Simeulue Island Mangrove Rehabilitation Assessment Gina Rae LaCerva and Dr. Brian G. McAdoo

Transcription

1 Simeulue Island Mangrove Rehabilitation Assessment Gina Rae LaCerva and Dr. Brian G. McAdoo Vassar College Department of Geology Poughkeepsie, NY USA

2 Mangrove Rehabilitation Assessment Page 2 of 12 Summary This report provides background information on the rehabilitation and conservation efforts concerning the mangrove ecosystems on Simeulue Island, Aceh Province, Indonesia. The tsunamis of December 2004 and March 2005 highlight their importance to costal dwelling communities. There has been a growing interest in the question of whether mangrove forests are effective buffers to tsunami wave energy and if they can be used to protect human establishments. By examining some past rehabilitation efforts in the Indo-Asian region we examine the conditions necessary for successful rehabilitation efforts and ultimately conclude that successful rehabilitation effort can help to protect coastal populations if located in the right place. We hope that this document will be a useful tool for organizations working on mangrove rehabilitation and protection efforts.

3 Mangrove Rehabilitation Assessment Page 3 of 12 Background and Ecology Simeulue Island is located just off the Sumatra mainland and was 30 km from the epicenter of the December 26 th earthquake (Figures 1 and 2). Despite its proximity to the epicenter, of Simeulue s population of 76,000 people, only 7 people died as the result of the tsunami. This was due largely to the local smong legend which recalls the deadly 1907 tsunami that hit the island. The residents knew what to do, and their quick evacuation from the lowlands following the earthquake undoubtedly saved many lives. Other factors influencing Simeulue s low death toll included propagation properties of the tsunami itself, and the ability of mangrove swamps to buffer some of the energy. Figure 1. Location map.

4 Mangrove Rehabilitation Assessment Page 4 of 12 Figure 2. Simeulue Island. There are currently a number of organizations (both non-governmental and UN) working on the island, and many have become interested in how to protect coastal inhabitants from future tsunamis. Mangrove conservation and rehabilitation is a potentially powerful tool to tsunami mitigation. Organizations such as Wetlands International are dedicated to preserving and rehabilitating the world s wetland ecosystems, including mangrove forests. They would like to initiate a project at the village of Alus-Alus which is exposed to the open ocean with very little vegetative buffer. It is their hope that an introduced mangrove forest can help shelter the community from future tsunamis. Other isolated mangrove plantings are present at Salur, but it is unclear who was in charge of that project. Mangroves forests are a type of estuary ecosystem. They grow in intertidal zones characterized by the alternation of tides and the presence of freshwater, at least occasionally (Baran 433). Of the Earth s tropical coastlines, 60-75% contain mangrove ecosystems and the Indo-Pacific region contains the largest number of tree species, with between present. Some of the more prevalent and understood species, all of which can be found on Simeulue are: Avicennia, Rhizophora, Bruguiera, Ceriops, and Nypa. A typical mangrove forest will contain a particular progression of these, the species composition changing as one moves from the land towards the sea. While variations in this zonation occur species composition varies widely, depending on a complex set of

5 Mangrove Rehabilitation Assessment Page 5 of 12 physical conditions, disturbances, and inter-species competition general schemes tend to exist by region. The following zonation pattern is common to Southeast Asia and is present on Simeulue (from sea to land): Avicennia at the water s edge, Rhizophora further inland, then Bruguiera and Ceriops, and finally, if a sandier substrate exists, Nypa (Figure 3). Ceriops Bruguiera Rhizophora mean high tide mean Avicennia low tide Figure 3. General mangrove zonation. Actual species composition may vary due to a complex set of physical, biological, and climatological factors. (source: Mangroves grow on low energy shores sheltered from wave action the minimal water movement allows the seeds to put down roots and are very dependant upon tidal action. Tidal changes help to exclude competitively superior species, transport nutrients, export wastes, disperse seedlings and prevent soil salinities from reaching lethal levels. Patches of mangroves can grow on rocky substrate or sand, and extensive forests are found on fine-grained sediment with high organic content and low oxygen levels, trapped by the mangrove roots. As an adaptation to these conditions, the roots send up extensions to the surface called pneumatophores, which allow the roots to receive oxygen. Mangroves are unique in that they can grow in this highly salient environment where most vascular plants cannot survive. Many species have salt glands that secrete or store salt, which helps to bring fresh water up into the tree. Other adaptations include prop roots (also called stilt roots) protrude from the main trunk of the tree to help stabilize the tree within the mud substrate and contribute to sediment accumulation (siltation) by further slowing water movement (Figure 4). These ecosystems are thus often associated with the production of new coastal land.

