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.
To begin with, we notice that the speaker is speaking in the first person to an imaginary audience.
Birches are trees with slender trunks and bark that peels off like paper. They can grow up to 50 feet tall.
Because birches have thin trunks, they bend pretty easily in the wind and under the weight of snow.
Also, some types of birches have white bark, so they stand out against "straighter darker trees."
When the speaker sees the birch trees bent to the ground, he imagines that a young boy was "swinging them." We can imagine that a birch would be bent a little after the swinging.
But swinging doesn't bend them down to stay As ice-storms do. Often you must have seen them Loaded with ice a sunny winter morning After a rain […]
How would you swing on a birch tree? Would you grab a hold of the trunk and move spiral around it?
From these lines we do that learn that whatever it is, swinging bends the tree down to the ground. But, swinging doesn't bend the tree enough to cause permanent damage like an ice-storm can.
During an ice-storm, the tree is covered with freezing rain. The rain coats the tree in a sheet of ice that is formed during a cold winter night.
The speaker expects you to have experienced this first-hand, but if you haven't we can assure you it is pretty cool to see the sun reflect off the ice. Here's a picture to help you visual what trees look like after an ice-storm.
[…] They click upon themselves As the breeze rises, and turn many-colored As the stir cracks and crazes their enamel
Not only does this sight of bending birches look beautiful, but a little wind can bump the ice-covered branches against each other, causing clicking sounds. Now we're involving senses besides sight (i.e., hearing).
This clicking action cracks the ice, but not all the way.
A "craze" is a poetic way of describing little cracks. They might look like veins or a small crack in a windshield that resembles a spider web.
"Enamel" is a glassy outer surface. You might have seen it on pottery, like a hand-made coffee mug, or you might have heard a dentist talk about tooth enamel. Either way, when we see the word, "enamel," we think of something that's hard, shiny, and glossy. In this case, the enamel is the coating of ice.