Study on Ecological Environment Succession Mechanism of Huixian Karst Wetland

According to the above analysis and research results, we can see that the succession of Huixian karst wetland or the change of ecological environment (wetland degradation) is the result of the comprehensive action of nature and human activities. However, in different regions, different stages of development and evolution or historical periods, the driving forces (or wetland evolution mechanism) leading to wetland evolution are different.

In the early geological history of wetland formation and development, that is, from the disintegration of Guilin inland basin to the early Holocene, the changes of natural conditions, especially climate change, played a leading role in the evolution of wetlands. Since Cenozoic, the climate change in Guilin has also experienced many alternating processes of cold and warm, dry and wet [17,58], which has had an important impact on the ecological environment change of wetlands. However, since human history, especially since the recorded human history after the construction of acacia trees in Tang Dynasty, human activities have participated in the evolution of wetland ecological environment to varying degrees, and the intensification of human economic activities in the last 40 years is the main reason for wetland degradation. According to the sedimentary cores, historical records and remote sensing research results of Hu Si Lake and Shizitan Lake, the ecological environment evolution of Huixian karst wetland in recent 2000 years can be roughly divided into three stages.

1. Early (prehistoric-the first year of longevity in Tang Dynasty)

During this period, large shallow lakes and swamps centered on Mudong Lake, Dulong Lake, Fenshui Lake and Hu Si Lake appeared in Huixian Town, Lingui County [57]. There is no record of human activities in this wetland, and the change of wetland ecological environment is mainly controlled by natural conditions, especially climate and change.

2. Mid-term stage (the first year of Tang Changshou-1950s)

This is a period when natural conditions and human activities jointly affect the evolution of wetland ecological environment, which has obvious regional zonation.

The core areas of the wetland-Mudong, Fenshuitang, Dulongtang and Li Antang in Huixian are obviously affected by human activities. Since acacia trees were built in Huixian karst wetland in 692 (the first year of longevity in Tang Dynasty), human beings began to intervene and influence the evolution of wetland ecological environment in this area on a large scale. Although the acacia tree was destroyed in 135 1 year (Yuan Dynasty) and expanded and maintained many times after the ninth year of Yongzheng in Qing Dynasty (173 1 year), this ancient great water conservancy project, together with Lingqu, communicated the traffic between the Central Plains and Lingnan and Southwest China, and played a role in ancient military and commercial exchanges. This influence includes: ① destroying the original hydrological pattern of wetland, resulting in the loss (or loss) of wetland surface water, and the unplanned blind development of wetland by human beings; (2) Impact on wetland hydrological process and water resources conservation; ③ Relevant immigrants and destructive development and utilization of water, soil, animal and plant resources. Therefore, it interferes with the evolution of the natural environment of wetlands to a great extent. These human activities are clearly reflected in the sedimentary records of the above wetland areas. Taking Shizitan, the control center of Xiangsidai Water Control Project, which is directly connected to the sub-reservoir, as an example, its sedimentary environment records reveal that 18 10 years ago was a gradual swamp environment, and TOC, TN and TP were particularly high, indicating that precipitation was obviously reduced or groundwater recharge was obviously reduced, and human activities were intensified. It may be related to the excavation, construction and maintenance of Acacia Belt 18 10 years ago (including digging an open channel between Feng Jia and Lion Mountain to directly draw Baxian Lake as the canal water source), which led to the decline of the wetland surface and groundwater level, the loss of water resources and the disconnection of the karst underground river in Lion Mountain; After the prosperity of Qing, Kang and Gan (168 1 ~ 1796), the Qing government gradually declined, bureaucratic corruption, internal and external troubles exhausted the national strength, and it was unable to maintain the operation of acacia trees, which was basically in a semi-abandoned state. Therefore, Acacia +08 10. After that (until 1990), although there was a brief eutrophication process during 1863 ~ 1872, it showed that the relative abundance of Amphibians with abundant aquatic vegetation remained at a high level, the species composition of Chironomids was stable, and the biodiversity index was high, indicating that the wetland was rich in aquatic vegetation and the ecosystem was stable during this period.

