Climate change and the future of mangroves

Table of Contents

Abstract

1 Introduction
1.1 Mangrove forests
1.2 Climate change in coastal zones and marine ecosystem

2 Impacts and Responses on Global Change
2.1 Relative sea-level change
2.1.1 Previous mangrove adaptations
2.1.2 General and current aspects of adaptive response
2.2 Biological responses to global warming
2.3 Storm effects

3 Conservation and Management
3.1 Aim of management
3.2 Advisories for protection
3.3 Restoration

4 Outlook

References

Abstract

Mangrove forests are severely endangered by human impacts all over the world. Extended destructions and degradations are results that can be observed. Besides overexploitation and destructive use, the global change is an increasing problem for mangrove systems. This includes phenomena like sea level rise, global warming and an increase in storms and hurricanes. Although previous developments show that mangroves always had to deal with changes in climate and sea level fluctuation and that they are able to adapt with biological properties or changes in distribution, the human accelerated climate change seem to be to fast for a natural response.

Therefore a management plan has to be implemented to give mangrove forests the chance to migrate landwards or to compensate changes in composition or destructive events. Management plans should be concentrated on protection and restoration and the inclusion of connected systems. Above that, a sustainable use should be a major aspect for management to maintain or reach a resilient system. Furthermore the participation of local people is very important in this context especially if they depend economically on mangroves (SAENGER 2002). In the end decisions have to be made which sites are worth to protect and which ones are suitable to be restored because of their chances to survive the global change. After McLeod and Salm (2006) questions like if landward migration is possible or if peat accretion keep pace with sea level rise may help for management decisions. This essay will provide an introduction to some selected impacts and an overview about strategies in management to outline the severe endangering of mangrove forests.

1 Introduction

The large debate about the worldwide human induced climate change includes especially coastal ecosystems. Phenomena that accompany climate change like sea level rise, global warming and an increasing number of storms and hurricanes regularly bring this topic into the awareness of the public. Mangrove forests have a special role in that context because they are both - responsible for coastal structures and sensible indicators for environmental changes. That is why questions are arising like, if and how mangroves could be influenced by the global climate change or if there is actually any observable change in distribution or species composition due to the phenomena of climate change. Besides, mangrove systems were threatened in recent times by an overexploitation that reached a loss of nearly 50% worldwide (UNEP 2006).

From an ecological point of view the destruction and loss or modification in size and extension of mangrove forests would mean a loss of a biodiverse ecosystem with a lot of important functions: it acts as habitat for many different animals, as water filter for linked ecosystems like sea grass beds and corals, as an important carbon sink and it provides shelter and important nursery sites. Moreover, the endangering of this ecosystem would mean the loss of income for many people as well as a natural coastal protection. Also, it is a big human resource for fishery, fire wood, textiles and recreational tourism (WWW.FAO.ORG).

Although it is assumed that mangroves have the capability to compensate impacts of climate change with migration or natural adaptations, management must be improved to enable them to respond and to give them a high degree of resilience (AGRAWALA 2003). Pollen analysis may help to understand how mangroves were able to react to climate changes in the past, how they used the ability to migrate (COHEN 2003) and to get hints how management should be done.

This essay will mention the most severe potential impacts that are related to climate change. Besides, it will figure out how mangrove systems are able to respond naturally. With looking at different management concepts, it will give advisories how sustainable management should be done in the future and how to prepare mangroves for the impact of climate change (SAENGER 2002).

1.1 Mangrove forests

Mangroves are a taxonomically diverse group of salt-tolerant, mainly arboreal, flowering plants (ELLISON AND STODDART 1991). They occur in saline wetlands (AGRAWALA 2003) and “are the characteristic intertidal plant formations of sheltered tropical and subtropical coastlines (see fig. 1, SAENGER 2002).” Mangrove trees have morphological, physiological, biochemical and reproductive characteristics that enable them to survive under special salt, pH and oxygen conditions in a harsh environment. Above all, they are able to adapt continuously to changing biological, chemical and physical conditions. They generally appear only in a certain temperature range (higher then 20°C, exception: Avicennia marina), in muddy soil, protected from wave action, in shallow shores, depending on a tidal range and certain ocean currents (SAENGER 2002).

