The Seed Behavior-trait Relationships Of Annual Plants Under Extreme-submerged Disturbance Habitats

In the context of global climate change,global damming not only alleviates the demand for water resources and clean energy,but also brings a serious ecological crisis.After dam operating,previous vegetation in its riparian zone had rapidly degenerated and replaced by a vegetation dominated by annuals.Seeds is the life-form of annuals that tolerant the submergence during the impoundment periods of big dams.Since seeds play important roles in dispersal,establishment and even in long-term persistent of the species,how annuals seeds respond to the extreme submergence disturbed habitat will affect the community assemblage and vegetation succession in riparian zone of big reservoir.There are a series of parameters that could describe the response behaviors of seeds under extreme submergence disturbed habitats,such as: Seed dispersal mode,seed buoyancy,seed intactness ratio,seed germination,germination speed and the evenness of seed germination,etc.Those parameters indicate the strategies of how annuals respond to the disturbed habitats.Currently,no studies that have systematically explored the seed behavior regimes of annual plants under extreme-submerged disturbance habitats.The natural regimes of environment factors determine seasonal occurrence pattern of plants.Based on that,annual plants can be categories into two types: Summer annual and winter annual.The former germinate in spring and fruit in autumn,the latter germinate in autumn\winter and fruit in spring\summer.Amount of studies indicated that there are significant differences in distribution pattern of those two kind of annuals in the riparian zone of big dam reservoirs.Theoretically,the differences in seed behaviors under the extreme submergence disturbed habitats are supposed to be the causality,but there is still no effective empirical evidence.Furthermore,the flow regime of different dam reservoir induced different seasonal disturbance habitats in their riparian zone,which might cause different seed response behaviors and the consequential ecological processes.But there is little information about whether seed behaviors of different types of annuals are different under extreme flooding habitats with different flow regime.Although the seed response behaviors play essential roles in community assembly and vegetation succession under disturbed habitats,little studies had applied seed behaviors in vegetation research.One of the reasons is that it is hard to systematically collect seed behaviors;another one is that seed behaviors is habitat-specific.The proxies of seed behaviors are seeking.Seed functional traits are supposed to be the best ones,given that it is easy to collection and generalized among species.Trait-based approach are widely used in how plants respond to new habitats,such as extreme drought,high temperature,but there is no related studies in human-regulated extreme submergence disturbed habitats in big dam reservoir.There are three types of seed functional traits: seed morphological traits,seed biophysical traits and germination traits.Seed morphological traits and biophysical traits,like seed mass,seed size,seed protein content,were evidenced to be strongly linked to seed behaviors under those habitats.The robust seed behavior-trait relationship is an essential precondition for applying trait-based approach into related vegetation studies.Currently,no research has reported on seed trait that is robustly linked to seed behaviors under extreme submergence disturbed habitats.Aiming to seek the robust seed behavior-trait relationships and select the valid seed functional traits to proxy the seed behaviors for applying trait-based approach in related researches,this study systematically tested the seed behaviors under different seasonal submergence disturbed habitats,and collected the seed functional traits of amount of annuals(with different life-history types),and analyzed their relationships.The main contents and results are listed as followed:Firstly,we collected the dispersal mode,seed buoyancy,seed submergence tolerance and post-submergence germination behaviors of 18 dominant annuals in the Three Gorges Reservoir(TGR).The results showed that the seeds of Bidens tripartita and Eclipta prostrata had very strong buoyancy(almost all their seeds could float on the water for more than 3 months),and the seed floating time of the other species was less than 1 month.Therefore,the seeds of the other species cannot escape the extreme flooding of the riparian zone in Three Gorges Reservoir by floating.The seed intactness ratio was higher than 80% and the post-submergence seed germination was about 60% for all the 18 annual species whose seeds had been submerged at the elevations of 170 m?165 m?160 m and155 m(with the maximum water depth of 5 m,10 m,15 m and 20 m,respectively)in the riparian zone of the TGR.Moreover,the seed germination index was increased with the elevation decreasing,all seeds tended to germinate at the beginning of the outcropping periods(“Growth window”).Although it is species-specific in the dispersal mode and buoyance for those dominated annuals,the uniform pattern in seed intactness ratio and germination behaviors indicated that the general strategies in “Quiescence” to tolerant the submergence stresses and in“Immediate germination” to establish in the limited “Growth window” for the