The formation of grassland degradation is influenced by many factors, including climate, landform, hydrology, soil, geology and biology, which are objective conditions for grassland degradation and desertification. However, inappropriate social and economic activities of human beings are an important reason for grassland degradation and desertification.

climatic factor

Climate change is an important natural cause of grassland change, especially the change of precipitation. Most temperate grasslands in China are located in semi-arid and arid areas, and the interannual and intra-annual variation of precipitation is obvious, which fundamentally determines the fluctuation of grassland productivity in the vast arid and semi-arid areas of China. The study of recent precipitation changes in northern China shows that the overall trend is drought, and the northwest region except Xinjiang shows obvious drought trend. According to the meteorological data of 1950 ~ 1990, the average temperature in the 1980s increased by 1. 1℃ compared with that in the 1950s, and the annual average precipitation decreased by 54 mm. However, in arid and semi-arid areas, due to the increase of temperature, the decrease of precipitation, the increase of evaporation and the decrease of humidity, the drought in grassland belt was aggravated. Based on the annual average precipitation of 195 1 ~ 1990, the precipitation in Inner Mongolia, Xinjiang, Qinghai, Tibet and other places in China decreased in the 1950s, except Inner Mongolia and Xinjiang. In the 1960s, precipitation was decreasing except in Tibet. In the 1970s, precipitation decreased everywhere; In the 1980s, there was an increasing trend. Therefore, from the late 1960s to 1970s, grassland degradation occurred in a large area in northern China. In northern China, the annual average temperature fluctuated and rose from 1950s to 1980s, especially since 1980s, the warming of temperature aggravated the loss of soil moisture, led to regional drought and accelerated the process of grassland degradation.

Generally speaking, the growth of plants needs air, sunlight, water and suitable temperature. Temperature also plays an important role in the growth and development of grassland. According to the adaptability of grassland to temperature conditions, it can be divided into two categories: cold season type and warm season type North China includes North China, Northeast China, Northwest China and Yunnan-Guizhou Plateau. The climate is mild, and the lawn grass is mainly cold-season grass. The optimum temperature for the growth of cool-season grass is 15℃ ~ 24℃. Although it has strong cold tolerance, it is not suitable for long-term survival under high temperature and high humidity conditions with a temperature exceeding 30℃. In contrast, warm-season species are distributed in the warm, humid and dry south, and the suitable temperature is 27℃ ~ 35℃. When the temperature is lower than 10℃, it often enters the dormant state. In addition to the surface temperature, the temperature of soil, the cradle of plant growth, is more important. Soil temperature is the temperature in the soil below the ground, which mainly refers to the shallow underground temperature directly related to the growth and development of flowers and trees.

(2) Geomorphological factors

Geomorphology macroscopically controls the distribution of climatic zones, thus determining the development pattern of vegetation. Topographically, China consists of three different steps. Daxinganling-the eastern plain area east of Taihang Mountain, with the altitude below 1000m, is the first level; A large area with an altitude of 1000 ~ 4000 m is the second step; The Qinghai-Tibet Plateau is the third step of China at an altitude of more than 4,000 meters. These three steps constitute the present earth outline of China, among which the sharp rise of the Qinghai-Tibet Plateau has the most important influence on the climate and vegetation in China. The uplift of the Qinghai-Tibet Plateau blocked the eastward advance of warm and humid airflow in the Indian Ocean, and promoted the formation of arid and semi-arid areas in China, thus greatly changing the vegetation landscape in China. In Miocene, a large area of savanna (a kind of herbaceous vegetation under the distribution of trees) appeared in northwest China. At the end of Tertiary, savanna was gradually replaced by grassland and desert. After Quaternary, a series of glacial periods and interglacial periods appeared, which caused climate fluctuation and promoted plant communication between north and south, mountains and mountains. In this way, the savanna in the western part of Northeast China, Inner Mongolia, northern Shaanxi and eastern Qinghai has turned into grassland, thus forming the outline of modern grassland areas in China. In the following time, the climate fluctuated and the distribution of vegetation changed to some extent. For example, during the Holocene warm period of 8.5 ~ 3.0 Ka B P (Ka B P), the boundary of grassland vegetation distribution moved westward, but its basic outline did not change. This basic outline is a large area of temperate grassland distributed in northern China, from the northeast plain through the Inner Mongolia Plateau and the Loess Plateau to the mountainous areas in the desert areas of Xinjiang. This temperate grassland is also the eastern part of the Eurasian grassland and the widest on the earth's surface. However, affected by the East Asian monsoon, warm and humid air flow in the Pacific Ocean and the precipitation brought by topography, there are three types of temperate grasslands in northern China: meadow grassland, typical grassland and desert grassland.

