from pg. 11 of the rule book(the side bar called "what is a trait):
"...Sometimes, this strength value has a plus or minus
sign. Multiple traits of the same kind with plus or
minus signs can be combined on the same object.
So, if a creature with Lightning +1 is enchanted
with a spell that gives Lightning -2, those two traits
combine and the creature now has Lightning -1.
If a trait does not have a plus or minus sign, it does
not combine with other traits that have the same
name. Only the trait with the highest value counts.
If the same unicorn was enchanted with a spell that
gave it Regenerate 3, it would only heal 3 damage
every round, not 5."
this would also apply to channeling.