A remote sensing / GIS database for forest management and monitoring in Sri Lanka

Transcription

1 A remote sensing / GIS database for forest management and monitoring in Sri Lanka N. JEWELL and C.A. LEGG Forest Management and Plantation Project, Forest Department, Colombo, Sri Lanka This paper was first presented at the 1993 ESRI User Conference for South East Asia, Kuala Lumpur, September 1993 Abstract. Sri Lanka s highly varied climate and topography supports a great diversity of natural and plantation forest. This resource, which includes many endemic plant and animal species, is under great pressure from agriculture and for timber. Accurate map information on the current distribution of natural and planted forest is essential as a baseline against which to measure future change, and as a forest planning and management tool. Landsat Thematic Mapper (TM) imagery, the primary data source, acquired between May 1991 and March 1992, and IRS-1 imagery where cloud-free TM imagery was unavailable, were used to produce forest cover maps of the island. The data were geometrically corrected to the Sri Lanka national grid, and visual interpretation of eight types of natural forest and four main planted species was carried out. The main objective was to produce a series of detailed maps covering the island at a scale of 1:50,000. Image interpretations corresponding in area to one 40 x 25 km 50,000 scale map sheet were converted to ARC / INFO coverages, combined with additional data, then plotted and field checked. When national coverage is completed in mid-1994, these data will form the basis of a GIS suitable for forest management applications. While traditional maps and statistics are still the main information requirement, there is a growing need for a flexible system capable of answering specific enquiries at national and local levels. Forest coverages and other data are transferred to desktop systems where data can be queried with read only software such as ARCVIEW. 1. Introduction Sri Lanka s great diversity of climate, topography and soils is reflected in the variety of natural forest cover. The combination of rugged topography, which reaches maximum elevations in excess of 2000 m, and the fact that the island receives both the north-east and south-west monsoons, results in climates ranging from semi-arid to extremely humid, and from very hot to relatively cool. These rapid spatial variations in climate result in great species diversity and endemism. Most of the natural forest is under severe pressure from an increasing population, both for fuelwood and for agricultural land. Many of the natural forest species yield high value timber, and commercial timber extraction has frequently caused forest degradation. The development of major irrigation schemes, in particular the Accelerated Mahaweli Development Programme, have also resulted in large scale forest clearance. In the central uplands, deforestation has led to increased soil erosion, land degradation and increased susceptibility to drought. While tree planting for watershed protection, land reclamation and timber production have been carried out for decades, it has become especially important in the past 20 years. Eucalypts (Eucalyptus grandis, E. microcorys, E. robusta) and conifers (Pinus caribaea) are extensively grown in the highlands. The establishment of large conifer plantations, increasingly recognised as environmentally damaging, has been controversial and is linked with accelerated rates of soil loss, particularly on steep slopes. Teak and some eucalypt species are grown very successfully in the drier lowlands, and mahogany plantations grow in lowland areas with intermediate amounts of rainfall.

2 There is considerable need for a forest information system which can be frequently up-dated to take into account the rapid changes in forest cover. The high rate of deforestation in Sri Lanka renders maps of natural forest cover obsolete within a short time span. Forest plantations, meanwhile, have increased rapidly in area and although various Government agencies concerned with forest plantations keep records of their own, there is a no single database of all plantation areas. Remote sensing and GIS techniques are appropriate means of providing this information. In the case of Sri Lanka, where 30% of the island is inaccessible owing to civil disturbances which preclude the preparation of new forest maps by conventional techniques, the approach is particularly relevant. Detailed maps of some areas are being prepared by air-photo interpretation, but this is a slow process, and up-to-date air photography is often not available for many areas. This remote sensing and GIS project is part of the Forest Management and Plantation Project (FORMP) which is one of three technical co-operation projects within the forest sector in Sri Lanka funded by the UK Overseas Development Administration (ODA). FORMP seeks to strengthen the Forest Department's ability to maintain management objectives for conservation, plantation programmes, controlled selective production from natural forest (logging), and community forest programmes. In order to fulfil these objectives there is a fundamental need for an accurate and consistent database of the island's forest resources. 2. Image processing Two 386 PCs run ERDAS software for image analysis. One PC also runs ARC /INFO and is dedicated to map production and GIS tasks. Two tape drives, a digitising table, and raster and vector plotters have also been installed. A substantial archive of satellite imagery (Landsat TM, SPOT XS and IRS-1 LISS-2) has been purchased. A 486 PC for staff training has been installed. This runs RChips image analysis software and ARCVIEW. Demonstration projects and courses are held to train local staff and ensure a continuing remote sensing and GIS capability at the Forest Department (FD). The satellite data used have been mainly 1991 and 1992 Landsat Thematic Mapper (TM). While other multi-spectral data, such as SPOT and IRS-1 are available, for practical discrimination of different vegetation categories, especially forest types, imagery acquired in the mid-infrared (1.5 micron wavelength) portion of the spectrum is essential. Landsat TM data, covering the 1.5 micron region, is therefore the main data source used for production of the maps. TM imagery, acquired between June 1991 and March 1992, covering 95% of Sri Lanka, was purchased from the Indian National Remote Sensing Agency, Hyderabad. The remaining areas were covered with 1988 TM