On the contrary, Hu Si Lake is located in the Xiang Si River Basin (the middle reaches of Qingshui River) in the west of Huixian karst wetland. Its water system is independent, and it is basically unaffected by the water conditions of Xiang Si Lake. It has been deposited in lacustrine facies for nearly 450 years, and the sedimentary records mainly reveal the influence of climate environment on wetland ecological environment. The results show that: ① during the period of 1562 ~ 1703 (especially during the period of 1574 ~ 1630), the phosphorus content was at the lowest and stable state, TOC and TN were low, and eutrophication did not occur, reflecting that the lake was in the natural evolution stage at that time, and human activities were not obvious. The fluctuation of the ratio of Mg/Sr and Mg/Ca from high to low reflects that the region experienced many climatic fluctuations of cold, warm and dry during the Little Ice Age, in which the characteristics of less precipitation and dry climate were obvious, which was not conducive to the development of wetlands. In the warm fluctuation period of the Little Ice Age, although the precipitation decreased, the concentration of Ca ions in the water was high, which brought abundant Ca elements to the lake. ② During the period of 1703 ~ 1894, the Mg/Sr ratio of Hu Si Lake rose slowly and slightly and remained stable, indicating that the temperature changed little, and the Mg/Ca ratio also rose slowly and slightly, indicating that there was more precipitation (precipitation reduced the concentration of Ca ions), and the climate as a whole was cold and wet (1850 ~ 650). Conducive to the development of wetlands. The decomposition of plant residues is slow and well preserved, and the TOC and TN values are relatively high, while the phosphorus deposition content is slightly increased but still low, especially after 1895, the TOC, TN and TP values are low, indicating that the lake is still in the natural evolution stage.

3. Recent stage (from the early 1950s to the present)

In the second half of the 20th century, despite the continuous warming of the climate, especially the continuous drought of 1955 ~ 1964 and 1985 ~ 1990 (Figure 5- 1, Figure 5-2), the changes of wetland ecological environment, wetland ecological environment, wetland ecological environment, wetland ecological environment, wetland ecological environment, wetland ecological environment, wetland ecological environment, wetland ecological environment, wetland ecological environment and Specific performance in:

The study of 1) sedimentary chronology shows that the average sedimentary rate of Hu Si and Shizitan is 1.4 ~ 1.7 mm/a, which indicates that the sedimentary background rate of the whole area is low (this may be related to the fact that the rocks in the wetland and its surrounding areas are pure carbonate rocks, lack of detrital materials and poor soil-forming ability). The average sedimentation rate of Hu Si Lake in 1952 ~ 1963 reached 3. 1 mm/a, which may be due to the land reform in 1952, the iron and steel smelting in 1958 ~ 1960 and the Great Leap Forward Movement.

