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

University of British Columbia

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. The fallow is traditionally believed to act to restore and sustain soil fertility and control weed populations, but may also be important for the maintenance of upland rice productivity by maintaining soil structure and in other ways. Presently, this system involves one year of cropping followed by five years of crop-free fallow. The national Thai government is trying to change shifting cultivation to fixed-field agriculture, but some ecologists and social scientists oppose the idea using arguments about the ecological and cultural integrity of this traditional farming practice. There has been little empirical research to examine the advantages and disadvantages of the system. 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. These decreases are attributed to nutrient uptake by the fallow vegetation and the decline in the effect of the burning at the end of the previous rotation. The largest changes in soil conditions took place when the 5-year fallow field was slashed, burned, and cropped. Standing-tree biomass increased gradually during the fallow period. Chromolaena odorata dominated the first two years of the fallow period, and it accumulated about 7 t/ha of aboveground biomass within one year after rice harvesting. Fertilizer trials of the regular first-year and the experimental second-year upland rice crop showed that N was the most deficient nutrient in upland rice productivity, which support the nutritional role of the fallow. Growing rice for a second-year also revealed that soil pathogens may play an important role in decreasing upland rice productivity in consecutive-year cropping. Interviews with farmers were explainable in an ecological context; for example, the variability within and between crop fields which reflects variability of soil fertility, which in turn depends on localized topography and physical characteristics of the soils. The farmers responded that the fallow period could be reduced to a minimum of two to three years, and the data support that this might be associated with increased weed competition and less ash. 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. Quantitative assessment of other pathways of nutrient inputs (e.g. N fixation and soil weathering) and outputs (e.g. erosion and leaching) are needed for a complete bipgeochemistry of the ecosystem of the forest-fallow shifting cultivation in order to examine its sustainability. 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.

Thesis/Dissertation

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2009-09-23

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http://hdl.handle.net/2429/13134

Forestry

University of British Columbia

UBCV

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