Answer:
Tavy works well and does not permanently destroy the forest as long as field sizes are small and farmers leave adequate time for re-growth. However, if farmers return to the fallow fields too quickly, as they do when human population densities increase, the soils become exhausted. And if little forest is left in between fields, then there are no parent trees to provide seeds and seedlings to restore the forest. Eventually large areas of forest are transformed into wastelands, upon which nothing can grow—neither rice nor forest. On these areas, farmers pasture a few cattle and continue to burn the grasslands each year, to provide "greener grass" for the cattle.
