Dioecious Allanblackia stuhlmannii, in East Usambara Mountains becomes a farmland tree when farmers leave the species after clearing the forests. This study examined the process of continuous thinning on sex ratio. Density in natural forest was estimated to be 34–74 mature trees per hectare with an average male:female ratio among reproducing trees observed to be 1:0.59. Density in farmland was 0.5–20 trees per hectare with male:female ratio ranging from 1:0.48 on recent farmland to 1:2 on old farmland. Our results suggest that the reduction of Allanblackia trees in new clearings was not sex biased; however, subsequent cutting targeted mainly males leaving farmland to be populated predominantly with females. Initial sex segregation of A. stuhlmannii is not known, but a 10-year old gene bank of related Allanblackia parviflora in Ghana suggests an even segregation. Allanblackia is gender stable albeit there are few observations of plants changing from male to female during the first year’s flowering and occasional fruit production in males. Average diameter at breast height for first flowering of 44 males was 13.3 cm and that of 17 females 16.3 cm. Decline in Allanblackia trees in farmland from 2011 to 2016 was estimated at about 13% (range from 0 to 23% in monitoring plots) and most of these were male trees.