Factors controlling succession following slash-and-burn agriculture in Amazonia.

Vegetation dynamics during 5 yr of succession following slash-and-burn agriculture in the upper Rio Negro region of the Venezuelan Amazon Basin were elucidated. The main study plot was dominated by grasses and forbs during the first year of succession, then by pioneer trees, particularly Vismia spp. Pioneer trees exhibit greatest establishment in protected farm-site microhabitats, such as under fruit trees and among slash. By 5 yr, Vismia mortality exceeded establishment and pioneer trees of the Melastomataceae were growing in the spaces vacated by Vismia. Primary forest species represented only 7% of all stems at this time. Due to burning and to decomposition of forest wood and root residues, there is a dramatic decline in carbon stocks during slash-and-burn agriculture. After 5 yr of succession, 86% of the plant mass from the pre-existing forest had disappeared from the main study plot. Biomass accumulation during 5 yr of succession only added 38 t ha^-1. Total site C stocks at 5 yr were well below half of the pre-burn forest stocks. Total above-ground production averaged 1258 g m^-2 yr^-1 over the study period - a value almost identical to that measured for mature forest. Pioneer trees colonizing this site grew 1-2 m in height and 0.5-2.0 cm in diameter each year; growth rate declined with tree age. Primary forest species grew c1 m in height and 0.7 cm in diameter each year and showed no age-related growth decline. -from Author