In addition, the landform controls vegetation not only at the macro level, but also different landform types and micro-landform units play a decisive role in the development and growth of vegetation. Taking the grassland vegetation in Xilin Gol League as an example, the grassland in Xilin Gol League is an integral part of the Central Asia-Asia sub-region in the Eurasian grassland region and one of the main grasslands in Inner Mongolia Autonomous Region. Its vegetation is mainly typical grassland xerophytes, and the zonal vegetation is Stipa grandis grassland, which is widely distributed in well-drained plains and forms a large-scale community. In addition, Stipa krylovii is also the basic formation in this area, and its distribution is also relatively common. It always exists near grasslands and traffic lines that are often grazed. In addition, there are meadows, swamps, desert vegetation and semi-hidden sandy vegetation in the League. In the low humidity environment, there are all kinds of meadow and swamp vegetation. In a sandy environment, due to the loose matrix. The stability is very poor, and the degree of soil development is relatively low, so the desert vegetation is relatively developed, including desert herb community, semi-shrub community, desert shrub community and desert elm forest.

(3) Hydrological factors

The first limiting factor for the growth of pasture is water. Under drought conditions, the extension and growth of plant cell wall are inhibited due to lack of water, and the differentiation is advanced, and the content of growth inhibitory hormones such as ABA in the body is increased, which leads to the miniaturization of plants, such as shorter plants, narrower leaves, shorter internodes and shallower root distribution. This is the expression that plants adapt to the arid environment. Therefore, water stress is an important reason for grassland degradation. Natural pasture is the main material basis of grazing and animal husbandry. Its growth dynamics and yield formation are influenced and restricted by surface water and groundwater conditions, which largely determines the geographical distribution of plants and the level of biological productivity. If the grassland community is in a state of serious water shortage during the whole growth period, the growth of plants will be greatly inhibited, and the species will be few and monotonous. The aboveground biomass, height and coverage of the community are affected by soil moisture, which changes in the direction of increasing → decreasing → increasing → decreasing.

In the past decades, great changes have taken place in the surface hydrological conditions in the arid and semi-arid areas of northern China, including the shrinkage of rivers, the shrinkage or drying up of lakes and the temporal and spatial redistribution of water in river basins. Moreover, with the change of hydrological situation in the basin, there is a widespread phenomenon of water quality deterioration, which is manifested in the continuous salinization of natural water bodies in the middle and lower reaches, and some lakes show an obvious salinization trend.

Soil factor

The natural conditions in grassland distribution areas are mostly dry and windy climate conditions and loose soil parent materials. The grassland surface composition in the grassland belt is mostly composed of physical sand, Quaternary loess or saline substances, so under the condition of drought and windy, the soil is easily eroded by the wind, resulting in grassland degradation and desertification.

(5) Geological factors

Mainly reflected in the influence of groundwater on grassland development. Groundwater, especially phreatic water in soil vadose zone, is the main water source for pasture growth, which largely determines the growth and development of pasture. In recent years, the groundwater level in many areas of northern China has been declining continuously, which has seriously affected the ecological balance of grassland.

In some degraded grassland areas, groundwater is buried deeply, which often leads to soil water shortage. After planting some leguminous grasses with developed roots, the water shortage will be aggravated. Continuous planting for many years will lead to soil drying and form the bottom dry layer of biological soil, which will affect the land water cycle. In this way, on the one hand, pasture consumes a lot of water, on the other hand, the lack of soil moisture further limits the yield of pasture, which eventually leads to grassland degradation.

