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.

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.

The main themes for Robert Frost's poem entitled "Birches" are:

The Interrelationship between Imagination and Reality

The speaker draws parallels between the tree and himself as he recalls being a boy and swinging on the birch tree branch. Further, he enjoys the image of ice as "broken glass to sweep away," and his imagination takes full play as he perceives the ice, the "crystal shells" shed from the tree as

Such heaps of broken glass to sweep away
You'd think the inner dome of heaven had fallen.

After a few lines, the speaker imagines that the bent branches in the summer have their leaves

               ...trailing on the ground
Like girls on hands and knees that throw their hair
Before them over their heads to dry in the sun.

The Relation Between Youth and Nature

Whenever he sees birch trees and their branches that have been weighed down by the ice, or bent by the wind, the speaker would rather imagine that some boy, much like himself as a youth, has been swinging on that branch. 

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

This memory is a release for the speaker, a return to days of youthful freedom and flights of fancy. For, when the speaker relates problems connected to life as an adult, he continues to employ nature imagery:

It’s when I’m weary of considerations,
And life is too much like a pathless wood
Where your face burns and tickles with the cobwebs....
I’d like to get away from earth awhile
And then come back to it and begin over.

Nature as an intermediary between the Human and the Divine

In both Nature and in religious beliefs, there are mysteries that seem inexplicable. Indeed, there is a sense of the spiritual in the contemplation of the mysteries of Nature. Yet, man's reasoning, as well as intuition, sometimes fails in reaching complete understanding of these mysteries in religion. However, in Nature, there is often that proverbial "leap of faith" that can be made. In this case, it is the climb and the swing of faith on the birch branch. 

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,...
That would be good both going and coming back.
One could do worse than be a swinger of birches.

In the spiritual swing of the tree branch toward heaven, there is an experience which seems to enlighten the speaker as he advises in the final line that one can do all right as a swinger of birches, both coming and going. For, swinging can open one's mind to other mysteries.

Robert Frost's "Birches" is a rather nostalgic and rueful conversation between the speaker and the reader, whom he addresses as "you." With the metaphor of swinging, the speaker takes the reader through a series of "swings" from the imagination to reality. He begins with a flight of fancy: 

When I see birches bend to left and right
Across the lines of straighter darker trees,
I like to think some boy's been swinging them.

But, realistically, the speaker knows it is ice storms which have crippled these limbs although his imagination wishes to reject the reality and return to his youthful innocence:

But I was going to say when Truth broke in--...
I should prefer to have some boy bend them...
Whose only play was what he found himself,
Summer or winter, and could play alone.

To swing freely as he has done in his youth would release from the speaker some of his earthly burdens as he would transcend the mundane--"when I'm weary of considerations"--and imaginatively return to his boyhood innocence. Lines 41-42 express this loss of innocence:

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

In reality, though, the speaker knows that "Earth's the right place for love" and he must accept his current station in life as a man despite his yearning "to go by climbing a birch tree." After all, "One could do worse than be a swinger of birches"; one needs that innocence of youth.

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.