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4 TRENDS IN FIELD EXPANSION AND SOIL DEGRADATION, IN
RELATION TO CROP YIELD AND POPULATION PRESSURE
4.1 INTRODUCTION
In the tropical forest area, agriculture has always been pointed out as the main factor of land
degradation because of its effects leading to soil degradation and deforestation. In Chapter 2,
various agricultural systems practised in the study area have been reviewed and their effects on
soil behaviour have been evaluated in Chapter 3. The food cropping systems appeared to bear the
main characteristics of shifting cultivation as result of important changes in soil properties during
the cropping period. This shifting effect of agricultural fields is cause of continuous deforestation.
This chapter aims to evaluate the expansion rates and trends of these fields and analyses the
relationships with the causal factors and their effects on the spatial sustainability of forest
management.
4.2 METHODOLOGY
Trends in agricultural field expansion are best studied by multi-temporal remote sensing images.
In this study, only the 1984 - 1985 aerial photo interpretation landscape ecological map was
available showing undisturbed forest land and land under agricultural activities (Touber, 1993).
This landscape ecological map was digitised and computerised using arc-info software. To obtain
the proportion estimate of the agricultural field expansion, the maps of the 290 agricultural fields
population sample (see Section 2.2.4) opened between 1995 and 1997 were overlaid on the 1984 -
1985 aerial photo-interpretation landscape ecological map using arc-view software. Fields that
were in 1984-1985 still undisturbed forestland were grouped to compute the sample proportion of
the forest that has been transformed into agricultural land during the 13-year period. The
following formula was used to determine the total land area (A 13 ) under agriculture in 1997:
A n* A 0
A 13 = -----------
A n0
Where:
A 13 = total land area under agriculture in 1997
A 0 = total land area under agriculture in 1984
A n = total measured agricultural sample fields population in 1995-1997
A n0 = total measured agricultural sample fields population under agriculture in 1984
The agricultural field expansion into the forest after 13 years is defined by A 13 - A 0
Stepwise multiple linear regression analysis was used to evaluate the relationships between soil
property changes and crop yields.
4.3 RESULTS AND DISCUSSIONS
4.3.1Trends in agricultural field expansion
From the Landscape Ecological Survey of the study area (van Gemerden and Hazeu, 1999) based
on aerial photograph of 1984 - 1985, over a total area of 167,350 ha and 51,180 ha were used for
low and high intensity agriculture respectively. This agricultural area represented about 31% of
the total TCP area (Table 4.1). A sample of 94.5 ha was mapped and used proportionally to the
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