the middle of the Youjiang River Valley is a narrow river valley plain. The two sides are hilly and mountainous areas. The quadrats were mainly based on the center of the valley. Several quadrats were set up in villages and towns separated by tens of kilometers from the eastwest length of about 130 km. For example, quadrats were set up between Sitang Town of Youjiang District and Tianyang District and between Xiangzhou Town to Silin Town of Tiandong County; and a certain quantity of contrasting quadrats were set in the mountainous areas with high altitudes in the north and south, such as Longhe Village, Daleng Township with an altitude of over 740 m, and Yufeng Town with an altitude of about 540 m, to fully understand the distribution of A. philoxeroides at different altitudes and in different habitats. The size of each quadrat was 1 m2 or 2 m2, and the coverage, height and ecological position of A. philoxeroides in the plots were recorded, so as to analyze the frequency and distribution in this area.

The frequency of occurrence of A. philoxeroides refers to the ratio of the number of quadrats with A. philoxeroides in the specific habitat of the investigation area to the total number of quadrats in the area［11］:

F=Number of quadrats with A. philoxeroides/The total number of quadrats in the area

Results and Analysis

Through three months of intermittent onspot investigation and the "two lines and two branches" investigation, the distribution of A. philoxeroides in the districts and counties and its main growth areas were initially understood. The investigation results show that A. philoxeroides is distributed in different directions in the Youjiang River Valley, especially widely distributed in the valley plain (the grain and vegetable planting areas), and congregated growth is observed around 83% of the villages.

The A. philoxeroides in Youjiang District is mainly distributed on the ridges of paddy fields and roadside ditches in Sitang Town, less in Daleng Township and Bailan Township, and occasionally in roadside wetlands; the distribution of A. philoxeroides is observed all around Tianyang District, ranging from the ridges of rice paddy by Youjiang River with altitudes as low as 90 m and the riverside of Yufeng Town with altitudes as high as 554 m, and Baiyu Town has a high density of distribution, forming a large area of A. philoxeroides population easily; Tiandong County has a wide distribution, but the total area of distribution is smaller than that of Tianyang District, and The Heheng Village and Naping Village around the county has a large area of congregated growth, in which A. philoxeroides growing on roadsides and vegetable fields was an absolute dominant species of the environmental herb layer; Pingguo County covers a smaller area of the plain and a large area of rocky mountains, and A. philoxeroides in its territory is mainly concentrated in Xinan Town, scattered in the drainage channels and rice fields in other villages and towns with a small area, and is basically not distributed in 73% of the hills.

The occurrence of A. philoxeroides in the Youjiang River Valley is mainly concentrated in habitats such as ridges of paddy fields, residential drainage ditches, clean water ponds and wetlands, and the occurrence frequency of these habitats is 100%. Due to differences in geographical environments in various counties, the frequency of occurrence in various habitats varies, showing the smallest value in mountainous areas and the largest value in residential ditches. The A. philoxeroides in living water ditches grow vigorously, and the population advantage is very obvious, which is often characterized by developed stolons, thick leaves in fresh green color, large thickness of the cushionshaped part, and growth density generally greater than 383 plants/m2.

According to the analysis on the investigation data and the onspot investigation, the National Highway 323 and 324 running through the Youjiang River Valley are the main landbased propagation channels of the terrestrial type A. philoxeroides; the artificial greening areas in the urban construction process have accelerated the largespan transfer and propagation of A. philoxeroides; and A. philoxeroides, which is propagated through natural pathways, tends to spread slowly and cannot become an absolute dominant population in the community.

It can be seen from Fig. 1 that A. philoxeroides has invaded most of the plain area of the Youjiang River Valley, north to the Wangdian Township of Youjiang District, and south to the Xinan Town of Pingguo County at the end of the river valley, spanning about 130 km. The valley plain in Tianyang District and Tiandong County has the densest distribution of A. philoxeroides, and the hilly areas in Pingguo County are the least distributed with A. philoxeroides; and in the highaltitude mountainous areas, A. philoxeroides can be observed in the habitats such as riversides, residential drainage channels, and ridges of sugarcane fields.

