― Do you know what mangroves are?

― Of course. You are talking about the trees that look like an octopus, right?

Nowadays, the term ‘mangrove,’ which indicates ecosystems observed only in tropical-subtropical coastal areas, has become a widely known word. ‘Mangrove’ is often used in environment-related reports, and the spreading recognition of the word ‘mangrove’ is likely related to the recent serious decrease and degradation of this type of ecosystem. Actually, the area of mangrove ecosystems has decreased worldwide. In the Indo-Pacific region, where the most diverse mangrove species exist, the current mangrove area has disappeared at a rate of approximately one to two percent per year for the past 30 years. This indicates that mangrove ecosystems in this region are likely to disappear within a hundred years, bringing us awareness of the precious value of this fascinating environment.

When you make your way into a mangrove forest, you will surely be able to appreciate encounters with living things. On the ground at low tide, mudskippers hop, colorful crabs dance as they wave their claws, and waterfowl peck at them. If you are lucky, you may encounter a simultaneous spawning event of sea snails. At high tide, fish swim swiftly through intertwining tree roots in the submerged forest. When you look up, you might find the brilliant blue of a kingfisher as it flies from one branch to another. You might also find monkeys watching you. Large mammals such as deer and tigers are also inhabitants of mangrove forests. Human beings are no exception. Local people have been receiving blessings from mangroves for countless years. They have the knowledge and philosophy to coexist with mangroves, which have been handed down from generation to generation.

What is supplying energy to all these living things, including human beings, is the mangrove plants’ production of organic matter. Mangrove plants have accomplished adaptation to intertidal environments, where it is physically and chemically harsh for plants to grow. They needed to develop a system to grow in environments characterized by high salinity, anaerobiosis and tidal turbulence. Mangrove forests have another significant role to play as a place for absorbing carbon dioxide. This astonishing tree has continuously intrigued mangrove researchers. Disappearance of mangrove ecosystems means the disappearance of all living beings surrounding the mangroves. What would happen in a world without mangroves? Now is the time we should seriously think about the future of this unique and scarce ecosystem.

This special issue is compiled with reviews, one miscellaneous note and several original papers on the latest findings on mangrove ecosystems. Topics range from scientific issues to practical case studies on forest management and reforestation. Readers can get knowledge on mangrove forest distribution patterns, physical processes supporting mangrove environments, nitrogen cycling in mangrove ecosystems and functions of benthic animals in mangrove ecosystems from the review articles. The miscellaneous note introduces findings from long-term mangrove restoration activities. The original papers contain original data on functions of nitrogen-associated bacteria in mangrove soils, productive dynamics of mangrove forests, human studies associating mangrove ecosystems and buffering functions of mangrove forests against high waves. We hope this book deepens the readers’ understanding on mangroves from various viewpoints and stimulates further discussion on mangrove ecosystems.

Tomomi INOUE and Kiyoshi FUJIMOTO

This paper describes, as a background, events that led to the publication of the 1997 World Mangrove Atlas and the 2010 World Atlas of Mangroves. The 2010 Atlas produced updated country-by-country man grove distribution maps using remote sensing techniques and assessed the status of mangroves in each country. The current area occupied by mangroves has been estimated to be 152,360 km² and about 0.7% of that is lost annually. It is hoped that the 2010 Atlas will serve as a baseline for future gain/loss assessment. Largely based on the findings of the 2010 Atlas, this paper highlights the world mangrove distribution, value of mangroves, impacts and threats to mangrove ecosystems, management and conservation of mangroves, restoration and afforestation efforts, and international cooperation for preservation of mangrove ecosystems. Although the rate of mangrove deforestation has declined in recent years, much needs to be done towards balancing man grove utilization and conservation and achieving the goals of sustainable management.

