Fangorn, Tree Shepherd — ruling 1

You can keep unspent green mana indefinitely while Fangorn, Tree Shepherd is on the battlefield. That means if you add a green mana during one step or phase, you can spend it during a later step or phase, or even a later turn. Other types of mana will be lost as normal as each step and phase ends.

Fangorn, Tree Shepherd — ruling 2

If a green mana you add has certain restrictions or riders associated with it (for example, if it was produced by Great Hall of the Citadel), they'll apply to that mana no matter when you spend it.

Fangorn, Tree Shepherd — ruling 3

Once Fangorn, Tree Shepherd leaves the battlefield, you have until the end of the current step or phase to spend your unspent green mana before it is lost as normal. There is no penalty associated with this other than the loss of the mana.

Fangorn, Tree Shepherd : règles, fonctionnement et interactions (MTG)

Fangorn, Tree Shepherd is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Treefolk you control have vigilance. Whenever one or more Treefolk you control attack, add twice that much . You don't lose unspent green mana as steps and phases end.

How it works : Fangorn, Tree Shepherd

Original English text — Wizards of the Coast / Scryfall

#1June 16, 2023· Wizards of the Coast
You can keep unspent green mana indefinitely while Fangorn, Tree Shepherd is on the battlefield. That means if you add a green mana during one step or phase, you can spend it during a later step or phase, or even a later turn. Other types of mana will be lost as normal as each step and phase ends.
No other card is named in this ruling.
#2June 16, 2023· Wizards of the Coast
If a green mana you add has certain restrictions or riders associated with it (for example, if it was produced by Great Hall of the Citadel), they'll apply to that mana no matter when you spend it.
No other card is named in this ruling.
#3June 16, 2023· Wizards of the Coast
Once Fangorn, Tree Shepherd leaves the battlefield, you have until the end of the current step or phase to spend your unspent green mana before it is lost as normal. There is no penalty associated with this other than the loss of the mana.
No other card is named in this ruling.