Experts and BHG readers answer.
injury to tree trunk
If the bark is gone from an area of the trunk, the best thing that you can do for it is to leave it open and exposed to sun and air so that disease will not enter the wound. Keeping it covered holds in moisture and is more likely to promote rot. Willow is a rapid grower, so there is a reasonable chance that the tree may grow around this wound and recover. However, if the wound extends more than half way around the stem, it will likely be permanently stunted. It might be better to completely remove this side branch if such is the case.