Successional pathways of post-milpa fallows in Oaxaca, Mexico

The milpa is a millennial agroecosystem commonly practiced in the Mesoamerica mountains. For soil recovery, the milpa depends on the fallows that develop in lands left unsown after the cultivation stage, but their successional dynamics are little known. We studied the successional dynamics of post-milpa fallows in the mountains of Mixteca Baja, north-western Oaxaca, Mexico, where this cropping system is common. We use the chronosequence approach in fallows ranging from ≈20 to 100 years old of three nearby watersheds that differ in their degree of exposure to Pacific Ocean winds, the leading source of rain and moisture. We analyzed information from field plots and unmanned aerial vehicle imagery to detect successional trends in milpa fallows. We find that around two decades after cultivation, the fallows are forests dominated by pines or alders. Plant density, vegetation cover, and diameter at breast height show little change afterward, suggesting that the forests reach a saturating point very early during succession. Subsequently, the main changes are in composition and diversity. Shade-tolerant species may replace pioneer species, species composition is moisture-related, and diversity shows contrasting patterns depending on the watershed. Old-growth vegetation is typically an oak forest in the driest watershed and a tropical montane cloud forest in the most humid. Species dominance tends to decrease with succession. β-diversity is significant and can be ascribed to microclimate and successional processes associated with orography and the milpa practice. Older stands within the same watershed, tend to diverge in composition relative to younger stands, suggesting a lack of convergence towards a climax community. Combining young and old-growth forests might positively impact biodiversity and landscape resilience and helps explain the millennial persistence of milpas in Mesoamerica.