Way back when, the first combo I built a deck around was Force of Nature with a Spirit Link on it. If, at the beginning of my upkeep, I have less than 8 life and I don't pay the upkeep, will I lose the game before the life gain kicks in?
104.3b If a player’s life total is 0 or less, he or she loses the game the next time a player would receive priority. (This is a state-based action. See rule 704.)
When the Force of Nature damages you, Spirit Link's trigger goes on the stack. If, between sustaining the damage and getting 8 life back from Link, your life total hits 0, you'll die.
This is confirmed by the Oracle text for Spirit Link:
You only gain the life when the triggered ability resolves. If you are reduced to zero life before the ability resolves, you will lose before gaining the life.
Luckily, you can avoid this problem by replacing Spirit Link with Lifelink.