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Hollow Trees enters the battlefield tapped.
You may choose not to untap Hollow Trees during your untap step.
At the beginning of your upkeep, if Hollow Trees is tapped, put a storage counter on it.
{T}, Remove any number of storage counters from Hollow Trees: Add {G} for each storage counter removed this way.
Counters are not lost if the land is changed to another land type. They wait around for the land to change back.
If the land is tapped by some external effect, no counters are removed from it.
It is only considered “tapped for mana” if you activate its mana ability and generate mana from it.
Whether or not it is tapped is checked at the beginning of upkeep. If it is not tapped, the ability does not trigger. It also checks during resolution and you only get a counter if it is still tapped then.
