The evolution of rocky desertification
Guizhou karst rocky desertification occurs and is formed under the simultaneous action of many natural and social factors. The main processes are deforestation - steep slope land reclamation - water and soil erosion - rock exposure. The fragile ecological environment provides conditions for the rocky desertification process, and undesirable human activities accelerate this process. From the perspective of the ecological environment succession law, the rocky desertification process is actually a habitat reverse succession process. It includes the degradation of the land environment, vegetation environment and water environment. That is, it consists of a stable geological foundation, thick soil, Land with higher productivity and good vegetation coverage has degraded into rocky desertification land lacking vegetation coverage, water and soil resources replacement, unstable geological foundation, large areas of bedrock exposure, and reduced land productivity.
The succession of rocky desertified land in karst mountainous areas is extremely complex. Due to the historical development of various factors that make up the land under the action of internal and external forces, they can all cause changes in the land. The variability of each factor is quite different, resulting in asynchronous and difficult to determine land changes. Landform is a key factor in determining the characteristics and types of karst land, and its changes ultimately determine the succession of land. From the perspective of Guizhou’s karst plateau area, the landform succession follows the following rules: karst plateau → peak cluster mountains → peak cluster low mountains → peak cluster low mountains and hills → peak forest hills → residual hilly lysate → lysogen. This change process is an extremely long geological historical evolution, so rocky desertification caused by landform succession is the longest change. Each state in the succession process is caused by factors such as vegetation, soil, and water environment. Therefore, the succession of land rocky desertification refers to the changes in vegetation, soil and other factors caused by human actions. This succession has two directions:
(1) From the perspective of optimization, karst land Starting from the bare rock state, the migration, settlement, and development of organisms (plants) are used as clues to develop sequentially, eventually forming a karst forest landscape. The process is: bare rock surface → shrub (grass slope) → karst forest. In Guizhou, the corresponding vegetation is roughly karst moss → rubus, thorn shrubs → eucalyptus or wolfberry, elm shrubs → horn oak, fragrant or sweet oak, etc. and coniferous (cypress) Forest. The vegetation types vary in different regions. Eastern and southern Guizhou end with evergreen forests, while western and central Guizhou mostly end with deciduous or mixed forests. On the contrary, the soil also undergoes succession: intermittent black lime soil → red (yellow) lime soil → red (yellow) soil. During the entire succession, if there is insufficient forest area or the forest is too far away, the shrub-grass slope state will remain for a long time and cannot continue the succession, and may even deteriorate.
(2) From the perspective of deterioration, in karst areas, if the native vegetation is excessively damaged, deterioration and succession will easily occur. Its evolution starts from the forest and ends with rocky desertification: karst forest → evergreen deciduous shrub → thorn shrub → (grass slope) → bare rock surface. If the damage does not continue, it can remain in the sub-peak state for a long time after reaching the thorn bush. Regardless of whether it is viewed from the perspective of optimization or deterioration, the intermediate links are all vine thorn shrubs. Therefore, this type is most common in karst areas and has existed for a long time because it is extremely adaptable to the environment. The main changes in the soil during the deterioration process are the thinning of the soil layer, the loss of active ingredients, the soil cover becoming more and more sporadic, and even the complete loss of soil. However, the soil conditions in the karst peaks and depressions do not necessarily change in this order. It can remain in the black lime soil stage without major changes, which is also suitable for the optimization process. From the current point of view, due to long-term human activities, the karst forest land type in Guizhou has become extremely rare except for a few areas such as Maolan. In the vast limestone areas, forests are mostly preserved near villages, existing in the form of Feng Shui forests, and dotted in the form of "green islands" on the extremely exposed karst land.
In short, the evolution of karst rocky desertification has two directions: forward and reverse. Generally, forward evolution is a gradual process and lasts a long time, while reverse evolution mostly involves sudden changes and lasts a short time. The occurrence of mutations will have a severe, even disastrous impact on the ecological environment.
It is precisely because of this strict selectivity that rocky desertification evolves very slowly. Under protection, it takes about 10 years to evolve from bare rock to xerophytic shrubs, and about 30 years to evolve into thorn shrubs. , and it takes decades or even hundreds of years to form karst forest vegetation.
