In August 1963, 135 yellow birch trees (Betulaalleghaniensis Britt.), averaging 5.08 cm in diameter at 1.37 m aboveground were pruned to two-thirds of their total height. Larger pruning wounds had a faster rate of closure but required more time to close than small wounds. The width and length of the pruning wound, as well as the diameter growth of the tree, were significantly related to closure rate. Although shade had no significant effect on closure rate, it reduced formation of epicormic branches.