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I'm not sure what you are referring to by cards being discarded from play by default.... but in reference to The White Tree, it has a card erratum which states: "Discard the Sapling of the White Tree."
But is there anything that indicates it must be discarded from play?
I think it is a matter of common sense. I understand that rules and rulings can be placed under a microscope and dissected to sometimes find unintended methods of use which are technically legal. However, I think sometimes taking a step back and asking what makes obvious sense is the correct approach.
The White Tree states: Playable only if a Sapling of the White Tree is at Minas Tirith.
The CRF states: "Discard the Sapling of the White Tree."
If you are making the argument that a stored Sapling which is out of play could be discarded (since the CRF doesn't state "from play"), then you might as well say it is perfectly fine to discard the Sapling from your hand. This makes no sense of course, so it should be dismissed.
Thematically, I don't have a problem allowing the discard of a stored Sapling at Minas Tirith, as it would make sense. However, the rules appear to forbid this. What I do have a problem with is allowing something that doesn't make sense to occur (discarding the Sapling from anywhere simply because the CRF doesn't state "from play.")