(vi) Biological factors

Rodent damage is another important factor to promote grassland degradation. According to statistics, from 1978 to 1999, the average annual rodent infestation area in northern China was nearly 200,000 km2. The increase of rodents aggravated grassland degradation. First, rats and livestock compete for food, which intensifies the contradiction between grass and livestock. The daily food intake of rodents is equivalent to 1/3 ~ 1/2 times of their own weight. The average fresh grass quantity of Brandt's vole is 14.5g/ mouse, and that of plateau pika is 66.7g/ mouse. According to estimates, there are at least 600 million plateau pikas on the Qinghai-Tibet Plateau in China, which consume150,000 tons of fresh grass every year, which is equivalent to the food intake of150,000 sheep, resulting in a serious shortage of grass for livestock on the Qinghai-Tibet Plateau. The second is to destroy the grassland. Digging holes and digging holes are the habits of mice. Digging holes and eating grass roots destroyed the root system of pasture, resulting in the death of pasture in pieces. The soil dug by the mouse was pushed out of the hole, forming many caves and mounds. The soil suppressed the grassland vegetation, which also caused the grass to die and became a secondary bare land. The black soil beach on the Qinghai-Tibet Plateau is caused by rats. According to statistics, the degraded area of black soil beach grassland caused by rodent damage in the source area of the Yellow River has reached 20,000 km2, and some grasslands have lost grazing value.

(vii) Human factors

The influence of human activities on grassland degradation is very far-reaching, which can be said to be one of the most important factors to promote grassland degradation except extreme drought. With the increase of population in grassland pastoral areas and frequent overgrazing and reclamation activities, the degradation process of grassland in northern China has greatly intensified. Other unreasonable resource development activities, such as vehicle rolling, dredging and quarrying, indiscriminate digging of medicinal materials, unreasonable exploitation of mineral resources, and shortage of heating energy, have also seriously damaged the grassland and caused grassland degradation.

1. Grassland reclamation

Since liberation, due to one-sided emphasis on grain production, violation of natural laws, blind reclamation of grasslands, planting grain without protective measures and irrigation conditions, and extending the reclamation area to areas with precipitation less than 250 mm. Over the past 50 years, China has reclaimed 60,000 square kilometers of grassland. Take Inner Mongolia as an example. Since the founding of New China, there have been three waves of reclamation. The grassland area reclaimed twice by 1958 ~ 1962 and 1966 ~ 1976 reached 20,670 km2. From the end of 1980s to 1990s, the cultivated land area in 34 counties of five leagues in the east only reached 9708 km2, and most of the cultivated land was mountain meadow, meadow grassland, low-humidity meadow and typical grassland with good conditions. With the large-scale reclamation of grassland, the wind erosion of land has intensified and the desertification area has expanded. In recent 30 years, the grassland desertification area in Inner Mongolia has expanded by 1 10,000 km2.

After the founding of New China, the population of China increased sharply. In order to solve the food problem, grassland was reclaimed on a large scale. The National Plan of Returning Farmland to Grassland (200 1 ~ 20 10) points out that the cultivated grassland in China is about 193000 km2, accounting for nearly 5% of the total grassland area in China at present, that is, 18.2% of the existing cultivated land in China comes from grassland. After the founding of New China, Xinjiang has reclaimed 33,333 square kilometers of grassland. At present, only 1.80 km2 is actually cultivated, and nearly half of the land is abandoned due to secondary salinization. The grassland reclamation area in Qinghai Province is 3800km2, of which 2 125km2 is concentrated in Qinghai Lake area. At present, the arable land area around Qinghai Lake is only 1060 km2, which is only 50% of the arable land area. Overexploitation of grassland is an important reason for grassland degradation and desertification.

2. Overloading and overgrazing of livestock

In 1960s, the stocking capacity of natural grassland in Inner Mongolia in warm season was 93.0387 million sheep. In the late 1980s, the carrying capacity of grassland in warm season dropped to 54,750,500 sheep units. In 2000, the stocking capacity of the whole region in warm season was 42,975,700 sheep units, a decrease of11774,800 sheep units compared with the same period in 1960s. At present, in the typical grassland area, the available grassland area per sheep has decreased from 0.033 km2 in 1980s to 0.0084 km2 in 200 1 year.

