New Neighbors Claiming Control Over Part Of My Family's Homestead. They Want To Tear Up Mature Trees To Take Government Money To Plant New Ones (il).

(Posting this for a friend, who I don't think knows how to use MS Paint... sorry!!)
My family has owned a rectangular residential property plot of about an acre in rural Illinois. It is a single family home bordered in the east by a road, on the south by an uninvolved plot, and on the north and west by a single other plot that forms an enveloping L shape around my family's plot and that consists mostly of empty field being rented to tenants as farmland, until this year, when the owner seeded the L plot with new pine trees.
The west border of our property is on open field and the north border area is populated with dozens of mature trees, including a couple very large oaks and pines. We have owned the lot for about a decade and have used and developed all of it for our personal use over this time, including gardening and home projects within the forested area.
This week, though, I came home to find surveyor's flags on what we thought were within our north and west borders. I was told that the owner of the L-plot had gotten a survey done and found that their borders encroach further onto our plot than anyone had assumed. They are saying they have a DNR grant from the government to plant trees and intend to use the reassigned area to plant as many as possible. The strangest part of all this is that they're claiming a right to the forested north border area and its many full sized trees, as though they intend to tear up the existing growth to make room for saplings!
I have no idea if my family has any recourse here. Our family has always understood the forested area to be on our property and we have used it that way since its purchase -- and the idea that they might bulldoze beautiful trees to fulfill a planting subsidy requirement makes me heartsick. Are we helpless to give into the survey or do we have any options?
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