Key Words : Mangrove ecosystem, Thailand

Abstract
The primary objective of the study on coastal zone environment management with emphasis on mangrove ecosystems was to identify an effective approach for sustainable mangrove forest management using Remote Sensing and GIS technology. The study was conducted by 1) assessing forest land use in 1973, 1987, 1993 and 1998 based on visual interpretation of satellite imagery at the scale of 1:50,000; 2) evaluating forest land use area and change using GIS; and 3) developing a plan for the sustainable forest land use based on additional relevant thematic data.

The major results of this study were the compilation of a relevant thematic database; assessment of forest land use in 1973, 1987, 1993 and 1998; assessment of forest distribution in 1987, 1993 and 1998 as well as the change of land use between 1987-1993 and 1993-1998 and development of a forest land use plan. The mangrove forest can be divided into three principal classes as follows : 1) class I is near to the seashore where the mud is very soft and wet. The dominant species in this class is Avicennia alba and A. officinalis 2) class II It is the are to the first class. The dominant species are Rhizophora apiculata and R. mucronata. 3) class III It is mostly consists of mixed species including Bruguiera cylindrica, Ceriops tagal, Excoecaria agallocha and Xylocarpus granatum.

Introduction
Mangrove forest is one of the most valuable coastal resources, important for its multiple economic, ecological, scientific and culture resources for present, and future generations. It is an important component of Thailand's coastal zone ecosystem. In addition, mangrove forests are utilized as a source of fuelwood and pole production, they provide a nursery and breeding ground for many commercial fish and prawn species. They also protect against coastal erosion and provide a habitat for some protected species of birds and animals.

In recent decades, the coastal zone has been subjected to the effects of a growing population and economic pressure manifested by a variety of activities such as an aquaculture, tin mining, waste disposal, construction and industrialization. Major coastal resources in Thailand, especially mangrove forest, beaches, coral reefs and marine water have been deteriorated or degraded. In addition, the area of mangrove forest in Thailand has decreased rapidly due to the expansion of shrimp farming.

In order to use coastal resources on a sustainable basis, a proper management planning process is necessary. remote sensing and GIS are the major tools for undertaking the plan. Since 1972, Thailand has used a remotely sensed satellite data for assessment of natural resources in many fields, proving that remote sensing is a useful tool for assessing and monitoring the country's coastal resources, especially mangrove forests. Mangrove forest assessment and monitoring has been conducted continuously in Thailand since 1979.

In 1990, high resolution data of Landsat-TM and SPOT-PLA, which can provide more effective information than Landsat-MSS, was used for mangrove forest land use zoning monitoring. It was found that the Landsat-TM bands 4-5-3/R-G-B with linear stretching technique can be used to differentiate mangrove forest from other vegetation and SPOT-PLA shows a better pattern for shrimp farm and built-up areas. In addition, the combination of TM4-TM5-PLA/R-G-B via digital classification can easily depict the mangrove forest zonation which corresponds to the dominant species (Ratanasermpong, S. and C. Silapathong, 1991)

Therefore the integration of remote sensing and GIS for mangrove forest management is considered as an important tool for the development of effective plans by natural resource managers and planners. The main objective of this study is to demonstrate an effective approach for sustainable mangrove forest management by using remote sensing and GIS technology.

Study Area
Ao Sawi - Thung Kha is located between the latitudes 10° 15¢ and 10° 30¢ N and the longitudes 99° 7¢ and 99° 20¢ E, at the mouth of Khlong Sawi and Khlong Thung Kha, in Chumphon province on the south Eastern seaboard of Thailand. It has a total surface area of about 675 sq. km.

The study site has a tropical monsoon climate with two main seasons (wet and dry). The average annual rainfall is 2,010 mm. The peak rainfall month is in November, while the driest month is in March. The wet season is from May to January. The mean annual temperature is 26.90° C, and the highest relative humidity is in October. Tides are diurnal (one high and one low tide a day) with an average amplitude of 1.04 m. The monthly mean sea level is the highest in November and the lowest in March. The monthly discharge from the Chumporn rivers is the highest in August and the lowest in March. The monthly runoff is the highest in November.

The present land use/land cover in the study area is composed of mangrove forest, tropical evergreen forest, freshwater swamp forest, paddy field, rubber plantation, oil palm plantation, mixed orchard, coconut plantation, villages, wetland, wasteland, water bodies and shrimp farms.

Methodology
The main tasks were to classify the forest land use in 1987, 1993 and 1998 based on visual interpretation of satellite imageries at the scale of 1:50,000. The assessment of land use/land cover and change was then conducted using GIS. Finally, the proposal for sustainable forest land use plan was developed based on extracted and relevant thematic data. A schematic flowchart methodology is shown in Figure 1.