The Lisu practiced swidden (slash-and-burn) agriculture. In conditions of low population density where land can be fallowed for many years, swiddening is an environmentally sustainable form of horticulture. Despite decades of swiddening by hill tribes such as the Lisu, northern Thailand had a higher proportion of intact forest than any other part of Thailand. However, with road building by the state, logging (some legal, but mostly illegal) by Thai companies, enclosure of land in national parks, and influx of immigrants from the lowlands, swidden fields can not be fallowed, can not re-grow, and swiddening results in large swathes of deforested mountainsides. Under these conditions, Lisu and other swiddeners have been forced to turn to new methods of agriculture to sustain themselves.