| With the completion of China's Three Gorges project,the Three Gorges Reservoir(TGR)with a total area of 1000 km~2 was formed,and a vast area of land was drowned by the reservoir's water impoundment.Aside from climate factors,such as rainfall and river runoff,the water level fluctuation of TGR was mainly controlled by the reservoir's operation scheme.The water level of TGR was manipulated at a lower elevation of145m during summer to a higher elevation of 175m in winter,resulting in formation of the drawdown zone with a total area of 350km~2.In TGR area,huge areas and different kinds of drawdown zone are distributed alongside the Yangtze river and it's tributaries.In some gentle slope river valley area,the specific Bay or Lake Basin drawdown zone was widespreadly dispersed.Generally,the drawdown zone bay was naturally cultivated and farmed by local people,as the arable land in TGR,especially in drawdown zone,is not nearly enough to ensure local farmer's agricultural production.However,aside from social and economical benefits,the excessive reclamation and farming of the drawdown zone land has also brought great pressure and damages to local ecological environment.The extensive land-use ways and mass use of chemical fertilizers and pesticides have not only aggravated the non-point sourse pollution,but also deteriorated both the environmental health and ecological safety of the drawdown zone.As influenced by both the water impoundment and land use activities,the drawdown zone bay ecosystem was developing with a relatively simpler structure,an unstable function,and a lower biodiversity compared with some off-site ecosystems,which eventually leading to a decrease of ecosystem services supply.In this study,we presented a comprehensive research about the land use impact on the bay ecosystem in water level fluctuation zone of TGR.We selected two different kinds of drawdown zone bay ecosystems,one is undertaking with human disturbance,namely Wuyang Bay(under land use activities),the other is without human activities,namely Baijiaxi Bay(under natural succession).Then,we used a combination of theories and methods,including environmental ecology,landscape ecology,eocsystem ecolgoy,and ecological economics,to learn the dynamic changes of landscape pattern,ecosystem structure,and ecosystem service function and its value of different drawdown zone bay ecosystems.And on this basis,we used emergy analysis method to evaluate the ecological and economic benefits of different kinds of land-use ecosystems in the drawdown zone of TGR.The main conclusions are summarized as follows:(1)According to field investigation in 2014 and 2015,Wuyang Bay was mainly composed of cropping,poultry and livestock raising and fish raising system.Therefore,agricultural production was the uppermost land use mode in Wuyang Bay ecosystem.As for Baijiaxi Bay,there is no human disturbance in this drawdown zone,so the land is under natural succession and gradually evolved into grassland ecosystem.As deeply influenced by the water level fluctuation of TGR,the agricultural acitvities in drawdown zone was differentiated from the elevation.The land use pattern of Wuyang Bay ecosystem was mainly composed of paddy fields and drylands,of which the paddy field located in 160~175m region and dryland mainly distributed between 165m and175m area.The dry land types in Wuyang Bay are ranging from corn filed,vegetable field,to peanut field and potato field.The water area refers to the ponds that dispersed in the lower altitude areas with elevation below 160m in Wuyang Bay,and the ponds are usually used to breed fish during summer.The land types of Baijiaxi Bay ecosystem are relatively little than Wuyang Bay,including grassland,shrubland and forest land,within which the grassland dominated in this drawdown zone area.(2)We also studied the landscape pattern dynamic of the two drawdown zone bay ecosystem.The results indicated that the landscape type of Wuyang Bay was mainly influenced by cropping productions in summer time,as different kinds of crop planting diversified local bay's landscape pattern.In winter,the water impoundment caused the land being drowned for nearly three to five month time,so the landscape types tend to be more simple.As for Baijiaxi Bay,the drawdown zone ecosystem's landscape type was mainly affected by the water level fluctuation of TGR.The landscape pattern indices analysis showed that both the landscape metric indices and class metric indices differentiated between Wuyang Bay and Baijiaxi Bay ecosystem.From the point of landscape metric indicies,the patch number and patch density of Wuyang Bay were much higher than that of Baijiaxi Bay,indicating that the landscape heterogeneity and landscape diversity are stronger in Wuyang Bay ecosystem.From the perspective of class metric indicies,the patch number and patch density of respective landscape type of Wuyang Bay were also higher than Baijiaxi Bay,indicating that the degree of landscape fragmentation of Wuyang Bay was more serious than Baijiaxi Bay ecosystem.Generally,the land use activities and human disturbance compelled the landscape pattern of Wuyang Bay ecosystem becaming more complicated than the natural succession ecosystem in Baijia Bay.