Damage types

While playing my Ranger I noticed that the monsters appeared to be not receiving a lot of the damage from my firey bow. I am assuming that is because they have some fire immunity.

My question is whether a Poisoner's bow deals strictly poison damage or does apply poison need to be used and the bow just increase the length of time the poison lingers? Also, does apply poison to a bow that does physical damage stack on top of the physical damage or just change the damage type?

What if I use apply poison to my firey bow? Will it deal both types of damage?

Thanks

Apply Poison doesn't change the damage type at all, it just makes your enemies poisoned. If you use Apply Poison with your fiery bow then you will still be dealing fire damage, but you will also poison your enemies. A Poisoner's Bow makes the poison effect from Apply Poison last longer, and it does not change the damage type (there is no such thing as poison damage.)

a poisoner's bow doesnt deal poison damage, still regular physical damage, just increases the poisoning length. poison (be it apply poison or poison arrow) just piggy-backs on whatever damage you're doing.

apply poison just causes your arrows to carry poison with them, there's no inherent change in the type of damage. if you use it with your firey bow, you'll be shooting poison tipped fire damage :P, still just fire element, but again the poison piggy-backs on it

and yes, some mosnters have better resistance to some elements.