Titan Forest

A Titan Forest (巨大樹の森) is comprised of tall trees (generally around 80 meters tall), which have enormous trunks, large branches, and can grow larger than almost most Titans. Due to their enormous sizes, humans can stay atop the upper branches of the trees and remain safe from Titans, which do not possess the intelligence to climb trees. They are used by the Survey Corps for certain goals on expeditions. These forests are excellent places to use the vertical maneuvering equipment, as there are many tall tree trunks and branches to which the steel cables can secure to, improving versatility and mobility. At the same time, it is a good place for Titans due to the large tree trunks concealing them and the denseness of the forests reducing visibility of incoming Titans.

The Female Titan arc
During the 57th expedition to the lands beyond Wall Rose, the entirety of the Survey Corps' outer right flank was wiped out, due to the Female Titan apparently leading an army of Titans. Commander Erwin, the leader of the expedition, signaled the center column to enter the pathway to the Titan Forest, located somewhere between Wall Maria and Wall Rose.

The remaining units, who were not brutally killed by the Female Titan, were ordered to station themselves on the borders of the forest and stop any Titans that attempted to enter. At the other end of the forest, as expected, the Female Titan followed the center column into the forest. She pursued Levi's squad in the forest, brutally killing the Survey Corps members who attempted to kill her.

Soon after, the Female Titan is lured and falls into Erwin's trap, and is held in the forest while protecting the back of her neck, with the Survey Corps' hopes of capturing the human inside. She later cries for Titans surrounding the forest to eat her alive as to escape her temporary confinement. After the whole of Levi's squad, with the exception of Eren and Levi himself, is brutally murdered by the Female Titan, Eren transforms to battle the Female Titan. After Eren is defeated and kidnapped, Levi and Mikasa attempt to attack the Female Titan and Levi successfully rescues him.

Clash of the Titans arc
Hange Zoë deduces that Reiner Braun and Bertolt Hoover must have stopped by at a Titan Forest, located somewhere between Wall Rose and Wall Maria, because they just had an exhausting fight with Eren and some members of the Survey Corps and would require some rest in their human forms. The upper branches of the forest would serve as excellent protection from other Titans while they recovered.

Later, Reiner expresses that he, Bertolt, Eren, and Ymir have no choice but to wait until nightfall to move out. After spotting the Survey Corps in the distance, Reiner and Bertolt decide to leave; strapping the two captives to their backs and escaping through the trees. In order to capture Historia Reiss, Ymir transforms into her Titan. The Survey Corps notice this and head into the forest. After coming into Ymir's proximity, Historia is swallowed by Ymir, who uses the trees to escape. Reiner transforms into the Armored Titan and exits the forest, linking up with Ymir to start running towards Shiganshina District. Meanwhile, Commander Erwin has the military ride through the forest, attracting Titans to use for his plan.

Unnamed final arc
Levi takes Zeke to a Titan Forest to be temporarily confined from society. Levi tells him that the area is suitable for him, as the dense forestry limits the usefulness of his Titan. Marveling at the sight, Zeke asks Levi if he can show the forest to Gabi and Falco, but is told that it depends on his own behavior. Zeke is kept in the forest to be watched over by Levi and thirty other Survey Corps soldiers.

Following Zackly's assassination, a Corps messenger sends word of the situation to Levi and the other soldiers. Levi tells him to inform Pixis of his plan to have Zeke fed to any captured Yeagerist. Meanwhile, Zeke is watched over from within the trees, with soldiers ready to swoop down if needed. As Levi is lost in his thoughts, musing how his vow to Erwin will soon be fulfilled, Zeke quickly begins scuttling away. Before anybody can react, he lets out a scream, transforming the Corpsmen in the trees into Titans.

Levi is forced to engage and kill his now Pure Titans subordinates and quickly catches up to Zeke, who in turn transforms into the Beast Titan. Levi uses the trees to hide from the Beast Titan's projectiles then begins cutting down branches to use as diversion, which allows him to defeat Zeke shortly after. Having captured him, Levi drags Zeke away from the forest.

Trivia

 * There are several of these forests in the region between Walls Maria and Rose and they originally functioned as tourist spots.
 * According to Zeke, the Titan Forest is a rather unique place in the world.

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