I SPOT the hills With yellow balls in autumn. (1-2)
The speaker is doing the "spotting" with himself. So right from the get-go, we see man and nature as one, since people don't usually look like "yellow balls."
And I am called pumpkins. (5)
Man's made a habit of naming the things he sees in nature. So our punch line here is even better, because it's man naming nature as something natural.
I light the prairie cornfields Orange and tawny gold clusters (3-4)
Since the pumpkin also happens to be a poet, we get this neat description of nature with all the amped up effects that vivid imagery can offer. So nature is describing itself with the benefits of having a poet's language.