A tree harvester assembly (5) has a pivoting tower (10) which secures the trunk (7) of the tree (8) and may pivot to impart a bending moment upon the tree (8), thereby permitting easier cutting by a chain saw (205). The tree harvester (5) also has a pivot chain saw guide (200) which permits the harvester (5) to cut trees on a hillside at an orientation parallel to the terrain of the hillside at a low stump height. The tree harvester (5) also has clamping arms (405, 410) which may be selectively pivoted to secure a tree (8) against the tower (10) and to selectively release the tree (8) in one direction or another. A chain saw pivoting and drive apparatus (600) permits the chain saw (205) to be pivoted about a desired range and simultaneously driven to perform a cutting operation. Finally, the tree harvester assembly (5) includes a chain saw tensioning device (800) utilizing floating dovetails (810) urged apart by a spring element (805) which acts to urge the chain (612) of the chain saw (205) away from the drive sprocket (627), thereby tensioning the chain (612).
