Why is there less vegetation in Yunnan-Guizhou Plateau?

During the Yuan, Ming and Qing Dynasties, the unification of China's territory and the wider and deeper governance of the frontier made great changes in the environment and society of Yunnan-Guizhou Plateau. It is generally believed that there are two main reasons for this great change: first, with the reform of the land and the return to the current during the Yuan Dynasty to the Qing Dynasty and Yongzheng Dynasty, the central and state systems and policies in Yunnan-Guizhou Plateau have gradually deepened, which are reflected in politics, military affairs, economy and culture; First, the three generations of immigrants and the regional development activities triggered by them have generally made the Yunnan-Guizhou Plateau present a situation of population growth and economic development. (1) Due to these factors, the agricultural development of Yunnan-Guizhou Plateau in the Yuan, Ming and Qing Dynasties greatly exceeded that of the previous generation in breadth and depth, and fundamentally, the interaction between man and nature in this area was further strengthened. It is an important part of the study of regional development and sustainable development to comprehensively investigate the agricultural reclamation and land use activities that have an important impact on the historical development and environmental changes of Yunnan-Guizhou Plateau and explore the evolution of energy exchange system including human activities and natural geographical environment changes.

Natural environment for agricultural development

The Yunnan-Guizhou Plateau is mainly composed of the central Guizhou Plateau in the east and the Yunnan Plateau in the west. The central Guizhou Plateau, south of Zunyi and north of Miaoling, has an average elevation of 1 100 meters, which is mainly composed of low mountains, wide valleys and shallow basins. Yunnan Plateau is about 2 000-2 500 meters above sea level, bounded by Diancang Mountain, Ailao Mountain and Hengduan Mountain in the west, and the western Sichuan Plateau in the north, which is mostly mountainous and basin. Traditionally, Yunnan is divided into two geomorphic regions: the eastern Yunnan Plateau or the eastern Yunnan Plateau, and the whole Yunnan-Guizhou Plateau. [2] Yunnan-Guizhou Plateau "has no three-mile level" and "all dimensions are mountains. Those who want the so-called plain are not allowed to stretch for dozens of miles"; (3) Mountain plains and mountains account for about 87% of the total area of Guizhou Province, hills account for 10%, and basins (dams), valley terraces and plains only account for 3%; The mountains and plateaus in Yunnan account for 94% of the total area of the province. [4] There are many mountains and few plains. Historically, grain producing areas were mainly concentrated in Pingba area with good agricultural conditions, and the cultivated land area was limited, "eight mountains, one water and one field". Mountainous areas have also caused difficulties in internal and external transportation in Yunnan-Guizhou area, which has been relatively closed for a long time. The geographical environment of Yunnan-Guizhou Plateau has obvious internal differences, and the regional ecosystem presents different landscapes under the influence of vertical topography and climate (height affects energy distribution). "There are four seasons in one mountain" and "different days in ten miles" are by no means simple meteorological landscapes, but also suitable for describing the distribution of animals and plants, land use and ethnic settlements. The karst landform in Yunnan-Guizhou region covers an area of 222,835 square kilometers, accounting for 39.5% of the total land area of the two provinces. [5] The limestone landform is dominant, the soil quality of agricultural land is poor, the drought problem is serious, and due to the enhancement of human activities, vegetation is gradually lacking, and it is heading for rocky desertification (rocky desertification), which is more obvious in the hinterland of the plateau. The soil in Yunnan Plateau is mainly red soil, and the primary vegetation is mainly subtropical evergreen broad-leaved forest (now the secondary vegetation is mainly Yunnan pine forest or pine-oak mixed forest and grassland). The red soil distribution area has good hydrothermal conditions, and dam areas, river valleys and hilly areas dominated by paddy soil have become the main agricultural areas in Yunnan. Guizhou Plateau is dominated by yellow soil and calcareous soil, and the primary vegetation is mainly subtropical evergreen broad-leaved forest and stony xerophytic shrub (secondary vegetation is mostly evergreen deciduous broad-leaved mixed forest and shrub, etc.). ). The yellow soil distribution area is an important agricultural and economic forest planting area for rice-wheat double cropping in Guizhou. The diversity of vegetation and soil types in Yunnan-Guizhou Plateau provides conditions for developing a variety of agriculture and forestry management, but due to long-term natural and human factors, soil erosion, poor soil properties and low maturity are widespread. [6] The average annual rainfall in most areas of Yunnan-Guizhou Plateau is about 1 100mm, which is abundant, but the rainfall in the hinterland of western Yunnan Plateau is relatively closed, such as Binchuan-Yuanmou, Eshan-Xinping, Weining-Hezhang-Bijie. Although the annual precipitation can meet the demand of double cropping system, the rainfall season and geographical distribution are uneven, and agriculture is vulnerable to drought. [7] Yunnan Plateau is warm in winter and has a long frost-free period; The temperature in Guizhou Plateau is mainly lower in winter and slightly higher in summer. On the whole, however, due to the high altitude and insufficient high temperature in summer, the large-scale cultivation of heat-loving crops and double-cropping rice is limited. Affected by topography, climate and hydrothermal conditions, plateau agriculture is in a three-dimensional distribution state. However, under the condition of less mountains and more people, some unreasonable land use destroyed the agricultural natural environment and caused ecological and structural poverty. [⑧]

