Are there novels where the protagonist is reincarnated as a plant or non-human living thing?

Yes, and this subgenre has developed a surprisingly devoted readership. While most reincarnation stories drop their protagonist into a human body in a fantasy world, a smaller branch of the genre goes further by placing them into something entirely non-human: a monster, a slime, or in some cases, a plant.

The reborn-as-a-plant premise sounds absurd at first, but it tends to produce stories with a genuinely different rhythm than conventional isekai. "Evolution From the Big Tree" is a good example. The protagonist wakes up as a willow tree in a world undergoing spiritual qi recovery, and the early chapters have to grapple with something most fantasy novels skip entirely: what does it mean to exist as something that cannot move, cannot speak, and experiences the world through roots and branches rather than eyes and hands? The answer the genre arrives at is usually that even a tree can accumulate power, expand its influence, and eventually become something that reshapes the world around it.

What keeps readers engaged is the sheer novelty of the premise combined with the familiar satisfaction of watching something utterly powerless become something extraordinary. The willow tree that cannot fight back against a passing woodcutter in chapter one may be worshipped as a world tree by later chapters. That transformation from utter vulnerability to cosmic significance is a specific kind of power fantasy that these stories deliver in a way human-protagonist novels rarely can.

The slower pacing and unusual perspective tend to filter out readers who want fast action, but for those who enjoy a more contemplative kind of progression story, they are genuinely hard to put down.