Shifts in swidden agriculture alter the diversity of young fallows: Is the regeneration of cloud forest at stake in southern Mexico?

For centuries, the milpa shifting cultivation, a maize-bean-squash polyculture in itinerant fields has been an essential component of the landscape (MS), in tropical montane cloud forest (TMCF) areas managed by Mesoamerican indigenous communities. At present, milpas are being replaced by a maize based semi-permanent system (SP) characterized by tillage, fire suppression, application of synthetic fertilizers, and short fallow periods. The effects of this substitution on post-cultivation fallows are unknown, but may have critical consequences for biodiversity and forest regeneration. In the TMCF area of Sierra Norte, Oaxaca, Mexico, the fallowed areas resulting from both cultivation systems are intermixed across the landscape. We compared the composition, richness, and diversity of vascular plant species in 2–3-year-old MS and SP fallows in this area, and explored the possible role of the adjoining forested areas on fallow plant species richness and density. Tree and herb species composition in the fallows separated into two clusters, one belonging to MS and the other to SP. Pioneer and late-successional TMCF tree species were distinctive of MS fallows, whereas native perennial grass and forb species were distinctive of SP fallows. The composition of other life forms could not be distinguished based on cultivation system. The species richness and diversity of trees, tree seedlings, shrubs, and vines were higher in MS fallows, whereas herb species richness and diversity was similar in both fallow types. In MS and SP fallows, the tree seedling species richness increased with the proportion of adjoining forested areas that were >20 years old, but not with forests younger than that age. Tree seedling density increased with the proportion of adjoining forested areas >20 years old in SP fallows but not in MS fallows. The density of trees and shrubs were significantly higher in MS fallows than in SP fallows, whereas herb density was significantly higher in SP fallows than in MS. We concluded that the replacement of MS by SP on the one hand, reduces plant diversity in early fallows and the fallow potential for TMCF regeneration; on the other hand, increases the dependence on adjoining >20-year-old forests as propagule sources.