Oudomxay province had a population of about 291,000 persons in 1990 (Mekong Secretariat, 1993). Three major ethnic groups and 26 tribes comprise the population of the province. The largest ethnic group is the Lao Theung, constituting about 55% of the provincial population, while the Lao Lum and Lao Sung represent 31% and 13%, respectively. The majority of the Lao Theung live in Xay, Houn, Beng and Pak Oudom districts. The Lao Lum live in smaller towns and in valleys, while the Lao Sung are dispersed in Xay, Nga, Houn and Pak Beng districts. There are 261 number of villages being identified in 1983, 1:100,000 topographic map within the chosen study area. Out of this, 181 villages were considered as large villages and the remaining 59 and 21 villages were accounted as small and abandoned categories, respectively. The main occupations of the population are farming and livestock raising. Slash and burn cultivation remains the pre-dominant agricultural practice in the study area.

Comparative result among the northern provinces of Laos, shows that Oudomxay exhibits the highest population growth of 55% from 1985 to 1990. Local forestry official ascertained that migration from the nearby provinces (e.g. Luang Prabang and Phongsaly) intensifies the provincial population increase. Additional data appeared in Table 2.1 manifests that Oudomxay province covered one of the highest hectarage subjected to slash and burn practice in northern provinces of this country.

2.6 Land Utilization Practice

According to the data available from the Provincial Agriculture and Forestry Office of Oudomxay, forest area covers 420,500 ha. of the province. The "forest" has been further classified into four categories viz. preserved forest (230,400 ha), improved forest (66,200 ha), production forest (81,200 ha) and unclassified forest (42,700 ha). Approximately, 30,000 ha of forest land is used every year for upland crop production, with an annual average rice crop production of 50000 tons (Mekong Secretariat, 1993). Hence, the province of Oudomxay is nearly self sufficient in rice production. However, due to the limited availability of both low and irrigated lands, slash and burn cultivation is the predominant agricultural practice in Oudomxay province.

In the upland system the main food crop, rice is cultivated for one year, then the land is abandoned to 3 to 4 years fallow. In isolated villages where the pressure of the population is low, the fallow may last 5 to 20 years. Maize, legumes and potato normally follow a similar cycle.

Land clearing starts in January-February by slashing of the under-brush and the higher trees. About six weeks later the dried material is burnt, during which special care is taken to prevent the spread of fire into the adjacent forest. Although upland rice in general is the main crop, other crops such as cassava, maize and chilli are also part of the cropping patterns. Crop calender of upland farmers starts one or two months in advance than that of the lowland farmers.

Table 2.1. Population and Forest Data of Selected Northern Provinces in Lao P.D.R.

Source: * - Phanvilay (1990)

** - Ministry of Economy Planning and Finance (1990)

Old shifting cultivators keep on rotating the same field according to the prescribed fallow period. However, new cultivators normally migrate from adjacent villages to create new opening in the forest. On the other hand, if the clearing is already too far from their original village a new small village will be established. One family with five members normally holds at least five hectares of plot.

Solicited field information indicated that upland cultivators preferred to open new forest than to cultivate long time abandoned fallow fields despite of the enormous task entail in removing tall and big trees. The possibility of obtaining large amount of ashes after burning together with stored organic matter within the forest soil became the major reason for them to prefer this site for their clearing practice. Besides, the availability of small to medium sized logs and abundance of firewood from a newly cleared forest were considered useful for housing construction or supplemental goods to be sold within the community.

2.7 Land Use/Land Cover Mapping Using Remote Sensing and GIS

2.7.1 Datasets and Methodology Used

SPOT multispectral satellite data acquired on March 26, 1992 was used as the principal source of information for the present study. Topographic maps and published information about the land use/land cover practices in the Northern Laos were also utilized. An intensive field trip of four days was organized on December, 1994 approximately three months earlier than the month of the acquisition of the satellite data. During the field trip, informal interviews were taken with the government staff and local people. A part of the study area was visited by establishing number of sample plots. In each sample plot information regarding shifting cultivation, forest types, topography, slope etc. were noted.