Chengbihu Reservoir is one of the important reservoirs in the Baise area. The surrounding villages and towns are threatened by the invasion of A. philoxeroides, which is concentrated in the paddy fields in Yongle Township and the riversides of the Laihao Village downstream to the hydro project, where the thickness of the cushionshaped part is up to 42 cm and the growth density reaches 557 plants/m2. Due to the lack of effective treatment methods, the trend of intrusion has gradually increased, and the same situation occurs in small and mediumsized reservoirs located in other counties and their surrounding tributaries.

Occurrence frequency of A. philoxeroides in the Youjiang Valley

Through the field investigation of the Youjiang River Valley, A. philoxeroides is commonly found in ridges of rice fields, living water ditches, urban greening areas, roadsides, lowlying wetlands, and riversides. In the above habitats, the frequency of living water ditches, ridges and roadsides exceeds 80%, and A. philoxeroides is prone to aggregation with a large distribution area. Among them, such four habitats as ridges, roadsides, living water ditches and riversides were mainly investigated in various districts and counties.

Through the field investigation of 55 quadrats and the onspot investigation, it is concluded that the frequency of A. philoxeroides in Tianyang District and Tiandong County is significantly higher than that in other regions, and the frequency of occurrence in different habitats of the two counties is higher than Youjiang District and Pingguo County. In particular, it is distributed in large quantities and large areas around the villages and towns with centralized residence, and widely observed in the vegetable fields, roadsides, and drainage ditches. The occurrence frequency of rice field ridges is the largest, while sugarcane fields have a small frequency, and grow with A. philoxeroides in scattered distribution having smaller growth areas than other habitats. There are a small number of aquatic A. philoxeroides plants in the Youjiang River and some tributaries (the Baidong River), while the riverside embankments (the riverside of Baiyu Village) are mostly distributed with terrestrial A. philoxeroides in large areas. The terrestrial A. philoxeroides in the valley area is higher than the aquatic type in both frequency and distribution area.

Occurrence frequency of A. philoxeroides in Youjiang District

Youjiang District is located in the upper part of the valley plain. The microclimates are obviously different in difference areas. The sunshine and rainfall are abundant. The annual average temperature is 18.5-22.5 ℃, and the average annual precipitation is 1 053.38 mm. The forest coverage rate is 68.5%.

The A. philoxeroides in Youjiang District mainly grows in the ridges of rice fields and along vegetable fields and roads, occasionally in urban greening areas, among which the greening area in Bandao Park of Baise City is the most representative. The occurrence frequency of rice field ridges in Yongle Township and Huarun Township around Chengbi Lake and the vegetable field ridges in Sitang Town is high. There are fewer plants in Daleng Township at higher altitudes, mainly in the living water ditches and sugarcane field ridges.

Occurrence frequency of A. philoxeroides in Tianyang District

Tianyang District is located in the middle of the Youjiang River Valley. The temperature is high and the rainfall is sufficient. The annual average temperature is 22 ℃, and the annual rainfall is 1 100 mm. Sugarcane and vegetables are grown in large areas in this region. A. philoxeroides mainly occurs in rice field ridges, sugarcane fields and living water ditches in the region. In Toutang Town and Tianzhou Town where the terrain is relatively flat, there are a large amount of A. philoxeroides plants growing in ridges of rice fields. The frequency is small in Napo town of the southern Shishan District, and only a small amount of A. philoxeroides is distributed in sugarcane land along the river.

Occurrence frequency of A. philoxeroides in Tiandong District

The terrain of Tiandong County is high in the north and south, low in the middle, and is undulating. The annual average temperature is 21.9 ℃, and the sunshine is sufficient. The frost and snow are less. The terrain is relatively occluded, and the climate difference is also large.

