Mechanisms Of The Effect Of Soil Seed Bank In Alpine Grassland Ecosystem Restoration In Tibetan Plateau

Climate change and anthropogenic activities have seriously affected different ecosystems around the world.Alpine ecosystems on the Tibetan Plateau are more fragile than other ecosystems,and these impacts have led to varying degrees of degradation of the alpine grassland ecosystems on the Tibetan Plateau.The research of the mechanisms of ecosystem resilience and degradation ecosystem restoration is important scientific questions.Soil seed banks are an important part of ecosystem resilience and a "reservoir" for maintaining species diversity of plant community,and it can recruit plant communities with seedlings after they are destroyed.Therefore,soil seed banks are generally regarded as important resources for the restoration of degraded ecosystems.However,up to now,the mechanism of soil seed banks in the restoration of degraded ecosystems remains unclear,especially,in the theoretical level,almost no research explore the role soil seed bank in alternative stable state model,ecosystem resilience(recovery and resistance),and in ecosystem warning systemWe conducted a large-scale,multi-site investigation almost on the whole Tibetan Plateau,covering three grassland types,including alpine desert,alpine steppe and alpine meadow ecosystems,while we also investigated the degradation sequence of alpine marshes in the eastern Tibetan Plateau.We collected the data of different components in each ecosystem,including spatial data(altitude,longitude and latitude),climate data(the mean annual precipitation and temperature),aboveground vegetation(species composition and structures),soil environmental factors(nitrogen,phosphorus,soil organic matters,soil moisture,etc.),soil seed bank(species composition and structures)and small vertebrates(only in alpine marsh,including bird species richness,abundance and density of rodents).In this study,we want to explore the dynamics of soil seed bank during alpine marsh ecosystem degradation on the Tibetan Plateau,and the effect mechanisms of soil seed bank in degraded alpine marsh ecosystem restoration;the mechanisms of state transition among alpine ecosystem,including alpine meadow,alpine grassland,alpine desert and shrubland ecosystems;Role of soil seed bank in ecosystem warning system,and effect mechanisms of soil seed bank in ecosystem state transition and restoration.Through these questions,we want to illustrate the effect mechanisms of soil seed banks in the restoration of alpine grassland ecosystem on the Tibetan Plateau.Based on above research,we want to supply a theoretical basis and scientific support for the typical and fragile alpine ecosystems protection and restoration in Tibetan Plateau.Our important findings are as follows.1.With the degradation of alpine marshes ecosystem on the Tibetan plateau,small vertebrates(birds and rodents)have the highest explanation for the variation of species composition of soil seed banks.Compared with other predictors,bird diversity had the greatest impact on the species composition of soil seed banks.Meanwhile,soil environmental factors have the highest explanation for the species richness of soil seed banks.Compared with other predictors,soil moisture had the greatest effect on the species richness of soil seed banks.Our results indicate that the species richness and species composition of soil seed banks are regulated by different mechanisms during the alpine marsh ecosystem degradation.Birds play an important role in maintaining the species diversity of degraded alpine marsh ecosystems(soil seed banks are potential providers of species diversity),and more attention should be paid to the protection and restoration of bird diversity in the future management and restoration of degraded alpine wetland ecosystems.2.Based on the analysis of 89 alpine steppe and meadow ecosystems on the whole Tibetan Plateau,we found that the main environmental driving factor affecting the total coverage of aboveground plant communities was annual mean precipitation(MAP).The total coverage of aboveground plant community and coverage of different functional groups(grasses,sedges and forbs)showed an obvious bimodal distribution.Moreover,after analyzing the total coverage of aboveground plant community,we found alpine esosytems changed from one stable state to two stable states,and then to one stable state along a precipitation gradient;after analyzing different functional groups,we found the spatial variability of coverage near the threshold is significantly higher than that far away from the threshold.The species composition of aboveground plant community and soil seed banks was significantly different between the two stable states,and the species composition of aboveground plant community and soil seed banks was significantly correlated.Our results indicated that the alpine grassland ecosystems of Tibetan Plateau have two stable states at different levels.The two stable states are separated by an unstable state,and it consistent with the prediction of system stochastic dynamics.The response of alpine grassland ecosystem to precipitation changes is discontinuous and abrupt.In the transition of alpine meadow to alpine steppe ecosystem,the species composition of aboveground plant community and soil seed banks also across the critical threshold.Such changes in species composition of soil seed banks may further promote the transition of alpine grassland ecosystems.3.Based on the analysis of 29 arid grassland and shrubland ecosystems in Qaidam basin on the northern Tibetan Plateau.We found ecosystem state transition from arid grassland to shrubland due to soil moisture decrease,and the change of species composition of plant community showed an abrupt change and also showed an obvious threshold along a continuously changing soil moisture gradient,while the species composition of the soil seed banks remained unchanged even though the aboveground plant community changed and across the critical threshold.Surprisingly,there is at threshold also emerged in similarity between species composition of aboveground plant community and soil seed banks along a continuously changing soil moisture gradient,ant the threshold coincided with the threshold region of the state transition from arid grassland to shrubland ecosystem.Our results indicated that the mechanism of the state transition from arid grassland to shrubland ecosystem is drought(soil moisture decrease).Species composition of soil seed bank keep unvaried before and after ecosystem state transition,and recovery/regeneration from soil seed banks to the aboveground vegetation was inhibited by drought,but not previous research suggested seed bank composition change or seed bank resources exhausted.The similarity of species composition between aboveground vegetation and soil seed banks can be used as an early warning signal of ecosystem transition.Our findings provide a theoretical support for the arid ecosystems management.