The Role Of Soil Seed Bank In Degraded Alpine Wetland Vegetation Restoration And Early Warning Research

In recent years,the alpine wetland ecosystems in the eastern part of the Tibetan Plateau have shrunk or degraded with varying degrees,losing important ecosystem services and functions.Research on the restoration and early warning of the degraded alpine wetland ecosystem is an important scientific question and also has important practical significance.As a potentially important resource for the regeneration of plant communities,soil seed banks are believed to play an important role in the restoration of degraded vegetation.However,the role of soil seed banks in the restoration of degraded alpine wetland ecosystems is still unclear.In addition,studying and finding early warning signals of ecosystem state transition is extremely important to prevent the loss of biodiversity and the catastrophic transition of ecosystems.The grass hill in the wetland ecosystems a long-formed wetland-specific landscape/microtopography,which is considered to be an important factor for regulating the wetland ecosystem.During the degradation of the alpine wetland ecosystems in the Tibetan Plateau,the size of the grass hills shows a regularly change,but whether it can be used as an useful early warning signal for predicting in biotic（aboveground plant communities and underground seed banks）,abiotic factors（soil moisture,SOC,TN,p H）,and the ecosystem state transition of s（from alpine wetland to alpine meadow）still not clear.We selected a typical degradation succession series in the alpine wetland on the eastern Tibetan Plateau,including six degradation levels,and carried out the following two aspect researches:1)we investigated the species composition and characteristics of plant communities at different degradation levels,as well as transient and persistent seed bank,soil physical and chemical factors,structural equation modeling（SEM）and non-metric multidimensional scale analysis（NMDS）were used to explore the direct and indirect of soil moisture change with alpine wetland degradation on seed bank（transient and persistent seed bank）,and the mechanism of seed bank in alpine wetland plant community regeneration and degraded vegetation restoration,respectively;2)The size of grass hills（including area and volume）was investigated in each of six degradation levels,and the segmented regression analysis was used to explore whether the size of grass hills can be used as an important early warning signal for predicting the biotic factor（plant community and the soil seed bank）,abiotic factors（soil moisture,SOC,TN,and p H）,and the state transition from alpine wetland to alpine meadow ecosystem.The main results are as follows:1)With the degradation of wetlands,the seed density and species diversity of the transient seed bank remained almost unchanged.The mechanism of this result was the reduction of soil moisture directly increases the species diversity and seed density of the transient seed bank,while it indirectly reduces the seed density of the transient seed bank by reducing soil p H and increasing the species diversity and abundance of plant communities（promote transient seed germination）.The combined effect of these two paths cased transient seed bank（seed density and species diversity）keep unvaried with alpine wetland degradation.2)With the degradation of wetlands,the seed density and species diversity of the persistent seed bank increased significantly.The mechanism of this result was soil moisture increases the species diversity and abundance（seed input）of plant communities by reducing p H,which indirectly increases the species diversity and seed density of the persistent seed bank.3)With the degradation of wetlands,the role of persistent and transient seed banks in aboveground plant community regeneration increases,but the role of persistent seed banks is greater than that of transient seed banks.4)There is a threshold（grass hill areas=0.13m~2）exist in plant community and soil seed bank species diversity（biotic factors）change during the process of alpine wetland degradation.When the area of the grass hill is higher than the threshold,the species diversity and biomass of the plant community increase with the decrease of the grass hill area,but when the grass hill area is lower than the threshold,the species diversity and biomass decrease significantly as the grass hill area decreases.The species diversity and seed density of the seed bank increase with the decrease of the grass hill area,and when the grass hill area is less than the threshold,the increasing trend is more significant.5)There is a threshold（0.2m~2）exist for the changes of soil moisture,SOC,and TN（abiotic factors）during the process of wetland degradation.Before the threshold,it decreases slowly with the decrease of the grass hill area,but the decreasing trend is more significant after the threshold.6)There is also a threshold（0.2m~2）exist for the ecosystem state transition during the of alpine wetland degradation.When it is lower than the threshold,the state transition happened from alpine wetland ecosystem to alpine meadow ecosystem.This study reveals the mechanism of the role of persistent and transient seed bank in the regeneration of plant communities and restoration of degraded vegetation during the alpine wetland degradation on the Tibetan Plateau.We also discovered for the first time that the area of grass hills is an easy-to-observe and effective early warning signal for predicting in biotic and abiotic factors change during the degradation of alpine wetland wetlands,and the ecosystem state transition.Our findings can provide a scientific basis for the protection and restoration of the degraded alpine wetland ecosystem on the Tibetan Plateau.