The effects of fire arrows...

[Arjos]

I don't think it stacks, but being surrounded maybe stacked with fire arrows...

[Olaf The Great]

No, but the pure damage from the arrows killing people would, and if one unit routs it causes the morale of nearby units to go down.

[Ludens]

My guess is the fire-penalty would not stack. What you observed was the effect of being surrounded, no nearby support, massive missile fire and casualties, and fire arrows. Since fire arrows are supposed to have reduced accuracy, you would probably have been better of by having only one or two units use fire arrows and the rest normal ones.

[Captain Jazzy]

Getting a unit of good archers like Cretans or Caucasians to shoot a unit at close range in the back with fire arrows destroys morale, Its seems especially effective on lower end phalanxes.

[vartan]

Getting a unit of good archers like Cretans or Caucasians to shoot a unit at close range in the back with fire arrows destroys morale, Its seems especially effective on lower end phalanxes.

Especially effective on any such units with <=10 morale.

[Duguntz]

And that's why half of your archers should ALWAYS shoot fire arrows! rout rout rout little roman! if I catch you I eat you! (sweboz lullaby)

[Cambyses]

Its why archers really are better than slingers.