On either side the river lie Long fields of barley and of rye, (lines 1-2)
Opening lines are a big deal in any poem. They set the tone, and focus our attention on the first details. It's definitely no accident that this poem opens with a description of the landscape. This poem isn't just about what's happening, but where it's happening. We get a lot of detail about the natural world; we can almost see the golden grain and almost feel the tug of the river's current.
Willows whiten, aspens quiver, Little breezes dusk and shiver (lines 10-11)
See what we mean about the importance of nature? These lines don't do a thing for the plot. They're almost like separate little nature poems in themselves, like a little haiku or something. Still, even if they seem separate from the story, they are full of life and movement and energy. In a way, they give an intensity and a rhythm to the larger poem. It's almost like the natural world is alive and singing in this moment. That's mostly thanks to the heavy meter. Hear it? WILL-ows WHI-ten, AS-pens QUI-ver. That, folks, is the magic of the trochee (see the "Form and Meter" section for more on that).
She saw the water-lily bloom, (line 111)
This is the moment when she looks out the window, the one thing she's not supposed to do. It's her moment of resistance, when she goes against the curse, whatever the consequences. The interesting thing here is that she doesn't just look at the studly Lancelot. She also looks at the blooming lilies. The natural world is part of what she's been missing. She wants contact with the handsome knight, but also with the blooming flowers.
In the stormy east-wind straining, The pale yellow woods were waning, (lines 118-119)
At this point, the natural world starts to reflect the mood of the Lady and the atmosphere of the poem. As her danger increases, as she gets closer to her doom, the sky gets dark and heavy, and a storm blows up. Making the natural world reflect human moods is a pretty old poetic trick, and Tennyson goes all out with it here.
The leaves upon her falling light-- Through the noises of the night (lines 138-139)
Although she won't make it to Lancelot, she does make contact with the natural world. The river rocks her, the wind blows her clothes and the leaves cover her. Even this close to death, she is feeling a kind of freedom and movement that was never there before. The story of her human isolation is really tragic, but maybe her meeting with the natural world gives it a little bit of a silver lining.