Succession and Nutrient Dynamics Following Forest Cutting and Burning in Amazonia

Plant establishment, plant mortality, vegetation productivity, nutrient accumulation, and nutrient leaching were studied during the first 5 yr of succesion following the cutting and burning of a forest plot in the upper Rio Negro region of the Amazon Basin. The speed and vigor of forest regeneration following disturbance of this type provide a measure of the recuperative potential of this ecosystem. The study plot (0.09 ha) was dominated by forbs during the 1st yr and then by the pioneer tree species Cecropia ficifolia Snethlage during year two. Tree mortality exceeded establishment during the 3rd yr because of C. ficifolia trees died nearly in unison. During the 4th yr, tree density increased sharply as successional and forest tree species grew in the space vacated by C. ficifolia. The canopy was dominated at that time by the pioneer species Vismia japurensis Reich. and Vismia lauriformis (Lam.) Choisy. By the 5th yr, all establishment space had apparently been preempted: there were only 0.07 new establishments/m² during year five, while 95% of the trees present at the end of year four survived through the 5th yr. There were 56 tree species (@<2 m tall) present on the site after 5 yr. More than half of these were primary forest species. Primary forest species were most common in the understory. These trees averaged >1 m in growth height each year and should eventually grow into the canopy. Hence, it appears that most individuals that will participate in the aggrading phase of forest development were already present on the site within 5 yr of burning. Biomass was only 66 g/m² at the end of the 1st yr, but °1000 g/m² were added in each subsequent year. The standing crop of biomass (shoot and root) at 5 yr was 4840 g/m², a value 16% that of the mature forest which occupied the site prior to cutting. Approximately 27% of the dead trunk and branch biomass from the pre—existing forest remained on the site after 5 yr. Litter production increased each year from 39 g/m² (year one) to 825 g/m² (year five) and tended to be inversely related to rainfall. Total aboveground production was highest for the 5th yr (1940 g/m²) and was probably close to a maximum for the site. Total live—plant nutrient stocks after 5 yr, as a percentage of the precut forest stocks, were 15% for N, 23% for P, 39% for K, 48% for Ca, and 45% for Mg. A large increase in the leachate concentrations of K, Mg, and NO₃ occurred during the first 2 yr following burning. By the 5th yr, concentrations were not significantly different from those in the undisturbed forest. After 5 yr soil nutrient levels were also similar to those of the preburn forest. Overall, nutrient losses apparently were not great enough to destroy the site's ability to return eventually to a state similar to that which existed prior to disturbance. This does not mean that a more severe treatment, such as conversion of forest to pasture, would not affect the regrowth ability of a site.