"The Poplar Field" by William Cowper is a critique of man's willful destruction of the environment. The speaker is in a nostalgic mood, thinking back to the time twelve years previously when he first saw the beautiful field where all his beloved poplar trees once proudly stood. But the trees are no more; they've all been cut down. Where once the speaker sat beneath the shade of a poplar, now he sits gloomily upon a tree stump, an indication of the environmental destruction that's since taken place:

Twelve years have elapsed since I first took a view
Of my favourite field, and the bank where they grew,
And now in the grass behold they are laid, And the tree is my seat that once lent me a shade.

The cutting down of the poplars has had a negative impact, not just on the speaker, but also on the animals that used to live among the trees:

The blackbird has fled to another retreat Where the hazels afford him a screen from the heat;
And the scene where his melody charmed me before
Resounds with his sweet-flowing ditty no more.

The blackbird has been forced to fly away to find another home. The speaker clearly misses him and his joyful, lovely song. The poplar field just isn't the same without him. But the destruction of this natural habitat has at least made the speaker realize something important: that the pleasures of this world are as temporary as the poplar trees themselves. Life is short, and this encourages us to pursue these fleeting pleasures at whatever cost, whether it's to other people or to the environment. As such, we become selfish, and it's this selfishness that leads to so much environmental destruction:

'Tis a sight to engage me, if anything can, To muse on the perishing pleasures of man;
Short-lived as we are, our enjoyments, I see,
Have a still shorter date, and die sooner than we.

Critical analysis of  the poem "The Poplar Field" by William Cowper

The structure of this poem is in five stanzas, of four lines each.  The rhyme scheme is A-A-B-B, where each line rhymes with its successive line.  For example, “shade” and “colonnade” (lines 1 and 2) rhyme with one another, while “before” and “more” match (lines 11 and 12) with each other. 

The surface meaning of the poem is about the speaker who notices that a certain set of trees (Poplar, as the title suggests) has shed its leaves.  This triggers a flurry of reflection about the meaning of change, seasonal and personal.  The speaker discusses how the shedding of leaves impacts a “farewell to the shade,” and a lack of song “in the leaves” that are no longer. 

This moves into a personal realm when the speaker realizes that, like the tree, he, too is becoming older: “My fugitive years are all hasting away.”  This culminates in the last stanza when the speaker connects the experience of maturation in the natural world to the personal domain:  “To muse on the perishing pleasures of man.”   The symbolic meaning of the poem is that age and maturation are experiences that cannot be overlooked or evaded.  Just as seasons change, and plants wither, and animals die, so shall we. 

There is a reflective melancholic tone struck in the poem, as the speaker realizes that growing old is a part of both natural and human life:  “With a turn of my breast and a stone at my head/ Ere another such grove shall arise in its stead.”  The theme of the poem is that the speaker, and by extension, we, as human beings, must embrace that natural growth and change of things, while ensuring that the time spent is one worthy of our fleeting condition:  “Short- lived as we are, our enjoyments, I see,/ Have a still shorts date, and die sooner than we.”

If we were to examine this theme in a positive light, it would be akin to ensuring that we make the best of what we have and what we do, for mortality is but a temporary condition.  If we were to examine this theme in an opposite light, it would be that revel as we might in our accomplishments, they are fleeting and temporary.  Simply put, our greatness is fleeting, like the leaves on the poplar tree.  This might be where the appreciation statements of this poem lie, depending on how one chooses to read it.