| The mangrove community is one of the important wetland types in the coastal area and has important social,economic and ecological values.Hainan,Guangdong,and Guangxi provinces(regions)account for more than 80%of the country's mangrove area,Guangdong Province is the province with the largest distribution of mangroves in China.The existing area of mangrove plantation is 1 2039.80 hm~2,while the Sonneratia apetala plantation is 2082 hm~2,accounting for 17.29%of the province's mangrove area.Whether such a large area of S.apetala threatens the survival and development of native mangrove species under its forest has become a concern.Therefore,the natural regeneration of native mangrove and its biology and non–biology relationship with the S.apetala forest are of urgent need for research.In this study,the S.apetala plantation in seven regions of Guangdong Province and the native mangroves nearby were studied.We searched the quantitative distribution of the seedlings(or hypocotyls)of the natural mangroves forests and interior and margins of S.apetala forest.We also explored the microenvironment factors,plant factors and soil factors of the S.apetala community to explain the distributed variation environment and selected the main influencing factors.Five main local mangrove afforestation species were studied,and field control experiments were carried out,and more gradient tidal simulation experiments were carried out to explore the suitable survive range of the five mangrove seedlings for light and flooded environments,providing a theoretical reference for the rural renovation project under S.apetala forests.The main findings are as follows:(1)The canopy closure of natural mangrove forests in Qi'ao Island,Gaoqiao,Leizhou,Dongzhai Port,Daya Bay and Yifeng River is 0.84–0.92.The seedlings coverage under the forest ranged from 6.45 to 26.56%.And Kandelia obovata,Aegiceras corniculatum,Bruguiera gymnorrhiza and Excoecaria agallocha were between 0.18 and 0.56 m in height.Due to the large degree of canopy closure of natural forest canopy,and all belong to the standing wood of stage II–III shrubs,making the seedlings in the forest margin and interior gradually diminish after several months due to lack of light.Only 1–4 months old seedlings are distributed,and light is the main limiting factors for the survival and development of seedlings.With the increase of distance from natural forests,the elevations of three tidal falts of S.apetala in all regions showed a decreasing trend.The order of the natural forest seedlings canopy closure in the forest was:S.apetala–I>S.apetala–II>S.apetala–III.The distance from natural forest and tidal flat elevation may be important factors affecting the number of seedlings and seeds under the S.apetala forest.Secondly,the number of natural forest seeds is also the influencing factor affecting the number of S.apetala seedlings in different sample regions.For example,the number of A.corniculatum flower seeds in Gaoqiao natural forest is9.34 times that of Daya Bay in Huizhou,and the the number of A.corniculatum flower seeds in S.apetala–III tidal falt is 8.04 times that of Daya Bay.(2)The number of seeds and seedlings in natural forests interior and margin were higher than those in S.apetala plantation.With the increase of distance from natural forests,the number of seeds and seedlings of forest interior and margin both decreased,and the number of seeds and seedlings of the margin was higher than that inside the forest.To improve the success rate of native mangrove seed and seedling migration to S.apetala forest,it is necessary to improve the migration channel between natural forest and S.apetala plantation,and improve the seawater circulation in natural forest and S.apetala plantation forest,making it easier for more seeds in the forest to diffuse out of the natural forest into the S.apetala plantation.(3)The effects of plant factors(number of S.apetala,diameter and height)on the species composition under the forest of Qi'ao Island and Yifeng River reached significant levels.And microenvironmental factors influence(tidal flat elevation and canopy closure)on species composition in the seven regions were significant,also the soil factors(organic carbon,total nitrogen and total phosphorus)on species composition under S.apetala forest in Qi'ao Island,Huizhou,Yifengxi and Cuiheng New Area all reached a significant level.