The journey of overgrazing grassland mainly comes from two factors. First, the reduction of grassland area reduces the absolute amount of grassland area. Second, the increase in the number of livestock has reduced the relative area occupied by livestock. Over the years, the development of animal husbandry is based on the increase of livestock quantity, not on animal products. Under the influence of these policies and viewpoints, the number of livestock in China has greatly increased compared with the early days of liberation. During the period of Inner Mongolia 1947, there were192.58 million sheep units, and each sheep unit occupied 0.04 1km2 of grassland on average, so the utilization intensity was very low. Since then, the number of livestock has increased year by year, reaching 73.278 million sheep units in 1965, which is 3.8 times that in 1947. Each livestock occupies an average of 0.009km2 of grassland, which has exceeded the carrying capacity of natural grassland. Since then, the number of livestock has fluctuated greatly with the climate fluctuation, always hovering around 70 million sheep units. At present, almost all natural grasslands in China are overloaded to varying degrees. At the end of 1980s, the grassland carrying capacity in Xinjiang was overloaded 1 times. Since 1978, the number of herbivorous livestock in Qinghai Province has always remained above 35 million sheep units, while the theoretical carrying capacity of grassland is about 25 million sheep units, with an overload rate of 40%. At present, the area north of Qinghai Lake is overloaded 140%.

3. The growth of human and animals

According to United Nations experts' estimation, the number of people living in the arid and semi-arid areas in the north is five per square kilometer, while the population density in the arid and semi-arid areas in China far exceeds this standard. The population density of Inner Mongolia Autonomous Region is 18.7 persons /km2, that of Xinjiang is 9 persons /km2, and that of Qinghai is 6 persons /km2. Population growth is the direct factor and key to the continuous growth of livestock and grassland reclamation. According to statistics, the population of pastoral areas in Inner Mongolia was 263,000 in 1949, and increased to1929,200 in 2000, an increase of 633.54%.

Second, the regional characteristics of grassland degradation in China

According to China's natural geographical pattern and regional characteristics of grassland distribution, combined with the severity, forms and causes of grassland degradation, China's grasslands are divided into six regions: northeast pastoral area, Inner Mongolia and North China pastoral area, northwest pastoral area, Qinghai-Tibet pastoral area, southwest pastoral area and south grassland (Table 4-5). Among them, the first five areas cover almost all the important pastoral areas in China, and the grassland resources are fully utilized and degraded seriously; However, the utilization rate of grassland in the south is small, and the grassland degradation is not obvious, and its degradation is only manifested in the problem of soil erosion on Caoshan grass slope, so the grassland in the south 13 province (region) is divided into one area.

(A) the current situation of regional grassland degradation

Pastoral areas in Northeast China mainly include Heilongjiang, Jilin, Liaoning and parts of Inner Mongolia. Grassland area 196 280.44 km2 is seriously degraded, accounting for 33.58% of the grassland area in this area.

Pastoral areas in Inner Mongolia and North China include six provinces (autonomous regions and municipalities) including Hebei, Ningxia, Shanxi, Henan, Tianjin and Beijing, as well as most areas of Inner Mongolia, Shaanxi and Gansu. * * * The grassland area is 8473 17.33 km2, of which the degraded grassland area has reached 3 10, 168+0.

The northwest pastoral area, including the whole of Xinjiang and parts of Gansu, Inner Mongolia and Qinghai, is one of the important pastoral areas in China, with a natural grassland area of 924 390.87 km2 and a degraded area of 2 17 882.49 km2, accounting for 23.57% of the grassland area in this area.

The pastoral areas in Qinghai-Tibet are mainly composed of Tibet, most of Qinghai and parts of Gansu, Sichuan and Yunnan, with grassland area of 1280 424.63 km2, degraded grassland area of 203328.0438+0km2, and degradation rate of 15.88%.

Pastoral areas in southwest China include Guizhou, Sichuan, Yunnan, Guangxi, Shaanxi and Hunan, and the grassland area is 3 14089.30 km2, which is one of the important pastoral areas in China, but the degradation problem also exists and is more serious. Degraded grassland has reached 47,458.02km2, accounting for15,438+05,438+0% of the total grassland area in this area.

Grassland distribution in southern China involves Henan, Yunnan, Guangxi, Qiong, Guangdong, Fujian, Jiangxi, Jiangsu, Zhejiang, Anhui, Hubei, Hunan and other provinces (regions), with an area of 365,823.75km2 and a degraded grassland area of 25,393.66km2, with a degradation rate of 6.94%.

