I've been having to look at historical aerial imagery with my job as of late. The goal is to identify and describe historic land use of the sites. Specifically, I've come across sites that were previous ag fields that have a common element that I've been unable to identify.
It looks like little dots in concentrated areas of the landscape, possibly trees, maybe stumps. In future photos, they are often not present suggesting that they are not living trees at least.
I've attached a photo with a couple example sites outlines by a green line.