Temporal Changes of Biomass Production, Soil Properties and Ground Flora in Eucalyptus globulus Plantations in the Central Highlands of Ethiopia

Blue gum (Eucalyptus globulus) plantations are managed as a successive short rotation coppice system for more than a century in the central Ethiopian highlands. Consecutive cutting cycles of these plantations may alter forest site productivity, soil physico-chemical properties, and forest floor environment. To test this hypothesis, ten plantations ranging from 11-60 years (2-10 cutting cycles), with 1, 4, 5, 7 and 9 years old coppice-shoots were selected in the central highlands of Ethiopia, where the most extensive and oldest Eucalyptus plantations exist within the country. The thesis summarizes the temporal dynamics of above-ground biomass and nutrient partitioning (Paper I & II), soil physical and chemical properties (Paper III), ground flora composition and diversity (Paper IV) in a number of Eucalyptus globulus plantations along a chronosequence. Mean above-ground biomass ranged from 11 to 153 ton ha-1 in 1 and 9 year old plantations, respectively. In contrast, above-ground biomass production declined markedly across the total plantation ages (cutting cycles). On average, stand biomass production was reduced by 14% from the second and third cutting cycles to the sixth and seventh cycles, and by a further 86% from the second and third to the tenth cutting cycles. Although non significant, a similar declining trend was observed for the vegetation cover, species composition and diversity of the ground flora in consecutive cutting cycles. The macro-nutrient concentrations (N, P, K, Ca and Mg) in above-ground tree biomass showed significant differences between tree components. The nutrient concentrations (N, P, K, Ca and Mg) were highest in foliage, while the lowest concentrations were obtained in stemwood and branch tree parts. The nutrient content (kg ha-1) followed the order: leaves > stemwood > stembark > twigs > branches. For most components, neither coppice-shoot age nor cutting-cycle number had a significant influence on the macronutrient concentrations. Although not significant, a decreasing nutrient concentration trend was generally observed with the coppice-shoot age. Soil nutrient concentrations (N, Ca and Mg) showed a statistically significant decline with the advancement of cutting cycles. Thus, the soil nutrient stock (kg ha -1) in the tenth cycle was the poorest of all the stands. The reduction of macronutrient stock in consecutive cutting cycles appears to be related to repetitive harvests in short rotations, whole-tree harvesting and nutrient loss caused by complete forest litter removal. Hence, deleterious anthropogenic practices associated with consecutive cutting cycles may eventually lead to yield decline and forest site degradation on a long-term basis. Therefore, it can be suggested that with appropriate silvicultural and management interventions, it could be possible to mitigate site quality decline over the successive cutting cycles of Eucalyptus plantations. For long-term site quality and sustainability of biomass production, prolonging the length of cutting cycles, and prohibiting or controlling recurrent litter raking appears to be imperative, because these practices may jeopardize the sustainable management of Eucalyptus globulus plantations in the central highlands of Ethiopia.