6 Mangrove Rehabilitation Assessment Page 6 of 12 Figure 4. Prop roots protruding from the main branches of a Rhizophora tree (foreground and background). Mangrove forests play many important roles both environmentally and economically. They have an incredible rich and diverse flora and fauna association. By stabilizing coastal sediments, thus reducing erosion, they create habitats rich in organic matter making them important nurseries to shellfish, shrimp, spiny lobster, blue crabs, and baby fish, all of which attract migratory birds. Environmental services include preventing excess nutrients from entering the ocean and protecting the coastline from wind, typhoons/hurricanes, monsoons and sea storm surges, which reduces the need for maintaining sea dikes (Hoang 1998). Mangrove forests also provide wood resources and several studies support the idea that coastal fish communities can thrive in this estuarine environment, although controversy remains about the degree of dependence and importance (Baran 1999). We were not able to find any studies, which show how or if they mitigate tsunami wave destruction, but surveys of structural damage and interviews with tsunami survivors indicate wave velocities are slower, but flow depth and amplitude are not affected. This was especially true at Labuhan Bajau, where buildings remained standing, and relatively undamaged, despite wave heights of up to 2 meters. It is possible that damaged or fragmentary mangrove stands may actually cause more damage because as they are not as well buffered, they can be more easily ripped out, and increase the debris coming on land. Rehabilitation: Past Efforts and Challenges Site Considerations. There is a tremendous amount of literature on rehabilitation projects undertaken over the past decade. The need for rehabilitation of a mangrove

7 Mangrove Rehabilitation Assessment Page 7 of 12 ecosystem implies that the area under consideration has been altered or degraded in some way that it is no longer functioning in a desired way. As a result, efforts tend to focus on areas that have been destroyed by human activity, such as places used for intensive shrimp farming and other aquaculture (which generally have a short lifespan but seriously degrade the environment), fuel wood gathering, or access to the sea (fishermen). Recovery of these disturbed forests are ideal for rehabilitation because they already contain many of the physical requirements of a site. It is also then desirable to use seedlings from surrounding tree species. Not all rehabilitation efforts require a preexisting mangrove site, though, and there are some key physical factors to consider when sighting a project: a gentle slope, tides that can regularly inundate young plants but not to the extent that they drown, stable soil at sufficient depth for planting, and sheltered sites because young seedlings cannot withstand strong winds or fierce currents (Field 1999). Moreover the presence of seagrass may indicate an appropriate site. In areas that have experienced a high level of erosion, perhaps due to the loss of mangrove forest, it has been estimated that a m intertidal mangrove belt must be planted in order to re-initiate the sedimentation process, thus restoring a favorable habitat for a larger mangrove forest (Winterwerpt et al, 2005). One must also be aware of the microbial community that accompanies healthy mangrove stands. A complex symbiotic relationship works to recycle nutrients, and while it isn t completely understood, it is very important to consider nutrient inputs when recreating a functioning and healthy mangrove ecosystem (Holguin et al, 2001). The rate of sedimentation at the site is a critical factor to measure, as it has been shown that excess input of sediment to mangroves stifles growth, and can even cause death of trees owing to root smothering (Field 1999; Ellison 1999). Incoming rivers may become overloaded with sediment if there is extensive logging occurring upstream. Rapid clearcutting leads to dangerous amounts of sediment that is swept into mountain streams and eventually brought to the ocean. Thus the health of a mangrove forest can be directly linked to the health of other kinds of forests that occur at some distance from the coast. Another example of rapid environmental change that may be detrimental to mangroves is tectonic action. Sudden subsidence or uplift of the ground during earthquakes (as occurred on Simeulue on December 26 th and March 28 th ) may remove the mangroves from the intertidal zone and eventually kill a forest. Liquefaction of the earth could cause trees to topple. As the landscape changes the tidal range it may be necessary to replant with a species that can grow successfully at slightly higher elevations in areas where the tide no longer reaches with the same intensity (Ellison 1999). In fact, uplifted reef flats may be good places to plant mangroves, as they are slowly being covered with sediment and often meet many of the physical requirements for seedlings, including shallow tidal movement (Figure 2). It was noticed that the uplifted reef at Linggi had small mangrove seedlings naturally beginning to take root (Figure 5).

