Der Einfluss des Brandrodungsbaus auf das Gefüge des Tropenwaldes und die Wasserführung der Ströme, untersucht am Beispiel Nordthailands

Abstract

This paper deals with the effects of shifting cultivation on the composition of the tropical forest in the northern provinces of Thailand seen from the aspect of forestry. The evaluation of the data of the 1956/57 forest census leads to the following conclusions. 1. Of the areas covered by the two types of forest, mixed deciduous monsoon forest and semi-evergreen broadleaf forest, 55 °/o is at present unproductive land as regards forestry as a result of shifting cultivation by burning. 2. Tectona grandis (teak), economically the most important tree species for export, is unable, or at best only very slowly able, to recolonize the sites lost through shifting cultivation by burning. The secondary forest of the type mixed deciduous monsoon forest which establishes itself on the burnt land is for decades, if not centuries, void of teak. The phenomenon of island-like occurence of teak within its appropriate type of forest is a result of earlier clearing by fire. Tectonia grandis may be taken as an example to show that shifting cultivation not merely reduces the quantity of timber production for a long time, but that a most valuable type of timber is gradually obliterated from its natural habitat. The present area oft the mixed deciduous monsoon forest which contains teak comprises merely 45 %» of the original. 3. Of the areas above 1,000 m of altitude which originally were certainly almost completely wooded, only a third can now be considered as unimpaired mountain broadleaf or coniferous forests; two-fifths are clearings at present used agriculturally or old clearings of nomadic mountain tribes, and a quarter are savannas whose origin is probably largely a result of earlier shifting cultivation. Shifting cultivation by burning, which at these altitudes has been greatly on the increase over the past 25 years, has already had its effects on the regimes of the rivers. Even if the damage resulting therefrom is as yet not directly tangible, since rice cultivation could be adjusted to the changed regimes of the rivers, the evident increase of floods already shows that an extremely dangerous situation will develop in the future. From the aspect of forestry the judgement on shifting cultivation by fire is thus as follows: the damage of shifting cultivation on the composition of the forest has already reached a disastrous degree as far as Thailand is concerned, and from a hydrological point of view future danger is also clearly discernible. At the North Pacific Congress of Science in Bangkok in 1957, the subject under consideration was shifting cultivation and the damage it causes. Two points of view emerged quite clearly. On the one side it was said that shifting cultivation would certainly not give rise to the unfavourable consequences attributed to it, on the other side the detrimental changes it causes in soil and climate were emphasized. In view of this clash of opinions it seems important to draw attention to two points in particular. 1. When speaking of shifting cultivation by burning a distinction should always be made between that with an established rotation cycle and only a one year utilization for agriculture, and another one without an established cycle and an agricultural utilization of possibly more years. As far as shifting cultivation is practised reasonably, systematically and with protection of the soil in mind, as for instance in the Philippines (5), the islands of Melanesia (1), the Ivory Coast (3), in Sarawak (7) and the Belgian Congo (24), a long-term damage of the soil cannot be discerned directly and such shifting cultivation can possibly be pursued for centuries without causing serious damage to agriculture. If, however, an unsystematical shifting cultivation, merely governed by momentary considerations, is practised, allowing only short recovery periods of the areas between the periods of agricultural utilization, which in turn sometimes even last for a number of years without interruption, then anirreparable damage of the soil is the result. 2. Seen from the aspect of forestry, shifting cultivation is always seriously criticised since it involves the burning and wasting of valuable timber. This may not be important as long as wood is plentiful and the potentiality or necessity of a regulated uitilisation of the forests does not yet exist. As far as the mixed deciduous monsoon forest of northern Thailand is concerned, the author hopes to have shown that in this case shifting cultivation must be seen from the aspect of its damage to forestry because of the importance of the timber resources for the further economic development of the country. To act according to the principles of forestry means to plan the utilization of woodlands on the basis of a longterm programme of lasting maximum production of usable timber. But the thinking and acting according to the principles of forestry have made their appearance only fairly recently. It is true that forestry authorities do exist in the developing countries of the Far East, but principles of forestry have not yet become an integral part of the national life of Thailand. Amongst the city dwellers as well as the country people the prevailing attitude still is, one hostile to the woodlands. Even in parliament it is difficult for the forestry authorities to find sympathy for their memoranda to set aside forest reserves, to restrict the completely unregulated shifting cultivation and unlicensed tree felling. There is a resistance to forestry as there has been to the introduction of technical advances of any kind, and only a general rise in the level of education will, after a time, be a remedy for that. The author is quite aware that particularly in Thailand shifting cultivation will continue and cannot be stopped during the next few decades. It might, however, still be possible to prevent the danger of a complete extermination of teak if means and ways are found to limit shifting cultivation to the mixed deciduous monsoon forest without teak trees and to urge the people to plant young teak trees in the clearings which could be made obtainable from yet to be established nurseries of the Ranger Stations; that is the so-called Taungya Plantation system which was developed in Burma. Only the future can tell whether it will be possible to apply such measures in Thailand successfully, or whether the haphazard shifting cultivation will reduce the teak production of Thailand more and more.