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The key is where the tree saplings are coming from. If they are seedlings you can wipe a weed killer like glyphosate (Roundup) on the foliage or cut them off and dab the herbicide onto the cut surface. If the saplings are coming from root sprouts they you don't want to treat them with a herbicide or the surrounding trees can be damaged. In such a case there are few options other than cutting them off repeatedly or using the landscape fabric.
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