The succession of rocky desertification land in karst mountainous area is very complicated. Due to the historical development of various factors that make up the land under the action of internal and external forces, all of them can cause changes in the land. The variability of each factor is quite different, which leads to the unsynchronization and uncertainty of land change. Geomorphology is the key factor to determine the characteristics and types of karst land, and its change ultimately determines the succession of land. Judging from the karst plateau area in Guizhou, the landform succession follows the following rules: karst plateau → peak cluster Zhongshan → peak cluster low mountain → peak forest low hill → peak forest hill → residual hill dissolution → dissolution. This change process is a very long geological historical evolution, so the rocky desertification caused by landform succession is the longest change. Every state in the succession process is caused by vegetation, soil, water environment and other factors. Therefore, the succession of land rocky desertification refers to the changes caused by human factors such as vegetation and soil, and this succession has two directions:
(1) From the optimization point of view, karst land starts from the bare rock state, followed by the migration, settlement and development of organisms (plants), and then develops in turn, finally forming karst forest landscape. Its process is: bare rock surface → shrub (grass slope) → karst forest. In Guizhou, the corresponding vegetation is generally karst moss → raspberry, prickly shrub → yueyueqing or citrus, elm shrub → Quercus anserina, Toona sinensis or Castanopsis eyrei and conifer forest. Its vegetation types are different in different regions, with evergreen forests in the east and south of Guizhou and deciduous or mixed forests in the west and middle. On the contrary, the soil has also undergone a succession: discontinuous distribution of black calcareous soil → red (yellow) calcareous soil → red (yellow) soil. In the whole succession process, if there is a lack of sufficient forest area or it is too far away from the forest, the state of shrub-grass slope will last for a long time, and it will not continue to succeed or even deteriorate.
(2) From the point of view of degradation, in karst areas, if the original vegetation is excessively destroyed, it is easy to have degradation succession. Its evolution began in the forest and ended in rocky desertification: karst forest → evergreen deciduous shrub → liana shrub → (grassy slope) → bare rock surface. If the destruction does not continue, it can maintain the sub-climax state for a long time after reaching the vine shrub. No matter from optimization or degradation, the middle link is liana shrub, so this type is the most common in karst areas and has existed for a long time because it is extremely adaptable to the environment. In the process of metamorphism, the main changes of soil are the thinning of soil layer, the loss of effective components, and the increasingly sporadic or even complete loss of soil cover. However, the soil conditions in karst peak-cluster depressions do not necessarily change in this order, and they can always keep the black calcareous soil stage without great changes, which is also suitable for the optimization process. At present, due to long-term human activities, except for a few areas such as Maolan, Guizhou karst forest land types are extremely rare. In the vast limestone area, forests are mostly kept near villages, existing in the form of geomantic forests and dotted on extremely exposed karst land in the form of "green islands".
In short, the evolution of karst rocky desertification has two directions: positive and negative. Generally, forward evolution is a gradual process with a long duration, while reverse evolution is mostly sudden and has a short duration. The occurrence of mutation will have a serious or even catastrophic impact on the ecological environment. However, the ecological environment of karst rocky desertification develops in stages, whether it is positive succession or reverse evolution, that is, the top rocky desertification is strong and fragile, the intensity of serious rocky desertification is fragile, the moderate rocky desertification is moderately fragile, the mild rocky desertification is slightly fragile, the dangerous rocky desertification is potentially fragile, and it is not fragile without rocky desertification. Under the complex man-land coupling system in karst area, various factors influence and restrict each other, and the evolution of rocky desertification is also extremely complicated. Comprehensive analysis shows that the evolution process mainly shows the following characteristics (Figure 4- 1 1):
Figure 4- 1 1 Karst rocky desertification evolution model diagram
(1) The time series of forward and reverse evolution coexist and are complementary in space: the main stage of forward evolution is rocky desertification land → xerophytic shrub → evergreen deciduous shrub → karst forest; The main stage of reverse evolution is karst forest → evergreen deciduous shrub → xerophytic thorn shrub → rocky desertification land, which is a pair of reverse evolution. In fact, both evolutionary models exist. In some typical sample areas, the staged achievements of the positive evolution of rocky desertification, such as the typical karst virgin forest in Maolan Nature Reserve, the secondary "Feng Shui forest" near the villages in the vast karst areas and the karst artificial ecological forest built in recent years, are typical representatives of the positive evolution. There are more types of reverse evolution, such as a large number of low coverage xerophytic shrubs in Maguan area of Puding County, and karst grass slopes everywhere in the upper reaches of Huangguoshu Waterfall in Zhenning County. As far as its spatial distribution is concerned, in a small watershed, there are both stages of rocky desertification's forward development and different representatives of rocky desertification's backward evolution, which constitute a mosaic landscape of rocky desertification. No matter from forward to backward, the middle link of evolution is liana shrub, so this type is the most common in karst areas. The whole evolution shows that the two evolution series coexist in time and complement each other in space.