There is little published research on the paleobotany of mangroves. According to Rollet (1981), from 1600-1975 the number of papers on the topic was 126, which was 2% of the 5,653 papers on mangrove research published in that period. Research on mangrove paleobotany since 1976 has also been minimal. We collected and reviewed papers on the paleobotany of mangroves, visited museums to obtain information on fossils, mainly pollen, and analyzed the collected data. The results are described below.

The first mangroves appeared during the Upper Cretaceous era, 100-65 million years ago (mya). The evolution of mangroves is thought to have been strongly related to sea-level change (transgression-regression cycles) in geological times. Some woody angiosperms growing in freshwater swamp forests succeeded at growing in tidal areas and acquired salinity tolerance when the vast extent of those areas underwent marine transgression. That event happened not only once but likely several times after the Upper Cretaceous, and each time the biodiversity of mangroves became richer. The origin of present mangroves can be divided into two groups, the Old and New (Worlds, and we predicted that the two mangrove groups could also be found in the period of the Upper Cretaceous. There were several questions we could not resolve, for example, how the expansion of their distribution proceeded. Mangrove propagules are spread by sea currents and they expand their habitats. The world’s seas were connected during the time from the Upper Cretaceous, when the first mangroves appeared, to the Miocene, when the Tethys Sea closed. Nevertheless, we assume that for over 80 million years, the Old and New Worlds did not mix. Genetic research on the migration and evolution of mangroves will be essential for complementing fossil studies.

Various physical processes support the mangrove environment. In this paper, a synopsis of hydraulic systems such as tidal flow, sea waves and groundwater in mangrove areas is introduced. Further, in order to preserve the natural environment of mangrove areas, it is proposed to connect quantitatively these physical processes and the mangrove ecosystem in execution of interdisciplinary studies.

Recent advances necessitate a critical reappraisal of nitrogen cycling in mangrove ecosystems. Nitrogen concentrations in soil, water and vegetation are low, but N-use and recycling efficiencies are high and N residence times are short, driven by rapid forest production and the need to conserve N. Most mangrove- associated fauna preferentially assimilate benthic algae, phytoplankton, algal detritus or complex mixtures of these forms to meet N demand. Consumer preferences, locations and types of habitat, and the relative availability of other foods determine the trophic significance of mangrove litter. A nitrogen mass balance model of the world’s mangrove forests indicates that: (1) 2687 Gg N yr^-1 is required to sustain global man grove net primary production (NPP); (2) N burial is ≈ 25% of total N input; (3) N2 fixation is ≤ 5% of total N input; (4) production in roots and litter account for 40% and 50% of mangrove NPP, respectively; (5) tidal export equates to ≈ 55% of N input; and (6) denitrification and N2O effluxes account for < 10% of total N losses. Despite proportionately large tidal losses, the global flux of nitrogen in mangrove forests is roughly in balance due to multiple adaptations and strategies to efficiently use and retain nitrogen.

This paper reviews the current knowledge of macrobenthic animals in mangrove swamps, focusing on their spatial organization and ecological functions. Spatial patterns are described separately for arboreal animals and animals living on the bottom, in dead wood, and in rivulets. Some sesarmid crabs and snails graze on higher proportions of mangrove litter in various parts of the world. Positive influences of macrobenthic animals on mangrove plants have been identified: sesarmid crabs and fiddler crabs are beneficial to the growth of mangrove trees, and some fouling and boring animals enhance the growth of mangrove prop roots.

The Al Gurm Research Centre started to develop mangrove planting techniques in the Middle East in 1981. Since 1994, it has been conducting reforestation activities in Southeast Asian countries under the name of Action for Mangrove Reforestation (ACTMANG) as a non-governmental organization. So far, mangrove ecosystems have been restored on a few thousand hectares in Vietnam and Myanmar.

