There are two ways you can get content into games. You can hand craft and design your entire environment, painstakingly the drawing trees, rocks, grass and sky. Or you can get a program to create all this for you. In the first approach you get exactly what you want, but it takes a while and you have to do all the work yourself. In the second you use carefully managed randomness to get something which "grew" by itself.
First thing this morning we were at a talk by the developers of FarCry 2, which is due for release later this year. They've gone for the programmatic approach to making their jungle. Their trees really grow, based on parameters and design settings from the graphics designers creating the game world world. They don't actually create the scenery when the game runs, instead they use a whole bunch of programs to make it before it is fed into the game. The end result was really impressive, with very realistic trees which wave in the breeze, and even come to bits when the weather hits storm force.
I expect that in the future more games will work this way, as the power of the consoles increases and the increasing amount of detail in the game environments makes it harder to make this stuff by hand.