The A. philoxeroides in the county mostly grows in the drainage ditches of the residential areas and the rural still water ponds (for example; the drainage channel of the Peoples Hospital of Tiandong County, abandoned ponds on the roadsides of Silin Town), and less along roadsides in villages and towns around the G323 line. There are large areas along the roadsides of Datang and Heyin Village around the county, and the frequency of roadsides is as high as 75%. There are a large number of A. philoxeroides plants along the G323 line and in residential drainage ditches in Silin Town, and the frequency in the rest of the habitats is also higher than that in other villages and towns in Tiandong. There are a large amount of A. philoxeroides aggregated in the artificial green belts of Xiangzhou Town Government, and the paddy fields have small areas of distribution but not dominant populations.

Occurrence frequency of A. philoxeroides in Pingguo District

Pingguo County is located in the middle and lower reaches of the Youjiang River. It is a county dominated by industry. It has a high temperature, rainy and subtropical monsoon climate. The annual average temperature in the whole county is 22.09 ℃, and the annual average rainfall is 1 313.76 mm. The rainfall is mainly concentrated in June-September.

Pingguo County has a small proportion in the Youjiang River Valley. The river flows fast and is narrow and slender in the county, and the mountainous area of the karst landform occupies a large area. Therefore, the frequency of A. philoxeroides in this area is smaller than that of other districts and counties. The main habitats are vegetable field ridges and roadsides. There are few plants occurring in ditches, and the probability of occurrence in mountain land is the smallest. A small amount of A. philoxeroides plants are distributed in artificial green belts in urban construction, such as Jiangbin Park in Pingguo County, but there is no obvious population, the distribution is scattered, and the growth advantage has not yet been shown.

Occurrence frequency of A. philoxeroides at different altitudes

The occurrence of A. philoxeroides in the districts and counties of the Youjiang River Valley varies greatly due to changes in altitude. The investigation of villages and towns at different altitudes revealed that in lowaltitude areas, the frequency of A. philoxeroides is maintained at a high position, and the frequency fluctuates smoothly; and in highaltitude villages and towns, the frequency of A. philoxeroides shows a significant downward trend, and it is not prone to the formation of a population and does not constitute a threat of invasion.

Occurrence characteristics of A. philoxeroides in different habitats

The narrowlyshaped Youjiang River Valley is a plain formed by alluviation. It has a large span and a small average width. The terrain in each district is extremely undulating, usually gentle in eastwest direction, but distributed with continuous mountains in northsouth direction on both sides, and the maximum altitude difference can reach 1 400 m (the Dawangling Mountain in Youjiang District is about 1 500 m above sea level, and the riversides of Youjiang River in Tianyang County is 89 m above sea level). Due to the special geographical location of the Youjiang River Valley, the growth of A. philoxeroides in different habitats has been significantly different, which is highlighted in the thickness of the cushionshaped part and density.

The thickness of the cushionshaped part of A. philoxeroides growing in the living water ditches can reach 53 cm, which is significantly higher than that of other habitats, followed by roadsides (42 cm) and field ridges (37 cm). The growth environment where A. philoxeroides grow vigorously with large density is generally cool and possesses fertile soil and abundant eutrophic water, which is in line with the prominent characteristics of the growth environment of A. philoxeroides, and A. philoxeroides shows an absolutely advantageous ecological position in the community, and serves as a constructive species. The growth environment of A. philoxeroides with low growth density is opposite to the above, and as most of the plants grow in poor soil (dry land, sugarcane land, etc.) with insufficient water, the growth of A. philoxeroides is limited to a certain degree, showing very limited thickness of the cushionshaped part and density.

Thickness of the cushionshaped part of A. philoxeroides in different habitats

The growth of plants is largely determined by the environment in which they grow. This is especially true for A. philoxeroides with strong invasiveness. Different growth environments determine its growth status and the maintenance of population advantages. Due to the large span and the special climate of the Youjiang River Valley, A. philoxeroides has different growth states in different habitats in each district and county, mainly in the thickness of the cushionshaped part of the population.

The thickness of the cushionshaped part of A. philoxeroides refers to the thickness from water surface to the canopy leaf tip in river channels and ditches and that from land surface to the canopy leaf tip in paddy fields and dry land［12］.

