Floods, coasts, and mangrove forests

I have been away for almost a total of 8 months from home. And it seems that anywhere I go there are always comments about the weather – particularly how much it has changed. I have never experienced the hottest summer here in the Philippines. I heard and read that it reached 36° C to 38° C! But I have most definitely experienced heavy rains and low temperatures in Spain during spring. The intense heat and dry air in Italy and most especially the heat wave in Athens that reached 46° C in June! Ah yes, I believe the weather or better yet the climate is really changing.

The Inter-governmental Panel for Climate Change (IPCC) stated that global climate change is caused by global warming. Global warming is defined as the increase in average temperature of the Earth’s near-surface air and oceans in recent decades and its projected continuation. Warming is caused by the rapid rate of emissions of greenhouse gases or GHGs (e.g. carbon dioxide, methane and chlorofluorocarbons) from human activities such as burning of fossil fuels, tropical deforestation, etc. Factors with the greatest direct effects on estuarine and marine ecosystems are sea-level rise, wind patterns and hydrodynamic shifts, storminess, temperature change and availability of water from precipitation and run-off (IPCC TAR 2001).

The Philippines, being an archipelagic country has been deemed to be one of the countries most prone to the effects of climate change. Storm surges and riverine flooding is aggravated by the increasing frequency and intensities of tropical cyclones and other types of environmental degradation. Low lying areas are severely affected and high economic losses result from the damages caused by typhoons. Sea-level rise will then exacerbate the already flooded areas (IPCC TAR 2001).

Figure 1. Flooded low-lying areas in the Philippines will be further aggravated by rising sea-levels. (From Rommel Maneja)

Even if there are people who still contest that climate change has not occurred, we Filipinos must consider that with or without climate change, our natural barrier for natural disasters in the coastal zone such as the mangrove ecosystem has been severely exploited. In 1918, mangrove forest covers were estimated to be around 450,000 hectares (Brown and Fisher 1918). While according to the Department of the Environment and Natural Resources 1995 Statistics, the mangrove forest covers have been significantly reduced to 117,700 hectares. This decline resulted from human activities such as mangrove conversion to fishponds and salt beds, mangrove reclamation for various developments, pollution, siltation and overuse further add stress on the mangrove forests.

The Asian Development Bank (ADB) estimated the total annual value of production from a 1-hectare holding at about Php 24,950 per year for plantations (beginning at the 12th year) with Php 11,500 coming from mangrove wood products and Php 13,450 in forestation (ADB Report 1990). On the other hand, White and Cruz-Trinidad (1998) estimated USD 600 per hectare as the acceptable economic equivalent when mangroves are converted to other uses.

Indeed mangrove forests have high economical value that’s why it is widely used. However, I believe that the economic returns are less than the ecological benefits when we maintain the mangrove ecosystem. The mangrove ecosystem interacts closely with the seagrass and coral reef ecosystems. Its primary ecological functions, as a stand alone ecosystem and with relation to seagrass beds and coral reefs, are:

1. It supports fisheries production by serving as nursery grounds for various finfish and shellfish species;
2. Its leaf litter and detritus are valuable sources of food for associated fauna;
3. It prevents soil erosion by stabilizing sediments with extensive root systems;
4. It aids in water quality maintenance;
5. It helps in organic biomass production and pollution reduction in near shore areas by trapping or absorption; and
6. It protects coastal areas and near shore communities from storm surges, waves, tidal currents and typhoons.

Mangroves are our natural barriers for sea-level rise. But then with reduced forest covers, it can also be affected by the rising sea-levels. Studies in the Caribbean, Florida and the Pacific Islands showed that mangrove responses to sea-level rise depend on the rates of submergence vs. sedimentation (Doyle 2003, McKee et al 2007, UNEP Report 2006, Society of Wetland Scientists 2006). These responses can either result to no changes in mangrove position and distribution, mangroves move land ward or mangrove loss. Figure 2 shows the migration patterns relative to sea-level rise and the presence of land obstruction.

