The poem's speaker sees the influence of nature on humans as of vital importance, and for this reason he mourns what might seem to others a minor act: the cutting down of a group of trees. To the speaker, the damage from this small act of removing a mere ten or twelve trees is great: they no longer will provide shade for those on the shore or shade the river's edge, and the beauty they provided has also disappeared.
The speaker uses language that is both mournful and lovely to describe the trees that are no longer there, such as by saying,
My aspens dear, whose airy cages quelled,
Quelled or quenched in leaves the leaping sun.
In other words, the "airy cages"—branches and leaves of the trees—quelled or stopped the sun from shining down too brightly on the earth below. They made the sunlight both more beautiful as it hit the leaves and less glaring.
Although the loss seems trivial, the speaker makes clear the pain he feels by comparing the removal of the trees to the way a minor "prick" will blind an eye. In other words, damage doesn't have to be on a vast scale to be devastating to the one impacted by it. The loss of the trees is "havoc" to this narrator, who mourns that future visitors will not be able even to "guess" how beautiful the natural scene once was. This loss is so profound it tolls like a death bell:
All felled, felled, are all felled.