Ecological studies of reduced forest-fallow shifting cultivation of Karen people in Mae Chaem watershed, northern Thailand, and implications for sustainability.

摘要

The forest-fallow system of shifting cultivation of upland rice and other food plants practiced by the Karen people of Mae Hae Tai village, Chiang Mai, northern Thailand, is changing due to increasing population and a resulting decrease in per capita arable land-base. This has resulted in a reduction of the fallow period, which was 10 or more years in the past.; Ecological studies were conducted to examine nutritional aspects of the forest-fallow shifting cultivation using field experiments and a chronosequence of fields. The farmers were interviewed about their traditional knowledge of shifting cultivation system management. The yield of the upland rice crop under this system was found to be about 1 t/ha, but is variable within fields, between fields, and between years. The chronosequence study revealed that during the five years of fallow there was an increase in soil organic matter and total N attributed to the addition of litterfall from the fallow species, but a decline in pH, available P, and extractable K, Ca, and Mg.; The biogeochemical studies of the forest-fallow shifting cultivation system showed that nutrient losses via slash burning and harvested rice grain are important outputs of N. P was found to be lost the most via harvested rice grain, while losses in erosion and leaching may be important for K, Ca, and Mg.; A series of carefully controlled and replicated field and pot experiments is needed to resolve the relative importance of the different contributions of fallow to the sustainability of upland rice. The following topics also deserve further research work: dynamics of N in the system, change in resource-allocation patterns between above- and belowground tree components, soil microbial activities and their effects on N cycling, and other roles of the fallow periods (e.g. maintaining good soil structure and providing useful plants and animals). The current fallow period of five years appears to be sustainable at the present landscape condition, but a further reduction in fallow length may pose a risk to the apparent sustainability of this forest-fallow shifting cultivation. Comparison of nutrient cycling between forest-fallow shifting cultivation and fixed-field farming by simple and/or computer models is needed to assess their sustainability. (Abstract shortened by UMI.)