If you are referring to a single tree cut down by chainsaw with no other aid to help then it is called felling. If a large area of trees are cut down, it is called deforestation. read more
Deforestation: When trees are cut down in mass to convert a forest cover to a commercial. Afforestation: The establishment of a forest in an area where there was no previous tree cover. Reforestation: It is the natural or intentional restocking of existing forests that have been depleted, usually through deforestation. read more