8 Mangrove Rehabilitation Assessment Page 8 of 12 Figure 5. An Avicennia mangrove plant naturally beginning to take root on an uplifted reef flat at Linggi. Institutional issues. Recovery time estimates for damaged forests is on the order of years, which presumable is about the amount of time needed for a successful forest to be grown. This means that there must be commitment for that time period to maintaining the site (Nybakken 1997). Because rehabilitation projects must be managed regular aerial photos, cataloging of physical conditions and species composition, etc. overlapping bureaucracies and a lack of clear lines of accountability have led to neglect. This partially explains why the most successful projects are community-based and supported locally. Although ultimately the needs and desires of the local population will determine the success and longevity of a mangrove ecosystem, political will and support at any level of government advances rehabilitation and conservation efforts (Field 1999; Primavera 2000). Alus-Alus. Mangrove rehabilitation efforts at Alus-Alus will be challenging indeed (Figure 6). This high-energy beach environment has never been a mangrove habitat, hence rehabilitation here is really an attempt at natural engineering. By trying to convert a high-energy beach into a low energy environment with a step-by-step build out towards the ocean may disturb a natural balance that is a function of many factors including offshore bathymetry, dominant wind and wave direction, etc. This area does not contain any of the optimal conditions for creating a mangrove ecosystem. There is significant wave action, large tidal changes, a sandy substrate, significant lateral sediment

9 Mangrove Rehabilitation Assessment Page 9 of 12 transport, and it is very windy. Due to the base-level changes the island experienced in March, many of the rivers on Simeulue are undergoing down-cutting as are the shorelines as new high tide marks are being established. Alus-Alus showed both of these characteristics resulting in increased erosion. Although the site does have freshwater, the exposed beach and rough water do not make a hospitable environment for seedlings. Furthermore, all the successful rehabilitation efforts in the past, even those where the mangrove area was extended artificially, initially had some area of naturally occurring mangroves. We were unable to find any reports about transforming high-energy beaches into low-energy mangrove systems, so such an attempt may be without precedence. Most importantly, this is a project that must be managed, monitored, and maintained for many years. Figure 6. Alus-Alus, site of a proposed mangrove forestation project. Note the high energy environment. Recommendations Considering these factors, we suggest planting efforts be focused on uplifted reef flats. Over the next 100 years, these uplifted zones will slowly subside, and sediment trapped by the mangroves may keep up with this subsidence, making an effective buffer against the next tsunami. Figure 7 shows areas of intertidal (green) and subtidal (red) reef as classified by satellite images. Following the uplift that resulted from the December and March earthquakes, the intertidal regions may now be supertidal, and better suited for species such as the Bruguiera that are viable in this zone. Subtidal reef is now intertidal in places (depending on the degree of uplift), and is the ideal location for mangrove planting efforts. These areas are low-energy and are naturally becoming covered with fine-grained sediment.

