Response And Adaptation Of Plant-soil System Of Alpine Meadows In Different Successional Stages To Grazing Intensity

The grasslands on Qinghai-Tibet Plateau have been degraded since 1990 s because of the increasing demand of quantity and quality of livestock products,and the overgrazing has become the most important form of human interference on Qinghai-Tibet Plateau.Alpine Kobresia meadows are the main body of the winter-spring pasture,which account for about 35% of the total area of Qinghai-Tibet Plateau.The status of its vegetation,affects not only the animal husbandry production,but also the ecological functions.Livestock grazing affects alpine meadows mainly through grazing and trampling.With the increasing of livestock grazing intensity,a part of alpine Kobresia meadows gradually degrade.In the past three decades,the degradation of alpine meadows has been getting more and more serious.The Kobresia meadow degradating has started to threaten the livelihood of pastoralists and the development of local economy.How to restore those degraded meadows and maintain the alpine Kobresia meadows sustainable is a serious challenge.Under the over-grazing disturbance,the dominant species have been changed from Gramineae and Cyperaceae plant functional groups to the Forbs in the plant community during the alpine Kobresia meadows succession process.In this process,characteristics of the surface patches(the mattic epipedon,soil biological crusts and so on),characteristics of the landscape and the nutrients cycling have been changed with the succession of plant community.The objects of this study are aimed to better understand: how is the soil factor changed along with the plant community succession? What are the coupling relationships between soil nutrient characteristics and plant community successional process? What is the mechanism and response of synergetic change among ecological factors in Kobresia meadow ecosystem during the succession process? How to use those findings to figure out a series of adapting management methods to restore the degradation Kobresia meadows in their succession process? These problems are still waiting for answers.And they are still the bottleneck of both research and practice in degradation restoration of Kobresia meadows.In this study,four key plant communities of Kobreisa meadow in degradation process were selected,including Gramineae-Kobresia humilis community,Kobresia humilis community,mattic epipedon thickening Kobresia pygmaea community and mattic epipedon cracking Kobresia pygmaea community.They were connected to each other in the same geographical area and owned by four householders respectively.The research area was mainly located in Huangcheng Township,Haibei County,Qinghai Province,China.The historical data indicated that the research area was Gramineae-Kobresia humilis meadow in 1995,after that time,it was divided into four parts,and fenced and managed individually.The only difference among those pastures was livestock carrying capacity,and they were 3.65 ha,7.50,8.25 and 11.25 sheep unit per ha in Gramineae-Kobresia humilis community,Kobresia humilis community,mattic epipedon thickening Kobresia pygmaea community and mattic epipedon cracking Kobresia pygmaea community respectively.After nearly 20 years of different utilization and management,the landscape features,characteristics of plant community composition and structure in those pastures were obviously differentiated,and a natural grazing succession gradient was formed.Eachpasture was divided into three subparts for three livestock grazing treatments,and they were "free grazing","half intensity grazing " and "zero grazing " respectively.The coverage and aboveground biomass of plant community were measured by the end of August or in early September from 2013 to 2015,and the quadrats were 0.25m×0.25 m.The underground biomass of the plant community in different succession status was measured by drilling in 0-5cm,5-10 cm,10-20 cm,20-30 cm,30-40 cm soil layers,and soil samples were collected at the same time.The main results are as follows: 1)The Kobresia community succession process was a process with the differentiation characteristics of aboveground biomass in different plant community.The average aboveground biomass of Gramineae-Kobresia humilis community status,Kobresia humilis community status,mattic epipedon thickening Kobresia pygmaea community status and mattic epipedon cracking Kobresia pygmaea community status were 257.1 ± 27.4 g/m2,248.9.3 ± 21.8 g/m2,225.9 ± 24.2 g/m2 and 228.3 ± 18.8 g/m2 respectively in free grazing treatment.The inter-annual variance only contributed 9.2% of total variance of biomass from 2013 to 2015,which was much smaller than that of variance among different plant community successional statuses(83.6%)from Gramineae-Kobresia humilis community status to mattic epipedon cracking Kobresia pygmaea community status.Since four successional statuses are in same climate zone and terrain environment,the biomass difference among four Kobresia meadows should be caused by the difference of grazing intensity.2)The response patterns of plant functional groups to grazing disturbance intensity were different,and the variation of plant functional groups had a certain indication effect on livestock grazing intensity.According to the types of response patterns to grazing intensity,the plant functional groups could be divided into two types,the first one included Gramineae,Forbs and legume plant functional groups,which were grazing inhibition functional groups(grazing intensity would decrease the plant functional status).The second one was Cyperaceae plant functional group,which was grazing intensity-promoting plant functional group(grazing intensity would increase the plant functional status in a certain range of livestock grazing intensity).In this study,this meant if the livestock grazing intensity was kept less than 11.2 sheep unit per ha,the status of Cyperaceae plant functional group would be promoted by increased livestock grazing intensity,if it was more than 11.2 sheep unit per ha,the status and roles of Cyperaceae plant functional group in plant communities would be decreased.The change of difference between plant functional groups width and overlap had a certain coupling effects by the increasing of livestock grazing intensity,and the response type of the plant functional group to grazing disturbance intensity depended on their position and function in the plant community,and the response type of plant functional groups to livestock grazing led the plant communities into different Kobresia meadow succession status.The difference of niche breadth and overlap among different plant functional