Alliteration appears here and there in "Trees," adding to its general musicality. One important example is in the poem's third couplet, lines 5 and 6:
A tree that looks at God all day,
And lifts her leafy arms to pray;
The consonance of "all" helps sustain the sound between the lines here as well.
There are a couple of key effects produced by this particular sound. First, the /l/ here recalls an earlier prominent /l/ sound in the poem—"lovely" (in line 2). Remember, loveliness, as the speaker sees it, is the tree's defining feature—and /l/ is the defining sound of the word that denotes its defining feature. The /l/ sound is thus linked directly to loveliness, which, in turn, is linked to godliness. Just like, for example, the hard /k/ in "spikiness" seems to embody the attribute that it describes, the soft, gentle /l/ sound suggests to beauty, balance, and grace throughout the poem.
Second, the prominent use of alliteration here also helps depict the tree as in good, leafy health. Think about how leaves grow along the length of a branch; the many /l/ sounds here subtly suggest leafy abundance.
Later in the poem, the /l/ sound reappears in lines 9 and 10 ("lain" and "lives"), with consonance (here of "intimately") once again bolstering the effect. These /l/ sounds evoke the gentleness of snow coming to rest on the tree, and the close relationship—or natural harmony—between the tree and rain.
Another example of alliteration is in line 8's "her hair." Here, the speaker describes how robins come to nest in the personified tree during the summer months. The /h/ sound, which requires rapid exhalation on the reader's part, suggests the breathlessness of the hottest time of year—as though the poem itself momentarily tries to cool itself down.
The poem's final moment of alliteration is the shared /m/ of "made" and "me." The shared sound here reflects the fact that the speaker, as a poet, is a kind of creator—though the poem makes clear that the speaker's creative power pales in comparison to God's.