Since I started using 3d trees in my work it also became clear I must use instances as much as possible and for large numbers distribute them using the particle system. I have already written about distributing objects using particles and I think settings in that post still apply to the current Blender version (v. 2.65). This method can be applied to areas of random growing vegetation. You can take this one step further by placing the trees in a precise pattern, something I often use in architectural renderings, for trees in parking lots or rows along a road, but still keeping the random scale and rotation features that come with particles and improve the repetition issues.
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