In regard to this problem (dying forests):
http://www.simutrans-forum.de/forum/thread.php?threadid=5820
I think part of the problem is that initially forest are generated without trees of the old/dying age. Thus for a while there is no space to seed new trees in a dense forest, and the entire inner population of a forest then dies off at once.
I'd assume that generating trees with a better age mix in forests would avoid this problem, at least to some extend.
When increasing the maximum age of a tree to 704 month, the spawning routine was not changed accordingly: Trees only hat initial ages between 0 and 400 month. Since all trees reproduce maximum three times at 512 month age, the 40 year cycle of trees started. Now trees seems stable again over 200 years.
Good :) Thank you for the fix!