Crescimento, mortalidade e viabilidade técnica e financeira do desbaste de liberação de copas em uma floresta ombrófila úmida, no leste do Pará.

Abstract

Forest managers in the Brazilian Amazon do not apply post-harvest silvicultural treatments because they understand that this activity is not cost effective. In other words, the gains with the treatments would not compensate for their costs. This study evaluated the benefits of adopting liberation thinning using poison girdling as a post-harvest silviculture. Two research plots of 25.5 ha each were set up in a tropical rain forest in the municipality of Paragominas, State of Pará, Brazil. In one plot (control) logging was carried out as usual, but no further interventions occurred; in the treatment plot liberation thinning was carried out. In the treatment plot 115 potential crop trees (PCT) of commercial species were liberated whereas in the control plot 86 PCT were tagged for monitoring; in both treatments growth and mortality behavior were monitored for five years and their productivity and costs of field activities were determined. The periodic annual increment of DBH (PAIdbh) of the control and of the treatment plots were, respectively, 0.56 cm year-1 and 0.47 cm year-1 and showed no statistical difference at 95% probability. PAI in basal arca were 0.017 in 2ha-1 lyear-1 and 0.019 in ha-1 year-1 for the control and treatment plot respectively. Mortality of competing trees was highly significant by applying 33% concentration of glyphosate; mortality was 78.95% three years after poison-girdling and 91.23% five years later. The study revealed that the lowest productivities were related to arca preparation (2.16 ha per hour) and 2.32 ha per hour for poison-girdling of competing trees. Regarding treatment efficiency, the families Caesalpiniaceae, Burseraceae e Lecythidaceae requires longer response time to mortality. In general, poison girdling applying 33% glyphosate in water was highly efficient to eliminate competing trees. To treat one ha requires investment of R$ 53.23 (c. US$ 27.30) every ten years (years zero, 10 and 20) of the management cycle. From the financial point of view, liberation thinning had a good response, resulting in a net present value (NPV) of R$ 364.10 (US$ 186.72 >zero) and a proportional cost benefit of 1.38 (>1) and therefore costs should not be an impediment for adopting this silvicultural tool.