The Sub-fossils Of Leaf Fragments In Sediments:the Record Of Mangrove Development In The Yingluo Bay,Guangxi Over The Last 150 Years

Located in the intertidal zone of the tropical and subtropical coasts,mangrove forests are an important ecosystem in the global carbon cycle and serve as a protector of local seashores.Under the double impacts of climate change,especially sea-level rise,and human activity,mangrove forests around the world have faced degradation,against which the reconstruction of the historical development of mangrove forests using an effective indicator has been regarded as a necessary strategy for designing a predictable model.As the primary product of mangrove forest,it is reasonable that the content of leaf fragments?CLF?buried in sediments in the form of sub-fossils has the potential to have the same indicative function for the development of mangrove forests as that of mangrove pollen,which has been widely used as an indicator of mangrove development.In this study,the leaf fragments in two sediment cores?YLW02 and YLW03?drilled in the Yingluo Bay in Guangxi province were picked and weighted,and their percentage weight was used as an indicator of mangrove development after examination of parallelism and a statistical correlation of the leaf fragments with the concentration of mangrove pollen.The results clearly show that the vertical distribution of the CLF for the core taken from the inner mangrove forests?YLW03?only parallels that of the local mangrove species?R.stylosa?with a significantly positive correlation?r=0.56,P=0.05?,while the vertical distribution of the CLF for the core near the trunk of tidal creeks of the bay?YLW02?parallels the total concentration of mangrove pollen with a more positive correlation than that of YLW03?r=0.85,P=0.01?,indicating that the trunk of tidal creeks must have been the site where mangrove production from the overall forest gathered rather than from local production.The variations in the CLF of both cores indicate that overall the mangrove forests in the Yingluo Bay have increasingly flourished over the last150years except for the interval of 1940¹960AD in response to an increase in air temperature and decrease in rainfall,which would have resulted in an increase in seawater salinity;while the coupled extreme increases in air temperature and in rainfall in summer,which would have resulted in extreme decreases in seawater salinity,would be responsible for the relative degradation of mangrove forests in the interval of 1940¹950AD.Stable isotope methods provide a means to characterize discrimination of tree height and its relationship to nutrient dynamics for a given mangrove wetland.In this study,we present for the first time the tree-height-related discrimination of leaf?¹³C values of four mature pioneer mangrove species?Aegiceras corniculatum,Kandelia candel,Rhizophora stylosa,and Bruguiera gymnorrhiza?from the intertidal zone of Guangxi,southwestern China,and link this isotopic pattern to carboxylation efficiency via leaf nitrogen concentration.As previously studied for a single mangrove species,Aegiceras corniculatum,which was sampled with detailed tree-height gradient,the increase in specific leaf?¹³C value parallels to decrease in its tree height.In addition,the mean leaf?¹³C values of the four species correspond to decreases in mean tree heights.The increases in mean and specific leaf?¹³C values of the four species correlate to the increases in mean and specific leaf nitrogen concentrations of these species,respectively.These correlations imply carboxylation efficiency via leaf nitrogen concentration is one of predominant reasons for tree-height-related discrimination of leaf?¹³C values of pioneer mangrove species from the intertidal zone of Guangxi,SW China.Determined the?¹³C values of sub-fossils of leaf fragments in two cores in Yingluo Bay,Guangxi,based on the tree-height-related pattern of leaf?¹³C values and the range of?¹³C values of four mangrove leaves,reconstructed the succession process of mangrove community in Yingluo Bay over the last 150 years.