That being said, players still have a lot of discretion for using Powers in ways were some of the effects won't happen or won't happen at full strength:
Spirits can target a land where some effects or possibly all of them won't do anything. (e.g., a Power with a "Push 1 Town." instruction doesn't need to target a land with Towns.)
If an effect is explicitly optional, it can be ignored. (e.g., "You may instead Remove 1 Blight." doesn't need to remove Blight even if there is Blight in target land.)
If given a choice for how many pieces to affect (e.g., "up to 3"), 0 pieces is a valid number unless a minimum amount is specified. (e.g., "Push up to 2 Dahan." can Push 0 or 1 Dahan even if 2 Dahan are present.)
If an effect checks how many Elements you have – usually as part of an Elemental Threshold requirement – you may choose to apply fewer Elements so long as you're consistent for the entire Power. (e.g., if you choose to use 8 Fire for a "1 Damage per Fire" effect, you also meet and must use an "If you have 7 Fire" threshold. But you could alternatively use only 6 of those Fire for the Power in order to deal 6 Damage and ignore the 7 Fire threshold.)
If a threshold requirement includes an Energy cost, you may choose not to pay it in order to skip that threshold. (e.g., "If you have 3 Fire and pay 2 Energy" does not require you to pay 2 Energy even if you have 2 Energy to spend and are using 3 Fire for the rest of the Power.) Tags: Elements, Resolving Powers, Firestorm (Heart of the Wildfire), Heart of the Wildfire, 72 more...
Practically speaking, check Defend whenever Invaders deal damage (Ravage, usually), Dahan damage reduction when they counterattack as part of Ravage, and Invader/Dahan health reduction whenever they take Damage (or for already-Damaged pieces, when their Health is reduced). Tags: Health, Damage, Rain and Mud Suppress Conflict (Downpour Drenches the World)