From my experience, the only way is to make all trees invulnerable per trigger. D&D will damage everything, no matter how you change the allowed targets.
If you don't want that, I guess you'll have to trigger the spell.
You could use the default spell and trigger the damage only? That'll make the triggering much easier to do....but I'm not very familiar with the mechanics of death and decay, so I'm not sure how easy it would be to get the targets hit by it.
The forum software says we have 554 members online now. We did not have that many unique visitors the whole day today. Too many bots and the disconnect between the forum counts and the stats.