Imagine if we didn't have a title for this poem. What would we make of that first word, "these"? The title, just like everything else we get in the body of the poem, is part of the larger picture (or world), so to speak. It stands alone, just like the trees, but it's also never really alone and has neighbors, just like the trees.
Notice that Ashbery isn't naming specifics in his title, but rather leaves room for the imagination. The ambiguity of the phrase "some trees" therefore seems to get at the idea of how we come to identify things and then fill them with meaning, even if all that "meaning" doesn't amount to much. The same thing happens in relationships. We have to identify things (and people) in relationships, but how reliable are all these appearances and their supposed meanings? Can't we just take things for what they are rather than bang our heads against a wall looking for what they mean? At the end of the day, then, some trees are just… some trees, no matter how hard we try to make them mean something. Our title pretty much sums it all up.