Thank you for your comments.
Without your Private Message, I wouldn’t have noticed that this topic had been visited.
Here are some explanations :
The trees are animated in two parts :
1 - The whole trees (trunks, branches and leaves) are animated with a lattice, slightly bent using shape keys.
2 - The leaves are all in the same vertex group, separated from the trunk, and they are animated by a cloud texture used as a displacement texture. The texture is not rendered and in the Map To panel, I use Object as reference. The texture object used as reference is an empty, moving slowly on the Z axis.
The displacement level is adjusted very low, in order to give a slight wind effect on the leaves.
When I speak about the vertex group, it is because the trunk , the branches and the leaves are in the same mesh, so in order to apply the wind only to the leaves, you have to put them in a vertex group.
An important thing is that you have to use a Displace Modifier instead of enabling the Disp Button in the Map To pannel !
It is in this modifier panel that you type the name of the cloud texture used for displacement as well as the name of the Empty used to displace the texture ! You also setup the strength and average level of displacement in the same modifier.
Don’t forget to choose Object in order to open the field to insert the name of the empty.
A good thing is that the effect of the Displace modifier is directly visible in the 3D view, instead of beeing visible only at rendering time !
The grass is on a second ground floor used as hair emitter, not rendered, and placed under the visible ground. This emitter has 3 different Particle system for 3 kinds of grass. The emitter mesh is also animated with a cloud displacement texture with an empty as reference.
That’s all ! you know the tricks.
I hope that this makes more sense for you.