INTRODUCTION

The jhum cultivation or shifting cultivation, popularly known as cultivation of slash and burn, is the most prevalent form of cultivation in the hilly areas of tropical Asian countries. It involves cutting of patches forest in the month of February to March burning of the slashed, dried vegetation after one month and then sowing of crop seeds in April in small holes made throughout sloppy fields. Harvesting of the crops is done in succession as they ripe between July to December. Usually rice, maize, millets, sesame, cotton, ginger, cucumber, pumpkin and melon etc are grown. It is carried out on different plots by shifting the place of cultivation after 1-2 years after which the land is abandoned and allowed to regenerate. The fallow quickly recovers into secondary forest from coppies, underground rhizome, root suckers and the soil seed bank (Mantel et al., 2006). The crop mix of perennial and season crops in jhum cultivation allows phased harvesting ensuring food security throughout the year and also provides needed diversity for nutrition and food preferences (Chhauchhuak, 2004b). The life and culture of the ethnic peoples of hill areas depend to a great extent on jhum cultivation. In a study at Temenggor Lake, Malaysia it was observed that the tribal peoples (Orang asli) of the area also practice jhum cultivation and their jhuming practices are affected by the establishment of dams on the lake (Karim and Mansor, 2010; Mansor, 2010). The International Centre for Integrated Mountain Development (ICIMOD) recognized jhum cultivation as good practices in term of farming, forestry, soil and water conservation and bio-diversity management (Kerkhoff and Sharma, 2006). However, the intensity of jhuming varies with changing condition of rainfall, topography, accessibility and density of population (Raman, 2000). It differs from settled cultivation (e.g., terrace cultivation) in many aspects such as ecological, economic and socio-cultural condition (Arunachalam et al., 2002) (Table 1).

In Bangladesh, jhum cultivation is practiced in the hill areas of Chittagong Hill Tracts (CHTs). The CHTs are located in the southern part of Bangladesh bordering with India and Myanmar and are the home to 11 different national communities. Chakma and Tongchonga tribal community belief that they have to put jhum soils (as sacred soil) on the feet of the goddess in every religious occasion.

Table 1:	Characteristics of jhum and settled cultivation (e.g., terrace cultivation)
Source: Modified from Arunachalam et al. (2002)

Lying between latitude 21°11-23°45 N and longitude 91°42-92°42 E, the region occupies an area of about 1.8 m ha which constitutes about 12% of the total land of the country. The total population is 1,05,500 and 53% of the population are tribal (ACFOD, 1997; BBS, 2000). In India it represents the area of the north-western Himalayas, north Kashmir, covering Ladakh and Gilgit districts with an area of 15.2 m ha, occupying 4.7% of the total geographical area (329 m ha) of the country.

This study examines the gradual changes occurred in government policies which affected jhum cultivation in hills and impacts of jhum cultivation on the management of soil and water resources, biodiversity, forest crop and productivity and socio economic conditions of jhum cultivators of tropical Asia with special reference to Bangladesh.

Gradual changes in jhuming areas of CHTs: In the jhum cultivation areas i.e., CHTs significant changes have had occurred in land-use over several centuries which can be categorized as Pre-British colonial period, British colonial period, Post-colonial period and Independence of Bangladesh period (Rasul and Thapa, 2007). These changes give an idea about the impacts on the mountains and the socio-economic conditions of the jhum cultivators (Table 2).

Table 2:	Changes occurred in land-use and socio-ecological aspects during different historical periods
Source: Modified from Rasul and Thapa (2007)

Table 3:	Trend of reduction in jhum cycle (fallow period) in CHTs over centuries
Source: Modified from Chhauchhuak (2004a)

Impacts of jhum cultivation: The degradation of land and forest resulting from shifting cultivation are the serious threats to the people of CHTs of Bangladesh. Among the other ill impacts, invasion of exotic weed species in the jhuming areas is of vital importance. Obviously, weed invasion is influenced by the ecology of the target area (Karim and Mamun, 1988; Karim et al., 1999). The effects of jhum cultivation on soil and water resources, forest productivity, biodiversity and socio-economic conditions of the jhum cultivators are enumerated below: