Theft is defined under Section 378 of the IPC as the dishonest intention of taking any movable property out of the possession of any person without that person's consent, and moving that property in order to effect such taking. Explanation 1 to Section 378 states that a thing attached to the earth, not being movable property, becomes capable of being the subject of theft as soon as it is severed from the earth. In this case, the tree, once severed by cutting, becomes movable property. The act of cutting down the tree causes it to be moved (e.g., it falls). Since this movement is done with the dishonest intention of taking the tree out of B's possession without consent, the offence of theft is complete as soon as the tree is severed and consequently moved.