oa Biomass distribution and soil characteristics along the distance from the coast in a natural mangrove forest in Qatar

Mangrove is a woody plant which grows in intertidal zones. Mangrove forests are mainly distributed in subtropical and tropical regions, and are valuable for ecosystems and for society. Even in dry regions such as Qatar, mangrove forests provide ecosystem services despite their small area and low productivity compared with tropical mangroves. Carbon sequestration by mangroves is one of the main ecosystem services mitigating climate change, as mangrove forests are carbon-rich ecosystems. In mangrove forests, there is a natural gradient in soil environments between land and sea. Soil salinity and water availability are the major factors influencing mangrove productivity. The objectives of this study were (1) to investigate the change in biomass of Avicennia marina, the only mangrove species in Qatar, along the distance from coast and the relationship between biomass and soil characteristics, and (2) to estimate carbon storage of biomass in a natural mangrove forest in Qatar. Three plots were established in a natural mangrove forest of A. marina in Al-Thakira, Qatar (25°42»15.9»N 51°32»18.4»E), at a distance of 0 m (D0), 50 m (D50), and 100 m (D100) from the coast. Plot size was 2 ×  2 m2, 3 ×  3 m2 and 4 ×  4 m2 at D0, D50, and D100, respectively. Plant abundance was determined by counting the number of individual seedlings ( ≤ 1.3 m high) and trees (>1.3 m high) per plot. Diameter at breast height (DBH) was measured on trees in each plot, and above-ground biomass (AGB) and below-ground biomass (BGB) of trees were estimated using allometric equations for A. marina. Carbon storage in biomass was calculated by carbon fraction of 45.1%. Soil samples at 0-10 cm depth were collected at three random points per plot. Soil samples were air-dried and sieved through a 2 mm mesh screen, and then pH, salinity, water content and nitrogen (N) concentration of each soil sample were measured. Differences in biomass of trees, pH, salinity, water content and N concentration of soil at each distance were analyzed using t-test (SAS 9.4 software). Seedling abundance (no. m-2) was 2.4 ± 0.1 at D0, 7.9 ± 0.8 at D50, and 1.9 ± 0.2 at D100, while tree abundance was 0.9 ± 0.1 at D50 and 1.2 ± 0.1 at D100. There were no trees at D0, so comparisons with this site were excluded. AGB was significantly higher at D100 (41.4 ± 1.9 Mg ha-1) than at D50 (7.3 ± 0.9 Mg ha-1). BGB was higher at D100 (44.9 ± 0.6 Mg ha-1) than at D50 (19.4 ± 2.3 Mg ha-1), but there was no significant difference between two distances. Salinity, water content, and N concentration of soil were 125.1%, 196.3%, and 114.5% higher, respectively, at D100 than at D50 (all differences were significant). Soil pH was significantly 3.1% lower at D100 than at D50. It was reported that growth and biomass of mangroves increased slightly and then decreased continuously along the salinity gradient. However, salinity at the study site was low compared with that in other mangrove forests and might be within the range that biomass increases with salinity. A. marina required a large amount of water and nutrients in high salinity condition (Naidoo, 2009). High water content and N concentration of soil at D100 could meet the requirement for water and nutrient in high salinity condition, and thereby result in the biomass increment. In this study, carbon storage (Mg C ha-1) was 7.3 ± 1.1 for AGB, 9.7 ± 1.1 for BGB, and 18.1 ± 2.4 for total biomass of A. marina. These values are lower than those reported for A. marina in temperate regions (AGB: 57.7 Mg C ha-1, BGB 69.8 Mg C ha-1), subtropical regions (AGB: 49.6-73.1 Mg ha-1, BGB: 49.2-56.8 Mg ha-1), and even dry regions (20.7-66.6 Mg C ha-1). To conclude, biomass of A. marina increased as the distance from the coast and was affected along the gradient of soil characteristics. A better understanding of mangrove biomass distribution between land and sea will contribute to estimate biomass and carbon storage in intertidal zones. * This study was supported by Korea Ministry of Environment (2014001810002).