Scientific Report to Naucrates
Biology of Mangroves and GIS Mapping
Jennifer Morse and Mirco Boschetti
Research Assistants, July – August 2002
Mangroves are the main coastal wetlands in tropical and subtropical regions of the world (Mitsch and Gosselink 2000). Mangroves occupy important transition zones between terrestrial and aquatic environments and between marine and fresh waters. They are essentially terrestrial environments dominated by plants that are particularly adapted to the stresses of saltwater and tidal flooding. Mangroves occur along coasts with warm ocean currents (latitudes ~ 35？N or S), because they usually
cannot tolerate freezing temperatures (Hogarth 1999). They are found in shallow, low wave energy environments, such as protected shores or along rivers and tidal creeks.
Tidal flooding in mangroves creates extreme chemical conditions (low oxygen and salinity) that require special adaptation by plants. Many mangrove species have evolved pneumatophores (air-roots) that are adapted for gas exchange and aid in stabilizing the trees against flooding. To deal with salinity, most mangrove species have thick, nearly impermeable leaves and special membranes to exclude salt; some species also have salt-secreting glands in their leaves to get rid of excess salt. Most mangroves have a unique reproductive strategy, vivipary, that produces live propagules that begin germinating while still on the parent tree (Tomlinson 1984).
Trees dominate the mangrove ecosystem. The most abundant mangrove species in Thailand are the Rhizophora, recognized by their exceptional stilt roots (Tomlinson 1984). Different patterns of tree associations are formed in response to abiotic factors such as salinity and flooding gradients, geomorphology, and soil type, and through interspecific competition. The natural progression from young to mature forest (succession) also influences the distribution of species (Johnstone 1983).
Intact mangroves provide many ecosystem services. The waters sheltered by their roots provide nursery grounds for many commercial and non-commercial fisheries. Mangroves provide coastal protection, reducing coastal erosion and protecting low-lying areas from flooding and storm surges. Coral reefs also benefit from this reduction in erosion, as silt deposited on reefs kills the coral by smothering it; in turn, dead reefs result in decreased fish diversity abundance. Important biogeochemical processes in mangroves are the conversion of carbon dioxide to oxygen by photosynthesis, the removal of nutrients and pollutants from water, and the burial of organic matter in sediments (Hogarth 1999; Mitsch and Gosselink 2000). Mangrove products provide many benefits to local communities, such as Nypa palm
fronds for roofing thatch, many plants with medicinal uses, and shellfish and fish harvests (Aksornkae et al. 1985).
The overall aim of the preliminary botanic survey of the mangrove forest near the research station at Golden Buddha Beach Resort was to lay a foundation for ongoing ecological research on mangroves of Phra Thong Island. The objectives of the first field season were to develop and implement methodologies for 1) plant identification, 2) location and monitoring of sample plots, 3) data collection and input and 4) mangrove restoration areas, with a long-term study in mind. With this baseline information and methods, monitoring can be expanded to other areas of the island and perhaps to animal communities, to understand the natural dynamics of biodiversity and ecology, such as succession, expansion or mortality with changing sea level or storms, and human-induced changes such as clearing, cutting, or restoration.
In line with Naucrates’s conservation goals, we sought to interact with different communities of the island,
by giving presentations on mangroves to schoolchildren at the three schools on the island, to the Thai staff and to guests of the resort. We also wished to build a working relationship with Thai researchers from the Ranong Coastal Resources Research Station (RCRRS).
2Koh Phra Thong is a 100 km island (15 km long x 7 km wide), located about 5 km from the western
coast of Thailand in the Andaman Sea (9.03-9.17？N, 98.25-98.33？E; Figure 1). The western coast of the
island is primarily a high-energy environment with narrow strip of sandy beach, composed of marine sediments and bordered by sparse beach vegetation; the other shorelines are protected, low-energy environments characterized by muddy alluvial substrates and dominated by mangroves. The island’s
interior is an arid, savannah-like grassland with sandy soils and clusters of Melaleuca spp. trees. A tidal
channel named Khlong Ko Khat bisects the island from North to South. A tidal creek (~1.5 km) on the northwest corner of the island, near the Golden Buddha Beach Resort, was the site of intensive preliminary sampling during July and August 2002.
