Responses Of Ecosystem Services To Ecological Projects In Gully Regions Of The Loess Plateau

The gully region of the Loess Plateau,mainly consists of two types of landforms,the loess tableland and gully region and the loess hill and gully region.Complex geomorphic background combined with climate change and human activities has led to serious degradation of ecosystem services in this area.Therefore,a series of large-scale ecological restoration projects have been carried out in the gully region of the Loess Plateau,such as terracing,check dams and grain for green program.However,restoration projects increased ecosystem services while exacerbating trade-offs between them.That is,the increase in one ecosystem service may come at the expense of other ecosystem services.The enhancement pathways of ecosystem services are differ under different geomorphic contexts.Especially,under the multiple demands of ecological construction and socio-economic benefits in the new period,the problems of unreasonable spatial layout of existing ecological projects,poor synergy of ecological service functions and insufficient ability to withstand extreme climate events are becoming increasingly prominent.Therefore,a comparative study of the response mechanisms of ecosystem services of different landform units to ecological projects is of great scientific and practical significance to optimize the layout of ecological projects and make the best use to the function of ecosystem services.In view of the above,we use the remote sensing data with 30 m resolution,field observation data and field survey data from 1990 to 2020.This study takes Dongzhiyuan(a typical loess tableland and gully region)and the Yanhe basin(a typical loess hilly and gully region)as target research areas.It aims to(1)characterize spatial and temporal patterns of major ecosystem types and their dynamic changes in the loess tableland and gully region under different geomorphological conditions;(2)based on the CASA model,the In VEST model,the CSLE model and the image element decomposition model,we simulated and evaluated the ecosystem service functions of carbon sequestration,water production,soil conservation and food production in the Dongzhiyuan and the Yanhe basin;(3)based on Spearman’s rank correlation analysis,spatial statistics and regression discontinuity,we elucidated the spatial and temporal trade-offs and synergistic relationships among different ecosystem services and their threshold effects;(4)using the method of substituting space for time,we explore responses of ecosystem services to various ecological projects such as the Grain for Green program,check dam and terraces under different landscape conditions.The main findings of the paper are as follows.(1)Farmland,forest and grassland are the most important ecosystem types in the gully region of the Loess Plateau.Among them,farmland ecosystem is the dominant ecosystem in the high-plateau gully regions,while grassland ecosystem is dominant in the hilly and gully regions.The dominant ecosystem type also has the highest average patch size,but the fragmentation of the ecosystem gradually increases with the intensity of human activity disturbance.The degree of fragmentation,in descending order,is:grassland>farmland>forest>scrub>town>water.The aggregation indices of towns,farmlands and grasslands are all relatively high,close to 90%.The dynamic changes of regional ecosystem pattern are very strong,and there is interconversion of large areas of grassland and forest.In addition,the dynamic change process of ecosystem types found that the Dongzhiyuan vegetation restoration project preceded the Yanhe basin and the initial ecological restoration was stronger(1990-2000),while the dynamic evolutionary effect among ecosystem types in the Yanhe basin was stronger.(2)The four ecosystem services of carbon sequestration,water production,soil conservation and food production in the gully regions of the Loess Plateau showed a decreasing trend followed by an increasing trend,with an overall increase.Among them,the average annual carbon sequestration of Dongzhiyuan is about 250-400 g C/m~2,and the carbon sequestration capacity of farmland and grassland is slightly stronger than that of the Yanhe basin.The average annual carbon sequestration of the Yanhe basin is about 200-450 g C/m~2,and the increase of its carbon sequestration is greater.However,the increase of carbon sequestration service is not the result from expansion of vegetation coverage but the improvement of vegetation quality.The average water yield of Dongzhiyuan is 120-230 t/ha,and the loess surface,which accounts for 30%of the regional area,is the main runoff producing area,contributing to a higher average water yield of the whole Dongzhiyuan than the Yanhe basin(35-150 t/ha).The distribution pattern of water yield is basically consistent with the rainfall pattern,in which Dongzhiyuan shows an increasing trend from north to south,while the Yanhe basin gradually increases from the upper to the