dominated annual species in the riparian zone of the TGR.Then we recorded the seed intactness ratio during submergence,post-submergence seed germination and germination dynamics of 23 winter annuals and 20 summer annuals at different elevations in the riparian zone of TGR,compared those seed behaviors with that under non-submerged natural habitats,and compared the differences of those seed behaviors between summer annuals and winter annuals.Results showed that: Comparing with non-submerged habitats,the extreme submergence disturbed habitats improved the seed intactness ratio and germination in the same periods at all elevations in the riparian zone of TGR.This was unexpected and indicated that the submergence stress is not the causation of seed resource limitation and seedling limitation for the establishment of annuals in the riparian zone of big dam reservoir.Summer annuals was with more advantages in the competition with winter annuals in the re-establishment process,given higher seed intactness ratio during submergence and post-submergence germination,and faster germination process in the riparian zone of TGR.While there were significant differences in seed behaviors between summer annuals and winter annuals,the seed behaviors within the same life-history was species-specific.Aiming to explore the seed behavior-trait relationship under extreme submergence disturbed habitats,we collected the seed morphological traits,biophysical traits and germination traits of 49 annuals species.Combining the seed behavior under the extreme habitats at different elevations in the riparian zone of the TGR,we systematically analyzed their relationships.Results showed that: the seed mass,seed length,seed width,seed volume and seed oil content was linked to the seed germination under natural terrestrial habitats,but not linked to the seed behaviors under submergence disturbed habitats in the riparian zone of TGR.The phylogenetic signal was weak in seed traits and seed behaviors,except in seed protein content and seed oil content.The PGLS analysis also indicated that the seed biophysical traits were not linked to seed behaviors under submergence disturbed habitats.Luckily,the seed germination under natural terrestrial habitats were strongly linked with seed behaviors under submergence disturbed habitats.But this behavior-trait relationship were affected by life-history types of annuals.As timing is the metaphor behind the life-history type and seed germination dynamics under natural habitats,we argued that the coupling of timing of submergence and the germination process under natural condition determined the seed behaviors under the extreme submergence disturbed habitats.Finally,we collected the seasonal germination data,seed dormancy characteristics and seed germination under different temperature conditions of 22 winter annuals and20 summer annuals,and the seed behaviors under summer flooding and winter flooding of those species,then systematically checked the relationships of those seed behaviors under different seasonal submergence disturbed habitats and suits of seed germination traits.Results showed that: There was different seasonal germination and seed dormancy pattern between summer annuals and winter annuals.For summer annuals,the seed intactness ratio during submergence and post-submergence germination under winter flooding disturbed habitats was significantly higher than that under summer flooding disturbed habitats.But winter annuals showed contrary patterns to summer annuals.Those differences in seed behaviors between different flooding seasons was related to seed dormancy characteristics.Combining with life-history,the seed germination under natural habitats showed strongly related to seed behaviors under submergence disturbed habitats.The seed germination under simulated flooding season temperature offered an easy checked and robust relationship with seed behaviors under different seasonal submergence disturbed habitats.We concluded that: The “Quiescence strategy” during submergence periods and the “Immediate germination strategy” in the limited “Growth widow” were the general strategies for those annuals dominated in the riparian zone of TGR.The seed behaviors under different seasonal submergence disturbed habitats were significantly different between summer annuals and winter annuals,which was related to their different dormancy characteristics.Despite that they were closely related to seed germination behaviors under natural habitats,seed morphological traits,seed biophysical traits were not link to the seed behaviors under submergence disturbed habitats.The seed germination traits(seed dormancy,seed germination under natural habitats and under different simulated seasonal temperature conditions)were strongly linked with seed behaviors under different seasonal submergence disturbed habitats.While the seed germination under simulated flooding season temperature offered an robust relationship with seed behaviors under different seasonal submergence disturbed habitats,we recommended it to be the effective seed traits that could be applying in vegetation researches based on trait approach under extreme habitats.This study not only deepen the understanding in the mechanism of community assembly and vegetation succession in the riparian zone of big dam reservoir,but also could expand the usage of trait-based approach in plant ecology under extreme disturbance habitats.