How to FindGameObjectsWithTag then cast the array of GameObjects to an Array of Interfaces

I have a few GameObjects that implement a common Interface: IsTree. I need to call the function doTreeThing() on each of those IsTrees after getting a reference to each of them via FindGameObjectsWithTag("tree").

This line complains that it cannot implicitly convert from GameObject[] to IsTree[]

IsTree[] trees = GameObject.FindGameObjectsWithTag("tree");

So then I tried to leave the GameObject[] an iterate over it to convert each individual object inside to an IsTree.

GameObject[] trees = GameObject.FindGameObjectsWithTag("tree");
        foreach (GameObject tree in trees) {
            IsTree isTree = (IsTree)tree;
            isTree.doTreeThing();
        }

But this complains that it also cannot convert GameObject to IsTree.

Then I got a little desperate and tried to make my IsTree into a GameObject.

public class IsTree : GameObject {
    public System.DateTime getAge(){
        return null;
    }
}

GameObject is a sealed type. (I don't really know what that means besides that this does not work)

So how do I get this array of GameObjects into an array of SomeInterface?