(4)The independent effect of species variation interpretation rate of microenvironmental factors,plant factors and soil factors of S.apetala were 14%–29%,4%–15%and 3%–13%,respectively.The interpretation rate of interaction between plant factors and microenvironment factors is between 8%and 28%,the interpretation rate of interaction between plant factors and soil factors is between 2%and 3%,and the interpretation rate of interaction between soil factors and microenvironment factors is between 2%–5%,indicating that the independent effects of microenvironment factors and their interaction with plant factors have a major impact on species composition and variation under S.apetala forest.(5)With different forest canopy closure and tidal flat elevation as limiting factors,the interacton experiment of 12 months in S.apetala plantation in Qi'ao Island reveals that although the seedlings of five native mangrove plants are of certain survival rate in various treatment combinations and the survival rate of A.corniculatum,Avicennia marina and Rhizophora stylosa in the low tide zone is significantly lower than that in the high tide zone,but the stem height and leaf number of all species in the low tide zone are higher than that of the high tide zone seedlings.The maximum value is reached when the degree of closure is 0.3.The relative growth rate of low tide zone seedlings was significantly higher than that of high tide zone,and it increased with the decrease of canopy closure degree.Under the interaction of light and tidal flat elevation,the survival rate and relative growth rate of B.gymnorrhiza were the highest.B.gymnorrhiza is the most suitable native mangrove species for localization under the S.apetala plantation.At the same time,regular artificial pruning treatment in the forests with large canopy closure and increased planting density of mangrove plant seeds or hypocotyls in the low tide zone can ensure higher seedling survival rate.(6)The effect of light–flooding interaction on the height of A.corniculatum,K.obovata and B.gymnorrhiza seedlings was extremely significant(P<0.01),and the effect on the number of leaves of A.corniculatum flower tree seedlings was extremely significant(P<0.01).The effect on the seedlings base diameter of A.marina,K.obovata and R.stylosa was significant(0.01<P<0.05).The effect on seedlings root biomass of B.gymnorrhiza,K.obovata and R.stylosa was significantly(P<0.01),and the effect on seedlings stalk biomass of five mangrove seedlings reached a very significant(P<0.01)level.The effect on seedlings root biomass of A.corniculatum and K.obovata reached a very significant(P<0.01)level.The effect on chlorophyll a on the leaves of K.obovata and B.gymnorrhiza seedlings was extremely significant(P<0.01),and the effect on chlorophyll b on the leaves of K.obovata and B.gymnorrhiza seedlings was extremely significant(P<0.01).The effect on net photosynthetic rate of five mangrove seedlings reached a very significant(P<0.01)level.The effects of MDA content in leaves of B.gymnorrhiza and R.stylosa seedlings were significantly(P<0.01),and the effects on superoxide dismutase(SOD)activity in leaves of A.corniculatum,B.gymnorrhiza and R.stylosa seedlings were extremely significant(P<0.01).The effects on the activity of peroxidase(POD)in leaves of A.corniculatum were significantly(P<0.01).In the different flooding and shading conditions,the A.corniculatum can show more significant changes to adapt to the adversity of growth.It can increase the nutrient supply to the aboveground part by increasing the root biomass.It can also increase the number of leaves and stem height to get more light energy.A.corniculatum tree is a kind of afforestation tree species that is more suitable for high canopy density at low tide level.K.obovata and R.stylosa are more suitable for the environment of flooding time of 10–14 h while the canopy density is less than 0.6 during flooding period.A.marina is more suitable for the environment of flooding time of 10–18 h while the canopy density is less than 0.6 during flooding period.B.gymnorrhiza is more suitable for the environment of flooding time of 6–18 h while the canopy density is less than 0.9 during flooding period.It can be seen that B.gymnorrhiza has the highest aquatic range for light level and flooding time,and it is the most suitable species for rural renovation under the S.apetala plantations.(7)The inspiration of the soilless transformation of the S.apetala plantation:In the low tide zone,the selection of the B.gymnorrhiza,A.corniculatum and Avicennia marin were planted,and the seed planting quantity of the paulownia tree and the Avicennia marina was about 2.32-3.42 times;the high tide belt selects B.gymnorrhiza,K.obovata and R.stylosa,and the number of hypocotyls of K.obovata and R.Stylosa were about 1.42-1.73 times of the number of B.gymnorrhiza.The afforestation area is treated with a sapling of sea mulberry,so that the degree of canopy closure is less than 0.6. |
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