How does the type of rhyme (rising or falling) in the last stanza affect the tone and impact of the poem "Desert Places"?
In "Desert Places," by Robert Frost, there is a significant change in the rhyming words used in the last stanza of the poem. Even though the same pattern is kept with the first, second, and fourth lines rhyming, the type of rhyme varies.
If I understand your question correctly, the first three stanzas use masculine rhymes--all one-syllable rhymes. The final syllable of each line is accented. Note the words "fast," "past," and "last," for instance in the first stanza. Here Frost uses an iambic rhythm, or a rising rhythm.
The last stanza, however, consists of feminine rhymes--more than one syllable rhymes and the final syllable is not accented. Note the words, "spaces," "race is," and "places." This type of rhyme results in a falling rhythm. The effect of such a change is subtle. With the feminine rhymes, there is a lack of finality, a sense of continuing action--almost as if the speaker is drifting off--overwhelmed and engulfed by his own emptiness, giving the last stanza a more haunting tone as if there is no relief for the feelings that are evoked by the snow-covered field.