The Above-and Below-ground Processes Of Degradation And Restoring Efficiency Of Grazing Exclusion In Typical Alpine Grasslands On The Tibetan Plateau

Due to the high altitude and unique alpine climate condition,the alpine grassland ecosystem is highly fragile and has been suffering from severe degradation effected by climate change and anthropic activities since recent decades.Grassland degradation on the Tibetan Plateau has not negatively influenced the herdsmen's lives and limited sustainable development of animal husbandry,but also resulted in a reduction in ecological functions such as biodiversity provisions,sinks of atmospheric carbon,water and soil conservation,and cultural recreation,which has recently become a global concern.To systemly explore the process and underlying mechanism of alpine grassland degradation and restoration on the Tibetan Plateau,we did the works as follows:(1)Collecting and analyzing data from literatures on grassland degradation on the Tibetan Plateau;(2)Choosing and conducting field surveys in degraded alpine grasslands with seven sequent levels in Xainza and Hongyuan sites,respectively;(3)Carrying out one-year grazing exclusion(GE)in the seven sequent degraded alpine grasslands at Hongyuan site;(4)Studying the succession of severely degraded alpine grassland along time of grazing exclusion;(5)Measuring and calculating soil properties such as soil water content(SWC),soil bulk density(SBD),soil compactness(SCOM),soil p H,soil temperature(STEM),soil organic carbon(SOC),soil total nitrogen(STN),soil available nitrogen(SAN),and soil available phosphorus(SAP)in the 0-30 cm soil layer,and plant characteristics including coverage,aboveground biomass(AGB),belowground biomass(BGB),species richness(SR)and diversity.This study can provide technical support for protection and sustainable management of alpine grasslands on the Tibetan Plateau,which can be mainly concluded as follows:(1)On regional scales,SWC,SCOM,STN,plant coverage,AGB,BGB,and species diversity all exhibited decreasing trends with degradation increasing,while SBD and soil p H significantly increased.Compared with undegraded grassland,SWC,SOC,STN,STP,plant coverage,and AGB in both alpine steppe and alpine meadow with moderate and high degradation degrees all significantly decreased,whereas SBD apparently rose.Moreover,the variation ranges were wider in grasslands with higher degraded extents.(2)On site scales,SR,overall AGB,and AGB of graminoids were apparently reduced with increasing degradation gradients,while AGB of forbs slightly increased.The increasing degradation levels induced a significant increase in the trade-off value of forbs,which was negatively associated with SWC,SCOM,SOC,STN,and SAN.The mean root length of forbs was significantly longer than that of graminoids(P<0.05).Moreover,the mean root diameter of the top 1/3 part of forbs was remarkably thicker than that of graminoids(P<0.05).These findings indicate that the degradation-induced cohesionless soils with insufficient water and nutrients together with the divergent root morphological traits of graminoids and forbs determine the plant community structure shift with grassland degradation.(3)On site scales,SWC,SCOM,SOC,STN,peroxidase and urease activities dramatically decreased along the degradation gradients at site scale.The contents of soil C/N,C/P,and N/P consistently decreased with degradation gradients intensifying.The C/N,C/P,and N/P ratios were positively affected by the SCOM,SWC and SR of graminoids,while negatively affected by soil p H and the proportion of AGB of forbs.STEM had a negative effect on the C/N ratio but showed positive effect on the C/P and N/P ratios.The current study shows that degradation-induced changes in abiotic and biotic conditions such as soil warming and drying,which accelerated the soil organic carbon mineralization,as well as the increase in the proportion of forbs,which were difficult to decompose and input less organic carbon into soil,resulted in the decreases in soil C/N,C/P,and N/P contents to a great extent.(4)Alping grassland degradation caused a dramatic reduction of BGB in the 0-10 cm soil layer(BGB0-10),whereas a slight increase in subsoil BGB.It may be because soil properties in surface soil were more responsive to degradation than those in subsoil,indicated by the significant positive relationships between trade-off value of BGB0-10 with SWC,SCOM,SOC,STN,and SAN as well as a negative correlation to SBD in 0-10 cm soil depth.The degradation-induced changes in soil properties lead to a dramatic decrease in graminoids while a gradual increase in forbs,which directly affects BGB vertical distribution.The findings suggested that root biomass transfer to deeper soil in response to degradation might be an adaptive strategy to increase abiotic stress.(5)One-year GE significantly increased SOC,STN,SWC,and plant biomass,while decreased SBD regardless of the degradation degree.These findings suggest that short-term GE may be an effective way to restore degraded alpine meadow.AGB and BGB were significantly positively associated with SWC,SOC,STN,STP,and SAN,but presented negative relationship with SBD.This shows the complicated interaction between vegetation and soil physiochemical properties that regulates the grassland recovery process.The degraded alpine meadow in this rainfall-rich region could rapidly recover once grazing disturbance was excluded.The optimal time of GE for lightly and heavily degraded grassland in this area are 1 and 9 years,respectively.