Research On Traits And Mechanisms Of Alternative Stable States In Yangtze Estuary Coastal Wetland Ecosystems

Coastal wetland ecosystems occurring at the transition zone between land and ocean,has high service value in the world,including environment purification,providing habitats,coastal protection and climate regulation,with increasing pressure from global changes and human activities.Alternative stable states phenomenon widely exists in a variety of ecosystems which is closely related to ecosystem health and sustainable development.The study for alternative stable states of coastal wetland ecosystems has important theoretical and practical meaning.In our study,taking the coastal wetland ecosystem in Chongming Dongtan Nature Reserve as study area,we aimed to identify the alternative stable states types,explore the mechanisms by analyzing biogeomorphic positive feedbacks,and test above mechanisms on the landscape scale.The main results were as follows:1.The alternative stable states types in Yangtze estuary coastal wetland ecosystems were identified.Normalized difference vegetation index(NDVI)frequency distribution revealedobvious bimodality of plant biomass at intertidal saltmarsh pioneer zone.Transplant experiments indicated that propagule biomass threshold limited the establishment of saltmarsh plant patches.The presence of bimodality and biomass threshold demonstrated there are "mudflat" stable state and "saltmarsh" stable state with distinct structure and function in saltmarsh front zone of Yangtze estuary coastal wetland ecosystems.2.The formation mechanisms of alternative stable states in Yangtze estuary coastal wetland ecosystems was analyzed.In Yangtze estuary coastal wetland ecosystems,the biogeomorphic positive feedback was the critical mechanism of alternative stable states.Inundation related to elevation clearly influenced the survival,growth and reproduction of saltmarsh plants,and persistent and high depth inundation had a strong stress on the whole life history of saltmarsh plants,greatly limiting the "saltmarsh" stable state formation.The lush saltmarsh plants facilitated the sediment accretion via inducing wave and current intensity,and promoted the development of "saltmarsh" stable state.There was a positive correlation between sediment accretion and plant biomass.The elevation increase caused by sediment accretion reduced the influence of high intensity inundation on saltmarsh plants,and therefore promoted the transition from "mudflat"stable state to "salt marsh" stable state.In the Yangtze estuary coastal wetland ecosystems,the biogeomorphic process played an important role on the patch scale,leading to the formation of alternative stable state.3.Regime shifts of Yangtze estuary coastal wetland ecosystems on the landscape scale were discussed.On the landscape scale,the hydrodynamic conditions and sedimentary physiognomy in different transcts of Chongming dongtan showed distinct spatial heterogeneity.The north transect was characterized by the tidal current with the weakest hydrodynamic conditions and flow direction perpendicular to the vegetation zone.Current velocities and turbidities during flood flow were clearly higher than those of ebb flow,which was the most important factor responsible for cohesivesediments rapid accretion in spring and summer.The hydrodynamic conditions and sedimentary physiognomy of the mid transect were similar to the north transect,but the sediment accretion rate of muddy flat was slightly lower than that of the north transect.The south transect was influenced by river runoff and tidal current,with the strongest hydrodynamic conditions and flow direction parallel to the vegetation zone.Its wave intensity and current velocity was of the largest,but water turbidity was not obvious,which was not conducive to sediment accretion so as to result in the formation of silty flat with low accretion rate.On the landscape scale,although the geomorphic features of three transects were different,there were regime shifts between "saltmarsh" stable state and "mudflat" stable state.Analysise of long time regime shifts in Chongming dongtan revealed evidence for long-term alternative stable states.The transition from "mudflat" stable state to"saltmarsh" stable state in 2009-2015(12.25 km2)was obvious more than 2003-2009(10.16 km2),and the opposite transition(0.75 km2)was little than 2003-2009(2.22 km2).The phenomenon could be explained by the sediment discharge dynamics of Yangtze estuary.The sediment discharge decreased sharply in 2003-2009,but tended to be stable after 2009.The results of this study(biogeomorphic positive feedback and "threshold effects")could enrich regime shift mechanisms researches in theory and methods,and provide the scientific supports for coastal zone vegetation restoration and comprehensive management.