What is the main theme in "Birches"?

Frost's main theme in "Birches" is that life is beautiful and good, more desirable than heaven. He was 40 when he published the poem, and it reveals the feelings of a man in middle age looking both ahead toward death and backward to childhood.

Frost shows the beauty of nature in his imagery surrounding the birch tree. In the passage below, it has been bowed down by winter's ice, but now the weather is warm enough that the ice begins to melt and shatter:

Soon the sun's warmth makes them shed crystal shells
Shattering and avalanching on the snow-crust—
Such heaps of broken glass to sweep away
You'd think the inner dome of heaven had fallen.

The ice is so beautiful that Frost likens it to the dome of heaven falling.

This tension between earth and heaven, childhood and adulthood, runs through the poem. The permanently bowed rich trees might make us think of aged people, but Frost associates this bowed look with youth, such as of girls on their hands and knees, throwing their damp hair over their faces to dry in the sun.

In the end, the poem's main theme is a longing for the earth and a deep aching appreciation of what life has to offer. Even when living brings sadness, Frost doesn't want to die or go to heaven. Instead, he writes:

I'd like to get away from earth awhile


And then come back to it and begin over.

He also writes that he'd like to go "toward" heaven by climbing up a birch tree, but be set down again on earth:


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.

In "Birches," Robert Frost uses a natural phenomenon, the bending of birch trees when they become covered with ice, as a vehicle for expressing a deep and reverent love for living. In his imagination, he thinks of the birch branches being bent by a boy swinging on them and leaving his permanent mark as he conquers them one by one.
The poem takes a turn from admiring the beauty of the natural world, the sight of gracefully bent birch branches, to an examination of where the speaker is in his life:

So was I once myself a swinger of birches
And so I dream of going back to be.

He dreams of his carefree boyhood, gently lamenting that in adult life, life can feel "like a pathless wood" that delivers setbacks and painful, even if minor, injuries. The speaker wishes for the opportunity to live his life again and dreads the time when he will be taken away from Earth, "the right place for love," never to return.