Milpa imprint on the tropical dry forest landscape in Yucatan, Mexico: Remote sensing & field measurement of edge vegetation

The Yucatan Peninsula hosts part of Central America's largest remaining tract of tropical dry forest and has been identified as a region of critical landscape change. This study complements the extensive research on land cover conversion in the region by investigating a subtle but important aspect of forest modification. We examine changes in the spatial characteristics of milpa cultivation plots in the swidden landscape of Peto municipality in Yucatan state from 1988 to 2003 using remote sensing. We also test the hypothesis that milpa clearings create a discernible edge effect in terms of forest structure. Results indicate that spatial patterns of milpas have changed over time. The amount of milpa/forest linear interface increased over the study period. Both satellite-based vegetation indices and field-based canopy cover measurements indicated that forest buffering milpa clearings had significantly lower biomass than background forest, despite that the background forest is itself a mosaic of successional forest stages. In contrast, there was no difference in stand basal area for milpa edge forest and background forest. Multivariate models demonstrated that the milpa edge indicator was the most important variable in explaining differences of vegetation indices for milpa edges and background forest compared with other factors that create edges in the landscape. Models were relatively effective in explaining mean values of vegetation indices; but they performed poorly in terms of explaining measures of forest vegetation heterogeneity. Comparing model results from each date suggests that the importance of milpa edges decreases over time, possibly as a function of the accumulated land use history as milpas rotate through the forest matrix. Evidence supports the notion that the effects of milpa land use extend beyond the clearing itself and into adjacent forest.