2) In 1980s, the ratios of Mg/Sr and Mg/Ca in the sediments of Hu Si Lake fluctuated slightly and were basically stable, which reflected that the climate changed steadily after the Little Ice Age (the temperature rose slowly and there was more precipitation), and TOC and TN values also rose rapidly, especially after 1990, reaching a stable high value, and the phosphorus deposition increased rapidly (from 700mg/kg to nearly 2000mg/kg). The field investigation and data analysis show that during this period, the scope of human farming gradually expanded, and human activities such as raising ducks in lakes, topdressing organic fertilizer on cultivated land and domestic sewage pollution in surrounding villages were intense, which had a significant impact on the wetland ecological environment. The early 1970s was the turning point of the ecological environment in Shizitan, and the species composition of Chironomids in lake sediments changed obviously, indicating that the relative abundance of Chironomids pinnata with eutrophication level increased rapidly, reaching the maximum of 22.5% in 1990. However, the relative abundance of Chironomus anthrax and Platymonas sinensis, which indicate eutrophic lakes, continued to drop to zero, obviously in eutrophic state. The reasons need to be further analyzed, but the completion of 1973 acacia waterlogging drainage project (east dredging and west blocking) has undoubtedly had an important impact on the wetland ecosystem. On the whole, however, before 2004, the eutrophication level of Shizitan was low (the highest biodiversity index was 4.25), the aquatic ecosystem was relatively stable, the aquatic plants were flourishing and the water quality was good, which did not reach the threshold of obvious deterioration of water quality. However, after 2004, the water nutrition level of Shizitan decreased, the species number, biodiversity index and individual density of Chironomid decreased obviously, and the aquatic vegetation decreased, which may be due to the entry of some toxic and harmful substances or other factors, resulting in obvious changes in the aquatic ecosystem structure. Since the 1990s, the contents of TOC, TN and TP in Hu Si Lake and Shizitan Lake have increased rapidly, especially the content of phosphorus sediments, which shows that human activities in this area have intensified, such as the gradual increase in the use of chemical fertilizers in farmland, the increasing impact of domestic sewage and industrial wastewater on the regional environment, and the rapid development of lake eutrophication, which reflects that strong human activities have interfered with the natural evolution process, which is consistent with the regional development process in the past 20 years.

3) The statistical results of remote sensing survey (Table 5-4) show that the total area of various natural karst wetlands (excluding underground karst wetlands such as underground rivers and lakes) within the range of 130km2 in 2006 is about 4200hm2. The total area of all kinds of natural wetlands is only1456.5hm2 (the average annual decrease in 37 years is about 74. 15hm2), and * * * reduces all kinds of natural wetlands by 2743.5hm2 (including 2653hm2 for swamps and aquatic plants, and 1.93 hm2 for lakes and ponds). According to the statistical results of wetland succession in the core area (Table 5-5), 66% of the 2653hm2 marshes and aquatic plants that disappeared were developed into cultivated land, and about 25% were developed into fish ponds. 60% of the disappeared 193hm2 lake ponds were developed (enclosed) as fish ponds and farms, and 38.7% were reclaimed as paddy fields. Field investigation and remote sensing analysis show that there are three ways to destructively develop wetlands: ① Reclaiming farmland around lakes, enclosing ponds (or digging swamps) to raise fish (Figure 5-27); (2) Trenching → Draining and wetting surface water → Cutting or burning wetland vegetation → Reclaiming cultivated land (Figure 5-28); ③ Fill lakes (marshes) to build factories or build roads (Figure 5-29).

According to the above analysis, it can be seen that if natural conditions change (mainly global warming, rainfall reduction, disastrous climate, etc. ) has an important influence on the evolution of the ecological environment of Huixian karst wetland, which is one of the reasons for wetland degradation, so human activities are the direct cause or controlling factor of Huixian karst wetland degradation. Therefore, under the condition that the current climate continues to warm and the precipitation decreases, which is not conducive to the development of wetlands, it is even more necessary to strengthen wetland protection, strictly limit the interference and destruction of wetlands by human activities, take advantage of the situation, strengthen the construction of water conservancy projects to improve wetlands, and protect Huixian, a famous karst wetland in Guilin.

Figure 5-27 Wetland Development Model

Figure 5-28 Deforestation in Dryland and Wetland

The key to wetland protection and restoration is to formulate relevant protection measures or regulations. On the basis of protecting the existing wetlands to the maximum extent, through restoring the original hydrological pattern of wetlands, cooperating with corresponding water replenishment projects, moderately returning farmland to wetlands, returning fish ponds to lakes and limiting over-exploitation by human beings, the key technologies of wetland ecological restoration will be studied, and the wetlands will be developed in an orderly manner with green industries such as eco-tourism as the main means. The government should play an active leading role in wetland protection, planning and ecological restoration.

Figure 5-29 Wetland Development Model