(4) Gradient nature and leaps in the evolution process: In the evolution process of rocky desertification, it strictly follows a gradual process without human intervention, especially forward evolution cannot occur from low to low. A direct transition from state to high state, while reverse evolution rarely occurs during natural disasters. However, when human activities are involved, reverse jump evolution can easily occur, while the scale of forward jump evolution caused by active human activities is very small. The evolution of rocky desertification is a complex process that includes both gradual and jumpy evolution. The two most direct indicators for judging the degree of rocky desertification are vegetation coverage and soil coverage. The soil coverage rate is directly proportional to the thickness of the soil layer. In karst mountainous areas, the terrain is undulating. Without a thicker soil layer, it is impossible to have a higher soil coverage rate. Bare land without vegetation coverage, but as long as there is a thicker soil layer The high soil coverage rate still belongs to the land without rocky desertification. In the long-term evolution process, forests that have no soil or have extremely shallow soil layers but are well developed are still considered to have no rocky desertification or potential rocky desertification. The Maolan Forest Nature Reserve is a typical example. The karst forest grows on the almost soilless karst stone mountain. The litter layer formed by it covers and fills the surface karst fissure zone. Due to its good water absorption function, it not only weakens the It reduces surface runoff, and also forms a karst forest stagnant water layer. Although its supply runoff path is short and the water volume is small, it is dynamically stable, has many discharge points, densely distributed springs, and continuous water flow all year round, which is easy to use and is the best supply. It is one of the sources of lower karst water and plays a significant role in improving karst groundwater recharge and runoff discharge conditions. It also transforms atmospheric precipitation, groundwater, and surface water into each other, and regulates each other to create a virtuous cycle. It rarely occurs in exposed karst forest-free mountainous areas. Poor habitat: a poor habitat with a combination of drought and floods. However, in vast karst areas, due to shallow soil layers, large slopes, and poor vegetation development, most areas suffer from moderate or above rocky desertification. If a large amount of vegetation is destroyed by humans, a jump evolution will occur from karst forest vegetation → rocky desert or thorn shrub → rocky desert. This kind of evolution may be a sudden and catastrophic change for humans, and the negative impact it brings cannot be eliminated in the short term. The subsequent negative effects of ecological environment deterioration will last for several generations. In other words, the contemporary human race Not only the current generation has suffered the consequences of environmental pollution, but future generations will not be able to escape its "benefit".
(5) Anthropogenic dominance of reverse evolution: Anthropogenic activities are the fundamental reason for the accelerated evolution of rocky desertification. Under purely natural conditions without human activities, due to the special regional background of the karst ecological environment, forward and reverse evolution will also occur under the comprehensive drive of various natural factors. In particular, forward evolution may be a faster process under the virtuous cycle formed by light, heat, water, soil, etc. Reverse evolution is local degradation caused by sudden natural disasters (such as landslides and debris flows), which has little impact on the regional environment. However, the participation of human activities has not only caused large-scale destruction of existing forests and land resources, but also greatly changed the driving factors of the positive evolution of rocky desertification, causing the originally fast evolution speed to rapidly slow down or even stop. However, the reverse process is accelerating, and the regional environment is rapidly deteriorating. From the relevant analysis conclusions, it can be seen that among the 9 dominant factors, the natural factors include karst area, river network density, average altitude, ≥25. For the area of slope land, the sum of the contribution rates to rocky desertification is 45, while the man-made factors include vegetation coverage (affected by humans), unused land, cultivated land, land reclamation rate and grassland, the sum of the contribution rates to rocky desertification is 55 , and human factors mainly use their destructive effects to reverse the evolution of rocky desertification, so human factors play a leading role in the reverse evolution of rocky desertification.
In the karst environment, the energy conversion pathway of the ecosystem is fragile and sensitive. For example, once the forest is destroyed, the material and energy exchange of the ecosystem will be temporarily interrupted, the ecological balance will mutate, and even lead to the emergence of a harsh environment that eliminates the possibility of human survival. The karst forest - The soil layer is the key to maintaining a virtuous cycle of the ecological environment. Once the vegetation is destroyed and the land becomes rocky desertification, it is extremely difficult to restore it, which will lead to an imbalance in the material and energy exchange balance of the ecological chain, interrupt the positive feedback benign effects, and cause a sudden increase in positive entropy. Moreover, the consequences of disorder are far more serious than those in non-karst normal forest vegetation areas. The rocky desert is essentially the top stage of the reverse evolution of rocky desertification. It is no longer possible for humans to continue to survive in this environment.