I think you should not use Terrains for trees. I'd use terrains only for the ground. Today I think the best solution would be to use a high number of little terrains and use them as a grid byside eachother, eg. 10*10 Terrains, and each of these Terrains uses it's own texture that is stretched across the complete terrain. SOu you would have 100 Textures, which can be highly detailed.
If I was you, I would do the following: Create a HUGE Heightmap, maybe 640*640 Pixels. Then slice it to 10*10 parts, each 64*64 Pixels and store them as heightmaps, bitmaps or whatever you like. Then write an application in Blitz which will load each heightmap and create a 512*512 pixel texture for it. Depending on height (=brightness) of each pixel it will draw parts of some textures, grass, rock, seaground, sand, mud, whatever. You can also use transitions to mix textures, and add a random factor to make it look interesting.
However, this way you can store a texture for each part of the Heightmap Grid. SInce Terrains are using LOD (Level of detail optimation), distant Terrains are automaticly reduced to a small number of Polygons and the framerate could remain pretty high, compared to the usage of meshbased big terrains.
If I was you, I would do the following: Create a HUGE Heightmap, maybe 640*640 Pixels. Then slice it to 10*10 parts, each 64*64 Pixels and store them as heightmaps, bitmaps or whatever you like. Then write an application in Blitz which will load each heightmap and create a 512*512 pixel texture for it. Depending on height (=brightness) of each pixel it will draw parts of some textures, grass, rock, seaground, sand, mud, whatever. You can also use transitions to mix textures, and add a random factor to make it look interesting.
However, this way you can store a texture for each part of the Heightmap Grid. SInce Terrains are using LOD (Level of detail optimation), distant Terrains are automaticly reduced to a small number of Polygons and the framerate could remain pretty high, compared to the usage of meshbased big terrains.