| The Loess Plateau is the most severe soil erosion area in China and globally,and vegetation construction is the most active and effective measure for soil and water conservation and ecological restoration.Plantation has played a key role in soil erosion control and ecological environment improvement on the Loess Plateau.However,in the process of vegetation restoration in this area,the large-scale artificial vegetation is dominated by monocultures and relatively few mixed plantations,ignoring the water limitation and the exertion of various ecological functions,resulting in many ecological problems,such as poor structure,low growth and poor stability in plantations.Focusing on the prominent problems of a single structure and low ecological function of plantations on the Loess Plateau,this study systematically studied the evolution characteristics and regional differentiation laws of soil water storage,carbon sink,soil and water conservation,and diversity maintenance under different afforestation patterns(mixed plantations and monocultures)through field survey sampling(172 plots)and literature data integration(375 plots).Then this study revealed the trade-off/synergy relationship and driving mechanism among various ecological functions and proposed a scheme for regulating plantation structure based on those trade-off relationships,which may provide a scientific basis for the sustainable management of plantations and the improvement of ecological function quality and efficiency in the Loess Plateau.The main findings are as follows:(1)The soil water storage of the tree-shrub mixture was generally better than that of the corresponding monocultures on the Loess Plateau,and this characteristic was most obvious in the plantation age > 20 years(16.81%).Overall,the soil water storage of mixed plantations in the 0-500 cm horizons was significantly higher than that of monocultures by 8.06 %,and the water storage capacity was related to the mixed type.Compared with the corresponding monocultures,the soil water storage of the mixtures of Robinia pseudoacacia-Hippophae rhamnoides and Pinus tabulaeformis-Hippophae rhamnoides increased by 9.32% and 5.13%,respectively,while the mixture of Robinia pseudoacacia-Pine tabulaeformis decreased the soil water storage by 2.73%.Treeshrub mixtures had more advantages than tree-shrub mixtures in maintaining the soil water storage,and this positive effect was more obvious in the plantation age > 20 years.Soil layer depth was a crucial factor affecting soil water storage change,and with the increase of soil layer,the improvement effect of mixed plantations on soil water storage decreased significantly.The effect of mixed plantations on soil water storage decreased with mean annual precipitation and mean annual temperature.Different afforestation patterns significantly consumed soil water storage,and the consumption rate of monocultures was higher than that of mixed plantations.(2)Compared with the corresponding monocultures,the tree-shrub mixture was more conducive to improving the plant carbon sequestration and soil carbon storage of0-200 cm.Overall,the plant carbon sequestration and soil carbon stocks(0-200cm)of mixed plantations were 14.17% and 14.31% higher than those of monocultures,respectively.For different mixed species,the mixture of Robinia pseudoacaciaHippophae rhamnoides was more beneficial to increase vegetation carbon sequestration(17.15%)and soil carbon stocks(18.81%).Plantation age was the main factor affecting plant and soil carbon sequestration.Soil carbon stocks increased significantly with the plantation age and were more obvious in the soil layer of 100-200 cm,while changes in plant carbon sequestration tended to stabilize after 50 years.In addition,while increasing soil carbon stocks,mixed plantations also significantly increased plant carbon sequestration.Converting farmland to forest significantly increased soil carbon stock,and the carbon sequestration rate of mixed plantations was significantly higher than that of monocultures.Suppose all the plantations in the suitable area of the Loess Plateau are transformed from monocultures to mixed plantations.In that case,the total carbon sequestration of vegetation and soil can increase by 106.25 Tg,accounting for about 1.07% of the total carbon emissions in the country in 2020.(3)The tree-shrub mixture containing Hippophae rhamnoides showed better soil and water conservation than the corresponding monocultures,and the mixed plantations were more effective in reducing the sediment yield than regulating the runoff yield.Overall,the surface runoff and sediment yield of mixed plantations were 30.20% and40.19% lower than those of monocultures,respectively.The relative impact of mixed type on runoff and sediment yield was the greatest.The effect of tree-shrub mixtures on runoff and sediment yield was higher than that of tree-tree mixtures.The mixture of Robinia pseudoacacia-Hippophae rhamnoides had the best impact on regulating runoff yield(-35.11%)and controlling sediment yield(-47.50%),followed by the mixtures of Pinus tabulaeformis-Hippophae rhamnoides and Armeniaca sibirica-Hippophae rhamnoides.Mixed plantations could significantly regulate the surface runoff while reducing the sediment yield,and mixed plantations were more effective in reducing sediment yield.(4)The mixtures of Armeniaca sibirica-Hippophae rhamnoides and Robinia pseudoacacia-Hippophae rhamnoides were more conducive to improving the understory plant diversity than the corresponding monocultures.For different afforestation patterns on the Loess Plateau,the Shannon-Wiener diversity index of understory plants in mixed plantations was 19.44% higher on average than that in monocultures,and this positive effect was more pronounced in mature stages(31.11%).In terms of different mixed types,the tree-shrub mixtures were more conducive to maintaining the understory plant diversity than the tree-tree mixtures.The mixture of Armeniaca sibirica-Hippophae rhamnoides had better maintenance effect on the understory plant diversity(22.71%),followed by Robinia pseudoacacia-Hippophae rhamnoides(20.83%).In addition,the response ratio of understory plant diversity decreased with the increase of mean annual precipitation and mean annual temperature,but this trend was not significant.(5)The mixture of Robinia pseudoacacia-Hippophae rhamnoides was more conducive to optimizing the trade-off/synergy between ecological functions.There was a trade-off relationship between plant carbon sequestration and soil water storage,runoff yield,sediment yield,and understory plant diversity,and a synergistic relationship with soil carbon storage.Among them,the trade-off between plant carbon sequestration and soil water storage was the highest.For forest stands in the Loess Plateau,it was particularly critical to alleviating the trade-off relationship between plant carbon sequestration and soil water storage.Mixed plantations could alleviate the degree of trade-offs among ecological functions and promote a synergistic increase in multifunctionality.Compared with monocultures,mixed plantations could increase soil carbon storage and soil water storage,and reduce the risk of soil erosion while improving plant carbon sequestration.Plantation age and rainfall were the key factors affecting the trade-off/synergy relationship.Mixed plantations were more effective in optimizing the trade-off/synergy relationship between ecological functions in the plantation age > 20 years and the arid grassland region(mean annual precipitation <450 mm).(6)It is an effective measure to optimize the structure and function of planted forests on the Loess Plateau to plant the tree-shrub mixtures in the arid and semi-arid forest-steppe region and plant tree-tree mixtures in the semi-humid forest region,which could greatly improve various ecological functions of the plantation.An ecological multifunctional deviation index was constructed based on the "zonal climax ecological background".In the forest-steppe region with the mean annual precipitation < 550 mm,the tree-shrub mixtures,especially the mixture of Robinia pseudoacacia-Hippophae rhamnoides,could better coordinate the functions of soil and water conservation and carbon sequestration while improving the soil water storage capacity.The mixture of Robinia pseudoacacia-Pinus tabulaeformis was the optimal afforestation pattern for the forest region with the mean annual precipitation > 550 mm.Overall,the results of plantation structure optimization in the Loess Plateau showed that,compared with the status quo of various ecological functions of plantations under the current configuration,soil carbon stock,plant carbon sequestration,and soil water storage had increased by32.04%,54.04%,and 22.38%,respectively,and reduced the surface runoff and sediment yield by 32.24% and 47.07%,respectively. |
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