"Multani's heart is a seed, and all of Yavimaya is its flower. There is as much life here as in the rest of Dominaria together."
—Karn

Yavimaya, Cradle of Growth

Each land is a Forest in addition to its other land types.

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Pippin thought he saw groves of trees moving. Could it be that the trees of Fangorn were awake, and the forest was rising, marching over the hills to war?

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Land cards not on the battlefield aren't Forests while Yavimaya is on the battlefield.

Yavimaya's ability causes each land on the battlefield to have the land type Forest. Any land that's a Forest has the ability "{T}: Add {G}." Nothing else changes about those lands, including their names, other subtypes, and whether they're legendary, basic, or snow.

Yavimaya, Cradle of Growth isn't a Forest while it's not on the battlefield.