I have learned many things from participating in these mangrove restoration activities in many areas, including arid coasts, over the past 30 years, and summarize my findings through those activities as follows:

1) There are two classes of factors in the deaths of young trees: physiological factors, such as lower air temperature and high salinity of seawater; and physical factors, such as adhesion of barnacles or algae, waves and tidal currents, browsing, fishing activities and oil spills;

2) There are two types of causes of forest degradation: natural factors, such as coastal erosion, typhoons and floods in arid regions; and human activity factors, such as shrimp pond construction, over-browsing by camels, causeway construction and dumping of dredged sediment;

3) Mangrove ecosystem services for local people include typhoon disaster reduction, pasturage for livestock, provision of building materials, firewood, green manure, edible fruit, fishing grounds, and resources for beekeeping and tourism;

4) Cases of our mangrove reforestation have included establishment of greenbelts serving as natural breakwaters against typhoons, restoration of abandoned shrimp ponds or paddy fields, reintroduction of extinct mangrove species, planting artificial habitat and community forestry.

Rapid changes, such as urbanization and artificial field expansion, are occurring in mangrove regions worldwide. To assess the characteristics of nitrogen-related processes in mangrove forests in the Mekong delta in relation to conditions in the surrounding environment, we measured water quality: inorganic nitrogen, inorganic phosphate and heavy metals; and nitrogen-associated bacterial activities: nitrogen-fixing activity and nitrogen-consuming activity. The water quality results suggest that key changes in Mekong delta man groves include a high input of ammonium, phosphorus and heavy metals from cities and aquaculture farms upstream. The current value of mangrove soil bacterial nitrogen fixation in the Mekong delta, 0.87-24.57 mg N m^-2 d^-1, has dropped to within the previously reported order of other mangrove regions. We determined that the addition of ammonium had reduced soil nitrogen fixation to less than half that of the control treatment. The addition of aquaculture pond water also suppressed soil nitrogen fixation. We postulated that if drainage from the artificial systems upstream continuously supplies the mangroves downstream, part of the mangrove soil nitrogen will shift from atmospherically-derived nitrogen to anthropogenically-derived nitrogen. If the nitrogen input from the upstream region exceeds the nitrogen-consuming capacity of the mangrove forest, effects on plant growth due to excessive ammonium and eutrophication of the surrounding ocean will be a concern.

The aboveground dynamics, biomass, productivity and carbon storage rate of major mangrove communities on Pohnpei Island, Federated States of Micronesia, were estimated using census data obtained from two 1-hectare permanent plots during about two decades. The aboveground biomass increased from 526 t ha^-1 in 1994 to 572 t ha^-1 in 2010 for the plot situated in a coral reef-type habitat, referred to as PC1, and from 637 t ha^-1 in 1994 to 744 t ha^-1 in 2011 for the plot situated in an estuary-type habitat, referred to as PE1. Both were higher than any other estimates for mangrove forests throughout the world, though the tree density decreased from 1,558 trees ha^-1 to 1,074 trees ha^-1 in PC1 and from 651 trees ha^-1 to 473 trees ha^-1 in PE1. The ratio of trees cut by local residents to all deceased trees was higher in PC1 (35%) than PE1 (15%) because of different accessibility. The potential rates of aboveground carbon storage were estimated at 1.66 t C ha^-1 yr^-1 for PC1 and 2.79 t C ha^-1 yr^-1 for PE1. The difference was considered to have resulted from different stages of succession and site environments, such as soil water EC, ground elevation and frequency of submergence by tides.

The authors analyzed the development of the size structure of some mangrove stands in Southeast Asia and how it changed with growth, productivity and growth performance, as a source of diverse benefits to sustain the livelihood of local residents. The similarity of the development pattern of natural Rhizophora- dominant stands to that of plantations suggested that the former were established as nearly evenly aged pure stands. Stand productivity varied according to dominant species, such that the pioneer Avicennia stand examined in Ranong was smaller than the late successional Rhizophora stand, reflecting low tree density with a smaller basal area. Regional or environmental differences were also apparent, e.g., the annual biomass growth rates in Kantang and some regions on the Thai Gulf side exceeded 15 Mg ha^-1 yr^-1 while those of Ranong and Merbok ranged between 10 and 15 Mg ha^-1 yr^-1, and stands in Bali showed smaller productivity, around 5 Mg ha^-1 yr^-1. The lack of a large river system supplying fertility from upper tributaries was the most likely factor affecting Bali’s low productivity. With biomass increase, forests release much plant mass as dead trees, and this loss sometimes reaches the same level of net gain in mangroves. The average tree weight of natural Rhizophora-dominant stands at certain stand densities showed lower productivity than those of plantations and this tendency was common over wide regions of Thailand, Malaysia and Indonesia.

