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An Analysis of the future productivity of Eucalyptus grandis plantations in the "Cerrado" region in Brazil : a nutrient cycling approach Ferreira, Maria Das Graças Moreira


Extensive areas of Eucalyptus plantations are being established in the "cerrado" region of Brazil for both pulp and charcoal production. These plantations are being managed intensively using short rotations and high stand density. Because the soils of the area are very infertile, there is concern over the sustainability of yields in the absence of repeated fertilization. Therefore, the present study was undertaken to evaluate the effects of intensive management of these plantations on the P and N status of the "cerrado" soils and the implications thereof for future productivity. Biomass and P and N content of each tree component (including the root system) were determined over age sequences of E. grandis plantations growing on good and poor "cerrado" sites in Minas Gerais State, Brazil. The dynamics of P and N were studied by analysing litterfall, forest floor accumulation, decomposition of branches and leaves, and internal cycling due to heartwood formation and the shedding of branches and foliage. The data that were collected permitted both a traditional static inventory assessment and a dynamic assessment of the effects of management on the site nutrient capital. However, because these two approaches are inadequate for the development of long-term sustained yield strategies for the management of these plantations, the thesis work included the development of conceptual models of two important aspects of eucalypt management in the "cerrado": coppice regeneration and phosphorus dynamics in the mineral soil. Studies of coppice sprouting were conducted to provide guidance for the development of the coppice model. By using information on both accumulation and dynamics of biomass and nutrients over age sequences of E. grandis growing on two different "cerrado" sites, the following predictions were obtained. Based on the static approach, the maximum yield over an initial 18 year period was obtained for a 54 month rotation, on both sites. However the use of this method to assess the effects of intensive management on site nutrient status would give misleading results because there is no feedback between changes in nutrient availability and yield. Therefore, a dynamic approach to the assessment was also used. This showed that over an 18 year period harvesting only stems on the good site is likely to cause a reduction in productivity of 48% while whole tree harvesting is likely to result in a reduction of 61%. On the poor site, there is likely to be only a 12% decrease due to whole tree harvesting, while harvesting stems only is unlikely to result in any decrease in yield. If whole tree harvesting is adopted on the good site, fertilizer quantities required to maintain the same level of productivity in the second rotation would be approximately 24 kg/ha of P while the amount required using stem harvesting only would be approximately 15 kg/ha. The amount of P required to produce one unit of biomass on the poor site was less than that required on the good site. In addition, the tree root systems on the poor site contain a greater proportion of total tree P as compared to those on the good site. This reduces the proportion of the total tree P that is removed at harvesting and gives an advantage to sprout growth on the poor site in subsequent rotations. Thus, intensive biomass harvesting on the poor site is expected to have relatively less impact on future site productivity compared to that on the good site. However, the good site can be expected to yield a greater biomass than the poor site. The data on accumulation and dynamics of biomass and P and N proved to be helpful in understanding the effects of intensive forest management on site nutrient status. However, the evaluation of long-term effects is rather complex because of the interaction of several factors when different management conditions are considered. Therefore, computer simulation models are required to evaluate the long-term effects of tree harvesting on site nutrient status. Because the growth strategies of a tree growing from seed are different from that of a tree developing from a stump, and the dynamics of P in mineral soil (the most limiting nutrient for eucalypt growth in the "cerrado" region) differ from that of N, coppice and phosphorus models were developed that could be implemented in the future in ecosystems models used to evaluate long-term effects of intensive management.

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