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.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.