Mapping area changes of mangroves using RS and GIS (Case study: mangroves of Hormozgan province)

Different coastal ecosystems, especially mangroves, are exposed permanently and almost simultaneously to multiple environmental stresses and disturbances (geological, physical, chemical and biological) which are varied in terms of their characteristics over time and space. The direct result of these stresses and disturbances is the reduction in area and health of mangroves and the subsequent exacerbation of global warming and other consequences of climate change, reduced coastal water quality, loss of biodiversity and degradation of coastal habitats. Considering the adverse consequences arising from the reduction in mangrove forest area, prioritizing management actions and providing appropriate infrastructure is essential to prevent a decrease in the extent of these ecosystems. Achieving this goal depends on having accurate and sufficient information and knowledge about the process of changes in the extent of mangrove forest habitats over time which can play an important role in the efficiency and success of rehabilitation programmes and the development of these ecosystems. Hence, the aim of this study was to investigate the long-term changes occurred in areas of mangrove forest of Hormozgan Province.
In this study, Landsat images from 1986, 2000, and 2016 were used to study changes in the extent of mangroves over time. The necessary pre-processing was doneconducted on images using Idrisi software. After extraction of the mangrove vegetation maps in the aforementioned years, changes in mangroves of the Khamir, Qeshm, Tiab, Sirik and Jask habitats were analyzed in ArcGIS software and the average values of areas of mangroves were calculated for two periods, namely pre- and post- 2000, and during a 30-year period.
The accuracy assessment of the classified images showed that the overall accuracy was equal to 92%. The results also showed that among the zones investigated (Khamir, Ghesh, Tiab, Sirik and Jask), Khamir habitat zone had the highest average annual value of area reduction (equal to -4.64 h/yr) and Sirik habitat zone had the highest average annual value of area expansion (equal to 23.5 h/yr) during the 30-year period. The results also showed that during period of 2000-2016, Sirik and Qeshm zones had the highest average value of expansion and reduction in area, respectively (equal to 32.47 and -21.38 h/yr, respectively). Analysis of mangrove area changes in different habitats showed that, despite the reduction in the extent of all mangroves in the period after 2000, the mangroves of Sirik habitat had significant increase in area in this period compared to the period before 2000 which was due to forestation with Avicenna and Rhizophora in the mud flats of this habitat in recent years.
The results of the study of changes in the areas of mangroves of Hormozgan Province showed that the trend of change the in area covered by the mangroves of Hormozgan Province over a 30-year period has been increasing from the coasts of the Persian Gulf to the Oman Sea; this indicates the impact of various environmental factors affecting the changes in the extent of the mangroves of the province. Finally it can be said that the results of this study, by providing up-to-date and accurate information about areas changes of mangrove habitat in Hormozgan Province, can help significantly to the planning of effective actions to protection and restoration of these habitats.