| Vegetation restoration was an effective measure to restore degraded land,mitigate climate change,regulate the hydrological cycle,and control soil erosion.As the most severe soil erosion area in China and globally,the Loess Plateau was also the area with the most significant effect of plantation construction.Artificial vegetation construction has played a critical role in soil erosion control and ecological environment improvement on the Loess Plateau.However,in the process of vegetation restoration in this area,a large number of artificial vegetation with soil and water conservation as the single goal had been built,ignoring the water conservation carrying capacity and water conservation functions of vegetation,resulting in many ecological problems,such as soil water deficit,runoff disruption,and imbalance of water supply and demand in the ecosystem.Therefore,adjusting the structure of the artificial vegetation to achieve a balance between the reduction of soil erosion and the improvement of water conservation has become an urgent need for the ecological restoration of the Loess Plateau.To clarify the formation and maintenance mechanism of the water conservation function of artificial vegetation,scientific evaluation of the water conservation function of artificial vegetation was a scientific question that needs to be answered on the Loess Plateau.For this purpose,this study selected different artificial types(Robinia pseudoacacia,Caragana korshinskii,Malus pumila,Pinus tabulaeformis,Medicago sativa)in the Loess Plateau as the research object to systematically study the loess through field positioning observation stations and sample points.The water conservation function and its driving factors of different vegetation types under different rainfall belts were systematically studied based on field investigation and long-term in situ observations,which revealed the formation process and mechanism of the water conservation function of vegetation under different spatial and temporal scales.Meanwhile,based on multiple eco-hydrological effect indicators of different dimensions,the evaluation index system of vegetation water conservation function was constructed to synthetically evaluate the water conservation function quality of different vegetation types and structures.This study proposed the optimal vegetation construction scheme based on water balance conditions in different study sites,which provided a basis for the sustainable operation of artificial vegetation and scientific management of water resources in the Loess Plateau.The main findings are as follows:(1)Artificial vegetation(artificial woodland,shrubland,and grassland)had greater water holding capacity in the vegetation layer and litter layer on the Loess Plateau.The water holding capacity of the vegetation layer and litter layer was controlled by vegetation above-ground biomass and litter accumulation.The maximum water holding capacity of the canopy layer in the artificial woodland(R.pseudoacacia,P.tabulaeformis,M.pumila)had enhanced by 146.95% than that of artificial shrubland(C.korshinskii).The maximum water holding capacity of the litter layer in the artificial woodland had increased by 78.55%,196.16%,and 319.66% to that of natural restoration grassland(abandoned grassland,enclosure grassland),artificial shrubland,and artificial grassland(M.sativa),respectively.Among the vegetation restoration types,the canopy and litter layer of P.tabulaeformis showed the highest water holding capability,approaching 20.37 mm and 53.79 mm,respectively.The water holding capacity of the vegetation layer and litter layer was affected by canopy structure,vegetation coverage,above-ground biomass,and litter thickness,among which,vegetation biomass and litter accumulation were the main influence factors.(2)Grassland(natural restoration grassland and artificial grassland)and shrubland in the semi-humid areas and grassland in the semi-arid areas on the Loess Plateau effectively increased the soil water content maintenance duration and depth and enhanced the vegetation’s comprehensive water storage.Compared with cropland and artificial vegetation,the natural restoration grassland showed a better soil water content maintenance effect.Under the condition of sub-rainfall amount ≤ 35 mm,vegetation restoration had increased the response time and response rainfall amount of soil water content by 21.83%and 33.31%,respectively in the 0-30 cm soil layer.At the monthly scale,vegetation restoration had increased the soil water content maintenance duration of 0-2 m depth.Compared with the cropland,the natural restoration grassland,artificial grassland,and artificial forestland(woodland,shrubland)had increased the soil water content maintenance duration of 0-2 m by 22.22%,11.11%,and 2.78%,respectively.At the annual scale,the natural restoration grassland and artificial shrubland in the semi-humid areas and the natural restoration grassland and artificial grassland in the semi-arid areas showed a deeper soil water content maintenance depth,which increased the vegetation’s comprehensive water storage by 50.96%,2.92%,29.72%,and 13.68%,respectively.The comprehensive water capacity of vegetation was affected by the water retention characteristics of the vegetation canopy,herbaceous layer,litter layer,and soil layer.The above-ground biomass and soil permeability properties were the key factors of the comprehensive water capacity of vegetation.(3)Grassland and shrubland in the semi-humid areas and natural restoration grassland in the semi-arid areas on the Loess Plateau effectively maintained the water balance of the vegetation.Precipitation,vegetation water demand,and soil water storage were the key factors affecting the water balance of vegetation.The water balance effect of different vegetation was different in different rainfall zones.Compared with the cropland,artificial woodland in the rainfall zone of 450-550 mm and artificial shrubland in the rainfall zone of ≤ 450 mm reduced the variation of deep(> 2m)soil water storage,which induced the imbalance of the ecosystem water budget.Artificial woodland,artificial shrubland,and natural restoration grassland increased the variation of deep soil water storage,which effectively maintained the ecosystem water balance in the areas with annual rainfall amounts larger than 550 mm,450-550 mm,and less than 450 mm,respectively.The characteristics of vegetation water balance were affected by soil moisture,rainfall,and evapotranspiration.Precipitation,vegetation water demand,and soil water storage were the key factors affecting the water balance of vegetation.Based on the regional rainfall amount,optimizing the layout of vegetation types and artificial management measures can increase the surplus of the water budget and improve the water conservation function of the vegetation.(4)Vegetation restoration had changed the ’quality-quantity’ relationship of water conservation function at the watershed scale on the Loess Plateau.The Jiuyuangou watershed with a high degree of ecological restoration and its early stage of revegetation showed a better water conservation function.Compared with the Peijiamao watershed,the runoff coefficient,sediment transport,and peak discharge of flood season in the Jiuyuangou watershed decreased by 55.71%,90.27%,and 85.47%,respectively,while the runoff days increased by 519.45%.Compared with the period before restoration,the runoff,sediment transport,and peak discharge of flood season decreased by 69.29%,91.79%,and 57.40%,respectively,in the period after vegetation restoration.The Jiuyuangou watershed with a high degree of ecological restoration and its early stage of revegetation had better effects on runoff regulation,water purification,and flood retention.The water conservation function at the watershed scale was mainly affected by vegetation coverage and rainfall amount.The land use/cover change caused by human activities was the main driving factor of runoff and sediment variation,contributing more than 70% and 80% to the runoff and sediment transport in flood season.(5)Vegetation restoration effectively improved the water conservation function of vegetation on the Loess Plateau.The reasonable allocation of artificial vegetation and the control of vegetation coverage were the key factors in improving the water conservation function.Compared with the cropland,water conservation function increased by 29.10% after revegetation,with artificial woodland,artificial shrubland,artificial grassland,and natural grassland enhanced by 38.10%,2.38%,35.71%,and 48.03%,respectively.In areas with annual rainfall amount larger than 550 mm,450-550 mm,and less than 450 mm of the Loess Plateau,planting artificial forests,artificial shrubs with reasonable density,and natural restoration grassland with human regulation,respectively,were the better choices to exert the function of vegetation water conservation and improve the quality of ecosystem water conservation.The increase in vegetation coverage had a negative impact on the runoff of the watershed.While under the native vegetation type dominated by natural restoration,the reasonable allocation of artificial species and the control of vegetation coverage in the watershed were the effective measures to achieve the ’quality-quantity’ balance of water resources of the watershed and promote the water conservation function of the ecosystem. |
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