10 Mangrove Rehabilitation Assessment Page 10 of 12 Figure 7. South Simeulue Island. Reds show subtidal reef that may be intertidal following the March 28 th, 2005 earthquake uplift, and green areas were intertidal pre-march 28 th, and now may be supertidal. We feel that the areas in red have the highest potential for successful mangrove plantings. Notice that Alus-Alus shows an expansive zone of subtidal reef- field observations suggest that if there is reef present here, it has little effect on buffering the wave action. It may be possible that the area was misclassified. Mangrove forests are disappearing rapidly, and Indonesia has lost much of its forests over the past decade. Thus, in a place like Simeulue, which still has many good forests intact, it is vital to stress to the population the importance of maintaining this resource. We observed healthy mangroves at Labuhan Bajau, Latiung, Linggi, and Teluk Dalan, many of which had already recovered from tsunami-related damage, even expanding upon uplifted reef patches, and this was only a small survey covering a limited area of the island (Figure 4). The greatest pressure on mangrove seems to occur in countries with brackish water shrimp and fish culture ponds, places that have a high demand for mangrove timber and areas with population pressures for living space and ocean-access, all of which cause people to clear-cut mangrove forests (Primavera 2000). Moreover, mangroves in the vicinity of other types of logging (hardwoods, etc.) are at risk of burial and suffocation by influxes of excess sediment. It will be important when determining policy to infer which of these pressures is most prevalent in Simeulue. Issues to be aware of include: governmental promotion of aquaculture as a path to development, low economic rent for

11 Mangrove Rehabilitation Assessment Page 11 of 12 mangroves (in other words, its monetary value is based only on the price received for its tradable products, without taking into account the services it provides, i.e. protects against wave action), conflicting governmental policies, and ineffective government management (Primavera 2000). In our opinion, in the case of Simeulue, it is better to spend energy and efforts educating the public about the benefits of healthy mangrove systems and protecting existing forests than trying to introduce mangroves to areas that never had any. These education efforts will not fall on deaf ears: we found that many of the communities, such as the people of Linggi, already recognize the worth of healthy mangroves. More Information: There are many other factors to take into consideration that could not be covered in this short review. For information on a case study involving sustainable management in Indonesia with the addition of aquaculture please see Atsuo Ida s article in Mangrove Management and Conservation, which has supplementary references and points to some published manuals that may be helpful. Also see Field s article which considers the most important factors of rehabilitation.

12 Mangrove Rehabilitation Assessment Page 12 of 12 References: Baran E. and Hambrey J. Mangrove conservation and coastal management in Southeast Asia: What impact on fishery resources? Marine Pollution Bulletin 37, no.8-12 (1999) p Ellison J.C. Impacts of sediment burial on mangroves Marine Pollution Bulletin 37, no.8-12 (1999) p Field C.D. Rehabilitation of mangrove ecosystems: An overview Marine Pollution Bulletin 37, no.8-12 (1999) p Hoang Tri N.; Adger W.; Kelly P. Natural resource management in mitigating climate impacts: the example of mangrove restoration in Vietnam Global Environmental Change, Volume 8, Number 1, April 1998, pp (13) Holguin G. ; Vazquez P. ; Bashan Y.; The role of sediment microorganisms in the productivity, conservation, and rehabilitation of mangrove ecosystems: An overview Biology and Fertility of Soils 33, no.4 (2001) p Ida, Atsuo. Sustainable mangrove management in Indonesia: Case study on mangrove planting and aquaculture, in Mangrove Management and Conservation. ed. Marta Vannucci,: United Nations University Press: New York pp Nybakken, James. Marine Biology: An Ecological Approach. Addison-Wesley Educational Publishers pp Primavera J.H., Development and conservation of Philippine mangroves: Institutional issues ; Ecological Economics 35, no.1 (2000) p Winterwerpt, Johan C, William G. Borst, and Mindert B. de Vriest Pilot Study on the Erosion and Rehabilitation of a Mangrove Mud Coast Journal of Coastal Research 21 no March 2005