(2) Non-equilibrium evolution: the diversity and variability of regional environmental factors in the evolution of karst rocky desertification lead to the complexity and diversity of regional site conditions, high ecological heterogeneity, great differences in the environmental conditions for the successful resurrection of biological communities, limited niche diversity and environmental capacity, and narrow niche, making it extremely difficult for species to jump from one habitat to another. Once the rocky desertification ecological environment is destroyed, it will quickly converge to a single drought and biochemistry, and reverse succession will occur. The species structure is in obvious contrast with the top community in karst evolution, which makes the reverse succession of karst ecological environment fast, and the evolution of community structure in different stages is quite different. Dynamic balance is an important law of a virtuous cycle of ecological environment. Only when the evolution of rocky desertification environment is basically balanced, can it develop in the direction of environmental optimization under the positive action of human beings. At present, under the strong interference of human activities, the spatial comprehensive driving force of forward evolution of rocky desertification is obviously less than that of reverse evolution, and the regional rocky desertification is dominated by reverse evolution, and the forward evolution is relatively weak, which indicates that the degree of rocky desertification is further aggravated, the quality of ecological environment is further deteriorated, and the whole evolution system is extremely unbalanced.
(3) Strict selectivity in evolution: The selectivity of karst rocky desertification evolution to environment is most obviously reflected in the strict selectivity to plants. The karst rocky desertification environment has strict selectivity for plants, and the growth of trees is extremely slow, because only those plant populations that are physiologically calcium-loving, drought-tolerant, stony and whose roots can adapt to climbing rocks and survive and absorb nutrients in crevices can grow and develop on calcareous soil with thin soil and easy drought. Therefore, when the ecological environment changes under the influence of external factors, plants with high habitat conditions, even many universal plants, are difficult to survive and grow. On the contrary, spiny shrubs with drought tolerance, barren tolerance and alkali tolerance can grow, and some plant species mutate to adapt to this ecological environment. For example, some herbs, such as bluegrass, can't absorb water and nutrients to the depths of rock crevices, so some stems will thicken, enlarge and mutate into "water storage bags" to store a certain amount of water, while leaves are leathery to resist the dry and hot habitats of rocks. Cactus form a cluster shrub, and trees (such as banyan) have developed roots, which climb on rocks and extend into crevices, and the root length can be several times higher than the height of trees. This environmental dislocation phenomenon is also a highly sensitive reflection of ecological environment variation. It is precisely because of this strict selectivity that the succession of rocky desertification is extremely slow. In the protected state, it takes about 10 years to evolve from bare rocks to xerophytic shrubs, about 30 years to evolve into liana shrubs, and decades or even hundreds of years to form karst forest vegetation.
