Forest succession in the Upper Rio Negro of Colombia and Venezuela

The Woody vegetation from 23 forest stands along the Upper Rio Negro of Venezuela and Colombia was sampled in 1982. Stands were selected from the tierra firme forests to represent a chronosequence of succession following "slash-and-burn" agricultural practices. objectives of this study were to examine the hypothesis that the Amazon forest has been largely undisturbed since the Pleistocene, to quantify vegetation development during different stages of succession following agricultural development, and to determine the time required for a successional stand to become a mature forest.

The ubiquitousness of charcoal in the tierra firme forest indicated the presence of fire associated with extreme dry periods and human disturbances. Charcoal amounts ranged from 3 to 24 t/ha, and the dates of charcoal from radiocarbon determinations ranged from approximately 6200 years before present (B.P.) to the present. Several sample dates coincide with dry phases recorded during the Holocene for central Amazonia. Ceramic shards were found at several sites, and thermoluminescence analyses indicated that their age ranged from 3750 to 460 years B.P. The substantiated age of the charcoal and shards confirms that the region has been subjected to fire and human disturbances.

Changes in species composition, vegetation structure, and woody Diomass were studied on 19 abandoned farms and four mature forest stands. The number of tree species per stand ranged from 33 to 96 in 2 900-m² plots. On 2.1 ha of cumulative stand area, ~ 290 tree species ≥ 1 cm dbn were identified.

The distribution of stems by size class showed that 76 to 95% of the trees are within 1 to 5 cm dbh. Trees ≥ 40 cm abh were found only in stands 40 years and older, representing < 1% of the total stems. Basal area ranged from 11.12 m²/ha for a 10-year-old stand to 2 36.95 m²/ha for a mature forest.

Living and dead biomass for the trees and their components was determined by regression equations developed from measurements of harvested trees. Total living aboveground and belowground biomass ranged from 51 t/ha for a 10-year-old stand to 336 t/ha for a mature forest. Belowground biomass ranged from 7.4 t/ha for a 10-year-old stand to 64 t/ha for a mature forest. Belowground root biomass was greater in mature forests than in any successional stands.

The rate of recovery of floristic composition, structure, and biomass following disturbance is relatively slow. Aboveground dead biomass remained high 14 years after the forest was disturbed by the agricultural practices. The lowest dead biomass is reached 20 years after abandonment, and the largest values are found in mature forests. Data analysis of 80-year-old stands showed that the species composition approached that of a mature forest. Approximately 140 to 200 years was required for an abandoned farm to attain the basal area and biomass values comparable to those of a mature forest. The results of this study indicate that recovery is five to seven times longer in the Upper Rio Negro than it is in other tropical areas in South America.