Factors Controlling Succession Following Slash-and-Burn Agriculture in Amazonia

SUMMARY (1) Vegetation dynamics during five years of succession following slash-and-burn agriculture in the upper Rio Negro region of the Amazon Basin were elucidated by combining (i) permanent plot studies of plant-replacement dynamics and productivity, (ii) field analyses of the resource-procurement strategies of successional species, and (iii) field experiments on the role of regeneration strategies, establishment microhabitats and nutrient availability in controlling succession. (2) The main study plot was dominated by grasses and forbs during the first year of succession and 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 five years, 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. (3) The number of tree species ( > 2 m tall) increased from seventeen (year 1) to thirtyfive (year 5). The number of pioneer species remained constant (about twelve) while the primary forest species increased slowly. Nevertheless, most primary forest species were represented by only one individual after five years. (4) A key factor retarding succession is the slow rate at which primary forest species become established on abandoned farms. Diaspores of forest species that were artificially dispersed to a farm site had high rates of removal by animals. However, the probability that the diaspore of a species would escape removal and germinate increased as diaspore size increased. If these diaspores do germinate, they stand a high chance of surviving; seedlings of five forest species transplanted to an abandoned farm showed over 90% survival after 4 5 years. (5) 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 five years of succession, 86% of the plant mass from the pre-existing forest had disappeared from the main study plot. Biomass accumulation during five years of succession only added 38 t ha- '. Total site carbon stocks at five years were well below half of the pre-burn forest stocks. (6) Based on measurements of tree growth and litter production, total above-ground production averaged 1258 g m-2 year- ' over the five-year study period-a value almost identical to that measured for mature forest. Pioneer trees colonizing this site grew 1-2 m