Mangrove floristics, forest structure and mapping of Neil Island (Andaman and Nicobar Islands, India) with emphasis on the diversity of Rhizophora species and the significance of small island mangroves

Abstract

Mangroves in small islands are critical resources for the stability of the island and the livelihood of local coastal communities. However, scientific inventories of mangroves in small islands are rare due to their limited distribution. Considering this, the present study was conducted during 2014–2015 to determine the species composition, biomass, vegetative carbon stock and spatial distribution of mangroves at Neil Island, one of the small islands of the Andaman and Nicobar Islands, India. The spatial locations of different species of mangroves identified from our extensive field surveys were compared with high-resolution images, and the coverage of mangroves (genus level distribution) was interpreted entirely based on the elements of tone and texture. The results show that the mangroves of Neil Island consist of 17 true mangrove species belonging to 12 genera in eight families. The co-existence of all three Indo-West Pacific mangrove species of the genus Rhizophora (Rhizophora apiculata, Rhizophora mucronata and Rhizophora stylosa), and their hybrids (Rhizophora × annamalayana, Rhizophora × lamarckii and Rhizophora × mohanii) shows that this island is a unique place for studying the hybridization and speciation of Rhizophora, a worldwide dominant mangrove genus. Rhizophora mucronata was found to be the dominant species in terms of density, basal area and biomass estimated from forest structural assessments using the quadrat method. The mean density and basal area were 1162 trees ha−1 and 28 m2 ha−1, respectively. The mean above-ground biomass was 271 Mg ha−1, while the mean below-ground biomass was 104 Mg ha−1. The total mean biomass of Neil Island mangroves was 375 Mg ha−1 and its corresponding vegetative carbon stock was 171 Mg C ha−1. The present study provides a practical approach to species-level mapping and assessment to gain site-specific knowledge of the mangroves of Neil Island. Since mangroves are vital for small islands such as Neil Island to cope with rising sea level and increases in natural calamities, the baseline scientific information provided by this study will be beneficial for Integrated Island Management.