Large scale felling of infected and dried sal trees in two forest ranges of Dehra Dun have been taken up by the UP Forest Corporation. About 15,000 trees have been felled already and at least double that number of trees are yet be felled over the next few weeks. The infection struck sal trees before the monsoons this year in at least five pockets of the Thano forest range in this district, the Chief Conservator of Forests (Garhwal), Mr M M Harbola, said.