There must be a total ban on planting big trees in the existing grasslands in the State's various forests so that the delicate balance of ecology is not disturbed, said Sri Anwaruddin Choudhury, a wildlife expert who has done a comprehensive project on preserving the forests of the State on behalf of the World Wildlife Fund for Nature's local chapter in Assam, Sri Choudhury was speaking about his experiences during the project at a seminar on Forestry of NE- problems and prospects' held here on the occassion of World Forestry Day at the Chemistry Department of Cotton College.