Responses Of Growth And Interactions Between Mangroves And Spartina Alterniflora Across Tidal Elevation Gradient

Mangroves is a kind of woody plant community grown in the tropical and subtropical intertidal zones,which has high ecological value.Under the impacts of global climate change and human activities,the exotic plant of Spartina alterniflora(S.alterniflora)have invaded into mangroves along the southeastern coast of China,and threated the regeneration and growth of native mangrove species.In order to better understanding the impacts of S.alterniflora invasion on the growth and distribution of mangrove spsecies and the mechanisms of interspecific interactions between them along environmental gradients,we investigated the response mechanism 'of mangrove species and S.alterniflora to flooding and interactions between invasive-native(S.alterniflora,Kandelia obovata),and invasive-exotic(S.alternifilora,Sonneratia apetala)along different tide gradient.We conducted 'marsh organ' to manipulate experimentally the elevation tidal gradient and conducted with or without competition from S.alterniflora in the Zhangjiang Estuary for two growing seasons.To analyze the effects of biotic and abiotic factors on the early regeneration and growth of mangrove along the tide gradient.The results showed that:1.With the increase of elevation(decrease of flooding time),the soil water content decreased,while the soil porewater salinity increased gradually.The soil water content ranged from 20.3 ± 0.9%to 50.3 ± 0.3%,and the soil porewater salinity ranged from 13.9 ± 0.5 PSU to 79.6 ± 10.2 PSU.Neighbor Spartina vegetation had no significant effects on the soil porewater salinity and the soil water content across different elevation gradient.2.Under different flooding time conditions,the survival time of mangrove and S.alterniflora was different.Under high inundation(15 h/d),the survival time of K.obovata was 14.7 months.Under low inundation and drought(flooding time 0.06 h/d),the survival time of the invasive S.alterniflora was only 11.2 months.However,prolonged flooding and drought did not directly affect the survival time of nonnative S.apetala.3.The responses of K,obovata and S.apetala(plant height,basal diameter,branches and leaf number)were identical across different elevation gradient.For K.obovata and S.apetala,we found a hump-shaped relationship between inundation and plant growth index along different tidal gradient.S.apetala responsed to flooding stress through increasing plant height and the number of branches,and low inundation(0.9 h/d)increased the number of leaves,which was responsing to the low soil water content and high soil porewater salinity.High inundation suppressed the branching and leaf growth of K.obovata.4.For K.obovata,S.apetala and S.alternifora,the biomass response of the three species was identical across different elevation gradient.They performed well at intermediate elevations,but grew and survived poorly in high-and low-marsh habitats.At the same time,the niche width of the three species along different tide gradient was different.K obovata had high drought tolerance and grew in the high tide,the growth niche width of K.obovata was 0.9-7 h/d along tide gradient.S.apetala had high drought and inundation tolerance,the growth niche width of S.apetala ranged from 0.9 to 10 h/d along tide gradient.S.alternziflora had high inundation tolerance but low drought tolerance,the growth niche width of S.alterniflora ranged from 3.5 to 10 h/d along tide gradient.5.The relative interaction intensity of mangrove species and S.alterniZora varied significantly along tide gradient.S.alterniflora significantly inhibited the growth of K.obovata and S.apetala across all elevation gradients.The relative neighbor interaction intensity index(R?)was positive correlated with biomass of S.alterniflora,but the neighboring S.alterniflora facilitated the survival and growth of K.obovata at the flooding time of 15 h/d.6.In the natural habitat of S.alterniflora.,S.alterniflora suppressed the growth of both native and exotic mangrove seedlings along all elevation gradients during the early establishment stage(two growing seasons),but the native Kobovata and the exotic S.apetalar showing a different growth trend.The native K.obovata was persistently suppressed by S.alternflora and had low growth rate,height of K.obovata was 71.1 ±5.1 cm and has no branching.In contrast,exotic S.apetala grew rapidly,plant height was 225.1 ± 14.8 cm,with taller of S.apetala plants than their S.alternifora neighbors within two growing season.These results indicated that S.alterniflora inhibited the survival and growth of mangrove seedlings and the nature and strength of competitive interactions between mangrove and S.alterniflora may vary across tidal gradient,species and then impact the vegetation distribution pattern and succession of mangrove-Spartina ecotone.These results can provide scientific evidence for mangrove conservation and management on southeast coast China.