The geographical environment and natural conditions of Yunnan-Guizhou Plateau cannot be generalized to develop agriculture. On the one hand, internal regional differences and ecological diversity can adapt to and accept a variety of human utilization activities, providing possible development space, but on the other hand, it also restricts the promotion of a single or a certain type of resource utilization and development model in a wider range. In the historical period, the degree of human transformation and utilization of the natural environment depends on the development level of population and productivity, and these decisive factors also show complex, diverse and changeable characteristics in the history of Yunnan-Guizhou Plateau. Together with the natural geographical environment, they have created a more complex ecosystem of Yunnan-Guizhou Plateau and its evolution process.

In traditional society, the most direct manifestation of the influence of natural variation on human society is agricultural disaster. Among all kinds of agro-meteorological disasters in Yunnan-Guizhou Plateau, drought is the one with the widest influence, the longest duration and the most serious harm. The drought in Yunnan-Guizhou Plateau is mainly caused by winter and spring drought and summer drought, among which the regional flower arrangement drought which is easy to form in summer has the greatest influence. According to historical records, there were 208 droughts in Yunnan-Guizhou Plateau 1949 and 595 droughts in14. [9] Historically, spring and summer are the main seasons of drought in Yunnan-Guizhou Plateau, and there is still a proverb that "spring drought occurs every year". Geographically, droughts in Yunnan Plateau and western Guizhou are mostly spring droughts, while summer droughts mostly occur in northern and southern Guizhou. This is similar to the systematic disaster records since the Republic of China: spring drought is the most common in the western Yunnan-Guizhou Plateau, with a wide area and great destructive power; Affected by the rainy season, the summer drought lasts for a short time and has little impact; Autumn drought is the least, and the spatial distribution is mostly in a small range, which has the nature of flower arrangement; The spatial distribution characteristics of drought in Guizhou are as follows: Like Yunnan, there are few summer droughts in the western regions (Bijie and Anshun), with little harm, and the incidence of summer droughts in other regions is higher.

