Wood Release

is a combined nature transformation kekkei genkai  originated from the Senju clan (in the anime from the Ōtsutsuki clan). It is made up of techniques that mix earth-based chakra in one hand and water-based chakra in the other to create wood as well as various plants, from simple seeds to even flowering trees. Wood and plants can be produced from any surface, including the user's own body, as the user's chakra is literally converted into a source of life. Wood Release techniques are commonly affiliated with the Snake seal.

Overview
Despite being known as a kekkei genkai, Hashirama Senju is the only known person to have naturally possessed Wood Release; all subsequent users have gained it by acquiring his genetic material — and even to then, their prowess with Wood Release paled in comparison to his. His ability to use Wood Release was one of the reasons he was named the First Hokage, and with Wood Release Secret Technique: Nativity of a World of Trees he created the foundation of Konohagakure. This technique also allowed him to completely reshape battlefields to his advantage. In the anime, Asura Ōtsutsuki also manifested it.

As Obito Uchiha noted, Wood Release is very hard to control due to the vitality of Hashirama's cells, which can potentially overwhelm any who are infused with them; when Danzō Shimura's chakra levels were depleted, he was forced to sever his right arm, which had been infused with Hashirama's DNA, in order to prevent being turned into a tree.

Wood Release has also been coveted by many for to its ability to suppress and control the chakra of a tailed beast and, by proxy, its jinchūriki. Hashirama's necklace, when used in conjunction with his Hokage-Style Sixty-Year-Old Technique — Kakuan Entering Society with Bliss-Bringing Hands technique, aided in this, but its exact purpose was unclear. After Naruto Uzumaki destroyed the necklace, Yamato, who had been experimented on and, as a result, was genetically identical to Hashirama, implied that he could no longer suppress the Nine-Tailed Demon Fox's influence. Nevertheless, when Naruto began transforming during his training to control the Nine-Tails at the Falls of Truth, Yamato stated that he would try to suppress the beast, though it is unknown whether this would have been successful due to Naruto overcoming the Fox's influence on his own.

Background
The first person to attempt to make Hashirama's power his own was his rival, Madara Uchiha. Having used their confrontation at the Valley of the End to acquire a portion of Hashirama's flesh, Madara later applied it to the wound he sustained in his chest. In addition, Madara used the Demonic Statue of the Outer Path as a catylyst to cultivate Hashirama's cells, resulting in the creation of a flowering tree made up of Hashirama's artificial body. The White Zetsu stored within the Demonic Statue were later pulled out through the tree by Black Zetsu, each gaining the ability to use a weaker version of Hashirama's Wood Release in the process. Madara later used Hashirama's artificial body to repair the otherwise fatal wounds sustained by Obito Uchiha during his mission at Kannabi Bridge, giving him the ability to use Wood Release as well. When Madara was later reincarnated in a state beyond his prime by Kabuto Yakushi, he once again gained access to Wood Release.

After Hashirama's passing, Konohagakure once attempted to replicate Wood Release through implanting Hashirama's cells into various shinobi, which consistently ended in failure and the subject's death. Ultimately, the research was ceased and deemed forbidden. Orochimaru, another who desired the First Hokage's abilities, injected Hashirama's DNA into sixty children, with the sole survivor gaining the ability to use Wood Release as a result. Orochimaru also performed a similar procedure on Danzō Shimura by implanting the First's cells into his right arm. While Danzō was capable of using this power, his version appeared cruder and caused a significant drain of chakra with each usage, causing Danzō to avoid using it as much as possible. Orochimaru himself would later acquire this coveted ability after taking over the body of one of White Zetsu's parasite clones.

After reviving the Ten-Tails in an incomplete state, Madara and Obito were able to improve their control of the beast through the use of Hashirama's cells. Obito also demonstrated the ability to channel his Wood Release techniques through the Ten-Tails' body while attached to it.

Trivia

 * Wood Release parallels the real-life ninja art . One example of real-life mokuton-no-jutsu is Tanuki-gakure: the practice of climbing a tree and camouflaging oneself within the foliage.
 * In the anime, the Moulding Mushroom appears to drain chakra from Wood Release users.
 * In the anime, Hashirama left behind a scroll detailing his various Wood Release techniques. Danzō later obtained this scroll and gave it to Yamato in the hopes that he would one day refine his Wood Release to Hashirama's level.
 * A replica of Hashirama's face formed on Danzō's right shoulder after Hashirama's cells were integrated into his body. Madara gained a similar replica of Hashirama's face on his chest after being reincarnated and modified by Kabuto.