(4) Evolution and jumping: In the process of rocky desertification evolution, under the action of human activities, it strictly follows the gradual process, especially in the forward evolution, it is impossible to directly transition from low state to high state, and jumping evolution only rarely occurs in the reverse evolution during natural disasters. However, when human activities are involved, it is easy to have a backward leap-forward evolution, while the forward leap-forward evolution caused by human active activities is very small. The evolution of rocky desertification is a complex process, with gradual evolution and jumping evolution simultaneously. The two most direct indicators to judge the degree of rocky desertification are vegetation coverage and soil coverage. Soil coverage is directly proportional to soil thickness. In karst mountainous areas, the terrain is undulating, there is no high soil coverage and bare land without vegetation, but as long as there is high soil coverage, it still belongs to rocky desertification-free land. In the long-term evolution process, there are well-developed forests, no soil or extremely shallow soil layers, which are still regarded as no rocky desertification or potential rocky desertification. Maolan Forest Nature Reserve is a typical example. Karst forests grow on almost soilless karst rocky mountains, and litter accumulates, covering and filling the karst fracture zone on the surface. Because of its good water absorption function, it not only weakens the surface runoff, but also forms the stagnant water layer of karst forest. Although the recharge runoff path is short and the water quantity is small, it is dynamic and stable, with many discharge points, dense distribution of springs and continuous water flow for many years, which is convenient to use. And it is one of the best water sources to replenish karst water in the lower layer, which plays a role in obviously improving the conditions of karst groundwater recharge and runoff discharge, and also makes atmospheric precipitation, groundwater and surface water transform and adjust each other, forming a virtuous circle, and there are few bad habitats in bare karst forestless areas, such as bad habitats and sharp changes in drought and flood. However, in the vast karst areas, due to shallow soil layer, large slope, poor vegetation development, rocky desertification above moderate level. If a large number of vegetation is destroyed by human beings, there will be a leap from karst forest vegetation to rocky desert or vine shrub to rocky desert. This evolution may be a sudden and disastrous change for human beings, and its negative effects cannot be eliminated in a short time. The subsequent negative impact of the deterioration of the ecological environment will last for generations. In other words, the environmental consequences planted by contemporary people will not only be tasted by contemporary people, but also unavoidable for future generations.
(5) Human-led reverse evolution: Human activities are the fundamental reason for the accelerated evolution of rocky desertification. Under the pure natural conditions of human activities, due to the special regional background of karst ecological environment and the comprehensive drive of various natural factors, forward and backward evolution will also occur. Especially under the virtuous cycle of light, heat, water and soil, positive evolution may be a relatively fast process. Reverse evolution is a local degradation under sudden natural disasters (such as landslides and mudslides), which has little impact on the regional environment. However, the participation of human activities, in addition to the large-scale destruction of existing forest and land resources, has also greatly changed the driving factors of the positive evolution of rocky desertification, making the original rapid evolution speed quickly slow down or even stop, while the reverse process is accelerated, and the regional environment is rapidly deteriorating. According to the conclusion of correlation analysis, among the nine leading factors, the natural factors are karst area, river network density, average altitude and ≥25. The sum of contribution rate of sloping land area to rocky desertification is 45%, while the sum of contribution rate of human factors is 55%, including vegetation coverage (affected by human factors), unused land, cultivated land, land reclamation rate and grassland, and human factors mainly make rocky desertification reverse evolution due to their destructive effects, so human factors play a leading role in the reverse evolution of rocky desertification. In karst environment, the energy conversion path of ecosystem is fragile and sensitive. For example, once the forest is destroyed, the exchange of material and energy in the ecosystem will be temporarily interrupted, and the ecological balance will suddenly change, even leading to the emergence of a harsh environment that loses human survival. Karst forest soil layer is the key to maintain a virtuous cycle of ecological environment. Once the vegetation is destroyed, it is extremely difficult to recover the rocky desertification, which leads to the imbalance of material and energy exchange in the ecological chain. The positive feedback effect is interrupted, the positive entropy suddenly increases, and the disorderly consequences are far more serious than those in non-karst normal forest vegetation areas. Stone desert is essentially the top stage of the reverse evolution of rocky desertification, and human beings can't survive in this environment.