(3)The ecosystem structure of different drawdown zone bay was also deeply influenced by the water level fluctuation and land use activities.The water quality,soil environment,and plant community structure were significantly changed between different land-use activities.The water quality parameters,such as total nitrogen,total phosphorus,ammonia nitrogen,and total organic carbon,are mainly affected by the long time water impoundment in the bay,whereas the agricutural farming activity takes a relatively small influence on water environment.However,the agricultural tillage and cultivation greatly affected the soil quality of drawdown zone.The soil organic matter and soil nutrients content,such as soil nitrogen,phosphorus,and potassium,were much higher in cropping field of Wuyang Bay than grassland in Baijiaxi Bay.The plant community structure comparison indicated that the annual herb plants,mainly rice and corn,were dominating in Wuyang Bay ecosystem,while the perennial herb plants,mainly Cynodon dactylon,Paspalum grass and Xanthium sibiricum,were the dominant species in Baijiaxi Bay ecosystem.The different plant species of respective bay ecosystem gradually developed an adaptive mechanism to accommodate the specific habitat change in drawdown zone,resulting in variation of phenological period,vegetation coverage,and height of the plant.A significant difference was also observed in biomass distribution between Wuyang Bay and Baijiaxi Bay ecosystem.In general,the herb plants'biomass in Baijiaxi Bay was greater than that of the crop plants in Wuyang Bay.At the same time,Baijiaxi Bay ecosystem showed a higher species diversity than the agricultural ecosystem in Wuyang Bay,and the species diversity was much higher in higher altitude areas of drawdown zone than lower elevation regions because of the relatively shorter inundating period.(4)The ecosystem service function was undertaking a huge impact from different land use option.As a matter of fact,cropping system,poultry and livestock rearing system,and fish raising system were the most common agricultural ecosystem that widely adopted in Wuyang Bay.The cropping system had a various kinds of ecosystem services,including food supply,primary production,soil conservation,air regulation,and flood dentention,etc.The poultry rearing and fish raising system were mainly providing food supply function.To concern Baijiaxi Bay,the grassland ecosystem,or we can say semi-natural wetland ecosystem,was the typical and most important ecosystem in drawdown zone.This kind of wetland ecosystem was thus owing abundant service functions,varying from biodiversity maintenance,primary production,soil conservation,water regulation,environment purification,and scientific research.The calculation of different bay ecosystem's service value indicated that the cropping system and the poultry rearing system in Wuyang Bay had a higher value of food supply service,while the fish raising system had a higher ecosystem servcie value of water regulation.On contrary,the total ecosystem service value of Baijiaxi Bay was superior to that of Wuyang Bay,within which the supporting and regulation service possess outstanding value,revealing that the natural succession ecosystem was with remarkable importance to the healthy and harmonious development of the drawdown zone.(5)In the last section of this study,emergy analysis was performed to evaluate the overall land use impact and benefit of the drawdown zone bay ecosystem.The emergy indicators analysis showed that the non-renewable purchased resource,mainly chemical fertilizers and pesticides,accounted for nearly 80%of the total emergy input in cropping system of Wuyang Bay.Duing to the heavy use of non-renewable purchased resources,the environmental loading ratio of cropping system was higher than the poultry rearing and fish raising systems,as the latter systems were mainly consuming renewable purchased resources such as fodders and straws.In contrast,the grassland ecosystem of Baijiaxi Bay was mainly relied on the renewable environmental resources,including solar energy,wind energy,and the rain energy.Hence,the environmental loading ratio was little and the environment pressure of the grassland ecosystem of Baijiaxi Bay was relatively lower than that of Wuyang Bay.For emergy yield ratio,the agricultural produciton system in Wuyang Bay showed a feasible value,meaning that the current cropping,poultry rearing and fish raising systems had a reliable resource exploiture efficiency.The emergy sustainable index of the three agricultural systems indicated that poultry rearing and fish raising systems had a better sustainable development prospect,but the current production was still in an undeveloped stage.The emergy evaluation of ecological and economic benefit showed that the grassland ecosystem had a superior ecological emergy value,indicating the natural ecosystem of Baijiaxi Bay had a higher contribution to the local ecological environment.The cropping system and fish raising system in Wuyang Bay had a moderate economic and ecological emergy value,meaning a favourable developing prospect in future.The poultry and livestock raising system had a considerable economic emergy,but there was no ecological emergy outcome,revealing a lower ecological benefit to the local environment.Above all,the current land use in drawdown zone have not only brought economic benefit for local residents,but also generated a great deal of ecological benefit to local environment.In future,it was proposed that some land use policies shoud be carried forward to optimize the current land use pattern in drawdown zone of TGR.The government should raise a moderate land use plan and policy supports to develop a sustainable agricultural production system in drawdown zone.The agricultural structure of current cropping,poultry and livestock rearing,and fish rasing system should be adjusted and the non-renewable purchased resources should be reduced to decrease the environmental loads for drawdown zone land.At last,the ecological and environmental awareness should be carried forward to cultivate the legal belief of the public.Through theses processes,a sustainable and ecological land use development could be realized in the drawdown zone of TGR. |
PDF Full Text Request