To put it simply, our speaker is a tree fan. Leaves, trunks, branches—he celebrates the whole package. Why else would he be so totally bummed about the chopping of "my aspens dear" (1)? That first line is interesting, and it tells us a lot about this guy. (We're just assuming the speaker's a he, because Gerard Manley Hopkins was a he, and we don't have any indication in the poem of who, exactly, the speaker really is.) Those trees are more than just background scenery. They're dear to him. He feels specially connected to them—hence the possessive "my."
Given this guy's passionate connection to the trees, it's not surprising that his imagination is charged with vivid, personified descriptions of these Binsey poplars. He sees them as soldiers and picnickers, but the speaker goes beyond that. He sees these trees as symbols of a natural environment that is fragile, tender, and irrevocably changed by human interference.
It's this big-picture view of things that most matters to our speaker. Sure, he loved those trees, but more than that he understood that those trees were just one small part of Nature that the world will never get back. The speaker understands that even attempts to "improve" Nature will forever change things. Once humanity starts meddling, Nature's never the same.
So sure, you might be tempted to think that the speaker is just a tree nut (an acorn maybe), but there's more to him than that. He's got a deep appreciation for the natural world and a perceptive view of how humanity's thoughtlessness can so easily and totally change our natural environment. This poem, then, is his way of giving us all a heads-up before there's nothing of the environment left unspoiled by human influence. Good lookin' out, man.