"Evangeline" starts in the forest and it ends in the forest. In between, we see meadows, oceans, rivers, fields, animals—a whole heapin' helping of natural imagery to accomplish a sense of place in the poem.
Part of the reason for all this natural imagery is because, well, the poem takes place a few hundred years ago in the seventeenth century. Folks were just closer to nature back in that day. We mean, you couldn't exactly hop in your minivan and take off cross-country on a superhighway. You had to saddle your horse and ride through the countryside, encountering the natural world every step (er, trot) of the way.
In this poem, we hear all about "the forests old, and aloft on the mountains," and how the "Sea-fogs pitched their tents, and mists from the mighty Atlantic/ Looked on the happy valley, but ne'er from their station descended" (29-31). Then, when the scene shifts from Acadia to Louisiana, we're treated to:
Water-lilies in myriads rocked on the slight undulations
Made by the passing oars, and, resplendent in beauty, the lotus
Lifted her golden crown above the heads of the boatmen.
Faint was the air with the odorous breath of magnolia blossoms,
And with the heat of noon; and numberless sylvan islands,
Fragrant and thickly embowered with blossoming hedges of roses. (807-812)
Longfellow really sets the scene for us here with this rich description. Given that so much of this poem takes place in or near the natural world, it seems reasonable that so much of the poem's imagery concerns that verdant space.