Table 4-5 Comparison of Degraded Grassland Area in Different Regions

(2) Regional law of grassland degradation

1. Northeast Pastoral Area

Pastoral areas in Northeast China have good water and heat conditions, and are areas with high vegetation coverage, good natural grassland quality and high yield. But at the same time, it is a large-scale grassland reclamation area; Grassland salinization and desertification are serious in the northeast and eastern coastal areas; Grassland pollution around some cities is becoming more and more serious, which poses a serious threat to grassland ecosystem; The rapid increase of livestock has caused overgrazing and overloading, which has made the utilization of grassland resources in some areas enter a vicious circle stage. In addition, the harm of rats and pests and the grassland area destroyed by a large number of medicinal materials are also increasing.

2. Inner Mongolia and North China Pastoral Areas

There are many reasons for grassland degradation in this area, but the main reasons are poor management and unreasonable utilization of grassland. This area, including the Loess Plateau, accounts for 70% of the soil erosion area, which is the main source of river sediment. However, the phenomenon of land reclamation on steep slopes has not been banned, so the area of grassland destruction has been expanding. There are many aeolian sandy soils in the Yellow River beach area, which are easily damaged by water erosion and wind erosion. In some areas, the grassland soil layer is shallow, the water retention capacity is poor, and soil erosion is also very serious; The local grassland is seriously overloaded and overgrazing, and predatory management causes grassland degradation. As residential areas are more and more concentrated along highways and rivers, grazing nearby is very serious, and grassland is not given the opportunity to recuperate, resulting in desertification, degradation and salinization of nearby grassland; Indiscriminate mining and digging make grassland desertification and degradation more and more serious. For example, the indiscriminate picking of Nostoc flagelliforme on the grassland of Alashan League in Inner Mongolia directly leads to grassland desertification and becomes the source of sandstorms in surrounding areas.

3. Northwest Pastoral Areas

The grassland types in this area are diverse and rich in resources, which provides a unique material basis for the development of grassland animal husbandry. However, because the northwest is located in arid areas, most of which are barren beaches and deserts, the ecosystem is very fragile. For a long time, some unreasonable human factors and some natural reasons have made grassland degradation quite serious. The main characteristics of grassland degradation in this area are: dry and windy climate, less precipitation, and the interruption or diversion of some inland rivers has aggravated the trend of grassland desertification; A large number of grasslands have been reclaimed, leading to land desertification and salinization. After being reclaimed, a large area of grassland near the water source was abandoned for various reasons, which completely destroyed the grassland ecosystem and even became barren. There has always been a serious problem of overloading and overgrazing in the pastoral areas of northwest China, with frequent rodents and pests, which has led to a significant decline in grassland vegetation coverage, serious soil erosion and sandstorm hazards, and deterioration of the ecological environment. The main form of grassland degradation is grassland desertification.

4. Pastoral areas in Tibet

Most areas in this area are above 3000 meters above sea level, with cold climate and sparse population. 70% of the grassland is alpine grassland, with fragile grassland ecosystem, short growth period and low grass yield. For a long time, due to overgrazing, unreasonable utilization, drought, rodent damage and human influence, grassland degradation in some traditional pastoral areas is very serious. According to the preliminary estimation of the agricultural department, the forage yield has decreased by more than 50% in recent 10 years. Especially in this pastoral area, the harm caused by rodents is particularly serious. For example, on the grassland of Qinghai Province, 7.826 billion kilograms of pasture is lost every year due to rodent damage, accounting for 65.438+065.438+0.8% of the total forage production in the province, and 5.36 million sheep can be fed a year.

5. Southwest Pastoral Areas

The reasons for grassland degradation in this area are mainly influenced by traditional farming ideas and population pressure, such as deforestation, extensive planting and poor harvest, which further aggravate soil erosion, frequent natural disasters and serious damage to grassland vegetation. In addition, a large number of indiscriminate exploitation of medicinal materials and indiscriminate killing of wild animals have also led to the spread of rodents, pests and toxic and harmful weeds in grassland.

6. Southern Grassland

The utilization rate of grassland in southern China is low and its distribution is scattered. The degradation problem is mainly the soil erosion problem of grass hill and slope. At present, the main hazards of grassland in southern China come from man-made destruction and soil erosion caused by various infrastructure construction, as well as blind development of a large number of barren hills and slopes caused by population growth pressure.

