Eslake, 'splain clipping and bounding, please? and if that is the issue, will putting the trees on upper floors help at all? (hope you're sensing how much I don't get it 
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Clipping for the layman.
Make some little paper puppets (southpart characters for example).
Take a big cardboard box, like a refrigerator box, and cut a little window in it, maybe 1foot x 1foot.
You can stand behind the box, and move the puppets in and out of the window area to have a puppet show.
They can move in and out of the viewable area, no problem.
You can move one puppet in front of another, no problem.
But when a computer is trying to do the same thing, there is no physical object to obstruct the view. if you pass one object in front of another, the computer has to determine what parts of the object further away are "clipped" from view or the images mix together and become unrecognizable.
Now.. to explain the tree problem..
Imagine if you made 3 different windows on your fridge-box puppet booth. Each on top of the other, so you could have 3 shows at a time. (floors of a house)
Everything is essentially the same. Puppets block other puppets, and so on.
But when you add something so tall that it reaches from the bottom window(floor) all the way up into the 3rd?
The game suddenly has to handle clipping on a single object on all 3 floors at once, for each tree present. This clipping is being handled not just on the objects Inside the house, but those in front of it, and behind it as well.
That is why older PCs will see a noticable increase in lag for large objects like these trees.
Bounding .. .. well you already get the idea of why the trees cause lag, explaining bounding might only serve to confuse the issue.
The short version is defining the space in which an object/objects can be drawn at all.
I actually hesitate to call it "lag" since most assume that means something to do with the connection or information being sent to and from the servers. But it sounds better than saying it Bogs Down.
