College of Forestry, Wildlife and Tourism

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Browsing College of Forestry, Wildlife and Tourism by Subject "Aboveground biomass"

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    Effects of field plot size on prediction accuracy of aboveground biomass in airborne laser scanning-assisted inventories in tropical rain forests of Tanzania
    (Springer, 2015) Mauya, Ernest William; Hansen, Endre Hofstad; Gobakken, Terje; Bollandsås, Ole Martin; Malimbwi, Rogers Ernest; Næsset, Erik
    Background: Airborne laser scanning (ALS) has recently emerged as a promising tool to acquire auxiliary information for improving aboveground biomass (AGB) estimation in sample-based forest inventories. Under design-based and model-assisted inferential frameworks, the estimation relies on a model that relates the auxiliary ALS metrics to AGB estimated on ground plots. The size of the field plots has been identified as one source of model uncertainty because of the so-called boundary effects which increases with decreasing plot size. Recent re- search in tropical forests has aimed to quantify the boundary effects on model prediction accuracy, but evidence of the consequences for the final AGB estimates is lacking. In this study we analyzed the effect of field plot size on model prediction accuracy and its implication when used in a model-assisted inferential framework. Results: The results showed that the prediction accuracy of the model improved as the plot size increased. The adjusted R 2 increased from 0.35 to 0.74 while the relative root mean square error decreased from 63.6 to 29.2%. Indicators of boundary effects were identified and confirmed to have significant effects on the model residuals. Variance estimates of model-assisted mean AGB relative to corresponding variance estimates of pure field-based AGB, decreased with increasing plot size in the range from 200 to 3000 m 2 . The variance ratio of field-based esti- mates relative to model-assisted variance ranged from 1.7 to 7.7. Conclusions: This study showed that the relative improvement in precision of AGB estimation when increasing field-plot size, was greater for an ALS-assisted inventory compared to that of a pure field-based inventory.
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    Nutrient and water dynamics in rotational woodlots: A case study in western Tanzania
    (Wageningen University, 2004) Nyadzi, Gerson Isaac
    Rotations of trees and crops on farms are considered as a potential technology to overcome the shortage of wood, reverse deforestation of natural forests and improve soil fertility for food security enhancement in western Tanzania, sub-Saharan Africa. However, overexploitation of soil water resources and depletion of soil nutrients have been suggested as possible negative effects of growing trees on farms in the semi-arid tropics. Such possible pitfalls undermine and even threaten a successful implementation of the woodlot technology at larger scale. Evidently, without proper understanding of the interactions and possible competition between trees and crops, the potential benefits of this agroforestry technology will not be realized. Therefore, this study was focused on improving the understanding of trcc-soil-crop interactions by examining water and nutrient dynamics of various trees planted in rotational woodlots with a 5-year cycle. Field trials were established with five-year rotational woodlots. Five tree species were compared with natural fallow and continuous maize. Inter-cropping of maize between trees was possible for the first two years of tree establishment without sacrificing maize yield. There was no evidence that trees were over-exploiting the water reserves after three years. Transpiration was greatest in A. crassicarpa and was related to stem diameter, size of the tree canopy and soil water availability. Trees depleted relatively more waler than continuous maize and natural fallow, but were able to store more water after rains. Acacia trees had high litter fall but the leaves were low in P and N, which led to N and P immobilization during decomposition. Trees retrieved leached inorganic N and made better use of it than natural fallow and continuous maize. Wood production at the end of the five-year growing period ranged from 30 to 90 Mg ha-1 while C sequestered in the aboveground biomass during the same period ranged between 13 to 30 Mg ha-1. The rate of biomass production was highest for Acacia species while the foliage nutrient accumulation was highest for Leucaena species. The wood component varied greatly among species, ranging from 32 to 85% of the total tree biomass. Maize yields were higher after growing trees than after natural fallow and continuous maize when no fertilizers were applied. The benefits of tree fallows compared to natural fallows were modest, in terms of maize yield increases. The increase in crop yields after woodlots was attributed in part to higher soil inorganic N. Maize responded to fertiliser N and P. The agronomic efficiency was about 30 kg grain per kg N applied at a rate of 50 kg fertilizer N, and 15 kg kg-1 between 50 and 100 kg N applied. Application of more than 20 kg ha"1 of P or K did not significantly increase maize yields. The benefit of woodlots expressed in terms of maize yield was more pronounced at mid and lower slope positions than at upper slopes. The significance of rotational woodlots in improving soil fertility is limited by substantial accumulation of nutrients in wood. Nutrient mining by wood exportation istherefore a major threat for the sustainability of woodlots, when the exported nutrients are not supplemented via external sources. Medium-term rotational woodlots have the potential to meet the domestic and industrial wood needs and at the same time to reduce deforestation in Sub-Saharan Africa.