Board Thread:Naruto Discussions/@comment-1512913-20170609150729/@comment-1043414-20170609152447

My guess is that as the Senju are closest direct descendants to Asura and Hashirama was his reincarnation, it made his powerful Senju chakra and life-force especially powerful, leading to the manifestation of the Wood Release. To which, even after his death, his cells retained the same potency, as to why Madara and every other user of Hashirama's cells were able to manifest his various physical capacities even after Asura's spirit was reincarnated into Naruto.