Comment by Manghuwa on The Player Hides His Past - Chapter 98

@Manghuwa the world tree doesn't grant him infinite mana, it just greatly restore his mana, but even then such has limitations, even a pool a finite amount of water,
Oh and
More mana input = the more powerful the spell
And some magic requires a certain amount of mana to activate to begin with

@Manghuwa also if he didn't quickly unlock the circle (by increasing his mana) he will die.