You've got to operate on evidence, there's an infinite number of things you can't falsify and you have no criteria for choosing which to believe or not.
In other animals we observe things consistent with pain such as long term behaviour modification in the absence of a persistent hormone. Things like avoiding places they were injured, becoming more cautious or less curious, even changes that destroy them like starving themselves to death.
Anyone that says "only humans feel pain" is a chauvinist ignoring stuff like rats giving up the will to live.
But trees or mosses or whatever do none of this. A tree will keep trying to grow towards a fence that damages branches in a storm, a tree never starves itself to death making thicker bark after teens carve lovehearts into it, a tree doesn't stop reproducing after 3 droughts kill all its children and so on. Leaves might change colour in response to periods of high or low sunlight but these changes are like tanning, they don't modify anything about how the tree trees.
We can't know is true, but we also can't know I don't have an invisible dragon in my garage. you should definitely not live your life thinking I have an invisible dragon in my garage. Why? you don't have any evidence to suspect it's real that is distinguishable from a random lie. We have no evidence of behaviour in trees indistinguishable from chemical signals we know are below the level of consciousness in ourselves.