There's no "trespassing" involved. The fence had been placed there illegally over a public right of way. It's not at all clear to me if cutting and removing the fence could be considered to be destruction of private property under Colorado law, but it most certainly could not be considered trespassing.It’s still private property. Trespassing is illegal, regardless of how much you feel you are helping others.
Here is the best discussion of the issue I could find.