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.