The places where I "hang out" (hammock pun) are completely tree-infested - the Pacific NW. One of the reasons I first tried the hammock is that the ground here tends to be lumpy, with lots of pointy things laying on the ground, and tons of critters crawling around on the forest floor :-)
I used the hammock once in Wyoming, where there were no trees in sight, but it was at a highway rest stop, and I strung it between two lamp posts in the parking lot.