Dynamics of biomass and nutrient accumulation in a clonal plantation of Eucalyptus in Congo

Since 1978 43 000 ha of clonal plantations have been planted around Pointe-Noire. Due to very low soil fertility in this part of Congo, getting maximum sustainable production is fundamental. The aim of this work was to obtain quantitative data on the biogeochemical functioning of an Eucalyptus ecosystem and to provide relevant information to forest managers. The dynamics of biomass and nutrient accumulation presented herein concerns an important flow in the nutrient cycle. It has been studied using a chronosequence approach for a single clone. Twelve trees were sampled in each of the five stands at ages 1, 2, 4, 5, and 7 (logging age). For the root system, three trees were studied in each of the 1-, 4- and 7-year-old stands. Tables were established to predict biomass and nutrient content from DBH and ages of the trees. They were applied to the inventory of the different stands to evaluate biomass and nutrient content for each component. The results exhibit different dynamics of biomass and nutrient accumulation: P and Ca accumulation is proportional to biomass of the trees throughout rotation; Mg is mainly accumulated during the first and the second year of the rotation, in connection with crown establishment; N and K are characterized by an intermediary dynamics, 50% of N and 65% of K have already been accumulated by the time the trees are two years old. The models were validated using 18 other trees sampled in the seven-year-old stand of the chronosequence. Nutrient removal by harvesting was also evaluated for different logging scenarios. The results confirm the need to de-bark on-site. This technique reduces the removal of Ca and ME by >50%. The quantities exported with pulpwood and firewood are ca. 100 kg N, 30 kg P, 39 kg K, 33 kg Ca and 19 kg Mg per hectare. (C) 2000 Elsevier Science B.V. All rights reserved.