3 which was used for initial mapping. Recent changes in forest extent were checked by reference to cloud-free IRS-1 imagery, covering the entire island, acquired in March In common with previous studies, image analysis has shown that of the seven TM bands available, bands 3, 4 and 5 contain the greatest amount of information on vegetation, especially woody vegetation. However, attempts at automated image classification of forest types using a variety of techniques proved unsuccessful. Significant misclassification errors occurred, particularly in the hill country. The use of band ratios as a means to eliminate the effects of differential illumination failed to produce accurate results. The plot of TM band 4 against TM band 3 (Red / IR) in Figure 1, illustrates this problem, showing the spectral overlap of many forest types. Figure 2 illustrates the additional information in TM band 5 distinguishing between forest classes. Three-band sub-sets were prepared from each of 21 Landsat TM quadrants as an initial step in processing. Contiguous quadrants acquired during the same satellite pass were joined digitally, and images were then geometrically corrected to the Sri Lanka national grid, using ground control points from published 1:50,000 and 1:63,360 scale maps. Interpretation was carried out on TM bands 3, 4 and 5 (B, R, and G) colour composites displayed on a high-resolution colour monitor; forest boundaries were digitised directly on screen. Nine categories of natural and planted forest were mapped, as indicated in Table 1. Previous research had indicated that spectral differences between the main types of closed-canopy natural forest (lowland rain forest, sub-montane rain forest, intermediate zone forest and dry-zone forest) are small, and that visual discrimination between them, especially since they are almost never in direct contact, was not possible. Major road and rail networks, district boundaries, elevation information (1000, 3000 and 5000 ft contours), and mean annual rainfall were digitised from maps produced by the Sri Lanka Survey Department.

4 3. The : 500,000 forest map Responding to an urgent requirement for a national map showing the current size and extent of the island's forests, work on a prototype forest map was undertaken in mid The 21 geometrically corrected Landsat quadrants and a number of IRS-1 scenes were visually interpreted for dense and sparse forest, plantations, and water bodies. Raster maps for each forest type were prepared and pasted into a master map of the whole island. The master raster map file was aggregated from a 30 m pixel to a 90 m pixel size, reducing the total file size and generalising boundaries as well as removing very small forest patches and plantations which would effectively disappear at small scale. Owing to the large numbers of polygons which exceeded the processing capacity of the PC ARC / INFO version then available, it was necessary to subdivide the final raster map into different forest categories before conversion to ARC/ INFO vector coverages. Dense natural forest, sparse forest, plantations and water areas were each processed separately. Coverages of elevation and rainfall were then used to subdivide the single closed-canopy natural forest category into lowland rain forest, sub-montane rain forest, moist monsoon forest and dry monsoon forest. Map compositions were generated combining forest boundaries, road and rail networks, contours, district boundaries and major towns digitised from 1 :250,000 scale maps published by the Sri Lanka Survey Department. All forest polygons with perimeters of less than 7 km were omitted for the sake of clarity from the forest map, although these were retained for extraction of forest area

