Secondary succession under a slash-and-burn regime in a tropical montane cloud forest: Soil and vegetation characteristics

Tropical montane cloud forest (TMCF) areas in southern Mexico are commonly used for growing maize and companion crops under slash-and-burn agriculture. As a result, the landscape is being transformed into a mosaic of crop fields, secondary forest and primary forests. Despite being a widespread process, very little is known about forest regeneration in TMCF areas. This chapter describes secondary succession in the TMCF of El Rincón Alto, Oaxaca, Mexico, with particular reference to soil and vegetation characteristics, taking advantage of successional chronosequences spanning a century of forest development. Cultivation practices cause severe losses of soil and carbon; the original vegetation and soil organic horizons vanish. Such effects are reversed, in part, during earlier stages of secondary succession. Soil layers, including those with organic horizons, begin to accumulate. Epiphytes, low-stature plants and shrubs begin to colonize very early during succession. Species richness of shrubs, geophytes and lowstature plants peak within the first 15 years after abandonment. The largest recorded decline in concentration of soil cations, and the highest annual rates of soil carbon sequestration also characterize this period. A pine-dominated community characterizes the first 10-75 years. Early successional species do not prosper under this forest canopy and an emergent stratum of broadleaf trees eventually replaces the former pine forest. Colonization of trees, lianas and climbing plants exceeds local extinction rates during the first century of forest development. The dominant groups of terrestrial and epiphytic plants are trees and liverworts, respectively, during all studied stages of forest development. Self-thinning of the first colonizing trees took place between 45 and 75 years after abandonment and coincided with a second increase in abundance of shrubs and other low-stature plants, but tree basal area did not decline significantly. Species of terrestrial plants typical of early stages are rare or absent in late successional stages. Thus, disturbances generated by autogenic processes such as natural tree fall are different from allogenic disturbances such as fires and landslides. Soil becomes increasingly acidic and infertile during forest development, a problem that is aggravated by the presence of soluble aluminium. Such acidity fosters mineral hydrolysis, releasing cations to the soil. Soil N/P ratios steadily decrease during forest development. We conclude that environmental changes derived from slash-and-burn processes increase landscape and species diversity under the long fallow regimes observed.