Forest Degradation Assessment Deforestation Monitoring

The pressure on forests is greatest in the developing countries. The primary causes of deforestation are encroachment of forest area for agricultural production and exploitation of forest cover for meeting housing and industrial needs. Deforestation leads to an increase in the loading of CO2 in the atmosphere. Increased albedo and change in aerodynamic roughness over deforested areas alter the energy balance bearing implications on atmospheric circulation patterns and rainfall statistics. Deforestation leads to soil erosion and gradual loss of biodiversity.

The amount of vegetation loss/deforestation due to encroachment can be estimated by the use of remote sensing technique. The impact of slash and burn during and after the 'jhumming' (slash and burn agriculture) operations is clearly visible from remote sensing imageries. The representative relationship between the population density and the percent of forest cover provides information about the rate of deforestation and thereby helps in formulating the mitigation plan. Utilization of remote sensing tool for stock mapping and growing stock estimation for forest management improves reliability. Assam is well known for its large forest tracts. The recorded forest area in Assam is 39.15% of the geographical area. These forests are repositories of a rich biological diversity. At the same point there is tremendous pressure on these forest lands. There has been an overall decrease of 1,031 sq. km of dense forest from 1997 to 1999 in Assam. This decrease is more pronounced in the Brahmaputra valley in the areas like Sonitpur and others. This study was undertaken after large-scale deforestation was observed in above district by IIRS team of scientists working in Arunachal Pradesh. The objective was to assess the large-scale deforestation and loss in biodiversity.

This study covers the entire Sonitpur district (5,103 km2) located in upper Assam valley. Land use within the area is divided primarily among tropical semi-evergreen forest, moist deciduous forest, riverain forest, pasture land, agriculture and tea gardens. Good quality Landsat-TM and IRS-1C LISS-III false colour composites of dry season pertaining to 1994, 1999 and 2001 periods were used to monitor the loss of biodiversity. All scenes were radiometrically and geometrically corrected and on-screen visually interpreted into forest and non-forest cover classes. The total number of plant species, species diversity, economically important and endemic species in similar forests in Assam were worked out in field to understand the type of loss incurred due to large scale deforestation.

The forest cover type of 1994, 1999 and 2001 are shown in Fig. 9. All three types of forests viz., semi-evergreen, moist deciduous, and riverain could be mapped from three data sets of different time periods. Results indicate that moist deciduous forests occupy the maximum area followed by tropical semievergreen and riverain. A loss of 86.73 km2 (1.68%) was observed between 1994-99 and 145.44 km2 between 1999-2001. An increase of 5.0 km2 area was observed in moist deciduous forest. The loss in semi-evergreen forests was found to be 0.52 km2 (0.01%) from 1994 to 1999 while between 19992000/2001 it was 2.04 km2 (0.04%). There was no loss in case of riverain forests. Table 5 gives the area under different forest types during different periods.

Figure 9: Deforestation monitoring in Sonitpur District of Assam

The results of field survey show that moist deciduous forests possess highest biodiversity (Shannon and Wiener Index -6.49) followed by evergreen (5.60) and semi-evergreen (5.45) forests.

Table 5. Area (km2) under different forest and non-forest categories in Sonitpur

Land cover




Net change

Moist deciduous














No change






Tea garden









No change










No change

The spatial distribution of different forest types from 1994 to 2001 shows that forest cover in the Sonitpur district undergoing massive reduction with time. The rate of deforestation in the district worked out to be 10.7% from 1994 to 1999 and 20% from 1999 to 2001. The overall rate of forest degradation was estimated to be 28.65% between 1994 and 2001, which may be the highest rate of deforestation anywhere in the country. The findings of the field survey suggest that we have lost very invaluable moist deciduous and semievergreen forests in the ongoing deforestation in Sonitpur district. Ironically, these forests happen to be the climax vegetation in the region, known for their immense ecological and economic value.

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