According to the onspot investigation and random sampling measurements along the G323 and G324 lines, it is concluded that the A. philoxeroides growing in the drainage ditches in the residential area of Tianyang District is very strong, and the thickness of the cushionshaped part is the highest in the sampling investigation, reaching 53 cm. The thickness of the cushionshaped part of A. philoxeroides growing along the National Highway 323 in Tianyang District is significantly higher than those in other counties, reaching an astonishing 42 cm, and the average value of A. philoxeroides growing in the crop field ridges is 29.5 cm. The sampling investigation shows the thickness of the cushionshaped part of aquatic A. philoxeroides under the Bailin Bridge in Baise City is as high as 38 cm. Aquatic A. philoxeroide communities rarely appear in the riversides of other counties, and the difference in the thickness of the cushionshaped part is not large.

Factors affecting the growth density of A. philoxeroides

The growth density of A. philoxeroides is different in different habitats in different areas of Youjiang River Valley. The density was the largest in the quadrats in Dinglu Village of Tianyang District, reaching 654 plants/m2, and the lowest in the quadrats by line 313 of Pingguo County, only 23 plants/m2. Among the many factors, there are two main factors affecting the growth density of A. philoxeroides, one of which is the soil type, and the other is the habitat of A. philoxeroides.

The results of the quadrat investigation shows that in the Youjiang River Valley, A. philoxeroides is still the dominant species in the habitats such as field ridges and artificial green belts, and is the constructive species in the roadside and ditch habitats in some areas, which is generally reflected by largescale aggregate growth. A. philoxeroides has formed a wide range of distribution throughout the Youjiang River Valley. The ecological location in the herbaceous layer has jeopardized the normal growth of native plants, resulting in the destruction of local plant diversity and the reduction of native plant species, and some native plants have gradually disappeared due to their inability to survive in the invasion of A. philoxeroides.

The growth density of A. philoxeroides varies rapidly with soil type and habitat. The combination of the two factors directly leads to the extensive growth of A. philoxeroides in the Youjiang River Valley, which threatens the growth of native plants and affects the ecological diversity of the herbaceous layer in the valley. Its destruction power is obvious.

Associated plants of A. philoxeroides

The A. philoxeroides growing in ditches is generally constructive species or a dominant species. The associated plants are generally: A. philoxeroides+Eichhornia crassipes, A. philoxeroides+Eclipta alba+Cuscuta chinensis, and A. philoxeroides+Acorus calamus. In ditches with sufficient water suffering from severe eutrophication, the strong reproductive capacity of A. philoxeroides can affect the survival of its associated plants, and the associated plants with weaker competitiveness (E. alba growing in the residential drainage channels) will not be able to absorb enough sunlight and finally die.

There are also large differences in associated species in different habitats. The influencing factors include altitude, temperature, soil pH, and regional microclimate, among which altitude and soil pH are the main factors affecting the associated species of A. philoxeroides. The associated species of A. philoxeroides along provincial highway 736, Longhe Village, Daleng Township, Youjiang District with a higher altitude (H: 744 m, E: 106°23.186′N:,23°44.558′) are as follows: Urena lobata+ Humulus scandens+Polygonum hydropiper+Spilanthes acmella+Anisomeles indica; the associated species of A. philoxeroides growing in the Youjiang River of Dongwu Shazhou in Longping Village, Tianyang District (H: 89 m, E: 106°53.876′, N: 23°43.342′) are as follows: Polygonum orientale Linn.+E. crassipes. The following are the associated plants that are common in different habitats during the survey.

Roadside: Wedelia chinensis, Bidens pilosa L., Cynodon dactylon (L.) Pers., Eleusine indica, Ageratum conyzoides, Alysicarpus vaginalis, et al.

Ditch: E. crassipes, Cleome gynandra L., A. philoxeroides, E. alba, Oxalis corniculata, Commelina communis, et al.

Ridge: Cynodon dactylon, Eleusine indica, Zoysia sinica, Amaranthus lividus, A. spinosus, Cyperus rotundus L., Euphorbia hirta L., et al.

