Alright:
I made an edit, adding several variables in order to, instead of simply removing the destructable, replace it with a burned version (trees). You'll need to add a condition to check whether the burning object is a tree or not, in order for the computer to know whether or not to create a burnt tree at that location.
Secondly, your system has one glitch: Because of the way you've triggered the fire to spread (although it is definitely the right way), the fire burns itself into isolated corners. Once it's there, since your fire is triggered to burn out only once it moves on to another tree, it remains on fire indefinitely. Although it's an unintended result, I actually really like the effect - it makes the burnt sections appear almost to 'smolder'. However, the fire needs to end eventually. I took a stab at creating an end to the fire in the trigger "Burnout Check".
Burnout Check (is intended to) logs the number of trees burning every 3 seconds, and saves the last 5 logged values as integer variables (BurnTimer[1-5]). Theoretically, once the fire works its way down to the final isolated trees, the number of trees burning will stop changing. When the second trigger, "Total Burnout", encounters a point where all 6 of these integer variables have the same value (i.e., the number of trees burning has stopped changing), it will replace all burning trees with burnt models. You could probably add some timer to make them burn out one by one instead of all at once, if desired.
If you decide to keep the burnt model addition, I would also recommend adding variables to log the scaling value and variation number of the tree being burnt, and replace it with a burnt model of the same scale and variation from the palette of burnt models (it will be clearly marked).
All of your triggers are marked using either point or destructible based variables (which are reused), so I don't see anything that would leak badly. If you're horribly afraid of leaking, I would reccomend you ask the trigger moderator (or Spinnaker) to check it. The other option is to load and start the game before you go to bed, and check in the morning for any symptoms of extreme lag.
Other than that, great effect, and I will definitely be finding some excuse to steal this (and give you credit) for some map in the near future!
Edited Map Pastebin Link:
http://www.hiveworkshop.com/forums/pastebin.php?id=jn0cpu