Rendering plants efficiently has always been a challenge in computer graphics, a relatively new technique to address this is Pixar's stochastic pruning algorithm. Originally developed for rendering the desert scenes in Cars, Weta also claim to have used the same technique on Avatar.
Although designed with offline rendering in mind it maps very naturally to the GPU and real-time rendering. The basic algorithm is this:
Build your mesh of N elements (in the case of a tree the elements would be leaves, usually represented by quads)
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