Growth Of Mangrove Seedlings And Its Response To Invasive Spartina Alterniflora-induced Above- And Below-Ground Competition

Coastal wetland is vunerable to biological invasion.Since the invasive Spartina alterniflora dominates the entire coastline,mixing with mangroves in southeast of China,the regeneration and growth of native mangrove seedlings are significantly inhibited.At the same time,the exotic mangrove,Sonneratia apetala shows a natural dispersal trend within the habitat of S.alterniflora,which is changing the vegetation distribution pattern of the mangrove-Spartina ecotone.As the soil-air interface creates a natural division and spatial separation between above-and below-ground competition,we know little about the contribution of the Spartina-induced above-and below-ground competition varies with species.The responses of mangrove seedlings to invasive Spartina-induced competition are affected by the above-and belowground competition,species characteristics and the biotic and abiotic factors over time.We conducted a field experiment in Zhangjiang Estuary in Fujian,China by using the "target technique" to separate above-and below-ground partition and to achieve four competition treatments,including neighbors' roots and shoots treatment,neighbors'shoots treatment,neighbors' roots treatment,no neighbor treatment.In the two continuous years,we measured the abiotic factors including the light intensity,soil water content,soil pore-water salinity and the growth of S.alterniflora.We also quatified the responses of one exotic(S.apetala)and three native(Avicennia marina,,Aegiceras corniculatum,Kandelia obovata)mangrove seedlings under different competition treatments,assessed the relative contributions and interactions of Spartinainduced above-and below-ground competition of different species,and analyzed the effects of variations of biotic and abiotic factors on mangrove seedling growth.The results showed that:1.The plant height and density of S.alterniflora in neighbors' roots and shoots treatment and neighbors' shoots treatment varied with seasons that the plant height and density were high in summer and autumn,and low in winter and spring.The aboveand below-ground biomass of showed significant differences among different competition treatments.The relative light intensity that mangrove seedling received was only 37.1 ± 10.8%,48.6 ± 2.2%under the S.alterniflora canopy.Neither soil water content nor soil pore-water salinity had significant differences among different competition treatments.2.During the first growing season,the survival time of every species showed no significant differences among competition treatments.The survival time of S.apetala was the lowest of four mangrove species.During the second growing season,the survival time of S.apetala in neighbors' roots and shoots treatment was the significantly lower than other three competition treatments(all survived).There were no significant differences between the survival time of native A.marina and A.corniculatum.Only the seedlings of K.obovata in no neighbor treatment survived.There was no significant difference among different species in every competition treatment.The survival rate at the second growing season of S.apetala was 42.5%higher than that of the first growing season.There was no significant difference between the survival time of two growing seasons of A.marina and A.corniculatum in every competition treatment,as well as there was no significant difference between the survival time of two growing seasons of K.obovata in no neighbor treatment.3.To those surviving mangrove seedlings,there were significant difference among different species and different competition treatments of the morphological indices including the plant height,basal diameter,and branch number and leaf number of mangrove seedlings at two growing season except the leaf number at the end of the second growing season among different competition treatments.By the end of the second growing season,S.apetala seedlings that under the shade of S.alterniflora exceeding the canopy height of S.alterniflora only in neighbors' shoots treatment.The native mangroves were under the canopy of S.alterniflora.Both above-and belowground competition inhibited the growth of mangrove seedlings during two growing seasons,especially the aboveground biomass.The aboveground biomass of S.apetala and native mangrove seedlings had no significant differences at the first growing season.At the second growing season,except for the neighbors' roots and shoots treatment,the aboveground biomass of S.apetala were significantly higher than the native.The aboveground biomass of different species of mangrove seedlings in different competition treatments at the second growing season were significantly higher than that of the first growing season except the aboveground biomass of A.marina in neighbors'shoots treatment and A.corniculatum in neighbors' roots and shoots treatment.4.Both the above-and below-ground competition imposed by S.alterniflora played key roles in inhibiting the growth of mangrove seedlings.The exotic S.apetala was mainly inhibited by aboveground competition.The native mangroves were inhibited by both the above-and below-ground competition.Moreover,the interactions of aboveand below-ground competition exacerbated inhabitation on the growth of A.corniculatum and K.obovata.The competitive intensity induced by S.alterniflora decreased as the mangrove seedlings grew up over time.The growth of mangrove seedlings was affected by the light intensity,above-and below-ground biomass of S.alterniflora.The mangrove seedlings grew better with the increase of light intensity and decrease of the biomass of S.alterniflora.In conclusion,the regeneration and growth of mangrove seedlings in the habitat of S.alterniflora were significantly inhibited by Spartina-induced above-and belowground competition,varying with mangrove species and time.The exotic S.apetala can emerged from the canopy of S.alterniflora due to its fast growth,except when the seedlings were under the competition of both above-and below-ground.The native mangrove seedlings grew slowly and cannot outgrow competition of S.alterniflora Therefore,we need different managements during different growth periods.Both removing the canopy of S.alterniflora and digging the roots in the early transplantation are necessary in the restoration and rehabitation of mangroves to replace the S.alterniflora meadows.These measurements may provide sufficient light,underground space and nutrient for regeneration of mangrove seedlings.Cleaning up the S.alterniflora that expand into the rehabitaiton habitat of the successful established mangrove seedlings is also necessary.To prevent potential threat of biological invasion,it is not recommended to use exotic S.apetala in the reforestation of mangrove in replacing S.alterniflora.