help with particle systems

[rjacobson2]

i want to know if you can apply a particle system to a certain material, and if so how to do it. one of the objects in my scene has a material applied to it with a cloud texture combining a stone like look with a green grass type color. and i want to give some trees coming out of just the green part, not the stone. so is there a way i can use the particle system to have the trees only coming out of the green? if so how can i do it?