Third, the main hazards

Grassland degradation will lead to the deterioration of grassland ecological environment, the decrease of grassland biomass, the decline of excellent forage quality, the increase of toxic and harmful plants with poor palatability and low nutritional value, and the decline of grassland utilization performance and quality. But its harm does not stop there. Grassland degradation is also closely related to the change of soil properties.

As we all know, there is a very close relationship between grassland plants and soil. Different vegetation types directly affect the amount and depth of soil organic matter accumulation, and have a very obvious influence on the location, thickness and hardening degree of soil calcium accumulation. Human overgrazing is easy to degrade grassland, and more serious overgrazing will destroy the surface structure, erode the soil and gradually thicken the soil. With the decrease of natural grassland area and the increase of livestock, the natural grassland in pastoral areas will be further degraded and gradually become sandy land, bare land or saline-alkali land.

Fourth, the development trend.

Generally speaking, the grassland in China tends to be degraded in different degrees, especially in the arid and semi-arid areas in northern China. Judging from the climatic factors, the ecotone between agriculture and animal husbandry in northern China is ecologically fragile, with warmer climate and large precipitation variability, which will be the main reasons for grassland degradation and desertification in these areas; From the aspect of soil, the soil in the traditional pastoral areas in the north is mainly chestnut soil and aeolian sand soil, with loose structure and poor soil formation. Excessive reclamation or grazing will directly destroy the soil structure and expose loose sandy sediments to the surface, thus providing opportunities for soil wind erosion and land desertification; In addition, the rapid growth of population, over-exploitation and utilization of resources and other high-intensity unreasonable grassland development will lead to further degradation and desertification of grassland.

Verb (abbreviation of verb) prevention and control measures

Degraded grassland can be restored to some extent after enclosure and rotation grazing. However, it takes a long time to restore degraded grassland only by relying on nature. Only by taking some restoration measures to improve the moisture and soil conditions of grassland rejuvenation can the restoration of degraded grassland be accelerated.

(A) improvement measures of grassland water conditions

Mainly includes grassland irrigation and elimination of grassland water accumulation.

1. Grassland irrigation

Grassland irrigation should be adapted to local conditions, and different methods should be adopted in different areas and conditions, mainly including flood irrigation, furrow irrigation and sprinkler irrigation.

2. Elimination of accumulated water in grassland

There is too much moisture in the grassland and the ventilation condition deteriorates, which makes the reduction in the soil dominant and accumulates substances harmful to plants. It weakens soil microbial activities, accumulates a lot of organic matter, causes changes in grass layer composition, reduces excellent forage and reduces grassland quality. The drainage method of open ditch or underground ditch is generally used to remove the accumulated water in grassland. Open ditch drainage can not only exclude groundwater, but also exclude groundwater. Underground ditches have high drainage efficiency, do not hinder livestock grazing and mechanical operation, and can improve soil ventilation and nutrition.

(2) Measures to improve grassland soil conditions

Mainly includes loosening the soil and fertilizing the grassland.

1. Lawn scarification

After a long period of natural evolution and human production activities, the surface layer of soil is hardened, the ventilation and water permeability of soil are weakened, the activities of microorganisms and biochemical processes are reduced, the excellent forage grass is reduced from the grass layer, and the grassland productivity is reduced. The purpose of loosening grassland soil is to improve soil air condition, increase soil microbial activity and promote soil organic matter decomposition.

2. Fertilization on grassland

Fertilization is an important technical measure to improve forage yield and quality. We should not only consider the effect of fertilizer on forage yield, but also pay attention to the effect of fertilizer on forage composition. Rational fertilization can promote the development of excellent forage grass in grass group. Grassland fertilization usually uses organic fertilizer and inorganic fertilizer. Organic fertilizer is a complete fertilizer, which contains not only nitrogen, phosphorus and potassium, but also other trace elements. Organic fertilizer can also improve the physical properties of soil and contribute to the formation of soil aggregate structure. Mainly including green manure and so on. Inorganic fertilizer contains no organic matter and is rich in fertilizer components, which is absorbed and utilized by vegetation. It mainly includes nitrogen fertilizer, phosphorus fertilizer, potassium fertilizer, lime, gypsum and trace element fertilizer.