| The exploitation of coal mine has caused severe damages to the ecological environment, such as destroying farmlands and vegetation, decreasing species diversity and land productivity, and eventually destroying the structure and function of local ecosystems. The degenerated environment has put forward great challenges for sustainable economic development. Consequently, ecological reconstruction, aim at increase plant coverage and soil nutrients in coal mine area, is of great significance for improvement of local environments.Gujiao coal mine is a example of ecological reconstruction in Shanxi province, where has rich mineral resources.There were three methods used in ecological reconstruction in Gujiao coal-gangue area, including artificial restoration by planting staghorn sumac (Rhus typhina) and oriental arborvitae (Platycladus orientalis), and natural restoration. Based on the survey of vegetation and soil properties, the changes of community structure and species diversity throughtout the succession process were analyzed. Also, the interactions between vegetation succession and soil conditions were discussed using TWINSPAN classification and CCA ordination methods. In order to evaluate the results of ecological restoration, three restoration methods were compared with the regional vegetation. Further, some suggestions are put forward, which can provide a scientific basis for ecological reconstruction in the mining area.The results showed that great changes occurred in community structure, species composition and diversity, coverage after 12 years of succession of staghorn sumac. Pioneer species such as Salsola collina and Setaria viridis played dominant roles at the early stage of succession, and they were replaced by dominant species Poa sphondylodes after 5 years. Because of the fast growth rate, canopy coverage of staghorn sumac community increased quickly. In the meantime, light radiation on the forest floor evidently decreased, leading to the decrease of species number and diversity in the herb layer. Due to its vigorous sprouting reproduction, staghorn sumac could colonize the new habitat and expanded the population within a short time. After 12 years, staghorn sumac became an exclusive edificator in the community and other species were greatly depressed and disappeared.Our study showed that light intensity was an important factor affecting species diversity and succession of staghorn sumac community. At the mature stage, weak light under the community canopy restricted the sprouting and dispersion of clonal ramet of staghorn sumac, as well as the growth of herb species. Thus, as a heliophytes species, the succession process and self-renewal of staghorn sumac was impeded to a great extent. Also, staghorn sumac was able to disperse into adjacent oriental arborvitae and shrub community and quickly expanded its population. The average height of clonal ramet of staghorn sumac was higher than that of oriental arborvitae and shrub. So, it was likely that the growth of oriental arborvitae and shrub would be depressed by staghorn sumac invasion in the future.It was showed that oriental arborvitae displayed a slower growth rate, with the height and coverage increasing gradually. Based on the results of TWINSPAN, annual pioneer species such as Salsola collina and Setaria viridis firstly occurred at the beginning stage of succession, then other dominant species such as Artemisia giraldii, Artemisia vestita and Poa sphondylodes appeared in turn through the oriental arborvitae succession. After 14 years, kinds of shrub appeared in the community. Species diversity in oriental arborvitae community was also increased during the whole succession. The thickness of soil added on the coal waste piles was a main factor affecting the growth of oriental arborvitae. There were significant differences in tree height, basal diameter and canopy coverage of oriental arborvitae among three soil types. When the thickness of soil was lower than 30 cm, some leaves growed sere and falled off, leading to higher mortality in oriental arborvitae community.Based on the results of TWINSPAN, species composition and diversity in the naturally restored communities experienced great changes during the succession period, with the dominant species changing from annual to perennial and from low-class to high-class. The dominant species such as Salsola collina, Salsola collina, Artemisia sacrorum, Poa sphondyl odes, Bothriochloa ischaemun and Lespedeza bicolor occurred in turn, and made up a complete succession series. After 25 years, shrub like Lespedeza bicolor played a dominant role in the naturally restored communities. It took a longer time for the naturally restored vegetation to develop to an advanced and steady state, and the restoration rate was much slower.Soil organic matter, total N content, available P content were all improved under three restoration modes, while soil bulk density and pH value were decreased. Therefore, soil conditions could be improved by three restoration methods. However, the improvement was very limited because soil fertility at the end of the succession were at a lower level according to the soil fertility classification standards of the second national soil survey in 1980s. The analysis of correlation and CCA ordination suggested that soil conditions were main factors influencing the change of species diversity and vegetation succession. Therefore, it is necessary to improve soil condition in ecological reconstruction in Gujiao coal-gangue area.There were significant differences in community structure, species composition and diversity, coverage among regional vegetation type and restored vegetation types in the coal-gangue piles area. Margalef and Shannon-Wiener indexes of four vegetation types decreased in the order: regional vegetation> naturally restored vegetation> oriental arborvitae plantations> staghorn sumac plantations. The highest Margalef and Shannon-Wiener indexes occurred in the regional vegetation type, which could be attributed to that regional vegetation was at the advanced stage after longer succession. Pielou index decreased in the order:naturally restored vegetation> regional vegetation> oriental arborvitae plantations> staghorn sumac plantations. This was because that environmental factors distributed evenly in naturally restored vegetation due to a lack of tree stratum. Soil nutrient and soil carbon stock of three restored vegetation was significantly lower than that of regional vegetation, while soil bulk density and pH value were significantly higher than that of regional vegetation. So improving soil properties will be helpful for vegetation restoration in coal-gangue area.Among three restoration modes, species diversity index was highest in naturally restored vegetation, which can imply that natural restoration is likely an ideal way in coal mining area. However, it was difficult for natural restored communities to develop to an advanced stage within a short time. So the natural restoration method is inefficient to improve environment quality. Therefore, it is necessary to introduce tree species in ecological reconstruction in coal coal-gangue area to improve community structure and function, and eventually to accelerate the vegetation succession.In the short term, the planting of staghorn sumac could increase vegetation coverage quickly, and community could reach to a coverage of 80% after 5 years. However, in the long term, staghorn sumac depressed not only other species but also its sprouts at the mature stage, which could bring great risks of degradation to the plant community when there existed a ecological stress. Staghorn sumac community could not develop to the climax stage. So the community coverage, species diversity, soil conditions and succession tendency should be regarded as main indexes in reconstruction evaluations.Therefore, caution should be taken when planting staghorn sumac to a large extention in ecological reconstruction in coal-gangue area. Whereas, planting indigenous species such as oriental arborvitae, combined with natural restoration, would be a better choice for vegetation restoration in coal-gangue piles. In the restoration processes, enriching the community structure, increasing species diversity and vegetation coverage are all needed. Furthermore, the improvement of soil condition is also important to accelerate the vegetation succession. Because soil conditions are main limiting factors for vegetation restoration, and the interactions between these main ecologlcal components ensure the self-maintenance of the ecosystems.In sum, there is a long way to achieve the purposes of ecological reconstruction in coal-gangue area. Based on the self-restoration of ecosystem, introducing suitable tree species and improving soil condition properly are absolute needed for ecological reconstruction in coal-gangue area. According to the principle of the interactions between vegetation succession and soil properties, accelerating progressive succession of vegetation and improving the structure and function of the ecosystem are the main approaches of ecological restoration and reconstruction in coal-gangue areas. |
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