I just tested this out on warden.
Equip a fire staff and a lightning staff and at least one AoE and one single target ability to test without a source of minor berserk.
On each bar the corresponding abilities are buffed(lightning buffs AoE and fire buffs single target)
Now equip a source of minor berserk. I used Bird of Prey.
The numbers on both types of abilities will stay the same regardless of which staff is used as long as minor berserk is being applied.
I think Ancient Knowledge is coded to give the same buff, minor berserk, but only to either single target or AoE depending on the staff type used and therefore cannot be applied further with an actual source of minor berserk.
This means that my ice staff is equal to fire and lightning for dps as long as I have minor berserk in my build.
Edit: this actually makes ice the best staff type for warden for pure single target dps