Impacts of pine and eucalyptus plantations on carbon and nutrients stocks and fluxes in miombo forests ecosystems

Abstract: Knowledge of how commercial pine and eucalyptus plantations affect soil carbon and nutrient status is important in Mozambique, where incentives are available to increase the area of forest plantations and also to conserve mature miombo forests. Tree species growing on similar sites may affect ecosystem carbon differently if they allocate carbon to aboveground and belowground parts at different rates. Moreover, changes in ecosystem carbon and nutrient status are closely correlated. This thesis examined the effects of first-rotation (around 34 years old) commercial plantation of Pinus taeda L. (Loblolly pine), Eucalyptus grandis Hill ex. Maiden and Eucalyptus cloeziana F. Muell in Mozambique on carbon and nutrient stocks, carbon fluxes (CF) and net primary production (NPP) compared with adjacent miombo forests. At three study sites (Penhalonga, Inhamacari, Rotanda) in the Western Highlands of Manica Province, Mozambique, plots of Pinus taeda, Eucalyptus grandis and mixed-deciduous miombo species were delineated to investigate soil carbon and nutrient status (0-10, 10-30, 30-50 cm depth). Additional plots with these three stand types were established at Inhamacari to determine aboveground and belowground carbon stocks, CF and NPP. Aboveground carbon stocks were estimated using allometric biomass equations (ABEs) developed in this thesis, while belowground biomass was estimated using root-shoot ratio values from the literature. Tree carbon stocks, net annual rate of carbon accumulation by trees, carbon allocation to litterfall and to fine root production and NPP were all significantly higher in Pinus taeda and Eucalyptus cloeziana plantations than in Miombo forest. Total (0-50 cm) soil carbon stocks, but not total soil nitrogen stocks, increased significantly following Pinus taeda and Eucalyptus grandis plantation. However, soil nitrogen stocks were higher in the top 10-cm layer of plantation soil than Miombo forest soil. Plantation of Pinus taeda had no effect on soil acidity, but Eucalyptus grandis increased soil pH (10-50 cm) and reduced total stock of extractable soil phosphorus (0-50 cm). Thus overall, after around 34 years, tree carbon stocks, aboveground and belowground carbon fluxes, total carbon stocks and NPP were higher in commercial plantations than in natural Miombo forest. The commercial plantations affected soil acidity and soil nutrient status differently, apart from total soil N stocks. Another novel contribution of the present work was development of ABEs for high-altitude Miombo forests and commercial plantations of Pinus taeda, Eucalyptus cloeziana and E. grandis in Mozambique.