The Big Culprit Behind
Land Deterioration Agriculture
(Page 3)

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By Professor Nils Nykvist

When agriculture becomes problematic (continued)

Owing to newly constructed roads made for the transport of timber, farmers can more easily reach nutrient rich soils in more remote and formerly pristine rainforests. This is probably one of the main reasons that previously undisturbed rainforests now are being destroyed so fast, especially in Brazil and Indonesia, where the cultivation of tropical forests has been a way to reduce population pressures in other, more densely populated parts of the country.

Repeated burning of the same area leads to colonization of the previously cultivated areas by vegetation consisting of bushes, grasses, and herbs that benefit from fire.

How fire worsens the problem

Usually slash and burn agriculture does not ignite the surrounding closed rainforest, because the dead plant residues on the soil surface are too damp to ignite, and the amounts are small due to the rapid decomposition in the warm and moist climate.

However, if there are gaps in the forest, as there can be after selective logging, fire danger can be high, even in normal dry seasons due to faster drying out of the litter, and in the case of selective logging, also because more combustible plant material remains.

In a tropical rainforest in Venezuela with an annual rainfall of about 3500 mm, the superficial layer of dead plant debris in a forest gap was found to burn after only 4–6 rain-free days. In a nearby closed forest, however, it proved impossible even after 41 days to set fire to dead plant debris lying on the ground. [1]

In very dry years, however, fire from slash and burn agriculture can spread further into the surrounding forest, as had happened.

For example, in Southeast Asia during the dry years of 1982–83 and 1997–98, very large areas of forest were destroyed, and smoke paralyzed air traffic in the region. With the help of satellite images it has been estimated that many fires in the years 1982 and 1983 destroyed about 4.5 million hectares of forest in Borneo [2]. In the Amazon region a rainforest area equivalent to seven times the area of Switzerland was ravaged in 1988 [3].

During dry periods that can occur even in areas with a rainforest climate, the grass can be very flammable, and the rainforest over time replaced by a grassland that is very unproductive and unsuitable for grazing.

These grasslands, in which one of the most well known species of grass is Imperata cylindrica, are therefore often called green deserts (Fig. 4). The distribution of such grassland in Indonesia is estimated to be about 10%, and in the Philippines about 17% of the total land surface [4, 5].

In areas with low rainfall, the forests are not as dense and humid as the rain-forests and therefore do not provide equally good protection against fire from slash and burn agriculture. These climate zones are also more attractive for people than the humid rainforests, which has led to cultivation over a much longer time than in rainforest areas.

An example of the great impoverishment of vegetation and soil due to fires is the plant community called miombo woodland in southern Africa, where the original forests with time have been replaced by vegetation consisting of low-growing shrubs and trees that have the ability to endure fires (Fig. 5). This plant community today covers more than 3 million square kilometers in southern Africa, which is more than eight times larger than Sweden’s total land area (Fig. 6).
Figure 5. Frequent fires have in large parts of Zambia transformed the original forest into a low-productivity plant community consisting of low-growing shrubs and trees called miombo woodland.

Figure 6. The distribution of the plant community miombo woodland in southern Africa [6].

See the references for this article.

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