Dandelumion
A tall seedhead whose drifting, luminous down is what lights the Dark Forest. Drain one and the plant never seeds again.
The Dark Forest has no sky worth the name. What light it has comes from the Dandelumion: a slender stalk carrying a globe of luminous down, each filament holding a startling amount of stored light for something so small. The globes shed constantly, and the spores ride the forest's slow air currents in drifting shoals, lighting a stretch of understory as they pass and going out somewhere further on. Everything in the forest is timed to them. Luxbushes near a spore lane sit permanently withdrawn; bushes the shoals never reach have closed over the paths entirely.
A spore holds more light than it has any business holding, and Moonhead can take it. Doing so empties the parent: the stalk survives, but it will never raise another globe. The barren stalks stay standing where they were emptied, dark and thin, for the rest of the game. The forest as a whole keeps its light (other stalks go on seeding, and released spores drift on regardless), so nothing becomes unpassable. The loss is not mechanical. It is the small ecology that had gathered in that plant's glow, drifting apart and going elsewhere.