groups indicated that the adaptation strategy in plant community was the basis of plant community numerical characteristic in Kobresia meadow succession process.On the other hand,it reflected the ability of plant community to maintain ecosystem stability by self-organization.3)The value of the diversity index(Shannon-Wiener index)in plant community diversity was significantly reduced from 2.58±0.01 to 2.03±0.05 from Gramineae-Kobresia humilis community status to mattic epipedon cracking Kobresia pygmaea community status.The Simpson index increased significantly from 0.31 ± 0.01 to 0.44 ± 0.01,while the JSW index decreased from 0.88 ± 0.01 to 0.78 ± 0.01 during the degradation succession process.The number of species decreased by 28.2% to 35.5% when the grazing intensity increased.The characteristics of plant community evenness were become weakening,and the richness of the community species was decreasing when the grazing intensities increased.4)In different alpine Kobresia meadow succession status,the soil nutrient content did not change in the same layer in different succession status.The soil nutrients content in transition soil layer and the surface layer were more sensitive to livestock grazing intensity.The results showed that alpine Kobresia meadows under different grazing intensities for long time could result in the difference of soil nutrients contents in soil profile transition layer and surface layer.5)The ecosystem function components changed while the grazing intensity increased.It could reduce the biomass of 28%,reduce the niche breadth width 24.2%,and the niche overlap between different plant functional groups even be changed,when the grazing intensity was increased by 2 times.But the soil nutrients contents were more stability than plant community characteristics when the grazing intensity increased,once the stability of soil nutrient characteristics was destroyed,it would become the bottleneck factor of grassland restoration.6)It was possible to reduce the soil compactness,increase the soil moisture and reduce the soil temperature by reducing the grazing intensity in Kobresia meadow succession process.The restoration effects were affected by the initial state of the Kobresia meadow.It could significantly reduce 2.2% to 51.4% of soil compactness in 0-40 cm layer in Gramineae-Kobresia humilis meadow status,Kobresia humilis meadow status and mattic epipedon thickening Kobresia pygmaea community status.However,the effect on reducing soil compactness was uncertainty under zero grazing treatment.While grazing prohibited could reduce 5.4%-51.6% soil compactness compare to freedom grazing area in Kobresia humilis meadow status.But grazing prohibited had no effect or little effect on reducing the soil compactness on other Kobresia meadow succession process except in Kobresia humilis meadow status.7)The effects on improving soil available nutrients depended on Kobresia meadow initial status.The results showed that the content of ammonium nitrogen increased by 33.2% and 29.5% in 0-5cm and 5-10 cm soil layers undergrazing reducing or zero grazing treatments in Gramineae-Kobresia humilis community status.And the effect in increasing the content of ammonium nitrogen in grazing reducing treatment was higher than that of zero grazing treatment.Proper grazing was beneficial to improve the soil nutrient content in Gramineae-Kobresia humilis community status.The contents of ammonium nitrogen and available potassium in 0-30 cm soil layers in Kobresia humilis meadow were increased under grazing reducing and zero grazing treatments,and zero grazing performed better.It had better effect on improving soil ammonium nitrogen and available phosphorous contents on mattic epipedon thickening Kobresia pygmaea community status under the grazing reducing or zero grazing treatments.And zero grazing treatment performed better.The effects on improving soil available nutrients were less in Gramineae-Kobresia humilis community,Kobresia humilis meadow and mattic epipedon thickening Kobresia pygmaea community.It showed that the effect on improving soil available nutrients in mattic epipedon cracking Kobresia pygmaea community status was limited in 2 years if only the grazing reducing and zero grazing treatments were carried out.The Kobresia meadow statuses reducing and prohibiting grazing could increase aboveground biomass 0.42 times to 3.5 times in community aboveground biomass,and could increase 1.0 times to 4.3 times in aboveground biomass of Gramineae plant functional groups,and could increase 0.4 times to 1.5 times in Leguminosae aboveground biomass.The phenomenon was that the better healthy the meadow was the more improvement effect on the Kobresia meadow.The long-term grazing forbidden in a natural Kobresia meadow would significantly reduce the bearing capacity of ecosystem,especially for the less degraded meadow,so to maintain reasonable grazing intensity was more conducive to the sustainable development of Kobresia meadow.9)Kobresia humilis meadow status was a critical period of the succession series of alpine meadow.At this stage,the richness of meadow was the largest,and the competition among resources and plants was more intense.The plant community could be effectively restored under reducing grazing treatment.So the state becomes the demarcation point of the self-recovery in the alpine Kobersia meadow degradation succession process.10)It was possible to reduce the humidity and increase the soil compactness in Kobresia meadow by reducing grazing and prohibiting grazing.Those treatments also had the effects on enhancing the biomass of Gramineous and Legume functional groups,but it would reduce the biomass of Cyperaceae plant functional group.Reducing grazing could control the biomass of Forbs,while zero grazing had the effect to enhance the biomass of Forbs.Zero grazing had obvious ability to restore vegetation and surface features in Kobresia meadow plant community,which could significantly reduce the area of black spot and white spot(the contraction rate of black spot reached 27.2 ± 3.1% in 2 years,the white spot shrinkage rate reached 23.9 ± 11.2% in 2 years),significantly increased the coverage of all functional groups in plant communities,especially in Gramineae and Legumes.Those phenomena indicated that mattic epipedon thickening Kobresia pygmaea community was another critical status in Kobresia meadow succession process.If Kobresia meadow degraded to this status,and the grazing intensity was not reduced,the difficulty degree forrestoring would be increased.The ecology carrying capacity could be enhanced by using grazing reducing or zero grazing methods,but further research work is still necessary.