Final answer:
Shifting cultivation was sustainable in the past due to techniques like slash-and-burn agriculture and crop rotation, which allowed for the rest and regeneration of the land's fertility.
Step-by-step explanation:
In many regions, shifting cultivation was considered sustainable in the past because it allowed for the rest and regeneration of the land's fertility. Techniques such as slash-and-burn agriculture and crop rotation were employed to maintain the bioproductivity of food-producing ecosystems. For example, slash-and-burn agriculture involved clearing a plot of land by burning vegetation and then planting crops on the nutrient-rich ashes. After a few years of cultivation, the land was left fallow, allowing the soil to recover and regain fertility.