Status and distribution of mangrove forests of the world using earth observation satellite data
Year Published:
Study Number:
29
Author:

C. Giri, E. Ochieng, L. L. Tieszen, Z. Zhu, A. Singh, T. Loveland, J. Masek & N. Duke

Abstract:

Aim: Our scientific understanding of the extent and distribution of mangrove forests of the world is inadequate. The available global mangrove databases, compiled using disparate geospatial data sources and national statistics, need to be improved. Here, we mapped the status and distributions of global mangroves using recently available Global Land Survey (GLS) data and the Landsat archive.

Methods: We interpreted approximately 1000 Landsat scenes using hybrid supervised and unsupervised digital image classification techniques. Each image was normalized for variation in solar angle and earth–sun distance by converting the digital number values to the top-of-the-atmosphere reflectance. Ground truth dataand existing maps and databases were used to select training samples and also for iterative labelling. Results were validated using existing GIS data and the published literature to map ‘true mangroves’.

Results: The total area of mangroves in the year 2000 was 137,760 km2 in 118 countries and territories in the tropical and subtropical regions of the world. Approximately 75% of world’s mangroves are found in just 15 countries, and only 6.9% are protected under the existing protected areas network (IUCN I-IV). Our study confirms earlier findings that the biogeographic distribution of mangroves is generally confined to the tropical and subtropical regions and the largest percentage of mangroves is found between 5° N and 5° S latitude.

Main conclusions: We report that the remaining area of mangrove forest in the world is less than previously thought. Our estimate is 12.3% smaller than the most recent estimate by the Food and Agriculture Organization (FAO) of the United Nations. We present the most comprehensive, globally consistent and highest resolution (30 m) global mangrove database ever created. We developed and used better mapping techniques and data sources and mapped mangroves with better spatial and thematic details than previous studies.

Main Results and Conclusions:
  • A comparison is made between this study’s data and that of other studies preceding it. The study showed that, in 2000, there was less mangrove coverage than previously thought: “Based on the first full assessment of all mangrove forests of the world (Fig. 1), we estimated that the total mangrove forest area of the world in 2000 was 137,760 km2 in 118 countries and territories. The total mangrove area accounts for 0.7% of total tropical forests of the world. This aerial estimate does not provide information about the quality of the forests… The largest extent of mangroves is found in Asia (42%) followed by Africa (20%), North and Central America (15%), Oceania (12%) and South America (11%). Approximately 75% of mangroves are concentrated in just 15 countries (Table 1)…Our estimate is approximately 12% smaller than the most recent estimate by the FAO (Fig. 2)” (3).
  • Patches of forest <900-2700 m2 were not visible to the satellite imagery used to collect these data, but it is unlikely that using high resolution imagery would cause a significant change in the total, global area of mangrove habitat (Wilkie and Fortune, 2003) (4).
  • Trends in mangrove habitat are summarized: “Our study confirms earlier findings that mangroves are generally confined to the tropical and subtropical regions of the world, with a few exceptions. Mangroves extend to 31°22′ N in Japan and 32°20′ N in Bermuda, and to 38°45′ S in Australia, 38°59′ S New Zealand and 32°59′ S on the eastern coast of South Africa (Spalding et al., 1997)…World-wide, species diversity, height and biomass are the lowest in the northern and southern extremes and increase toward the tropics. The best developed mangroves can be found in the Sundarbans, Mekong Delta, Amazon, Madagascar, Papua New Guinea and Southeast Asia”(4).
Works Cited:

Spalding, M.D., Blasco, F. & Field, C.D. (eds) (1997)World mangrove atlas. The International Society for Mangrove Ecosystems, Okinawa.

Wilkie, M.L. & Fortune, S. (2003) Status and trends of mangrove area worldwide. Forest Resources Assessment Working Paper No. 63. Food and Agriculture Organization of the United Nations, Rome.