Introduction
Swidden agriculture (also called slash-and-burn or shifting cultivation) including the length of fallow period and its intensity is continuously evolving or being transformed since the mid-20th century, however, it remains a widespread but controversial farming practice and/or land use category in tropical uplands (see the figure, panel A )(Cairns, 2015; Heinimann et al., 2017; van Vliet et al., 2012). Sometimes, this is a straw to clutch for millions of impoverished upland ethnic groups facing constantly-changing market and extreme climates (e.g. El NiƱo)(Cramb et al., 2009; Smith & Dressler, 2019). The evolution and/or transformation not only matter the swidden-dependent uplanders, but also trigger endless arguments (e.g. pros and cons) of swidden agriculture towards carbon emission and biological diversity(Fox, Castella, & Ziegler, 2014). After age-long coexistence with human beings, how will the traditional farming (or practice) develop in this century(Heinimann et al., 2017)? Will it demise or persist?