Figure 2. Possible mangrove responses to sea-level rise. (UNEP Report 2006)

Based on the studies in the modeling studies in the Pacific Island states, mangrove loss is the possible scenario that will occur in their areas. This is due to the presence of land obstructions such as tourist facilities and coastal communities. Moreover, most of the substrates in their area are made of limestone. The absence of sediments that mangroves need for attachment and food resources has been said to contribute to the further degradation of the habitat (UNEP Report 2006). Although there have been little to no similar studies done in the Philippines, I believe that the scenario that may occur in the Pacific Islands is also possible in our country. Similarly, our coastal areas are heavily populated due to the dependence on coastal resources. Also, some of our islands are also made up of limestone.

Mangroves are special and delicate ecosystems. This ecosystem is composed of the interaction of mangrove trees that serve as primary producers, associated aquatic and terrestrial fauna and physical factors of the environment (Figure 3). Mangroves are located at intertidal settings along river mouths of tropical and subtropical regions worldwide. They are composed of single overstory strata of relatively few tree species tolerant to fluctuating salinity. They require slow currents, no frost and plenty of fine sediment for roots attachment. Species specifity and distribution are based on the salinity and substrate gradients. Areas with high rainfall and upstream runoff, for example the Philippines, are associated with best-developed mangrove forests.

Figure 3. The mangrove forest is an ecosystem of interrelations of mangrove trees, associated fauna and physical factors of the coastal zone and estuarine environments. (Photos by Zazu Aquino – mangroves, Wikipedia – Nicobar pigeon)

The Philippines is endowed with one of the most diverse mangrove ecosystems, consisting of 35-40 mangrove species (Primavera 2002). Its fauna is made up of birds one of which is the threatened Nicobar pigeon, mammals such as rats and the long-tailed Macaque, reptiles, mollusks, crustaceans, polychaetes, fishes and insects. It can be used for other purposes other than conversion for other developments. It supports fisheries production. It is a source of tannin extract and can serve as recreational grounds for birdwatching and observation of other wildlife. It is threatened by climate change, but it has a vital role in carbon dioxide mitigation and coastal protection. Management, rehabilitation and scientific studies particularly on the establishment of rate of change and identification of most vulnerable areas for adaptive management are therefore important in order to maintain covers of mangrove forests.

References:

Asian Development Bank of the Philippines. 1990. Mangrove Development Project Feasibility Report. Final Report. Vol. 1 and 2. Manila.

Brown, W.H. and A.F. Fischer. 1918. Philippine Mangrove Swamps. Department of Agriculture and Natural Resources. Bureau of Fishery Bulletin. No. 7.

DENR Forestry Statistics. 1995. Environment and Natural Resources. Atlas of the Philippines. Published by the Environment Center of the Philippine Foundation. 1998. Manila, Philippines.

Doyle, T.W., G.F. Girod, and M.A. Books. 2003. Modelling Mangrove Forest Migration along the Southwest Coast of Florida Under Climate Change. U.S. Geological Survey, National Wetlands Research Center.

IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp.

Maneja, RH. 2006. Analysis of the Responses of Mangrove Ecosystems in the Philippines to Climate Change. Agusan River Basin Mangrove Wildlife Conservation Network Website. (http://www.freewebs.com/arbmwcnetwork/)

McKee, KL, DR Cahoon, IC Felle. 2007. Carribean Mangroves adjust to rising sea level through bioaccumulation or change in soil elevation. Glob. Geo & Biogeo.

Pacific Island Mangroves in a Changing Climate. 2006. UNEP Regional Seas Reports and Studies No. 179. United Nations Environment Programme, Regional Seas Programme, Nairobi, Kenya.

Primavera, J.H. 2002. Management and Conservation of Mangroves in the Philippines. Joint UNU-Iwate-UNESCO International Conference. Conserving Our Coastal Environment 8-10 July 2002

Proceedings of the Symposium on Mangrove Responses to Relative Sea-Level Rise and Other Climate Change Effects, 13 July 2006. The Society of Wetland Scientists 27th International Conference, Cairns, Australia

White, A.J. and A. Cruz-Trinidad. 1998. The Values of Philippine Coastal Resources: Why Protection and Management are Critical. Coastal Resources Management Project. Cebu City. Philippines. 96 p.

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