Mangroves Distribution Pattern Along The River And The Adapted Mechanism To Salt Water And Fresh Water Rotation

Why are the true mangroves only living in salinity habitat? There are two hypotheses to answer this question.The first one,true mangroves are facultative halophytes.The second one,true mangroves are obligate halophytes.The first one was widely accepted now.The most important evidence was that many true mangroves penetrate considerable distances upstream along river banks where water is permanently fresh and tidal fluctuations are small or absent.In order to test this hypothesis we studied the mangroves along a analogous river,Huangzhu River,in the monsoonal subtropics of southwestern China.We studied the mangroves distribution pattern along the Huangzhu River.We also monitor the water salinity,chlorophyll fluorescence and ion contents of a true mangrove species,Aegiceras corniculatum, during the dry and wet seasons at different sites of the Huangzhu River.We analysed the effects of salinity seasonal variation on photosynthetic performance of A. corniculatum and discussed the roles of the salinity to true mangroves and the adapted mechanism of the ture mangroves to seasonal freshwater condtion.The main results were as follows:1.From upstream through intermediate to downstream the water salinity increased gradually.In the dry season,water salinity decresed approximately linearly with distance from the mouth of the river.And the Huangzhu River was a freshwater-dominated river,the water salinity showed significant daily and seasonal variation.There was abundant rainfall in the wet season and the water salinity was generally low.The water salinity ranged from 0～0.4 ppt,0.1～4.8 ppt and 4.6～24.5 ppt at the upstream,intermediate and downstream sites respectively.The precipitation was small in the dry season and the water salinity was generally high.The water salinity ranged from 6.1～17.0 ppt,12.5～23.5 ppt and 29.9～31.2 ppt at the upstream, intermediate and downstream sites respectively.2.There was difference between the ture mangroves and mangrove associates distribution pattern.As for true mangroves,areas with moderate salinity(intermediate) had higher species richness than those with high salinity(downstream),and the areas exposed to extremes of freshwater conditions(upstream) had the lowest species richness.As for mangroves associates,intermediate had higher species than upstream, and downstream had the lowest species richness.3.The upstream penetration distance of salt water determined the range of true mangrove distribution,and true mangroves do not occur where freshwater is permanent.4.The photosynthetic performance of A.corniculatum also showed spatial and seasonal variations.In the wet season,the actual PSII efficiency(ΦPSII) and electron transport rate(ETR) at upstream site were significant lower than intermediate,and there were no difference between upstream and downstream sties.In the dry season,theΦPSII and ETR at upstream and intermediate sites were higher than downstream,and there were no difference between upstream and intermediate sites.TheΦPSII and ETR of downstream site was significant lower in both seasons.Quenching analysis indicated that there was a down-regulation photoprotection at upstream site during the wet season and a high degree of photoinhibition at downstream site in both seasons.5.The ion contents of soil also showed outstanding spatial and seasonal variations. In the wet season,the ion contents of soil were significant higher than the dry season, but the ion contents of A.corniculatum remained at a high level.At the upstream site, the ion contents of the soil were extremely lower in the wet season,but the Na,K and C1 contents of the A.corniculatum remained at a high level and there was no difference from the dry season(P＞0.05).The Ca content of the A.corniculatum was higher in the wet season than the dry season(P＜0.05) at all sites.And all the element enrichment ratios at upstream site were extremely higher in the wet season than the dry season.Our results do not agree with the hypothesis that true mangroves do not need salt. We suppose that true mangroves are not only facultative halophytes but also obligate halophytes.Salt is a necessary resource for true mangroves.And there was an optimum salinity for ture mangroves growth,too low or too high salinity both have negative effects on true mangroves growth.True mangroves can store salts in their plant body when the salinity was high(dry season) and use them for growth during the freshwater season(wet season).