"The Poplar Field" begins with a sudden, surprising revelation: the poplar trees of the poem's title have been cut down. What might have been a relatively simple nature poem describing some beautiful trees thus quickly becomes something more complicated. The poem's tone is elegiac; rather than celebrate the field's beauty, the speaker bids it "farewell."
The speaker's mournful feelings are reflected in the language of the very first line. This is divided into two neat halves by the comma after "fell'd," which creates a strong caesura. The weighty pause here intensifies the impact of the poem's very first statement: "The Poplars are fell'd." The comma seems to break the line, almost mirroring how the speaker's beloved poplar trees were broken when they were cut down. But the caesura also connects the two parts of the line and establishes a logical relationship between them. The observation of the lines first half is what prompts the "farewell" of the second. This link is further emphasized by the consonance (on the /f/ and /l/ sounds) and assonance (on the /eh/ sound) between "fell'd" and "farewell."
Yet the poem doesn't stay quite so somber for long. The speaker's "farewell" rushes straight into the next line, and this enjambment leads into a different musical and emotional atmosphere. Even as the speaker continues bidding the field farewell, the poem's beautiful language begins to recreate the pleasing music of the field that the speaker remembers.
The pleasant consonance on the /c/ and /l/ sounds in "cool colonnade," for example, mirrors the beauty of the distinguished row of trees. The /l/ sound, especially, echoes the consonance of "fell'd" and "farewell" from line 1—but now, the sound is emblematic of peace and beauty, not destruction and the sadness of saying goodbye. Similarly, the /p/ sound in "Poplars" comes back in the word "whispering," suggesting that even though the poplars are gone, they still whisper sweet music in the speaker's mind.
Finally, let's take a look at the meter of these first two lines. The meter of line 1 is mixed:
The Pop- | lars are fell’d, | farewell | to the shade
The line opens with iamb (a foot with an unstressed-stressed syllable pattern) and then an anapest (unstressed-unstressed-stressed). Then comes another iamb-anapest pair, for a total of four metrical feet. This is called tetrameter. Compared to the anapests, which feel light and musical, the iambs seem heavier and more serious, even somber. This makes sense—the iambs give weight, emphasis, and dignity to the "Poplars" and the speaker's mournful "farewell" to them. The alternation between iamb and anapest in the first line prepares a central tension in the poem between somber reflection and happy memory.
By contrast, notice how each foot in line 2 is an anapest:
And the whisp- | ering sound | of the cool | colonnade,
This is called anapestic tetrameter, and it is the dominant meter of the poem. The sudden change of meter (following the enjambment) makes this line feel light and happy, especially compared to the more serious, somber first line. The quick da-da-dum rhythm of the anapests echos the "whispering sound" of the wind in the trees and reflects how good the pleasant memories of the "cool colonnade" make the speaker feel.