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fig. 1: Global distribution of mangroves ( HTTP :// SITEMAKER . UMICH . EDU 2006)

Mangrove systems use several specific features to deal with the conditions they live in. They are able to deal with high salt concentrations by dissolving it with their roots (Rhizophora, Sonneratia, Avicennia, Osboria), extrude it with salt glands (Avicennia, Aegiceras, Acanthus) or store it in their leaves (Sonneratia, Avicennia, Bruguiera). In addition, mangrove plants are able to take up water against an osmotic gradient and show xeromorphic characteristics (thick cuticle, few stomata, occurrence of hair) to deal with tidal water fluctuations (WWW.VCBIO.SCIENCE.RU.NL/).

Mangroves occur in waterlogged muddy soils that show anaerobic conditions. Adaptations to provide the plant with enough oxygen are a special shallow root system (knee roots, stilt roots, buttress roots, aerial roots, with lenticles inside and the presence of aerenchymatous tissues like pneumatophores (Avicennia, Xylocarpus molussensis, Heritiera fomes, Sonneratia). Also, the reproduction cycle of mangrove species is adapted to the tidal cycle. Mangroves release a viviparous large seedling (propagule) which is equipped with a buoyancy system to swim in the water. These embryos have a characterizing rapid rooting that enables them to use the small time span at low tide to settle down (WWW.NHMI.ORG/MANGROVES/REP.HTM).

Mangrove forests have strongly connected communities that show mutual, antagonistic, competitive and parasitic interactions between plants, microorganisms and animals. Especially plant-animal interactions are important for an intact ecosystem. An example is bioturbation of sediments (crabs, mud lobster, callianassid shrimps, fiddler crabs, mud skipper, mud crab etc.), which is necessary to mix, aerate and drain soils. Beyond that, mangrove forest are habitats for several migrating (birds, bats, brown pelicans) and resident species (mud skipper, fiddler crab) that spend either their whole life or only a part of their life span there and are needed for pollination (SAENGER 2002).

The dominance of mangrove species follows a typical shoreline (seaward to landward) or upriver zonation. For example Rhizophora as a primary succession plant has the ability to colonize new land (SAENGER 2002).

1.2 Climate change in coastal zones and marine ecosystems

The human induced climate change is primarily due to the rise in the burning of fossil fuels and changes in land cover. Human activities are changing “the concentration of atmospheric constituents or properties of the earth’s surface that absorb and scatter radiant energy (IPCC 2007).” The increased concentration of green house gases (e.g. carbon dioxide, methane, ozone) and aerosols are the reason for the observable increase in temperature. These changes lead to changes in precipitation patterns, and other climate events that have consequences for the natural environment (IPCC 2007).

One of the expected events that will affect coastal ecosystems is an increase of the sea surface temperature and the mean global sea level. This is because of expansion of seawater and a loss of glacial ice due to global warming (MCLEOD AND SALM 2006). The observable change in sea level over the last century has already been between 10 and 20 cm. That also includes an observable increase of the tight gauge (about 1.8 mm/a) over the last 50 years (UNEP 2006).

Besides, it could be observed that since 1880 the temperature has increased by about 0.8 °C. Forecasts say that it will rise by 2-6 °C till 2100. The increase of air temperature has also a positive correlation with the surface water temperature of the sea (MCLEOD AND SALM 2006).

A change in precipitation is another response to global warming. It is predicted that there will be an increase of 25 % by 2050, although the direction of change is pretty uncertain. A significant change in rainfall, more El Niño events and periods of dryness will lead to a large precipitation difference between the years (AGRAWALA ET AL. 2003).

Above all, climate change will lead to an increased level of flooding events, accelerated erosion and a seawater intrusion into freshwater sources. The extent and severity of storm impacts, floods and shore erosion that is already observed nowadays is related to climate change too. It is predicted that wind and tropical storms will increase in intensity and frequency (MCLEOD AND SALM 2006).

Changes in salinity are usually related to the phenomena discussed beforehand. In addition to this sea level rise pushes back mouths of rivers that change the time and place of salt and fresh water mixing in estuarial regions. Inundation from a sea level rise would result in an increase of salinity. An increase in precipitation can also lead to a change of salinity. Other climate change induced phenomena that cause salinity changes are an increase in frequency and level of high water events, floods, strong winds and heavy rainfalls due to an increase in storm events (UNEP 2006).