Carbon sequestration and the effect of different densities of mixture stands on Hypsipyla Robusta attacks and growth of African mahogany

Establishment of tree plantations, like mahogany, in the tropics has been suggested as a way of reducing the rate of increase in atmospheric carbon providing valuable timber for commercial purposes by decreasing the dependence on natural forests for timber. The African mahogany in plantations is often assailed by a number of pests of which Hypsipyla robusta is the most destructive. Mixed species plantation is likely to be effective in managing H. robusta infestation by hindering the host finding ability of the pest and possible abundance in natural enemies in mixture stands. A ten hectare experimental plot at Sarmartex Timber and Plywood Company limited located in the wet evergreen forest zone of Ghana was planted with Khaya grandifoliola and Khaya ivorensis in mixture stands containing Heritieria utilis, Terminalia superba, and Entandrophragma angolense in densities of 100%, 60%, 50%, 40%, 20% and 10%. Growth measurements and damage by H. robusta was assessed for Khaya grandifoliola and Khaya ivorensis in pure and mixture stands. Amount of carbon which can be sequestrated was also assessed for all species in the mixture stands. Overall, the study showed that Khaya grandifoliola had a better growth performance than Khaya ivorensis. After two years of planting, there were not much difference shown between growths displayed for different planting densities for K. grandifoliola; however, the 10% density mixed stand had the best growth rate. Differences were shown between growths displayed for different planting densities for K. ivorensis trees, with the best growth exhibited by the mixed-species stands. The study also showed that forking occurred in all densities planted with multiply shoots with K. ivorensis having the least number of shoots. For K. grandifoliola, there were no significant difference between attack levels of different planting densities; however, the mixed species stands recorded the lowest level of H. robusta attack. On the other hand, there were clear differences between the attack levels of different planting densities with the pure stands having the highest levels of H. robusta attack for K. ivorensis trees. The presence of weaver ants (Oecophylla longinoda) affected the growth and damage by H. robusta attack for the African mahoganies. The presence of weaver ants on K. grandifoliola and K. ivorensis trees showed that the more weaver ants present, the lower the number of shoots attacked indicating that the presence of weaver ants can help decrease the intensity of H. robusta attack. The mixed species can serve as alternate hosts to the weaver ants which may impede H. robusta infestation with Terminalia superba having the most weaver ants as compared with the other species in the mixture stands. Carbon sequestration was projected for species planted using a forty year tree rotation. K. grandifoliola was shown to sequester more carbon and much earlier than the other species used, followed by H. utilis, T. superba, E. angolense and K. ivorensis. Due to the different species capability to sequester carbon at different rates and time, it indicated that mixed species plantations might have the ability to sequester more carbon than monocultures.

Source: KNUST