Can you use a land's mana ability to pay for a triggered ability after the trigger is already on the stack?

Mana abilities are special: unlike other activated abilities, they do not use the stack and can be activated whenever you need mana to pay a cost (CR 605.3a). This means you are not limited to activating lands only at specific priority windows — you can tap a land for mana in direct response to needing to pay for something.

The most common situation is a triggered ability with a mandatory or optional mana cost in its effect, such as a 'pay {2} or sacrifice this' trigger. When that trigger resolves and asks you to pay, you may activate mana abilities — including tapping lands — right then to generate the mana needed (CR 605.3b).

Similarly, if a triggered ability has an additional cost that must be paid as it is put on the stack (an 'as an additional cost' or a cost printed on the trigger itself), you may use mana abilities immediately at that moment as well, because mana abilities are always available whenever mana is needed (CR 117.3).

Example: Your opponent controls Smothering Tithe and you cast a spell, causing its triggered ability to go on the stack. When the trigger resolves and asks you to pay {2} or let your opponent draw, you may tap two lands for mana right at that moment to pay the {2}, even though you had no floating mana before the trigger resolved.

Key rules: CR 605.3a specifies mana abilities don't use the stack; CR 605.3b clarifies you may activate them whenever you need mana; CR 117.3 covers when players may pay costs, including using mana abilities freely during cost payment.