Terrain question

[STRIMIX]

Is there any way i can have my terrain take less memory without losing quality?

So what i meen is, can i make it so my terrain doesn't take allot of computer memory?

(here is my terrain with some pictures!)
http://img87.imageshack.us/i/greybox11.png/
http://img42.imageshack.us/i/screenshotadr.jpg/

[Pandazor]

Unload unseen object and load them when they are visible to the player. You might want to make low quality model version of the tree so only when the player near the tree load the higher quality of the tree. You might as well use texture stream to reduce memory cost.

Hope that can help you.

[Elleclouds]

When I add a terrain to a scene it doesn't have a collision model so I can walk on top of it. How do I change it so I can walk on the terrain rather than pass through it?