Study On The Characteristics Of Typical Vegetation And Soil Moisture Dynamic In The Loess Plateau Of Western Shanxi Province

The Loess Plateau is the most concentrated area of loess in the world with the most serious soil erosion and environmental problems in China.Ecological restoration and reconstruction are important parts of the ecological environment construction in the Loess Plateau.The Green for Green Project(GFGP)has played an important role in the restoration and reconstruction of vegetation and ecosystem restoration in the region.However,the low annual precipitation and uneven seasonal distribution on the Loess Plateau have brought huge challenges to the restoration and construction of vegetation.Soil moisture is a restrictive factor for vegetation growth in this area.Whether the vegetation ecosystem formed by GFGP exceeds the soil water carrying capacity of the area has currently become a research hotspot and an urgent scientific problem.In this research,we took the black locust,oriental arborvitae and Chinese pine as research objects.We measured the characteristics of vegetation community,soil physical and chemical properties,soil moisture dynamics using field experiments,indoor analysis and simulation methods and analyzed the forest-water interaction.The main conclusions of this study are as follows:(1)The vegetation coverage of Robinia pseudoacacia,Platycladus orientalis,and Pinus tabulaeformis reached 73.2%,70.3%,and 74.7%when the artificial forests were restored.The diversity index is 6.39,3.04,5.54 respectively.The maximum water holding rate of the litter was126.35%,129.33%,144.63%respectively.The root biomass is 22.49 t/hm~2,16.77 t/hm~2,24.67 t/hm~2respectively.Robinia pseudoacacia forest's richness index is 4.68 and species diversity index is 5.33,Platycladus orientalis has the highest uniformity index(1.60),and Pinus tabulaeformis forest has the highest dominance index(0.91).The plant diversity showed an increasing trend,and the plant community structure tended to be stable after GFGP.(2)the soil bulk density,capillary porosity and steady infiltration rate of Robinia pseudoacacia,Platycladus orientalis,and Pinus tabulaeformis forest land are higher than those of farmland,the unsaturated hydraulic conductivity is 3.06×10-3cm/h,2.67×10-3cm/h,and 2.23×10-3cm/h respectively,the soil permeability in GFGP forestland is higher than that of farmland,and the water holding capacity is better.The soil anti-scourability of Platycladus orientalis,Robinia pseudoacacia,Pinus tabulaeformis is 6.56 g/(L·m~2),4.17 g/(L·m~2),4.53 g/(L·m~2),respectively,which are higher than those of farmland.263.8 g/(L·m~2).The GFGP vegetation can significantly improve the soil's anti-scouring performance,but the anti-scourability of the GFGP forestland will rapidly decrease when the slope of the forest land is greater than 20°.Forest roots are significantly related to the anti-scourability,and the root density of<1mm directly affects the soil anti-scourability.(3)The annual average soil water content of each forest land is in the order of Platycladus orientalis(21.74%)>Pinus tabulaeformis(17.49%)>Robinia pseudoacacia(15.63%).The rainy season soil water content(16.05-20.68%)is 1.5 times of that in the non-rainy period(10.5-15%).The soil water dynamics in the 0-50cm soil profiles of forestland are highly consistent with the precipitation distribution process.The surface soil(0-20cm)of the Grain for Green Project forest land has the strongest response to rainfall.The soil water content of the 120cm-200cm soil layer is relatively stable but with a lag of about 8 days.The soil moisture in the 0-200cm soil layer of Platycladus orientalis is mostly easy-effective water;the soil water in the surface layer(0-40cm)of the Robinia locust forest is easy-effective water,and the deep layer(140-200cm)is medium-effective water,while the 120-200cm soil profiles is ineffective water.(4)Rainwater infiltration depth and infiltration rate of forestland converted from farmland under the same land preparation method showed a significant increasing trend with the increase of rainfall.Under moderate rain(19.66mm),the increment of soil water storage of these forest lands was 17%?22%,and the maximum infiltration depth was 70cm.The average rate of rainwater infiltration is 0.23?1.81mm/h respectively.Under heavy rain(31.87mm),28%?65%is the incrementof soil water storage in woodland.The depth of rainwater is range from 80-160cm,and the average rate of rainwater infiltration is1.15?2.63mm/h.Under heavy rain(65.77mm),the forests water storage increased by 62%?82%and the maximum infiltration depth reached to 130cm-180cm.The replenishment of rainwater and the depth of infiltration increased with the rainfall.The main replenishment depth of rainfall ranges from 0-120cm.Heavy rain can replenish soil water below 2m depth,which is the main rainfall type to replenish deep soil water storage.(5)Correlation analysis results of vegetation characteristics and soil characteristics show that soil characteristics and site environment have a direct decisive effect on the structure of plant community and vegetation growth.Soil moisture,root dimension,canopy interception,ammonium nitrogen and leaf area index are positively correlated(P<0.05),and soil bulk density is negatively correlated with leaf area index(P<0.01).Path analysis results show that soil moisture(P<0.01),above-ground biomass(P<0.01)and slope(P<0.05)have a direct effect on plant diversity index,while organic matter,available nitrogen and available phosphorus have a direct effect on plant diversity.Sexual index plays an indirect role.Soil moisture is the main determinant of vegetation characteristics such as leaf area index,root distribution,and diversity index of reclaimed forest land.The indirect influencing factors are soil hydraulic factors such as soil bulk density and saturated hydraulic conductivity.(6)The simulation effect of the Hydrus-1D model that modeled soil water movement in the 0-200cm layer of Robinia pseudoacacia forest is good.The NSE,RMSE and MAE of the simulation results reached0.859,0.028 and 0.035 respectively.The model parameters are suitable for the simulation of soil water movement in the study area.The hydraulic parameters that affect the accuracy of the Hydrus-1D model are the shape parameter n and saturated hydraulic conductivity Ks.The measured factors that affect the accuracy of the simulation are the leaf area index and root growth.(7)The water balance simulation results of Hydrus-1D model show that when the annual precipitation is 652mm,the annual loss of the Robinia pseudoacacia forest land and the infiltration replenishment are524.6mm and 616.8mm,respectively,and the increase in water storage reaches 64.1mm,The deep-seated leakage is 28.1mm;when the precipitation is 455mm,the annual loss and infiltration replenishment of Robinia pseudoacacia forest are 374mm and 401mm,respectively,the increase in soil water storage is only 25mm,and the deep-seated leakage is only 2mm.The precipitation of 455mm is the limit to maintain the water balance of the black locust forest land and prevent the formation of dry soil layer.