Every time I notice the sign, it causes me to think. First, of course: "What trees?" There are no trees: like I said, it is a marshy area. But then I suppose, that's the whole point. There are no trees. Not anymore. They were illegally cut.
However I don't think the sign is grammatically correct. The phrase "these trees" should refer to trees, not empty spaces, after all. The problem is that correcting the sign gets a little complicated.
"These ex-trees were illegally cut."
"The trees that used to be here were illegally cut."
"The trees which you can no longer see were illegally cut"
"The trees that were growing here before they were chopped down were illegally cut."
"Before this sign was here, there were some lovely trees, but then they were illegally cut."
Even beyond the syntactical issues though, I don't really understand the sign. I mean, what is it doing there? What purpose does it serve? Most road signs I see are there for a reason. "Dangerous curve ahead." "Speed Limit 70 MPH." "Prison Area, Do Not Pickup Hitchhikers." Each of these has a clear and obvious intent, and they make sense to me.
The "Illegally Cut" sign though.... I just don't get it. Was it erected just so that whoever did the illegal cutting will feel guilty whenever they drive by? If so, then it seems like a lot of money and effort for a single tree-cutter. Is it supposed to serve as a warning, so that we others won't be inspired to do any illegal cutting of our own? If so, then it's not very useful, because it doesn't actually indicate which of the current trees we oughtn't cut, nor how you can identify a tree which may or may not be cut. Is it supposed to inspire me to pull over to the side of the road and go Johnny-Appleseeding in that marsh? If so, then it's not working...but perhaps I'll toss a few acorns over next time I drive by.
Why is it there? I have no idea. Basically, all it has done is give me plenty to puzzle over as I drive my way home after work. It's quite the mysterious sign, really.