Tree of Perdition

[jaykaye05]

When you use Tree of Perdition:
Defender
{T}: Exchange target opponent's life total with Tree of Perdition's toughness.

Your opponent has 20 life before you tap Tree of Perdition. Assuming you tap this creature round 2 against a player and now the player's life total is 13, during your next turn, during your untap phase, you untap Tree of Perdition, does the player go back to 20?

I was told if it doesn't specific saying "until end of turn" then the effect stands. Where in magic should we assume this rule. Why wouldn't the assumption be "as long as creature is tapped" does ability with stand.

Thank you,
Jay

[Dice_Box]

When the ability resolves, Tree of Perdition’s toughness becomes the targeted opponent’s former life total and that player gains or loses an amount of life necessary so that their life total equals Tree of Perdition’s former toughness. Other effects that interact with life gain or life loss will interact with this effect accordingly.
Its not a temporary effect, its a life change effect. Life changing effects do not "Wear off" at the end of a turn.