Iambic Tetrameter
"Loveliest of Trees" is mostly written in a rhythmic pattern known as "iambic tetrameter." Say what who-meter? Here's what that means: Each line of iambic tetrameter contains 4 (tetra- means 4) beats called iambs. The iamb is the most famous type of beat in English poetry, and it consists of an unstressed syllable followed by a stressed syllable (daDUM), as you can see in line 8:
It only leaves me fifty more.
While iambic tetrameter is the most common type of meter in this poem, there are a number of lines that break the mold, so to speak, such as line 4:
Wearing white for Eastertide.
For starters, there are an odd number of syllables in this line, which is rather… odd (forgive us). Second, the first three beats contain a stressed syllable followed by an unstressed syllable (these are called trochees). The trochees in line 4 make the line stand out from the rhythm of the preceding three, and also bring out the alliteration with all those W words here. That emphasis makes sure we remember that it's just after Easter.
This rhythmic shake-up is there for another reason, too. "Loveliest of Trees" is a poem about change. The speaker keeps reminding us that it is the spring time, but that the spring won't last forever (hence the branches "hung with snow"). The shifting meter of the poem, then, mirrors the poem's themes of seasonal change (both in life, and in nature).
The poem's rhyme scheme is part of the same process. The poem is comprised of three quatrains, each of which in turn is comprised of two rhyming couplets. The regular rhyme scheme of AABB—where each letter represents that line's end rhyme—is like clockwork: it stays exactly the same, with no changes, for the entire poem. The regularity of the rhyme scheme balances the see-saw effect of the meter, and suggests that seasonal change (winter-spring-summer-fall, youth-old age-death) is a fact of life, one that is as regular as clockwork.