If Tree of Redemption isn't on the battlefield when its activated ability resolves, the exchange can't happen and the ability will have no effect.

When its activated ability resolves, Tree of Redemption's toughness will become your former life total and you will gain or lose an amount of life necessary so that your life total equals Tree of Redemption's former toughness. Other effects that interact with life gain or life loss will interact with this effect accordingly.

Any toughness-modifying effects, counters, Auras, or Equipment will apply after its toughness is set to your former life total. For example, say Tree of Redemption is enchanted with Lunarch Mantle (which makes it 2/15) and your life total is 7. After the exchange, Tree of Redemption would be a 2/9 creature (its toughness became 7, which was then modified by Lunarch Mantle) and your life total would be 15.