Old Growth Taiga

The Giant Tree Taiga, known as the Mega Taiga before 1.13, is a rare cold biome. It has four variations total.

Description
Much as the normal taiga can be considered a colder counterpart of the forest biome, the giant tree taiga can be considered a colder counterpart of the jungle biome. Giant tree taigas usually generate surrounded by normal taigas. Spruce trees grow all over the landscape, like regular taigas, but most of them are the large 2x2 variants that grow significantly taller than regular spruce trees, and rival jungle trees in height. The terrain is fairly flat, like regular taigas, but much of the grass is replaced by large swaths of podzol and coarse dirt; this is the only biome where podzol may be found naturally. Additionally, "boulders" made of mossy cobblestone frequently generate on the ground, and dead bushes may generate alongside the ferns and tall grass on the ground. Wolves may spawn here, as they can in all taiga variants, and rabbits may spawn as well.

As the climate here is cold, rainfall changes to snowfall at higher elevations, though this very rarely happens in even hilly variants, as the snowfall line is quite high up. There are overall more trees in giant tree taigas compared to normal taigas, leading to increased foliage density. This makes wood more abundant, though as a large portion of it grows as part of tall 2x2 spruce trees, it may be more difficult to obtain. Giant tree taigas are slightly more difficult for survival than regular taigas for this reason, though aren't so bad once acclimated and strategies are used to clear the tall trees; they may offer more unique views compared to regular taigas as well.

Variants
Giant tree taigas have three variants, for a total of four different biomes.

Giant Tree Taiga Hills
Like all other hills biomes, Giant Tree Taiga Hills (known as Mega Taiga Hills before 1.13) feature elevated, hillier terrain compared to the normal giant taiga, making the landscape less suitable for shelter. Podzol, coarse dirt, and boulders may all still generate on the hills, and wolves and rabbits may still spawn.

Giant Spruce Taiga
The rare Giant Spruce Taiga variant, known as Mega Spruce Taiga before 1.13, is almost indistinguishable from the regular giant taiga at first glance, though if one looks at the tops of the trees, they will notice that they have much more leaves on their tops. The giant trees in the regular giant taiga usually only have small amounts of leaves covering their tops, while the giant trees in mega spruce taigas usually have leaves surrounding much more of the tree. The much larger amounts of leaves increases the chances of saplings dropping when these trees are cut down, leading to easier sustainability. The terrain is otherwise essentially the same as the normal giant taiga, with podzol, coarse dirt, boulders; rabbits and wolves spawn in this biome.

Giant Spruce Taiga Hills
Giant Spruce Taiga Hills, known as Mega Spruce Taiga Hills before 1.13 though erroneously referred to in-game as "Redwood Taiga Hills M", is a variant intended to be a hillier version of the giant spruce taiga. However, due to a likely error in the way terrain height is calculated, there is absolutely no difference in terrain between Giant Spruce Taiga and Giant Spruce Taiga Hills. Specifically, the game uses internal values known as  and   when generating hills biomes, but these values are the same for both giant spruce taiga and giant spruce hills, resulting in no actual difference between the two. This is the only hills biome in the game with this issue.