with great difficulty
I would say you want 3 frames that clearly show a tree moving back before reaching the location of the 1st one on frame 4.
Do this for both sides but offset.

The problem you have here is that there are too many trees to deal with. Might be easiest to start without any trees, make one on a seperate layer and duplicate that. People aren't going to notice that they are the same tree.
( HappyToast B3ta Art Auction http://b3ta.com/links/749374, Thu 31 Jul 2008, 14:54, archived)

You star!
Top tip. Thanks very much.

( Fresh Water Mole - loves his baby boy more and more each day, Thu 31 Jul 2008, 14:55, archived)

make a few larger background copies and duplicate them all
then for each frame, show 2 backgrounds, 1/2 the loop length apart.
Fade the top bg layer out from 100% to 0% over the length of the loop.

or something like that!

( Bloop Fri 16 Jul, 22:10, Thu 31 Jul 2008, 15:06, archived)

Hurry . . .
there's no end to it !
( Valin I changed glasses while I wasn't looking, Thu 31 Jul 2008, 15:13, archived)