Studies on succession in cleared areas of Amazonian Rain Forest / Mário Dantas.

Tese (Ph.D.) - Oxford University, 1989

Amazonian primary rain forest plots, which had been trasnformed at different times within the last 20 years into man-made pastue, plantationsm or secondary vegetation were all shown to have undergone impoverishment in terms of floristic composition, species diversity, standing biomass and nutrient stocks. Ecosystem recovery started immediately after perturbation of the primary forest by means of clearfelling plus a variety of land clearance procedures. It was hypothesised that succession under all treatments, and in the absence of further disturbance, would proceed towards a floristic composition, biomass and nutrient stock level similar to that observed in mature primary forest. Floristic composition, species diversity, density and frequency were studied in primary forest and different stages of succession (early abandoned pasture; 2 year, 3 year, 4 year, 5 year and 10 year old capoeira). Species composition of primary forest, as represented by plants with a greater than 30 cm, comprised 115 to 221 species per hectare depending on location. Species diversity was high and the plant community was characterized by a few dominant species with many individuals and a large number of rare species. In successional vegetation, species composition changed over time showing three clear stages: herbaceous-graminoid, shrubby and arboreal. The diversity was high and tended to increase over time while number of individuals per species decrease. Frequency of occurrence patterns were similar to those found for primary forest. Standing biomass and nutrient content: litterfall, ash production and their potentially available nutrients were studied in selected stages of succession. Primary forest biomass at different locations ranged from 374 to 522 t dry weight.ha-¹ (mean ± SD = 468 ± 66 t.ha-¹) while in secondary vegetation it was more variable depending on age and previous land use (range: 5.54 t.ha-¹ in abandoned pasture to 53.2 t.ha-¹ in 2-year old capoeira). Nutrient concentrations were found to be higher in species characteristic of secondary vegetation but the nutrient stocks per unit area of the successional ecosystems were much less than those of primary forest. Similar amount of nutrients were released via ash from primary forest and a 20-year old capoeira; but the total nutrients in litterfall proved to be highest in primary forest. The impact of four methods of land clearence (slach + burn, slash + herbicide, slash + burn +herbicide and bulldozer) on seed banks and early stages of succession were also studied. Bulldozer was the most and slash + herbicide the least, deleterious treatment according to the attributes under study.