The main idea in the first half of Robert Frost's "Birches" is about the power of nature, presented for the most part by describing both the power of the ice-storm and the power of the birch trees in enduring that storm. In the second half of the poem, the main idea is perhaps that the relationship between mankind and nature is a necessary and spiritually fulfilling relationship.
In the first half of the poem, the speaker suggests that "You'd think the inner dome of heaven had fallen" when describing the aftermath of the ice-storm. The scale ("heaven") implies how powerful the storm was, as do words like "Shattering" and "avalanching." Despite the power of the ice-storm, the birch trees, though "dragged" and "bowed" by the wind, "seem[ed] not to break." The power of the trees—specifically the power to endure—is also emphasized by Frost's decision to personify them as "girls on hands and knees that throw their hair / Before them over their heads to dry in the sun." Personifying the trees in this way attributes to them a kind of childish indifference to the storm.
In the second half of the poem, the speaker imagines a boy climbing in the branches of the birch trees and then jumping back down to the ground. This boyish adventure is described in a wistful, fondly nostalgic tone. The boy, for example, flings himself from the branches "with a swish, / Kicking his way down through the air." The speaker wishes that he could be this boy once more and "get away from the earth awhile." This idea seems at first somewhat paradoxical. The speaker describes climbing in and jumping from the trees in almost a transcendental way, as if being amongst nature, and the physical world, is at the same time a means of transcending that world. Perhaps the idea is that, for the speaker, being at one with nature is a sort of meditative exercise and thus a way to forget about—or transcend—the ordinary problems of the real world. This is especially pertinent because the speaker is an adult ("I once myself [was] a swinger of branches . . . I dream of going back") reminiscing about his childhood. He, as an adult, is likely more mired in the real world now than he was then, so the transcendental relationship with nature that he perhaps took for granted as a child seems especially attractive to him now.