Survival and growth of mixed plantations of Milicia excelsa and Terminalia superba 9 years after planting in Ghana

Iroko (Milicia excelsa and M. regia) is a valuable hardwood from the humid tropics of Africa, and is currently under threat of extinction because of over-exploitation and poor regeneration. Attempts to establish Milicia plantations in Africa have been hampered by gall-forming psyllids of the genus Phytolyma. This study investigated the impact of Phytolyma on Milicia planted in 11:89, 25:75, 50:50, and 100:0% mixtures (by stem numbers) of M. excelsa (high-risk species) and Terminalia superba (companion species). At the forest site, M. excelsa survival was 10% in the short season plantation and 30% in the long season plantation. Mean diameter and mean height were significantly higher in the long season plantation than in the short season plantation 2.4 cm versus 0.43 cm diameter respectively, and 295 cm versus 74 cm in height. However, neither survival nor growth was influenced by density of Milicia in a plot. Survival of Milicia was even lower (6.3%) at the agricultural site (short season only). Survival and growth of T. superba were relatively higher than that of M. excelsa. The mean overall diameter growth of T. superba was significantly higher at the agricultural site than at the forest site.Shade from T. superba appeared to reduce psyllid galls on M. excelsa, though crop tree growth was slow, with the best mean height attained being 400 cm at 8–9 years in 50:50 mixtures. Continuing experiments will involve thinning and-or pruning T. superba so that low levels of galls and reasonably rapid growth of M. excelsa will be achieved.