If a Power has a conditional effect *(eg, Defend if a Sacred Site is present)*, does the effect persist when the condition changes?

If a Power has a conditional effect (eg, Defend if a Sacred Site is present), does the effect persist when the condition changes?

Yes, unless specified otherwise.
You evaluate the condition when the Power is executed, not when the effect happens. For instance, The Trees and Stones Speak of War (without the threshold) gives Damage and Defense per Dahan present at the time you execute the power, not Dahan present during the Ravage Step.
This only applies to Powers with a localized effect. “Everywhere” effects that modify an entire class of things operate differently.