Oh, I see what you're trying to do.
Yes, you probably can, err, maybe Dan can, ask over at the Cow Expression Forum:
http://forums.creativecow.net/viewforum/227
I think if you combine this position expression from Colin:
minX = 0;//Minimum x depth value
maxX = 1000;//Maximum x depth value
minZ = 0;//Minimum z depth value
maxZ = 2000;//Maximum z depth value
seedRandom(index, true);
x = random(minX,maxX);
z = random(minZ, maxZ);
[Math.round( x), position[1], Math.round( z )]
With an Opacity expression that is 100 when there is a Collision Detection, and 0 when not, from Dan's page:
http://www.motionscript.com/design-guide/collision.html
But I don't have AE CS3 or the expression finesse to do it.
Alternatively, you could just use Colin's Position expression above, with the min/max set to hit the island, and then turn off the eyeballs of all trees landing off-island, and shy them so they don't clutter up the stack. That way no collision detection necessary... er, actually you would be performing the collision detection, manually.
