The effect of fallow period length on the abundance and diversity of usable plant assemblages in shifting cultivation system (Swidden agriculture) in northern Laos

Shifting cultivation, or swiddening, involves alternating cycles of cropping and fallow periods. Rather than being abandoned, the fallow swidden fields provide a source of important plant products for remote, forest-dwelling human communities. In northern Laos, the traditional length of fallow periods used to range between five and 11 years, depending on the availability of land and quality of the soil. Fallow periods are currently being shortened because of population pressures and government policies. To determine the potential impacts of shorter fallow periods, 128 swidden fallows of various ages (up to 20 years), located at altitudes between 400 and 800 m a.s.l. in northern Laos were sampled for usable plants. The impact of fallow length on plant abundance and species diversity was analysed. The number of usable plant species was found to increase with the fallow age until it was 11-12 years old, and then decline, while the abundance of usable plants increased beyond that period. We conclude that shorter fallows will result in a significantly smaller usable plant abundance and species diversity for the villagers. We propose that 11-12-year-long fallows should be considered in order to fully exploit the greater plant volume of older fallows as well as the compositional shift of species in fallows over time.