Many years ago--15, at least--I read a book about a person who found himself in a lush, green world dominated by conscious plants. The plants ended up "absorbing" the person into themselves, increasing their consciousness. It's an older book... I would guess 1960's?
Credit goes to Daphne B for her comment above IDing this, but, having just read this story, I agree it sounds very much like Ursula K. LeGuin's "Vaster than Empires and More Slow" (1971). (Note that if you are certain that your story is from the 1960s, then Richard McKenna's novella "Hunter, Come Home" as proposed in another answer is a better fit; it was first published in 1963.)
An "Extreme Survey" crew made up of volunteers goes to an unexplored planet known as World 4470 in a ship named Gum. World 4470 is unusual in that it contains only plant-type life:
The biologist's hunch proved correct. When they began field analyses they found no animals even among the microbiota. Nobody here ate anybody else. All life-forms were photosynthesizing or saprophagous, living off light or death, not off life. Plants: infinite plants, not one species known to the visitors from the house of Man. Infinite shades and intensities of green, violet, purple, brown, red. Infinite silences.
One of the members of the survey team is a not-quite-telepath ("empathic") with an albino-like appearance, who by nature doesn't get along well with the others:
She stopped. Osden had come into the main cabin.
He looked flayed. His skin was unnaturally white and thing, showing the channels of his blood like a faded road map in red and blue. His Adam's apple, the muscles that circled his mouth, the bones and ligaments of his wrists and hands, all stood out distinctly as if displayed for an anatomy lesson. His hair was pale rust, like long-dried blood. He had eyebrows and lashes, but they were visible only in certain lights; what one saw was the bones of the eye sockets, the veining of the lids, and the colorless eyes. They were not red eyes, for he was not really an albino, but they were not blue or grey; colors had cancelled out in Osden's eyes, leaving a cold water-like clarity, infinitely penetrable. His face lacked expression, like an anatomical drawing, or a skinned face.
"I agree," he said in a high, harsh tenor, "that even autistic withdrawal might be preferable to the smog of cheap secondhand emotions with which you people surround me. What are you sweating hate for now, Porlock? Can't stand the sight of me?..."
It's later revealed that this animosity is because of a positive feedback loop of negative emotion that starts whenever someone meets him for the first time.
Everyone on the team, but most especially Osden, begins to feel an intense fear whenever they're in the forest. Osden separates himself from the rest of the team to minimize friction, but it attacked by something (another team member, it turns out) and brought back to base to recuperate. He explains that he thinks the fear is coming from the forest itself:
"There is something." He closed his mouth, the muscles of his lips stood out rigid.
"In the forest?"
[WARNING: STOP READING NOW IF YOU DON'T WANT SPOILERS!]
It's theorized that the whole forest may be one being, mentally:
"What about those root-nodes that we've been puzzling about for twenty days -- eh?"
"What about them?"
"They are, indubitably, connections. Connections among the trees. Right? Now let's just suppose, most improbably, that you knew nothing of animal brain-structure. And you were given one axon, or one detached glial cell, to examine. Would you be likely to discover what it was? Would you see that the cell was capable of sentience?"
"No. Because it isn't. A single cell is capable of mechanical response to stimulus. No more. Are you hypothesizing that individual arboriformes are 'cells' in a kind of brain, Mannon?"
The group moves its camp to another part of the planet. The feeling of fear is there, too. Osden speculates that the whole planet is reacting to the awareness for the first time of something other than itself. He wants to try to communicate with it somehow:
"If I gave into it," Osden mused, "could I communicate?"
"By 'give in,'" Mannon said in a rapid, nervous voice, "I assume that you mean, stop sending back the empathic information which you receive from the plant-entity: stop rejecting the fear, and absorb it. That will either kill you at once, or drive you back into total psychological withdrawal, autism."
