Not the most recent trees rezzed, it seems that the ones the rezzer rezzed first are preferentially deleted.
(How many trees can a tree rezzer rez if a tree rezzer could rez trees?

Just so you know, I'm not expecting the rezzer to continue working after I log off and there is nobody on the world. I let the rezzer run for a while when I am there, I wait for the forest to grow to a nice state, I tell the rezzer to stop (usually I delete it). I admire the forest. I log off. When I come back, most of the trees are gone!
Since I have to hang around and watch, I do tell the rezzer to rez trees at a fast rate: one every 0.5 seconds.
Is there something I can do to force these trees to persist? Like rez something else by hand before I log out? Slow down the rezzer?