Thanks guys. Yes, from a distance it's absolutely OK. You have to do some tests to get the correct settings of the power fractal, but, as you said, Ulco, it's probably easier than having to use a vast motion file. And there is of course a transform shader to move the PF.
One more thing: the trees have all the same motion of course, but since the instances are all rotated differently and the motion is rather subtle, it's not too obvious.