The poem begins with an epigraph that's like something you would see on a tombstone. Sure, we're not in the habit of burying trees (they're a lot more useful to us above ground), but a sad, mournful tone is set right from the poem's beginning.
My aspens dear, whose airy cages quelled, (1)
We move from a mournful epigraph to a poignant reminder of how meaningful these trees were to our speaker. They were dear to him, like friends might be. As such, their loss is rendered more impactful, and sadder, to us readers.
All felled, felled, are all felled; (3)
The repetition of "felled" in this line really drives home the speaker's mournful state. He's simply run out of words to describe the awfulness of losing these trees, yet he can't get his mind off of them.
Not spared, not one That dandled a sandalled Shadow that swam or sank On meadow and river and wind-wandering weed-winding bank. (5-8)
As if things weren't sad enough, the speaker comes up with this invented, personified backstory to the trees, who apparently lived once-happy lives, frolicking on the river bank. The fact that they can no longer enjoy themselves in this way makes their "deaths" even sadder.
O if we but knew what we do (9)
Here's a quick poetry tip for you, Shmoopers: anytime you see "O" in a poem, you're dealing with some intense emotion. The speaker's sadness really comes through in this line's lamenting of human ignorance. We're just the worst, and that's super-sad in his view.
Strokes of havoc únselve The sweet especial scene, Rural scene, a rural scene, Sweet especial rural scene. (21-24)
Just like in line 3, we get more repetition here at the end of the poem. Was Hopkins getting paid by the word? Doubtful—the repetition here is just another mark of sadness. We can practically see the speaker, numb with grief, holding his head in his hands as he repeats his mournful lament for the lost trees. It doesn't get much sadder than this.