That their merely being there Means something; that soon We may touch, love, explain.
First up, we have a consonance alert, what with all the R sounds going on back in line 9 ("are") and now in line 10 ("their," "merely," "there").
And speaking of sounds, it's time to note the conversational sound that the speaker seems to have in the poem. Even though the stanzas are divided into a pretty common kind of verse, all the enjambment makes it sound more casual than the poem's formal stanzas might look. This is a common style in Ashbery's work. Sometimes it's easier to just read the poem like you would any other piece of prose/narrative type work. When you do it that way, it tends to make more sense because we can more easily hear the pieces fit together. (Check out "Sound Check" for more on the poem's sound.)
Now back to the question at hand: just what in the Sam Hill are the trees trying to tell our couple? That's easy: "That their merely being there / Means something" (10-11). So, it does mean something, although the speaker's not going to give up just what that something might be. Are you rolling your eyes yet? Remember what we said earlier: don't expect any answers.
Although, we kind of have an answer, don't we? If the "trees being there" means something, well then… that's an acceptable answer. We don't know what that something is, but we know it's there.
So the speaker seems to be encouraging us to experience things, like trees, in the simplest way possible: appreciate that they're there and that the experience means something, but don't rack your brain for a neat little answer because it probably doesn't exist, or at least doesn't exist in the way you'd like it to. Feeling any better?
Lines 11-12 seem to be getting at the ways we tend to go about experiencing things: "soon / We may touch, love, explain."
Maybe the speaker is suggesting that the truest form of "experience" is in the way we relate to one another.
It's as if right now the relationship we're hearing about isn't touching, loving, or explaining things in the way it should. And that kind of goes with the whole performance idea we saw in the first stanza. There's something that's not entirely honest about how things are occurring in this relationship right now. But the speaker reminds us that "soon" this may all change. The "we" may also relate to a bigger sense of "we" as a people, human beings.
Finally, there's no enjambment between stanzas here. This one ends with a period. Why is that? How is this stanza different from the others? Is it more important, so it naturally needs its very own period? Let's read on to see…