Call it the lingering influence of 1973's The Secret Life of Plants if you like. A story idea today popped into my mind today with an intelligent tree as the central character.
The book I referenced above made many claims that plants observe what human beings and other animals do. That they are aware of other plants being destroyed and can recognize the particular human who destroyed another plant--so the book claimed. The book also legitimized things some people tended to do with plants anyway, such as talk to them or play Morzart to them. Because supposedly, the plants recognize this.
The book was largely debunked not long after publication. There is no proof trees or other plants really do such things.
But modern science does find that trees actually do have a number of sensory perceptions. Roots know which way up and down are (like the human sense of balance). They also seek out water, including the sound of rushing water when experiments are performed that exclude any extra moisture that would lead them there otherwise. So plants have a form of hearing. They react to light, which could be called a form of sight. Roots move around certain kinds of hard objects on contact with them, which could be considered a form of touch. Plants roots move toward soils with nutrients they need and away from toxic soil, indicating a form of taste or smell.
What plants lack is a central nervous system to funnel all these sensory perceptions into one "self" that would be able to coordinate these perceptions into a common picture of reality. Or do they? Some kinds of things plants react to send chemical signals through the entire body of a plant. Some electrical signal activity is also seen in plants. This is happens slower and not as coordinated as what happens in animals, but it is observed to exist.
Trees especially have long been thought to have spirits among animist religions. This is why fantasy, which often draws on legends to build its stories, not infrequently has trees with magic powers or trees with the ability to communicate in some way or other.
But I'm talking about a writing story from a science fiction setting in which a tree is an unexpected character. A setting like our world, in which trees are thought to be essentially inanimate objects, but in reality are not. Maybe this could be from alien trees in or from another world. Or better, trees on our own world with unusual properties--trees that have been genetically modified somehow, but nobody realizes the botanical impact of these modifications. Or maybe a story could take quite ordinary trees and give animistic concepts a fresh coat of paint to explain how trees could be intelligent without having a physical reason for being that way. For example, as a Christian writer I could craft a story in which trees as part of God's creation share a common spiritual heritage with the rest of nature in having a form of spiritual life, one found in all things, both living and non-living. This life would be unlike the spiritual nature of a human being, in that it would not be eternal. (I'm not saying I believe such a common spiritual nature to all creation is true, but there are truths about nature the Scriptures do not reveal, and there happen to be poetic parts of the Bible that talk about rocks crying out and waves clapping their hands and the like. And the idea could make an interesting premise for a story, even though I don't believe it.)
In any case, in a world of trees either unintelligent in fact or expected to be unintelligent, the story focuses on one particular tree that is observing the events in the lives of a family nearby over a long period of time. I'd write it so the tree experiences the passage of time much more slowly than we do, so that a year might seem like a week or something in tree subjective time. So there would be a lot of things the tree would not notice about the fast-moving creatures around it. But it would notice some things. Perhaps a child that lingered for hours at time in its branches would catch its attention. And then perhaps if this child were to be killed and buried, the tree's roots might find the corpse and recognize it.
Part of the story would be told from the tree's point of view, but I'd also feature human beings somehow discovering what the tree knows. Perhaps through an examination of this unusual tree's DNA and physical make up, cracking it's internal messaging code as it were. Or perhaps through some kind of message the tree manages to send in human language--a branch growing with phonograph ridges or something very unexpected like that, which someone learns to play or read. Or perhaps the story could show an angelic being in communication with the tree. Or reference such a spiritual communication. To the end that what the tree knows is found out.
I think a tree helping solve a murder has a natural resonance with me. But the tree could be a "watcher" in other ways as well. It's not as gripping, but a story could feature a tree having a long-time loving relationship with a family, or perhaps a particular human being...