im using craftbukkit with minecraft 1.2_02 but i have no pines or something like that (with the white wood)....
Exactly. If you still want new trees in old locations, you can delete as many chunks as you can (i.e. there are no important buildings or anything) and they will be recreated with new trees. From my tests on this, the world will look exactly the same as the randomization is done only when the world is created for the first time.