What does this mean?

I noticed near one of the roads I take regularly, they have what looks like a bunch of pvc pipes sticking out of the ground. It's an entire area of grass and these pipes are covering it. They are like 3-4 feet tall.

I went to this park that has a walking trail that I haven't been to in a while, and they have the same thing in one area. They have signs up saying not to mow the area and something about preserving, but it does not have any specifics at all.