An ethnobotanical study on the plant resources of mangrove forests was conducted in the village of Ashe Mayan in the Ayeyarwady Delta, Myanmar. Among 82 species of mangrove flora, 54 species including 19 mangrove plants, 20 mangrove associates and 15 non-mangrove plants were inventoried as useful plants for local subsistence. The cumulative number of plant resources stood at 119 species, consisting of 28 species applied for medical purposes or as poison, 22 species used in crafts, 19 edible species, fourteen species providing construction materials, ten species used for tying, seven species used for fuel, one species used in roofs and walls and 18 species with other purposes. Examining the meteorological and geographical environment, plant features and requisites for life, we assessed that utilization of plant resources of mangrove forests is harmonized with the endemic life culture.

Coastal zones of the arid tropics, where arid land is juxtaposed with mangrove and coral reef areas, show a contrast in extremes between the most unproductive land ecosystem and the most productive sea ecosystem. I analyzed resource patch accessibility and availability in arid tropical mangrove ecosystems, focusing on human-camel relationships among the Beja on the Sudanese Red Sea coast. Firstly, I made schematic overviews of the physical environment, biological environment and livestock grazing zones in a three-dimensional figure. Secondly, I elucidated the role of dromedaries (one-humped camels), outlining resource patch accessibility and availability by constructing a relationship web among human beings, livestock, coral reefs (the physical environment) and resource patches (the biological environment). As a result, I could show that re source patch accessibility and availability are determined by camels’ intervention, so that resource over exploitation in coastal zones of the arid tropics has been limited consequently. Therefore, in terms of sustainability of human resource use, the relationship between humans and dromedaries is the key interspecific relationship in arid tropical mangrove ecosystems.

In this paper, some findings on tsunami damage and the protective role of mangroves and other coastal forests are presented based on case studies of two tsunamis: the Great East Japan tsunami at Sendai Airport, Sendai Bay and Yamashita of the Sendai Plain in Miyagi Prefecture, Japan; and the Indian Ocean tsunami at Banda Aceh (Sumatra, Indonesia), and at Khao Lak and Nam Kem (Southern Thailand). From our studies at Banda Aceh in Indonesia, we found that mangrove trees with ≥10 cm dbh are not damaged by tsunamis with an inundation depth of ~3 m. More than 50% of mangrove trees with ≥10 and ≥30 cm dbh would survive tsunamis with inundation depths of 4 and 6 m, respectively. Mangroves and other coastal forests do serve to reduce the impacts of tsunamis by trapping debris and mitigating flow force effects. Mangrove trees growing on sandbars at Nam Kem in Thailand were an exception as they suffered severe damage during the Indian Ocean tsunami at 3-4 m inundation depth, suggesting that soil types and possibly root anchorage are parameters to be considered in future studies. Other factors such as land subsidence, raised ground water level and liquefaction, like that which occurred in the Sendai Plain of Miyagi Prefecture in Japan during the East Japan earthquake, are noteworthy as they weaken the protective function of coastal forests against tsunami damage. The destruction of houses and other property by drift trees with projecting roots is another interesting observation. It appears that mangroves and other coastal forests do contribute to protection against tsunami damage but if they give way, they add to the devastation. The relationship between tsunami flow forces and the protective role of coastal vegetation is complex and warrants further investigation.