middle and lower reaches.There are obvious spatial differences in soil conservation services under different geomorphic conditions.In comparsion with the high loess gully region,the soil conservation services in the loess hilly and gully region are stronger.In particular,the average soil conservation volume in 2020(1350 t/ha)is almost twice as much as that of Dongzhiyuan.In terms of agricultural production,Dongzhiyuan has a unique geographical advantage with a higher average annual grain yield(115～190 kg/ha),which is about twice as much as the average annual grain yield(80～95 kg/ha)in the Yanhe basin.Among them,the deep loess layer of Dongzhiyuan has built a large number of terraces,while the high density of gullies in the Yanhe basin has built a large number of check dams.Both have avoided in different ways the problem of reduced farmland and reduced grain yield caused by the Grain for Green program.(3)The study of trade-offs and synergistic relationships among ecosystem services at the regional and sub-basin scales found that Dongzhiyuan is generally dominated by synergistic relationships,and carbon sequestration has the strongest synergy with the other three services(water production,soil conservation and food production).However,there are high synergistic relationships among carbon sequestration,water production and soil conservation in the Yanhe basin,and all three have trade-off relationships with food production.In general,the synergistic relationship is weakening and the trade-off relationship is increasing in the Loess Plateau gullies.At the same time,there is a scale effect in this relationship,which gradually strengthens or weakens or even reverses as the scale increases.In addition,the trade-off or synergistic relationship between ecosystem services mostly has a threshold effect as ecosystem services increase,and this threshold effect is more complex in the Yanhe basin.For example,the synergies between water production and carbon sequestration shifts to trade-offs when the water production in the Yan River basin is around 160 t/ha,while the trade-offs between water production and food production shift to synergistic.When ecosystem services are small,fewer resources are needed to maintain such services,resulting in services that tend to exhibit synergistic relationships with each other.When ecosystem services increase further,the demand for various resources also gradually increases,leading to a weakening of synergistic relationships and even a shift from synergistic to trade-off relationships.(4)By comparing the differences of ecosystem services before and after the implementation of ecological projects and inside and outside the project area,it was found that before the return of Grain for Green,the ecosystem services remained stable or slightly decreased in general,and increased significantly afterwards,and did not cause any reduction in food production.Carbon sequestration and soil conservation were significantly better in the Grain for Green program area than outside the area,while water production was the opposite.Due to the vast difference in the number of check dams between the two regions,the construction of Dongzhiyuan check dams did not have a significant impact on ecosystem services.In the Yanhe basin,the enhancement of ecosystem services,especially in terms of carbon sequestration and soil conservation,became more pronounced as the number of years of siltation increased.Differences in carbon sequestration,water production and soil conservation between inside and outside the check dam project area are significant.Ecosystem services continued to rise after the construction of terraces,but the contribution remained stable for 10-20 years after the project implementation.Ecological engineering did not have a significant impact on food production because terraces and check dams compensated for the reduction in food production due to the fallowing and Grain for Green program.In general,the larger the number and scale of ecological projects,the more lasting the impact on ecosystem services.The ecological effects of check dams were maintained for the longest time,followed by reforestation,and finally by terraces.Four key ecosystem services such as carbon sequestration,water production,soil conservation and food production have been assessed based on the study of ecosystem pattern.The spatio-temporal trade-offs and synergistic relationships among the services have been clarified in the gully region of Loess Plateau.It found that such spatio-temporal correlations produce threshold effects with changes in ecosystem services.Finally,we clarify the mechanisms of changes in ecosystem services before and after the implementation of the projects such as the Grain for Green program,check dams and terraces,and the changes in ecosystem services inside and outside the project area,and propose a dynamic and sustainable management plan for the maintenance and comprehensive benefit enhancement of the relevant ecological engineering systems.