Hi ! I would suggest adding some native soil back to the base of your tree to fill the void. If you have a lawn it won't harm your tree to put some top soil around the base. To prevent this from happening again in the future, water gently and water further away from the base of the tree. You could also put some landscaping fabric around the tree and cover that with some mulch - this way you won't erode the soil around the tree when you water. If you want to respond with a picture of tree it might be helpful too - the team here may have a few more ideas for you. Best, MyWoodlot Team Member Heather