Riverside: Eichhornia crassipes, Polygonum hydropiper L., A. calamus, Phragmites communis, et al.

Highaltitude grassland: U. lobata, Ageratina adenophora, Prunella vulgaris L, Sida acuta Burm. f., Elephantopus scaber L., Emilia sonchifolia, et al.

In highaltitude areas, except for a small amount of A. philoxeroides distributed in human settlements and cropgrowing areas, there is basically no occurrence of A. philoxeroides in the forest margins and forests, and the ecosystem is relatively complete. However, with the deepening of human activities to the forests, whether A. philoxeroides will invade the forests, destroy the balance of the herb layer, and lead to the reduction and extinction of native plants is still a very important subject to be studied.

The soil pH suitable for growth of A. philoxeroides is between 5.5 and 5.8. In the acidic soil environment, the invasion trend is more obvious, the growth state is better than that in other soil types, and the plant is tall and has the wide and fat leaves and long internode distance. Therefore, the associated species of A. philoxeroides grown in acidic soils are common acidic plants, including: B. pilosa, Oxalis corymbosa, and Sphagneticola trilobata.

Coverage of A. philoxeroides in different habitats

Vegetation coverage refers to the percentage of the vertical projection area of the plant community or the aerial part of each body to the sample area, which reflects the degree of vegetation and the size of the photosynthesis area of the plant. The coverage of A. philoxeroides varies greatly in different habitats. In the living water ditches and riversides, the coverage is often over 80%, while on dry roadsides, the coverage is only 12%. The different coverage of different habitats visually shows the growth status of A. philoxeroides and reflects its ecological location in the population. A. philoxeroides can be the constructive species with coverage higher than 75%, the dominant species with coverage of 40%-65%, and generally associated species with coverage smaller than 25%, and in some more extreme habitats (artificial Eucalyptus robusta plantations), the coverage is only 8%.

Discussion and Conclusions

Factors affecting the occurrence frequency of A. philoxeroides in the intercounty space

Tianyang County and Tiandong County are located in the central hinterland of the Youjiang River Valley. They have most of the valley plain of the Youjiang River, and are gentle in terrain, low in average elevation. The rainfall is abundant all year round, and the soil contains more nutrients than other areas. A. philoxeroides in this area has more abundant water and nutrient supply, making it grow in large quantities and form a dominant community.

Both Tianyang and Tiandong are important vegetable and grain producing areas, and vegetables, grain and other crops have large planting areas and cover a wide area. With the extensive cultivation of crops, the artificial propagation of A. philoxeroides is fast, and it can obtain the largest spread area in the same time.

The proportion of Youjiang District and Pingguo County in the river valley is small, and they have rapidlyfluctuating terrains and large altitude differences. A. philoxeroides is not prone to aggregation in large quantities, and is easy to control and difficult to spread, and its propagation speed is thus slowed.

The scientific cognition of A. philoxeroides is different in different areas. The ordinary residents of Pingguo County and Youjiang District have a deep understanding of A. philoxeroides, and would consciously control it in the event of a small area of invasion, which makes it difficult for A. philoxeroides to become a dominant species and make its propagation blocked. Tianyang District and Tiandong County have large agricultural cultivation areas, and lack effective and thorough control methods, and it is difficult to eradicate the invasion of A. philoxeroides, resulting in a larger base number than other regions.

There are significant differences in microclimate within the same districts and counties. If the growth and reproduction of A. philoxeroides is constrained, there will be certain differences in propagation. For example, Daleng Township in Youjiang District is a mountainous area with high altitude, and A. philoxeroides in this area grows slowly due to temperature, and suffer from obstacles in propagation by natural means.

