By the way - you could double your terrain - and on this second terrain you could add an eco with just a leaf as an object and cover that ground, fully, driven by slope, heigth or a bitmap driven variable density or paint them on the terrain - or whatever - many, many options.
After the eco leafes are all covering this second terrain, go into this 'underlying' terrain and turn the transparancy to 100%, so the second terrain is invisible and the only thing that's left are the eco leafes. Now the first terrain with your mixed ground texture will be covered with nice leafes, because they are separate objects and they are fitting exactly the first terrain shape, like in a real forest, too ....