Some of the crew take Osden to the depths of the forest again, where they are overcome by fear and start to panic. Osden leaves alone and seems to succeed:
Osden moved suddenly and quietly, swinging out of the doorway, down into the dark. He was gone.
I am coming! said a great voice that made no sound.
Tomiko screamed. Harfax coughed; he seemed to be trying to stand up, but did not do so.
Tomiko drew in upon herself, all centered on the blind eye in her belly, in the center of her being; and outside that there was nothing but the fear.
Osden does not return, but seems to be communicating through control of the the body of a team member:
The mouth opened and spoke. "All well," it said.
"All well," said the soft voice from Eskwana's mouth.
"Where are you?"
A wind was rising. "I'll stay here," the soft voice said.
The rest of the team members are forced to leave, but they later return. They don't find him or his body. It's implied that he has merged his consciousness with that of the plants and finds it preferable to be vast and alone instead of constantly exposed to negative human emotions.
Hunter, Come Home by Richard McKenna 1963 Planet is covered by a planetary forest. The leaves are mobile, flying about like butterflies. The protagonist is part of a team from a macho culture whose male citizens are not considered to be adults until they have slain a large animal called a Great Russel. There is a Russel shortage on their home world. So they want to exterminate the forest and make a huge Russel game reserve.
A female character from a different culture thinks this awful, and the protagonist comes to agree.
The forest is so good at resisting extermination that the team is driven to use a dangerous germ warfare weapon. The weapon mutates, and the planet is put under quarantine. The team commits suicide to avoid being infected, except for the protagonist. He goes into the forest to die.
But the global forest has become sentient under extermination stress. It cures the protagonist and the woman, amplifies their intelligence, and makes the part of the planetary mass mind
Midworld (1975), Alan Dean Foster
It almost fits, although the absorbee is not the protagonist.
My guess is "The Engineer and the Executioner" by Brian M. Stableford. A man who has created a somehow intelligent biological system inside a small scientific asteroid faces its destruction, but enters the system itself before the asteroid falls into the sun, getting absorbed by the species he created.
It could it be The Right Hand of Dextra by David J. Lake.
The plants on the planet are a purple base, where all life has a DNA helix of life had a right-hand thread (earth biosphere had a left-handed thread) so humans keep trying to develop Earth crops etc on the planet and the planet reacts. The ending I think of is a man is inside a plant, like a womb, and it changes his genetic structure but that could be a different book the title of which is eluding me.
Is The Long Afternoon of Earth (aka Hothouse) the book the one you are thinking of?
Set in a far future, the earth has locked rotation with the Sun, and is attached to the now-more-distant Moon, which resides at a Trojan point, with cobwebs spun by enormous spider-like plants. The Sun has swollen to fill half the sky and, with the increased light and heat, the plants are engaged in a constant frenzy of growth and decay, like a tropical forest enhanced a thousandfold. The plants – many now omnivores – have filled all the ecological niches on the land and in the air, many evolving primitive nervous systems and, in some cases, eyes; of the animals in the forest only the descendants of four species of social insects remain - tigerflies (evolved from wasps), tree-bees, plant-ants and termights (from termites) - along with small groups of humans (a fifth of the size they are now); all other land and air animals have been driven to extinction by the vegetable kingdom, apart from a few shore dwellers. The humans live on the edge of extinction, within the canopy layer of a giant banyan tree that covers the continent on the day side of the earth.
BIOS, by Robert Charles Wilson?
In the 22nd century, humankind has colonized the solar system. Starflight is possible but hugely expensive, so humakind's efforts are focused on Isis, the one nearby Earthlike world. Isis is verdant, Edenic, rich with complex DNA-based plant and animal life. And every molecule of Isian life is spectacularly toxic to human beings. The entire planet is a permanent Level Four Hot Zone.
Despite that, Isis is the most interesting discovery of the millennium: a parallel biology with lessons to teach us about our own nature. It's also the hardest of hardship posts, the loneliest place in the universe.