5 statistics. Finally, overlaying digitised district boundaries with the forest coverages enabled forest cover estimates to be generated on a district basis. As this information had been gathered by district in previous forest surveys, estimates of change in forest area could be made. The statistics on forest cover of Sri Lanka are listed in Table 2. The area of closed-canopy natural forest, 1.33 million ha, or 20.2% of the total area of the country, can be compared with a 1956 value of 2.9 million ha (44%), and with a 1983 value of 1.76 million ha (26.6%). These figures suggest that the average annual rate of deforestation during the past decade, both planned (timber extraction and land clearance for new settlement schemes) and unplanned (illegal felling and encroachment), has been about 54,000 ha. The average annual rate of deforestation between 1956 and 1983 was about 42,000 ha, suggesting that the rate of deforestation is increasing rather than decreasing. It may, however, be inappropriate to consider deforestation in terms of an annual average during this period since a large proportion of recent forest loss was probably related to very extensive land clearance in preparation for new irrigation schemes of the Mahaweli Systems which occurred during the first part of the decade. A break-down of the major categories of closed canopy natural forest is presented in Figure 3. This shows that the remaining natural forest of Sri Lanka is dominated by the dry-zone forests (moist and dry monsoon forest) and that the lowland rain forest, the most biologically diverse category, occupies less than 10% of the total forest area. A quantitative assessment of the main areas of change in forest cover during the past decade is difficult the lack of reliable baseline maps and statistics against which to measure this change. The forest map produced in 1981 by the Centre for Remote Sensing at the Survey Department was interpreted from the much cruder satellite imagery that was available at that time, and may have omitted some significant areas of natural forest, notably along the north-eastern slopes of the Knuckles range. The land use maps have an important category of open forest, as well as a sparse forest and scrub category. While there have been some inconsistencies between previous forest surveys, it appears likely that the estimates of forest cover prepared for the 1985 FD /FAO inventory are the most reliable, since these drew on a range of different data sources and were subject to some field checking. The fairly close correspondence between these statistics and the 1992 data in areas where little change is thought to have occurred suggests that the definitions of' closed-canopy forest were similar in both these assessments. The 1983 forest cover statistics presented in the FD/ FAO inventory were thus used as the basis for a study of change in forest area by district (Table 3 and Figure 4).

6 The Polonnaruwa, Batticaloa, Ampara, Badulla, and Hambantota districts show the largest declines in forest area, all having lost more than half their forest cover between 1983 and Significant forest loss (between a half and a quarter of the 1983 area) has also occurred in the Moneragala and Nuwara Eliya districts, while forest reduction in the Vavuniya, Trincomalee, Kurunegala, Kandy and Kegalle districts has been more modest, ranging from one tenth to one quarter. The Kilinochchi, Mullaittivu, Mannar, Anuradhapura, Puttalam, Matale and Colombo districts do not appear to have suffered significant change, while the Jaffna and Gampaha districts appear to have been without any significant forest in both 1983 and It appears that up to half the total deforestation (210,000 ha) is directly or indirectly related to land clearance and settlement in the Mahaweli irrigation systems. Other areas of natural forest, such as that in the Floodplains National Park may have been cleared by unauthorised settlers. In Moneragala and Hambantota districts, and in the southern corner of Badulla district, large scale deforestation can be identified in the western extension of the Ruhunu National Park natural forest (north of Kataragama) and an area of once dense natural forest south-west of Welawaya. Both these forests have been extensively degraded since 1983, and much of the clearing appears to have been for chena cultivation. Mangroves constitute a spatially insignificant but ecologically critical forest type that has also shown extensive decline over the study period. No significant concentrations of mangroves were detected in the Jaffna, Colombo, Kalutara, Galle and Matara districts in the 1992 survey, although all had mangroves recorded in the 1983 census. Field observation confirms that some mangroves

