you can not kill a god.

@Starry Mind  
then rip gallus’s plan.

@greenleafy  
I don’t think so.
 
Here are the official rules of Death Notes. They only affect humans, but they would probably affect other species one was in the MLP world. I do not think that trees would be included though.
 
Also, I was wondering how long it would take for a death to happen after a name was written, and it would apparently be exactly 40 seconds, unless it was manually made faster by changing the time of death before then.
 
If Gallus is a fast writer, he could write “Tree of Harmony, now” to cause a death within fifteen seconds. The issue would be whether it would even affect the tree in the first place.
 
Wait a second… if the tree is over 124 years old, it could not be affected.
 
Rule XXIX (29)  

@PonyPon  
trees can die also.. so the tree of harmony can totally die i think.

I’m not sure the death note has that kind of power.

tight spaces would have really killed poor gallus..