Spatio-temporal variation of water salinity in mangroves revealed by continuous monitoring and its relationship to floristic diversity.

Salinity is among the most critical factors limiting the growth and species distribution of coastal plants. Water salinity in estuarine ecosystems varies temporally and spatially, but the variation patterns across different time scales and salinity fluctuation have rarely been quantified. The effects of salinity on floristic diversity in mangroves are not fully understood due to the temporal and spatial heterogeneity of salinity. In this study, we monitored water salinity at an interval of 10-min over one year in three mangrove catchment areas representing the outer part, middle part, and inner part respectively of Dongzhai Bay, Hainan, China. The number of mangrove community types and dominant mangrove species of the three catchment areas were also investigated. We found that the diurnal variation and dry-season intra-month variation in water salinity were driven by tidal cycles. The seasonal variation in water salinity was mainly driven by rainfall with higher salinity occurring in the dry season and lower salinity occurring in the wet season. Spatially, water salinity was highest at the outer part, intermediate at the middle part, and lowest at the inner part of the bay. The intra-month and annual fluctuations of water salinity were highest at the middle part and lowest at the outer part of the bay. The number of mangrove community types and dominant species were lowest at the outer part, intermediate at the middle part, and highest at the inner part of the bay. These results suggest that the temporal variation of water salinity in mangroves is driven by different factors at different time scales and therefore it is necessary to measure water salinity at different time scales to get a complete picture of the saline environment that mangroves experience. Spatially, lower salinity levels benefit mangrove species richness within a bay landscape, however, further research is needed to distinguish the effects of salinity fluctuation and salinity level in affecting mangrove species richness.

Increasing carbon and nutrient burial rates in mangroves coincided with coastal aquaculture development and water eutrophication in NE Hainan, China.

Mangroves sequester and store large area-specific quantities of blue carbon (C) and essential nutrients such as nitrogen (N) and phosphorous (P). Quantifying C and nutrient burial rates in mangroves across a centennial time span and relating these rates to mangrove habitat is fundamental for elucidating the role of mangroves in carbon and nutrient budgets and their responses to environmental changes. However, relevant data are very limited in China. In this study, we used the radionuclides (210Pb and 137Cs) to determine chronologies and C, N and P burial rates in two mangrove forests located at different geomorphologic settings in NE Hainan Island, China. We found that the temporal patterns of C, N and P burial rates since 1900 fitted a quadratic function with a notable increase after 1960s in both mangroves, which coincided with the rapid development of coastal aquaculture since 1960s in NE Hainan and the subsequent coastal water eutrophication in this area. Sediment accretion rate (SAR) and mass accumulation rate (MAR) stayed relatively steady in the open-coastal mangroves, while they increased exponentially in the estuarine mangroves since 1900. The estuarine mangroves had significantly higher SAR and C, N and P burial rates than the open-coastal mangroves. C, N and P burial rates averaged at 141.52 g m-2 a-1, 6.27 g m-2 a-1 and 1.14 g m-2 a-1, respectively in the estuarine core, and these rates averaged at 61.71 g m-2 a-1, 3.71 g m-2 a-1 and 0.43 g m-2 a-1, respectively in the open-coastal core. The results suggest that estuarine mangroves may be more capable of surviving accelerating sea level rise under climate change and play a greater role in C accumulation and nutrient filtering under anthropogenic nutrient enrichment than marine-dominated mangroves. Blue C burial may be enhanced by coastal water eutrophication, but such a relationship needs to be tested in further studies.

Niches of nine mangrove species in a Sonneratia apetala-colonized area of Dongzhai Harbor, Hainan Island, China.

The distribution of mangroves is influenced by the environment. We aimed to understand the ecological adaptability of various mangrove species within the range of the exotic species, Sonneratia apetala Buch.-Ham., in Dongzhai Harbor, Hainan Island, China. We used three niche breadth indexes (Simpson, Levins, and Shannon-Weiner) and two niche overlap indexes (Pianka and Levins) to quantitatively determine the niche characteristics of nine mangrove species. The results showed that the order of the niche breadth values of mangrove species was as follows: Aegiceras corniculatum (Linn.) Blanco > Kandelia obovata Sheue et al. > Bruguiera gymnorrhiza (L.) Poir. > Avicennia marina (Forsk.) Vierh. Hailanci > S. apetala > S. caseolaris (L.) Engl. > Rhizophora stylosa Griff > Ceriops tagal (Perr.) C. B. Rob. > B. sexangula (Lour.) Poir. Pearson correlation analysis revealed that the niche breadth of each population was significantly correlated with the importance value of the population in the whole sample (R1 = R2 = 0.771, R3 = 0.644, p < .05). The nine mangrove species were divided into three groups by Bray-Curtis cluster analysis; the groups were similar to the distribution of mangrove species in the natural state as determined by tide level. Niche similarity analysis showed that the niche similarity of most mangroves ranged between 0.5 and 0.8 and that the species pairs A. corniculatum-B. gymnorrhiza, A. corniculatum-Avicennia marina, and K. obovata-S. caseolaris were characterized by large niche similarity ratios. Although it had a moderate niche breadth, S. apetala had a relatively broad niche overlap with mangroves in the mid- and low-tide zones (S. caseolaris, A. corniculatum, K. obovata, and Avicennia marina), a moderate overlap with B. gymnorrhiza and R. stylosa, only a slight overlap with C. tagal, and no overlap with B. sexangular. There was no obvious linear relationship between niche width and niche overlap of mangroves. Due to its inefficiency in utilizing certain resources and relatively high degree of resource selection, it seems likely that S. apetala will not pose a threat to the survival of native plants, let alone completely replace native species.

Genetic diversity and population structure of Prunus mira (Koehne) from the Tibet plateau in China and recommended conservation strategies.

Prunus mira Koehne, an important economic fruit crop with high breeding and medicinal values, and an ancestral species of many cultivated peach species, has recently been declared an endangered species. However, basic information about genetic diversity, population structure, and morphological variation is still limited for this species. In this study, we sampled 420 P. mira individuals from 21 wild populations in the Tibet plateau to conduct a comprehensive analysis of genetic and morphological characteristics. The results of molecular analyses based on simple sequence repeat (SSR) markers indicated moderate genetic diversity and inbreeding (A = 3.8, Ae = 2.5, He = 0.52, Ho = 0.44, I = 0.95, FIS = 0.17) within P. mira populations. STRUCTURE, GENELAND, and phylogenetic analyses assigned the 21 populations to three genetic clusters that were moderately correlated with geographic altitudes, and this may have resulted from significantly different climatic and environmental factors at different altitudinal ranges. Significant isolation-by-distance was detected across the entire distribution of P. mira populations, but geographic altitude might have more significant effects on genetic structure than geographic distance in partial small-scale areas. Furthermore, clear genetic structure, high genetic differentiation, and restricted gene flow were detected between pairwise populations from different geographic groups, indicating that geographic barriers and genetic drift have significant effects on P. mira populations. Analyses of molecular variance based on the SSR markers indicated high variation (83.7% and 81.7%), whereas morphological analyses revealed low variation (1.30%-36.17%) within the populations. Large and heavy fruits were better adapted than light fruits and nutlets to poor climate and environmental conditions at high altitudes. Based on the results of molecular and morphological analyses, we classified the area into three conservation units and proposed several conservation strategies for wild P. mira populations in the Tibet plateau.