What is the theme of Robert Frost's poem "Birches"?

You asked two questions so I have had to cut it down to
one. In this poem the speaker contemplates a row of birches bent to the ground and he
imagines how the trees may have changed their shape in this way. When nature itself
bends the birches, during ice storms, they may stay bowed permanently. But the
speaker prefers to think of the trees as bent under the weight of a young boy repeatedly
climbing and swinging on them. The speaker describes this process on both a literal and
a symbolic level. As a boy the speaker also climbed birches, blissfully ascending toward
heaven but also gladly descending back to earth. As an adult the speaker remembers this
oscillation between heaven and earth and longs to be again a "swinger of
birches."

This climbing up the birches and then swinging
back down again obviously operates symbolically in the story. It seems that the theme
has to do with the human desire to accomplish something extraordinary, represented by
the climbing up to "heaven" on the birches, but at the same time the poem states the
necessity of returning to "earth" or reality, where love, which is the greatest human
joy, abides. Note how Frost describes his desire for both
states:

May
no fate willfully misunderstand me

And half grant what i
wish and snatch me away

Not to return. Earth's the right
place for love:

I don't know where it's likely to go
better.

I'd like to go by climbing a birch
tree,

And climb black branches up a snow-white
trunk

Toward heaven, till the tree could bear no
more,

But dipped its top and set me down
again.

According to Frost,
therefore, we as humans need both - we need to be climbing and reaching towards "heaven"
or the achievement of great deeds, but at the same time we need to return to "earth" to
experience love.