The speaker stands in a woods where tuliptrees have fallen, and the poem moves the way attention does on a slow walk: from what is seen to what is known, and back to what is seen. The quoted phrase "common though not abundant" sounds like a field guide, and so does the Latin name. These are the tidy labels people use to sort the natural world. But the poem quickly shows how little labels cover. Even how sap rises through such enormous trunks is "still a mystery."
The images stay close and physical: "massive trunks," bark patterned like water rippling over stones. The last lines are the quietest. A fallen tree lies so still that algae can grow on it. The speaker doesn't say what he feels outright, but the mood suggests humility and calm. There is a sense of respect for the dead trees, and a gentle acceptance that decay is also a kind of life.