The poem's epigraph lets us know that we're going to be looking at the intersection of man and the natural world. After all, the word "felled" suggests that those titular trees were cut down deliberately. They didn't just blow over in a stiff wind.
My aspens dear, whose airy cages quelled, (1)
The phrase "My aspens dear" is an interesting one. It tells us that the speaker had deep affection for these trees, and that's borne out in the rest of the poem. At the same time, though, "my" suggests that the poet felt some kind of ownership over these trees, and that's not really how unspoiled Nature is supposed to work—is it?
All felled, felled, are all felled; (3)
The repetition of "felled" here drives home the loss that the speaker feels at seeing the trees cut down. He's so stunned, it seems like he's run out of words—in this line anyway. He soon regains his composure, and then he has plenty to say.
O if we but knew what we do When we delve or hew— Hack and rack the growing green! (9-11)
Here the poem shifts from the first stanza to the second and, at the same time, it pivots to a bigger picture. We're no longer just talking about the poplars. The speaker has turned his attention to the natural world as a whole ("the growing green"). The real tragedy, then, becomes humanity's inability to see how much damage it's inflicting on the environment. Well, we're glad sure that changed completely in the last hundred years (sarcasm alert).
Since country is so tender To touch, her being só slender, (12-13)
Here the speaker personifies the natural world as a "her," saying how fragile and sensitive it is. Sure, this is a pretty backward view of the "weaker sex," but Hopkins is just using the prevailing chauvinism of his time to point out how important it is to take care of our natural world.
Where we, even where we mean To mend her we end her, When we hew or delve: After-comers cannot guess the beauty been. (16-19)
Want to "clean up" the environment? Well, too bad. You're still transforming it from unspoiled Nature into an area of human influence. Any attempts to meddle in the natural world, says that speaker, will change the nature of…Nature—forever. Anyone who comes after that moment will be unable to appreciate what was there before humanity came tromping along, ruining everything.
Ten or twelve, only ten or twelve Strokes of havoc únselve (20-21)
It doesn't take much for man to ruin the natural world. Only a dozen or so "strokes of havoc" can permanently change the environment. We're doing more than just damaging the place, says the speaker; we're causing permanent change: "unselv[ing]" Nature and converting it to an extension of human civilization.