Yeah, if the poem's title had you all set to enjoy a happy little poem about some trees, the epigraph sets you straight right from the jump. Change has come to these trees, and the natural world, thanks to some well-placed axe blows.
All felled, felled, are all felled; (3)
We'll say it again: the repetition of this line really drives home the profound impact that this change has on the speaker. He's totally stunned and at a loss for words.
O if we but knew what we do When we delve or hew— Hack and rack the growing green! (9-11)
In the second stanza, the speaker starts to realize exactly how harmful this kind of change can be. It's not just a few trees that he's talking about anymore. Instead, this loss represents an irrevocable change, one that can never be undone. The speaker laments the fact that folks just don't realize this.
Since country is so tender To touch, her being só slender, That, like this sleek and seeing ball But a prick will make no eye at all, (12-15)
If you've ever jammed a stick into your eye, well, first off, why? Second, though, you must have realized pretty quickly that your eyeball ceased to function. It was no longer an eye, really, since you couldn't see out of it. That stick changed the whole function of your eye. The speaker here is comparing that kind of profound change to the changes wrought by human intrusion into the natural world. By definition, humans meddling in Nature ceases to make it natural. Instead, it's now part of human civilization. That's a change that can't be undone.
After-comers cannot guess the beauty been. Ten or twelve, only ten or twelve Strokes of havoc únselve The sweet especial scene, (19-22)
The change brought on by the loss of these trees is permanent and profound. Whoever comes to the same spot after they're gone will never know the beauty that existed before. That's a pretty heavy realization. It really makes you want to tip-toe through the natural world so you don't change a thing.