Effects Of Environmental Gradient On Reproductive Strategy Of Salt Marsh Plants

Reproduction is a critical stage in the life cycle of plants,as they enhance their fitness by adjusting resource allocation patterns and life-history strategies to complete population regeneration under varying environmental conditions.Intraspecific and interspecific interactions also strongly influence the vegetative growth and reproductive strategies of plants.However,there is still a lack of systematic understanding of the variation of plant reproductive strategies along environmental gradients in salt marshes.In this study,we utilized a combination of field investigations,pot experiments,controlled environment experiments,and other methods to uncover the influence of elevation,salinity,and intraspecific and interspecific interactions on the reproductive allocation and germination potential of salt marsh plants.These findings illustrated the mechanisms of plant population regeneration and community assembly in salt marshes.The main findings of our study were as follows.1.The growth and reproduction of Phragmites australis were significantly influenced by elevation.Our research found that aboveground biomass,belowground biomass,total biomass,seed biomass,sexual reproduction allocation and rhizome biomass were at their highest in the middle marsh,whereas asexual reproduction was the lowest in this area.Total biomass of P.australis had a positive correlation with its sexual reproduction allocation,but a negative correlation with its asexual reproduction allocation.This suggested that P.australis could balance its sexual and asexual reproduction to allocate limited resources for optimal population reproduction in different habitats.The study also revealed that seeds produced in different elevations had varying germination potentials in salt or fresh water.Seeds produced in the low marsh had a higher germination rate in salt water,which might be related to the maternal effect.In addition,under 1% salinity treatment,the germination speed of seeds in different tidal zones was inhibited.2.The vegetative growth and reproduction of P.australis and Scirpus mariqueter were significantly inhibited by 1% salinity,while it promoted the vegetative growth of Spartina alterniflora.This indicated that S.alterniflora had a stronger salt tolerance compared with P.australis and S.mariqueter,which contributed to its competitive advantage over native species and rapid expansion along the Chinese coastal zone.Additionally,density had a significant impact on the vegetative growth and reproduction of these three species.High density treatment promoted the vegetative growth and reproduction of S.alterniflora and S.mariqueter,but it inhibited the vegetative growth and reproduction of P.australis.This might be due to higher resource demand and stronger intraspecific competition of P.australis ramets.In contrast,S.alterniflora and S.mariqueter require less resource as they have smaller size,and intraspecific facilitation is dominant during their growth.3.The germination response of S.alterniflora,S.mariqueter,P.australis,Suaeda glauca,and Tripolium vulgare seeds to storage and germination environment varied significantly.Field storage enhanced the seed germination rate of S.alterniflora and S.glauca,but decreased the germination rate of T.vulgare.On the other hand,refrigerated storage increased the germination rate of P.australis and S.glauca,but decreased that of S.mariqueter.Field storage shortened the germination time of S.alterniflora,S.mariqueter,and S.glauca.Meanwhile,refrigerated storage prolonged the germination time of S.alterniflora and S.mariqueter while reducing that of S.glauca.Seed dormancy at different sites in the field had little impact on the germination rate and time of these dominant species.However,with an increase in elevation,the germination rate of S.mariqueter and S.glauca seeds increased while the germination time decreased.During the field germination experiment,P.australis seeds did not germinate,possibly due to being buried deep in the soil.For the other species,germination was mainly observed in March.S.alterniflora seeds varied in germination rate with the highest germination rate in Tuanjiesha high marsh and Dashitou middle marsh.Salinity and inundation were the main environmental factors affecting the seed germination of S.mariqueter.Therefore,S.mariqueter seeds mainly germinated in the low and middle marshes of Tuanjiesha with higher soil water content and lower salinity.S.glauca and T.vulgare were sensitive to flooding stress,and hence their seed germination mainly occurred in Tuanjiesha high marsh.In mixture,S.mariqueter seeds inhibited the germination of S.alterniflora,and P.australis seeds inhibited the germination of S.mariqueter.The effects of S.alterniflora seeds on S.mariqueter and P.australis seeds on S.alterniflora varied among different sites.There is a trade-off between sexual and asexual reproduction of dominant plants in Chongming Dongtan,and the threshold for sexual reproduction is higher than that for asexual reproduction.The effects of dormant and germination environment on germination of salt marsh plant seeds differ,and the germination process is also affected by interspecific interaction.Therefore,we suggest using S.mariqueter seeds stored in wet cold conditions as the source for population recovery,which has the highest survival rate and can inhibit the germination of S.alterniflora seeds.Besides,ramet transplanting is recommended for P.australis population recovery.These targeted measures are helpful to improve the restoration efficiency of salt marsh ecosystems.