7 still survive in these districts, notably as narrow belts along the lower reaches of many rivers. It is likely that these belts are too narrow to be detectable by remote sensing, but it appears unlikely that these were the sole contributors to the originally significant areas. Decline in mangrove area is also apparent in the Kilinochchi, Mullaittivu, Puttalam and Gampaha districts, and field visits to the Negombo, Chilaw and Puttalam lagoons confirmed the active destruction of mangroves in the latter two districts. Mangroves in the Mannar, Trincomalee, Batticaloa and Hambantota districts appear to be relatively stable, while there is a marked apparent increase in the area of mangroves in Ampara district which cannot currently be accounted for. A few districts show an apparent increase in area of natural forest between 1983 and The most prominent of these is Mullaittivu district. Comparison of 1983 and 1992 maps reveals that the district boundary between Kilinochchi and Mullaittivu was redefined between the dates of the two surveys, and that a large area of natural forest which lay in Kilinochchi district in 1983 is now in the enlarged Mullaittivu district. When the two districts are considered as a single unit, there has been a very small overall decline in forest area. A smaller apparent increase in forest area is seen for Mannar district. No explanation can be given for this at present, although it may also be a result of boundary changes. Small apparent increases in forest area in the main lowland rain forest districts (Ratnapura, Kalutara, Galle and Matale) probably result from the misidentification of some forest gardens neighbouring natural forest. These could, in some cases, have been interpreted as dense natural forest and have resulted in an over-estimate of the actual forest area. Statistics for forest plantations are presented in Figure 5 and Table 4. Some very large discrepancies exist between the 1992 statistics and those collected for 1983, especially in the case of drier districts. No forest plantations at all have been mapped in Ampara district in the present survey, although this district had the greatest planted area in the entire country in the 1983 census. The areas of plantations mapped in Puttalam, Moneragala and Hambantota districts are significantly lower in 1992 than in Re-mapping at 1:50,000 scale using multi-seasonal imagery is producing much more accurate results and it is almost certain that the indicated areas of teak and dry-zone eucalypts will increase using the final database produced at 150,000 scale. The 1992 estimates for conifers and eucalypts are thought to be accurate representations of the area of mature, successful plantations in upland areas, and probably represent plantings prior to The estimates for mahogany enriched forest are also considered to be accurate.

8 The overall statistics derived from the map indicate a continuing, and even accelerating, decline in the area of natural forest in Sri Lanka. However, it appears that more than half of the total area deforested since 1983 represents land clearance for planned settlement schemes, or unplanned clearance by the new settlers in the vicinity of these schemes.

9 Conservation of forest in the Floodplains National Park and the Nilgala Iungle Corridor appears to have been ineffective since little or no dense forest remains in Forest clearing for agriculture, especially chena, appears to have been the major motive for deforestation in most parts of Sri Lanka during the past decade. Although the 1:500,000 map was known to contain certain errors, mainly in the detection of plantations, it represents the first demonstration of the use of GIS functionality such as map overlay and generating statistical summaries in a national forest inventory in Sri Lanka. 4. Map production at 1:50,000 scale While the 1:500,000 scale map has demonstrated the viability of the RS/GIS approach, the main element of this Project is the production of a series of semi-detailed 1:50,000 scale forest maps. Of a total of 92 map sheets, 28 have been checked and completed. These maps are already being used to plan and direct a national forest inventory, and a national forest biodiversity survey, among other uses. The map production procedure shown in Figure 6 uses very similar techniques to those employed for producing the 1:500,000 map. The 1:50,000 mapping includes important revisions following on from the 1 :500,000 work, In particular, it is expected that there will be higher mapping accuracy for smaller forest areas such as conifer plantations and eucalypts. On screen interpretation of forest boundaries is performed on x 2 magnified and enhanced TM band 3, 4 and 5 composites (Figure 7). This allows on-screen mapping at an effective scale of around 1: 25,000. ERDAS vector files are converted to raster files, recoded to fixed classes, then converted to ARC / INFO coverages. Logical operations (intersections) are then performed as shown in Figure 8, using elevation and rainfall data, to sub-divide the single category of dense natural forest into the following ecological types: Forest Category Lowland rain forest Sub-montane forest Moist monsoon forest Dry monsoon forest Elevation Constraints Elevation less than 1000 m Elevation greater than 1000 m Elevation less than 1000 m Rainfall Constraints Annual rainfall > 2500 mm Annual rainfall > 2500 mm Annual rainfall mm Annual rainfall < 1500 mm

10 A template corresponding to the Sri Lanka Survey Department standard map format has been prepared as a series of plot files. These files contain the l:50,000 scale grid, legend, and other descriptive information. Forest polygons, digitised map data, map name and number are added to the template which is then plotted. Field checking of map sheets takes an average of two days per sheet, although frequently, more time is required in the hill country. Field checking is a qualitative assessment of the presence or absence of forest or plantation types and does not involve physical measurement of the forest area. Accuracy in field checking has been considerably improved through the use of Global Positioning System (GPS) receivers. Subsequent editing of interpretations enable changes to be made where required, allowing for a much greater mapping accuracy than hitherto possible with the 1:500,000 map. This is particularly important for forest plantations, large areas of which had not been identified during initial interpretation. Table 5, which compares the area measurements from 1:50,000 and 1:500,000 scale databases for forest types in two districts, Galle and Matara, shows the improved detection of conifer mapping at higher resolution, probably with an accuracy better than 85%. This is with the exception of pines and eucalypts planted after 1987 which are not yet visible on satellite imagery. The most difficult forest plantations to map using remote sensing remain the teak and low-country eucalypts, which had been significantly under-estimated during the 1:500,000 mapping.

