The decline of the itinerant milpa and the maintenance of traditional agrobiodiversity: Crops and weeds coexistence in a tropical cloud forest area in Oaxaca, Mexico

The Mesoamerican milpa shifting cultivation (MS), characterized by slash-and-burn practices and long fallow periods, has being substituted by a semi-permanent system (SP), characterized by fire suppression, tilling, the application of synthetic fertilizers, and shorter fallow periods. We compared the composition, richness, and structure of crops and weed species associated with these two cultivation systems in a tropical montane cloud forest area in Sierra Norte, Oaxaca, Mexico. We studied how such changes are associated with crop species and varieties, weed life-cycle, and weed cover. We could detect three maize varieties of the local maize landraces (Zea mays L.) in MS, and only two in SP. The yellow variety was the indicator crop of MS, while the white variety was that of SP. In MS, maize density was lower but often intercropped with climbing bean species, whereas in SP, maize monoculture was common. The shift of cropping systems altered drastically the composition, richness, density, and cover of weeds. The plots of MS and SP were clearly separated in two clusters of weed species. In MS, the density of perennial weeds was higher, the total weed cover was lower, and the bracken-fern was the indicator species. Contrastingly, SP displayed a higher richness and density of annual weed species, a high weed cover, and several annuals, mostly Asteraceae, were the indicator species. We conclude that MS is a better reservoir of agrobiodiversity, and its substitution may accelerate the decline of climbing beans, maize varieties, and perennial weeds, by favoring maize monoculture and the infestation of the crop fields by annual weeds. Banning of shifting cultivation in indigenous communities can contribute to the decline of agrobiodiversity, reduce the nutritional apportionment of the crops to the local people, and favor land degradation by weed-infestation.