| Natural vegetation provides a variety of important ecosystem services for human beings,and the ecosystem services provided by different vegetation types are different.Terrain is an important factor affecting vegetation types and ecosystem services.However,not enough attention has been given to the effects of different topographic features on vegetation types and ecosystem services.Meanwhile,the characteristics of the nonlinear effects of vegetation change on ecosystem services are not clear,especially the determination of impact thresholds.Therefore,it is important to understand the effects of terrain gradients and vegetation thresholds on the ecosystem services of different vegetation types for ecological restoration management decisions and improving ecosystem services.Based on the In VEST 3.7.0 model,ecosystem services(water yield,carbon storage,soil conservation and water purification)assessment and spatial patterns were carried out based on vegetation types,climate,soil,terrain and NDVI data.Hierarchical cluster analysis was used to identify the similarities and differences of ecosystem services among different vegetation types.Variance analysis was used to explore the spatial differences of ecosystem services under different terrain gradients(altitude,slope,landform relief,and terrain niche).The impact thresholds of vegetation cover on ecosystem services under different vegetation types and terrain gradients were determined by elastic analysis.The results show as following:Spatial distribution of vegetation types and ecosystem services in the Chuan-Dian ecological shelter: Evergreen needleleaf forest,dry farmland and deciduous broadleaf shrub were the main vegetation types,accounting for 29.90%,12.12% and 10.93%,respectively.Evergreen needleleaf forest was mainly distributed in the south,dry farmland was concentrated in the east,and deciduous broadleaf shrub was mainly appeared in the north.The average water yield,carbon storage,soil conservation and nitrogen export were 450.35 mm,49.90 t/ha,143.48 t/ha and 6.54 kg/ha,respectively.The high value area of water yield is located in the mid-west and southwest,while the low value is located in the southeast and north.The carbon storage and soil conservation were higher in the southwest and lower in the northeast.The high value of nitrogen export was located in the northeast,while the low value was located in the middle and south.Natural forest had the highest carbon storage(86.95 t/ha)and soil conservation(178.43 t/ha),and the lowest water yield(310.78 mm)and nitrogen export(0.14 t/ha).The carbon storage,soil conservation and water purification of the natural forest were higher than those of the plantation forest,while the water yield was lower than that of the plantation forest.The water yield,carbon storage and soil conservation of natural shrub were higher than those of natural grassland,while the water purification of natural shrub was lower than that of natural grassland.The nitrogen export of cropland was the highest,and its ecosystem services were significantly lower than those of other vegetation types.Evergreen broadleaf forest,evergreen needleleaf forest and broadleaf and needleleaf mixed forest have higher ecosystem services and should be protected in future ecological construction.Spatial heterogeneity of ecosystem services under terrain gradient: The proportion of evergreen needleleaf forest was the highest at the altitude of 2400–3300 m(50.72%),that of deciduous broadleaf shrub was the highest at the altitude of 900–1700 m(19.23%),and that of dry farmland was the highest at the altitude of 200–900 m(33.73%).Natural forest first increased and then decreased with altitude and terrain niche gradients,while natural shrub first increased and then decreased with slope,landform relief,and terrain niche gradients.The plantation decreased with increasing slope and landform relief,but first increased and then decreased with increasing altitude and terrain niche.The natural grassland is mainly distributed in the high terrain region,while the cropland is mainly concentrated in the low terrain region.The maximum water yield(641.97 mm)was found at the altitude of 3300–7200m,which was significantly higher than that of other terrain regions.The maximum soil conservation(271.22 t/ha)was found at the slope of 31.12°–77.81°,which was significantly higher than that of other terrain regions.Nitrogen export was highest at an altitude of 200–900 m,a slope of 0–8.54°,a landform relief of 0–293 m and a terrain niche of 0.11–0.83(17.63 kg/ha–20.47 kg/ha),and decreased with the increase of terrain.Carbon storage and soil conservation first increased and then decreased with altitude and terrain niche,and increased with slope and landform relief.Water yield,carbon storage and soil conservation were higher in the high terrain region,while nitrogen export was higher in the low terrain region.Threshold effect of ecosystem services under the terrain gradient: At the regional scale,vegetation coverage was negatively correlated with water yield and nitrogen export(r=-0.350 and-0.036,p<0.01),and vegetation coverage was significantly positively correlated with carbon storage and soil conservation(r=0.355 and 0.146,p<0.01).The effect of vegetation coverage on ecosystem services varied with the terrain gradient.Vegetation coverage was positively correlated with nitrogen export in the low terrain region,and negatively correlated with water yield in the high terrain region.The positive effect of vegetation coverage on carbon storage was greater in the high terrain region,but less on soil conservation.Vegetation coverage promotes ecosystem services and has a threshold.The thresholds of vegetation coverage promotion in natural forest,natural shrub,plantation forest and natural grassland were 0.86,0.79,0.85 and 0.83,respectively.The response characteristics of ecosystem services to vegetation coverage and the impact threshold of vegetation coverage varied with terrain gradients.The threshold value of vegetation coverage promoting ecosystem services under low terrain regions was larger than that under high terrain regions.The promotion effect of vegetation coverage on ecosystem services tended to weaken after the threshold value was exceeded.The vegetation impact thresholds under different vegetation types and terrain gradients can provide theoretical guidance and decision support for large-scale ecological management and restoration. |
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