Council of Elrond

Is an item stored in a site considered to be in that site? In other words, a Sapling of The White Tree stored in Minas Tirith is considered in Minas Tirith for purposes of playing The White Tree?

No.
But people are playing The White Tree as if "if a Sapling of the White Tree is at Minas Tirith" was "if a Sapling of the White Tree is at or stored Minas Tirith".

The White Tree - Limited Edition wrote:Unique. Sage only at Minas Tirith. Playable only if a Sapling of the White Tree is at Minas Tirith. Minas Tirith becomes a Haven [H] for the purposes of healing and playing hazards. And the Tree that was withered shall be renewed, and he shall plant it in high places, and the City shall be blessed.-LotRVI

The White Tree - Unlimited Edition wrote:Unique. Sage only at Minas Tirith. Playable only if a Sapling of the White Tree is at Minas Tirith and is discarded. Minas Tirith becomes a Haven [H] for the purposes of healing and playing hazards. And the Tree that was withered shall be renewed, and he shall plant it in high places, and the City shall be blessed.-LotRVI

CRF, Errata (Cards), The White Tree wrote:Card Erratum: Add "Discard the Sapling of the White Tree."

Interestingly the text of The White Tree - Unlimited Edition does not look like The White Tree - Limited Edition with the errata applied.
"is at Minas Tirith and is discarded" (at the same time) does not make a sense.

Thanks for your reply Konrad. I apologize at the same time, because I did not understand you. I explain:

My company arrives to Minas Tirith and stores a Sapling of the White Tree in my organization phase. My company does not move to another site. Can I play The White Tree in my site phase or at the end of my turn (considering my company is in Minas Tirith and discarding the stored Sapling)?

This is the most important thing for me: Is an item stored in a site considered to be located in that site? I thought so far that yes

It was a first word of my previous answer: "No".
According to text of the card - no.
The rest was a digression.

Text of The White Tree - Unlimited Edition literally does not have the card any chance
Text of The White Tree - Limited Edition (with or without errata) makes them playable.

That does lead to some awkwardness with Mithril, as there is no "same company" as a stored item.

@ Amatar: This may be another case of sloppy wording on ICE's part, but the texts as they stand work as Konrad has stated. Compare with Wizard's Trove:

You may play The White Tree at one of your Wizardhavens if Sapling of the White Tree is stored there.

White Tree doesn't mention Sapling being stored there -- just being there.

Ok, I perfectly understand. Thanks for the answers.

Bandobras Took wrote: ↑Tue Sep 26, 2017 1:52 pm That does lead to some awkwardness with Mithril, as there is no "same company" as a stored item.

It is not clear what means "in the same company". It may be the same company that stored Mithril, or something else.