The scene before her reminded Minamoto Tamako of the iron wireworm, which parasitizes insects, devours their bodies, and ultimately controls them to head towards a water source...
To put it simply, the tree man was hollow a long time ago, with the surface merely maintaining a human shape.
It's akin to the maturation process of Cordyceps, where fungi from the family Clavicipitaceae parasitize bat moth larvae, growing and reproducing inside the larvae, absorbing their nutrients.
As the fungus grows, the larvae's body gradually gets consumed, eventually forming a "frozen insect" in winter. By the following summer, the fungus grows from the head of the frozen insect, emerging above ground like a grass-like fruiting body...
Just like the tree man.
Minamoto Tamako thought she was hallucinating again. Before she could grasp what was happening, the head of the leading tree man suddenly cracked open, extending tendrils that rushed at her!
