Study On The Evolution Of Ecosystem Service Function Of Robinia Pseudoacacia Forests On The Loess Plateau

In order to evaluate the evolution characteristics of ecosystem services on the Loess Plateau since the implementation of the“Grain for Green Project”,this study used two methods:Meta-analysis（Meta-analysis）and field investigation and analysis.The 8 a,18 a and28 a artificial Robinia pseudoacacia forests in Changwu（semi-humid continental monsoon climate,MAP=560 mm）,Ansai（mid-temperate continental semi-arid monsoon climate,MAP=505 mm）and Mizhi（mid-temperate semi-arid climate,MAP=452 mm）were studied.Four indicators of water conservation（soil moisture）,soil carbon sequestration,soil conservation and plant diversity were selected to evaluate the evolution characteristics of ecosystem services of artificial Robinia pseudoacacia forests on the Loess Plateau,and to clarify the correlation between ecosystem services.The main conclusions are as follows:（1）The artificial Robinia pseudoacacia forests on the Loess Plateau significantly reduced soil moisture in the profile,but significantly improved ecosystem services such as soil carbon sequestration,soil conservation,and plant diversity.The results of meta-analysis showed that the average soil water storage per centimeter was 1.51 mm and the relative change rate was-35.6%after the restoration of artificial Robinia pseudoacacia forests.The soil carbon storage was 2.85 kg/m²,and the relative growth rate was 81.3%.The plant diversity index was 2.57,and the relative growth rate was 32.6%.The results showed that the restoration of artificial Robinia pseudoacacia forests reduced the soil water conservation function,but the soil carbon sequestration and plant diversity function were significantly improved.In different climatic regions,the soil water storage in 0～10 m of artificial Robinia pseudoacacia forests.in semi-humid area was 660 mm higher than that in semi-arid area,the soil carbon storage in 0～1 m was 1.17 kg/m² higher than that in semi-arid area,and the diversity index increased by 18.5%.Under different restoration years,the soil water storage in 0～10 m of artificial Robinia pseudoacacia forests with three restoration years of≤15 a,15～30 a and≥30 a was 1700mm,1430 mm and 1410 mm respectively.The soil carbon storage of 0～1 m in the three restoration years was 1.85 kg/m²,3.03 kg/m² and 3.57 kg/m²,respectively.The diversity index of the three restoration years was 2.41,2.67 and 2.57,respectively.（2）In different climatic regions on the Loess Plateau,the soil moisture,soil carbon sequestration,soil conservation and plant diversity functions of artificial Robinia pseudoacacia forests gradually decreased with the degree of climate drought.Compared with Changwu,the soil moisture of artificial Robinia pseudoacacia forests in Ansai and Mizhi decreased by 50.5%and 58.5%respectively,and the soil water storage deficit ratio increased by 13.0%and 46.0%respectively.The soil organic carbon content of Ansai and Mizhi decreased by 39.3%and 47.9%,respectively,compared with Changwu area.The soil erosion modulus from Changwu,Ansai to Mizhi were 5.86,12.46,12.90 t·hm^-2·a^-1,respectively.The average weight diameter（MWD）of soil water-stable aggregates in Changwu and Ansai was1.14 times and 1.23 times that of Mizhi,respectively.The geometric mean diameter（GMD）of soil water-stable aggregates was 1.18 times and 1.21 times that of Mizhi,respectively.The percentage of>0.25 mm aggregates(R_0.25)in soil water-stable aggregates was 3.6 times and1.26 times that of Mizhi,respectively.The erodibility factor K value was 14.7%and 16.8%lower than that of Mizhi,respectively.Compared with Mizhi,Margale richness index increased by 53.2%and 61.0%,Simpson diversity index increased by 11.9%and 10.7%,Shannon-Wiener index increased by 45.6%and 31.1%,Pielou evenness index increased by13.7%and 19.8%,respectively.（3）Under different restoration years（8,18 and 28 a）on the Loess Plateau,the soil moisture of artificial Robinia pseudoacacia forests decreased first and then increased with the increase of restoration years.Soil carbon sequestration and soil conservation capacity increased with the increase of restoration years,and plant diversity decreased with the increase of restoration years.Compared with the 8 a artificial Robinia pseudoacacia forests,the soil moisture of the 18 a and 28 a artificial Robinia pseudoacacia forests decreased by 19.9%and10.3%,respectively,and the soil water storage deficit ratio increased by 64.7%and 47.7%,respectively.The soil organic carbon storage of 18 a and 28 a artificial Robinia pseudoacacia forests increased by 16.0%and 40.0%respectively compared with that of 8 a artificial Robinia pseudoacacia forests.Compared with 28 a artificial Robinia pseudoacacia forests,the annual soil erosion of 8 a and 18 a artificial Robinia pseudoacacia forests decreased by 100.1%and86.7%,respectively.Among the indexes of soil water-stable aggregates,the proportion of soil aggregates with>0.25 mm particle size in 18 a and 28 a artificial Robinia pseudoacacia forests was 1.36 times and 1.64 times that of 8 a artificial Robinia pseudoacacia forests,respectively.The MWD value,GMD value and R_0.25 value of 28 a artificial Robinia pseudoacacia forests were the highest compared with other restoration years,which were 1.39 times,1.13 times,1.12 times,1.05 times and 1.58 times,1.23 times of 8 a and 18 a artificial Robinia pseudoacacia forests,respectively.Compared with 28 a artificial Robinia pseudoacacia forests,Margalef richness index of 8 a and 18 a artificial Robinia pseudoacacia forests increased by 7.3%and 1.7%,Simpson diversity index increased by 9.6%and 8.8%,Shannon-Wiener index increased by 26.4%and 19.2%,Pielou evenness index increased by 45.5%and45.8%,respectively.（4）The trade-offs between soil moisture and plant diversity and between soil carbon sequestration and soil conservation were strong,while the trade-offs between soil carbon sequestration and soil conservation were weak.Under different climatic regions and different restoration years,the carbon sequestration and soil conservation services of artificial Robinia pseudoacacia forests are at the expense of consuming soil moisture,and soil carbon sequestration and soil conservation develop in a mutually beneficial direction.When Grain for Green Project on the Loess Plateau,the climate zone,recovery period and plant growth status should be considered comprehensively.Artificial Robinia pseudoacacia forest is more suitable for planting in semi-humid areas and the recovery period should not exceed 30～40 years.With the increase of restoration years,the consumption of soil moisture will increase.At the same time,forest degradation will also affect vegetation carbon sequestration,soil conservation and plant diversity.Appropriate thinning or planting plants with less water consumption,such as grassland and shrubs,is more conducive to the sustainable development on the Loess Plateau.