The island is largely undeveloped, and its infrastructure is limited to a minor network of unpaved tracks; there is no island-wide water, sewer, telephone, or electrical system. The island is experiencing development for tourism, primarily along the north-west shore.
The climate on Koh Phra Thong is influenced by the seasonal monsoon, resulting in a tropical savannah climate with three seasons: rainy or southwest monsoon (May to October), winter or northeast monsoon (November to February), and summer (March to April). General climatic data for the western coast of Thailand were obtained from the Thai Meteorological Department (Table 1; TMD 2003).
Table 1. Climatic data for the southwest coast of Thailand (TMD 2003). Rainy Season Winter Summer
27.3 26.8 28.4 Mean temperature (？C) Mean rainfall (mm) 1,914.7 429.5 380.0 Mean humidity (%) 84 77 76
To prepare for species identification, we became familiar with local mangrove plants through preliminary surveys and reference books, mainly Aksornkae et al. (1992), Tomlinson (1994), and Lovelock (1999). Staff from RCRRS provided guidance and assistance with plant identification. We used characteristics of root systems, leaf morphology, bark texture and color, propagules, and flowers (when present) to identify species.
Permanent Monitoring Plots
The tidal creek near the research station was designated for the preliminary survey of the mangrove ecosystem and as the first location for permanent monitoring plots. A rough, ground-based map of the channel area was first sketched by hand, using a compass; the map was then corrected and refined using precise coordinates obtained using a Garmin III GPS unit (Figure 1). The tidal creek was accessed by kayak, and interior areas of the mangrove forest were surveyed on foot during low tide.
The sampling method was developed in collaboration with researchers from RCRRS, to enable data sharing and comparisons with their monitoring efforts. We also consulted Bullock (1996), Küchler and Zonneveld (1988), and Snedaker and Snedaker (1984) for information on vegetation sampling. Three permanent monitoring belt transects were established perpendicular to the creek and extending to the mangrove/upland border on either side (Figure 1), to capture the longitudinal variability from ocean to inland and laterally from bank to upland. Each transect consisted of 10 m x 10 m plots extending from the bank to the mangrove/upland border or 50 m, whichever was encountered sooner. The corners and center point of each 10 m x 10 m plot were marked with labeled stakes. The landward limit of transects was
established a posteriori in the field, after ground observations showed little change in community +composition between 50 and 100 meters to the upland border. Transects were numbered with respect to their position along the channel (T01, T02, and T03, increasing away from the ocean), and each plot was assigned a code corresponding to its position on the hydrographic right or left of the channel and distance from the bank (RA, RB, RC; LA, LB, LC; increasing inland). Each plot, therefore, had a unique identifier: e.g., T01RE. We established a total of 26 plots.
In each 10 m x 10 m plot, each tree (defined as having a diameter at breast height, DBH > 4 cm) was identified to the species level; its diameter was measured directly with a caliper or estimated as circumference with a measuring tape then converted to diameter; and its height was estimated to ， 0.5 m.
Trees with multiple stems were treated as single individuals; the major stem was identified, and its diameter was measured at breast height or 5-10 cm above the highest prop root, where feasible. For the two palm species (Nypa fruticans and Phoenix paludosa, diameter was deemed an unreliable and rather
hazardous metric, because P. paludosa has long, sharp spines; only height was measured for these plants. In a 5 m x 5 m subplot at the left, inland corner of the 10 m x 10 m plot, we identified each small tree or sapling (defined as DBH < 4 cm and height > 1 m), measured its DBH, and estimated its height. In a 1 m x 1 m subplot, each seedling (defined as height < 1 m) was identified and measured. Observations of epiphytic plants, fauna, soil characteristics, visible human impacts, and dead trees were also recorded. From the center of each plot, canopy cover was estimated to the nearest 10%, and where signal reception was possible, GPS coordinates were also obtained.
2Trees: 100 m Landward ； 2Saplings: 25 m 2Seedlings: 1 m Toward creek ？ Figure 2. Schematic representation of a sampling plot