Vaster Than Empires And More Slow by Ursula K. Le Guin

A small expedition to a remote, vegetative planet discovers a single, planet-spanning plant intelligence whose intense, pervasive emotions amplify and reflect those of the human visitors; as the crew—particularly a deeply sensitive outsider—grapples with rising fear, loneliness, and mutual mistrust, they must learn to understand an alien, communal psyche and confront how empathy and alienness can both connect and destroy.