The SPOT image was spatially georeferenced to a UTM map projection using a first order polynomial and resampled with nearest neighbor algorithm. For the selection of ground control points (GCP), 1:50,000 topographic maps were utilized. A RMS (Root Mean Square) error of ± 1 pixel (±20 m) was accepted during the rectification.

Supervised classification using maximum likelihood classifier in ERDAS ( an image processing software) was performed by selecting number of training samples. Statistical analysis for these training areas were performed to find out their consistency. The classified image was converted to vector format for further GIS analysis in the ARC/INFO environment.

Village map was digitized from the 1:100,000 scale topographic map. Other information layers available in digital format were used in the study include, rivers, road networks, elevation and geology. These information were extracted primarily from the Digital Chart of the World (DCW) and were used for various GIS analysis with the classified image. The flow-chart of the methodology is shown in the figure below.

2.7.2 Land Use/Land Cover Categorization

Digital interpretation of the SPOT multi-spectral satellite data yielded eight major land use/land cover types for the study area. Forest category has been subdivided into three levels from its dense to a more open formation as affected by shifting cultivation. Likewise, varying stages of the shifting cultivation area were discerned from the newly cleared plot to that of abandoned shifting cultivated areas. Since majority of the study area consists of rugged mountains, along its narrow valleys lie paddy fields and on steep slopes topographic shadows abound. The following land use/land cover categories were discerned.
 

Mean value of representative training samples illustrates their varying vegetative intensity as demonstrated by the computed NDVI (normalized difference vegetation index) and the reflected infra-red channel (Band 3) of the satellite sensor. The weak NDVI values of the burnt agricultural plots (-0.03) particularly in the upland compared to the strong NDVI values of the close forest (0.3) were visibly shown in Figure 2.4. As part of upland farmer's field preparation, the exposed soil exhibits the brightest spots in the imagery, where a higher value of reflected visible light is also apparent.

However, series of similarities among the identified land use/land cover types were noted. This occurs between the lowland paddy field and the newly cultivated shifting cultivation where soil is exposed or recently left after harvesting. During the month of March, rice fields along the narrow valleys is still being left idle until the rain comes. In this respect, dried rice straw combined with exposed soil become the dominant cover of this paddy fields. Different forms of degraded forest and long time abandoned shifting cultivation also show difficulty in properly demarcating their boundary both in the imagery and on the ground. The effect of haze is also found visible for various stages of shifting cultivation especially along the valleys of Nam Tha and Nam Beng.

2.7.3 Land Use/Land Cover Assessment

The location as well as the size of shifting cultivation captured by the SPOT data exhibit a striking phenomenon of the study area. Majority of these cultivated areas are located just in between two major river systems. Large portion of abandoned and degraded forests are located in the left side of Nam Tha while the eastern side of Nam Beng is dominated by dense forest. In this regard, the proximity of these river systems where large villages are located, provided a strong link in terms of trading of goods and services. The presence of Road 2 along the Nam Beng river serves as the market pull in eastward direction for the villagers residing near the road. At present, the prominent steep slopes on the eastern side of Nam Beng yielded a natural barrier for the community not to expand this cultivation on this pristine and well-covered forest area.

The more or less rolling and less steep topography in the lower part of the study area approaching the Mekong River signifies larger plot size of upland cultivation compared to small patches in the hilly and steep slopes in the north. Large plot measures 650 to 200 ha., while smaller plot averages 2 to 3 ha. Small patches of cultivation were reported to contain 5 to 8 families in isolated distribution while large size cultivation accommodate 60 to 100 families. On the other hand, size of paddy cultivation increases as the river meanders southward.

Large proportion of the forest cover especially in the eastern side of Nam Beng is dominated with evergreen formation particularly the Dipterocarp group. The vast area in between Nam Tha and Nam Beng valleys where topography is somewhat rolling, exhibits a disturbed forest formation affected largely by shifting cultivation. The forest area in the western part of Nam Tha was reported to be subjected to such agricultural practice in long time ago. The presence of secondary forest with bamboo and tall grasses manifests the decreasing impact of human activity. On the other hand, a wide distribution of recently cultivated and abandoned shifting cultivation in the valleys of Nam Beng illustrate that the present utilization and intensity of cultivation of land is focused and more pronounce on this basin.