Improved fallows as alternative for shifting cultivation in Chiapas, Mexico

Background/Question/Methods

The transformation of natural forests to secondary vegetation and pastures has been the most common land-use change in tropical countries in recent decades. Shifting cultivation has been regarded as the main driver of land use change in tropical countries, especially in Mexico. Although shifting cultivation was formerly a sustainable practice, smallholdings and pressure on land use have shortened rotation cycles limiting the recovery of the site's productive capability. Previous studies have indicated that the functions of the ecosystem decline with each slash-and-burn cycle.

It has been argued that improved fallows may play an important positive role on ecosystem structure and functions.

The aim of this research was to assess improved fallows in relation to continuous maize and traditional fallows in farmer’s plots in Chiapas, Mexico.

Interviews and inventories were conducted in 24 farmers and plots in Bachajón, Chiapas Mexico. Timber trees (Swietenia macrophylla, Cedrela odorata, Ceiba pentandra, Cordia alliodora, Pinus chiapensis) were established on 2m-narrow lines alternating with 7m-line natural vegetation in five-year old natural fallows. Soil, structural and functional variables were assessed as indicators of site conditions, productivity, complexity, diversity, ecosystem services and economic value in plots arranged along a chronosequence.

Results/Conclusions

A general trend of increasing complexity from continuous maize to improved fallow and traditional fallow along the chronosequence was observed. At the ninth-year since establishment improved fallows showed a mixture of cultivated trees interspersed with coloniser trees and shrubs in a proportion of 40:60.  Sixty seven tree species were accounted for improved fallows.  A significant accumulation of aboveground biomass (R²=0.22), soil organic carbon (R²=0.29), carbon in dead organic matter (R²=0.97) and woody plant diversity (R²=0.41) were showed over time. Improved fallows showed an important economic value given through tree enrichment. All the three systems presented a decreasing carbon percentage with soil depth (0-30cm; R²>90). However, the variation of carbon percentage in soil depth was less pronounced in improved fallow than in both traditional fallow and maize, which may be attributed to tree pruning. The correlation between pH and time showed a slight negative trend in both types of fallows.

Results permit to suggest that improved fallows may play an important role to produce timber, reduce burning, to maintain ecosystem functions, to sequester carbon and to contribute to sedentarize agriculture, where shifting cultivation is the method commonly used by farmers in similar regions.