Flood is the most recorded disaster in the history of agricultural disasters in Yunnan-Guizhou Plateau. The floods in Yunnan-Guizhou Plateau mainly occur in mountainous areas and low-lying basins, and the flood characteristics in flash flood basins are obvious. Although flood is not as widespread as drought, it can cause serious impact in a short time, and it is easy to cause large-scale soil erosion in mountainous areas under the scouring of rainstorm and flood. However, waterlogging in low-lying areas, basins and lakeside areas caused crop failure, crop failure and famine due to flooding of fertile fields. According to the statistics of local chronicles, as of 1949, there were 340 floods in Yunnan-Guizhou Plateau with clear written records, and 754 floods in 174. Although floods in Yunnan-Guizhou Plateau mostly occurred in dry periods in history, floods generally occurred in mountainous areas, and waterlogging disasters mostly occurred in low-lying areas such as river valleys and basins. There were many floods and few waterlogging disasters, and global floods rarely occurred in Yunnan-Guizhou area. Most of the floods are concentrated in the rainy season, and most of them are caused by heavy rain in a short time. It is worth noting that from the historical records alone, it can be found that the flood disasters in Yunnan-Guizhou Plateau are mostly distributed in central Yunnan, northeastern Yunnan, eastern Yunnan, western Guizhou, northern Guizhou and eastern Guizhou (actually representing the areas where Yunnan-Guizhou developed earlier). Except for dam areas and river basins, the number of floods in mountainous areas in these areas is obviously higher than that in remote areas (such as western Yunnan and southern Yunnan, but of course, the uneven literature records and legacy factors are not excluded). [1 1] According to the study of flood records since the founding of the People's Republic of China, this situation has not changed. "The distribution trend of flood disasters is opposite to that of precipitation, that is, there are fewer flood disasters in the southern region with more precipitation, and more frequent flood disasters in the northeast and east regions with less precipitation. The main reason is that the relative height difference of valleys (basins) in Northeast and East China is relatively large, with less forest vegetation and more exposed ground. Therefore, the plateau "marginal area" with small relative height difference, less human activities and more forest vegetation has a strong interception effect on surface water flow and is easy to form floods. [ 12]

In addition to the above two main meteorological disasters, hail, low temperature, frost and heavy snow in Yunnan-Guizhou Plateau in history have also caused certain harm to agricultural production and people's lives. Due to the limitation of historical records, the items such as low temperature, cold freezing and heavy snow that we can sort out in ancient books are very few compared with those such as water and drought. However, modern research shows that frost, low temperature and rain are the main natural disasters in agricultural production in Yunnan-Guizhou Plateau, and what people call "cold in spring" and "turning green" generally endanger the growth of rice, spring crops, other tropical crops and cash crops. [13] There are many records of biological disasters such as epidemics in Yunnan-Guizhou Plateau in history, but the disasters caused by epidemics are the most serious, especially the large number of deaths of agricultural population. According to textual research, the plague epidemic occurred in Yunnan in the same year of Qing Dynasty and Xian Dynasty, and the death toll in Yunnan, Chengjiang and other seven provinces was as high as 6.5438+0.5 million. [14] According to the literature, the plague epidemic in Yunnan was serious in three periods: 1772- 1855, 1856- 1937, 1938+0949, and the death toll was very high. [ 15]

The occurrence of natural disasters is caused by human activities, except for irresistible natural internal causes. Therefore, natural disasters have become an important interface for people to examine the interaction between human activities and nature, and as a yardstick to measure the quality of regional ecosystems. From the above characteristics of natural disasters in Yunnan-Guizhou Plateau, it can be seen that most of the floods and droughts in history are basically caused by human factors, and the destruction of forest vegetation and the poor soil and water conservation are related to the excessive and unreasonable development and utilization of land; The spread of epidemics and other disasters is also related to the expansion of human communication activities.

Judging from the geographical environment and natural conditions of human development since the above history, the ecological chain between the development of Yunnan-Guizhou Plateau and environmental changes is more complicated. On the one hand, the diversification of natural ecological environment creates basic conditions for various economic development activities, but on the other hand, due to the uneven distribution of landform, climate, soil, society and population, development activities such as agricultural reclamation easily exceed the critical value of regional ecological balance, which makes the ecological environment enter a reverse evolution. From this perspective, the agricultural ecology of Yunnan-Guizhou Plateau