In terms of timeframe, "Binsey Poplars" begins at the end—at the end of the poplars, that is. Our speaker starts out by letting us know that all of his "aspens dear" (aspens are a kind of poplar tree) have been cut down (1). These weren't just any trees to the speaker; they were beautiful, joyful, and "fresh," arranged in a line almost like a military procession (4). Now, though, every single one of them has been chopped down.
It's not clear who's responsible for this heinous tree-i-cide, but the speaker lets us know that whoever cut these trees down had no idea what they were doing. They weren't just cutting down a few trees; they were fundamentally altering the nature of, well, Nature. Just like a sharp poke can totally change the nature of an eyeball (from an organ of seeing to a pain-filled blindspot), so too can cutting down a tree change the natural world. Once humanity steps in, even to "improve" things, Nature is completely changed—forever.
Folks who happen on this place in the future will never know its prior beauty, when the poplars were still standing. And it only took ten or twelve blows of an axe to transform the scene completely, which once used to be so sweet and special. Oh poplars, we hardly knew ye.