11 While field map checking for the one-third of the total area of the island is restricted because of security constraints, it has been found, in most cases, that natural forest has been correctly identified, although there has been a tendency to under-estimate areas of closed-canopy forest in the drier portions of the island. The open/ sparse forest category contains more errors since it includes a huge range of categories of different origins, such as naturally open forest and thorn-scrub, severely degraded forest, and some secondary re-growth. It appears that this category may have been significantly under-estimated, and that very sparse thorn forest, some savannah woodland and early stages of re-growth may not have been mapped. Special categories of dense forest, notably mangroves and montane (dwarf) forest, appear to have been mapped very accurately (better than 95%), although the distinction between dry-zone riverine forest and other closed-canopy dry monsoon forest is not always clear. After field checking, coverages are clipped and appended to the master forest coverage. 5. The Forest Department GIS The main function of the forest GIS will be for forest inventory and planning in the various forest administrative areas ( beats ). Further uses of the GIS are likely to be in strategic forest planning, zoning forests into different management areas either for total conservation, community forestry management, commercial (plantation) production or selective felling. The identification of specific areas with the authority responsible for their management is particularly important from an overall monitoring perspective in Sri Lanka where large blocks of forest may be the administrative responsibility of different Government departments. Other tasks are the identification and selection of suitable sites for afforestation and reforestation. The data may also be of some use in silvicultural planning and scheduling extraction of timber from forest plantations. In general, the main use of the database will be the provision of information about the very large tracts of natural forest in the country, particularly in the dry-zone. Forest information needs vary according to the different levels of user. The system is required to integrate information at national and regional level for which interactive querying is required, while range and divisional forest officers will make more use of the map data in the field. A list of information planned for the forest GIS is given in Table 6. Desktop systems with interactive software such as ARCVIEW will provide the main access to the GIS database for planning and forest management. In addition to other Government agencies, such as the Wildlife Department, there are a large number of NGO users requiring customised map products for use in the field.

12 Editing of database feature attribute tables to include manually gathered information will allow matching of traditionally gathered information on forest blocks, such as crown closure and species composition, with reliable spatial information. Current spatial information on forest reserves needs considerable revision, and maps and map boundaries are frequently of questionable accuracy. An important task of the GIS will be up-dating forest cover within nominally protected land. Although Sri Lanka has nominally one of the highest percentages of protected land (in reserves) in south-east Asia, these areas are often encroached upon, to the extent that some forest reserves are now entirely devoid of tree cover. An example of this is the Nilgala Jungle Corridor, established to preserve a route for elephant movement between the Senanayake Samudra and the Maduru Oya National Parks, which has been almost totally deforested. Of urgent importance is the identification of unprotected forest blocks lying outside gazetted national park boundaries, areas particularly at risk from unauthorised logging. An important task is the maintenance and up-dating of key data sets in the GIS, particularly for forest cover. A new TM derived forest inventory conducted at intervals of between 3 and 5 years will be required to maintain accuracy and provide further information on the rate and type of spatial change in forest cover. In addition to monitoring changes in natural forests, detection and monitoring of pine and eucalyptus plantations is an important future requirement. 6. Conclusion The need for information on the extent of forest in Sri Lanka is essential as a baseline for monitoring future changes and for planning and managing the country's forest resources. Satellite imagery has been used successfully to map the forests of Sri Lanka at 1:500,000 scale and at a more detailed level of 1:50,000 scale. GIS technology has been used to extract additional forest types from the forest coverages derived from the satellite imagery. By overlaying digital forest coverage with rainfall and elevation coverages, forest types such as lowland rain forest, sub-montane forest, moist monsoon forest and dry forest were derived. The GIS was also used to extract area statistics. Results suggest that rates of deforestation have increased. The final coverages produced will be used as part of a national forest GIS database. References SURVEY DEPARTMENT (1981). Sri Lanka Forest Cover Map. Centre for Remote Sensing, Colombo. SURVEY DEPARTMENt (1988). The National Atlas. NARESA (1992). Environ mental Synopsis of Sri Lanka. International Institute for Environment and Development, London. FAO/GOSL (1986). A National Forest Inventory of Sri Lanka,