Various habitats in which A. philoxeroide invades

Ditches are the habitat most vulnerable to the invasion of A. philoxeroides. They mainly have the characteristics of sufficient water (eutrophic in most ditches) and low flow velocity, and local aquatic plants growing therein are not competitive, which is easy to cause largescale outbreak of A. philoxeroides, which further blocks flow channels. Therefore, ditches serve as an important way for the waterway spread of A. philoxeroides. In the area of the Youjiang River Valley, almost all of the residents drainage channels in the villages in different areas show A. philoxeroides growth, and A. philoxeroides occurs frequently. Taking a village of 100 households as an example, the growth area of A. philoxeroides in the habitat of ditches is about 16 m2, and once the drainage channel of a household in the village grows with A. philoxeroides, it will spread to other drainage channels in a short time. A. philoxeroides is more capable of spreading through the canals than other native plants.

National Highway 323 and 324 are the main roads running through the Youjiang River Valley, and trucks from all over the country shuttle thereon every day, which accelerates the spread of A. philoxeroides in different areas and at different altitudes in the valley. The distribution of A. philoxeroides is observed in the section of the National Highway 323 from Tianyang to Yufeng in a small amount. Yufeng Town is a small town in the middle of National Highway 323, around which there are a large number of A. philoxeroides plants growing in the rivers, roadsides and vegetable field ridges. However, there is no A. philoxeroides in the mountains on both sides and mountainside villages. In the drainage ditches of all levels of roads running through the Youjiang River Valley, A. philoxeroides is distributed adjacent to vegetables and rice field ridges, and the propagation of A. philoxeroides is mainly based on road transportation, supplemented by natural spread of wind and rivers.

With the acceleration of urbanization, each district, county and town has focused on the greening areas of roads and parks. A. philoxeroides have also invaded the habitat of artificial greening areas with the footsteps of urban construction. The urban greening in Baise City is in its infancy, and a large number of green areas have not been managed in time. This has led to the largescale madness of A. philoxeroides, which has a large distribution area and is distributed nearly 1 200 m2 in the lawn and under green trees of Bandao Park in Baise City. The artificial greening area of the city and towns has been continuously expanded, and the spread speed of A. philoxeroides has also increased. It has spread over the street greening and park greening areas in the four districts and counties of the Youjiang River Valley.

The nature of A. philoxeroides as an alien invasive plant has been reflected in the Youjiang River Valley. In particular, when rice is planted, the canals are blocked, and in severe cases, the irrigation canals can be blocked to affect the irrigation of crops, resulting in reduced crop yields, a significant decline in the drainage capacity of the canals, and even loss of the function of the canals. In terms of ecology, some local plants are vulnerable to the exclusion of A. philoxeroides and lose their living space, gradually losing their original habitat.

The prevention and control of A. philoxeroides is a relatively weak link in the Youjiang River Valley. The general public and government agencies have no scientific understanding of the threat of invasion of invasive plants, and will only take measures when a largescale outbreak of A. philoxeroides occurs. There is a lack of professional prevention and control capabilities for plant invasion disasters and scientific publicity work for ordinary people. When it is difficult to obtain the desired effect with chemical herbicide on aquatic A. philoxeroides, underwater fishing method can be used to control it, and the fished plants should be buried or retted in time［13］. Terrestrial A. philoxeroides is often artificially or mechanically removed with spraying glyphosate as an auxiliary measure［14］.

After visiting the Dawangling virgin forest and other highaltitude mountainous areas, it is found that A. philoxeroides is less likely to invade by natural spread ways such as wind and river. The results of investigation and analysis indicate that the main reasons

for this phenomenon are the higher altitude, lower temperature and less illumination time under the forest; and the competitiveness of native plants in the forest herb layer is obviously superior, and the growth rate of A. philoxeroides is slowed down and the invasive ability is weakened. The above two factors have slowed down the invasion pace of A. philoxeroides. For an exotic species, its escape from natural enemies, favorable competition with indigenous species and new characteristics due to hybridization can temporarily compensate for the adverse conditions of its low genetic diversity［15］. In the investigation of forest margins and forests in other counties, the distribution of A. philoxeroides was not found, and there were a small amount of A. philoxeroides plants distributed in the residential drainage channels in the villages at the foot of the mountains. Whether it will invade natural forests or artificial plantation in the later period is still a subject that needs further research.

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