The story you are thinking of is "Alien Earth", a novelette by Edmond Hamilton, a writer seldom mistaken for Ursula K. Le Guin! It was first published in Thrilling Wonder Stories, April 1949, available at the Internet Archive.
As noted in your update, you misremembered some details. I'll try to set out the matches and mismatches in your description, for the benefit of anyone who may be looking for the same story in the future.
There is a scientist character in the story, a French botanist who was the first white man to take the native drug and commune with the forest:
Farris lifted Berreau. The man's body was rigid, muscles locked in an effort no less strong because it was infinitely slow.
He got the young Frenchman down on the stretcher, and then looked at the girl. "Can you help carry him? Or will you get a native?"
She shook her head. "The tribesmen mustn't know of this. Andre isn't heavy."
He wasn't. He was light as though wasted by fever, though the sickened Farris knew that it wasn't any fever that had done it.
Why should a civilized young botanist go out into the forest and partake of a filthy primitive drug of some kind that slowed him down to a frozen stupor? It didn't make sense.
The viewpoint character, however, is a teak-hunter:
His business here in easternmost Indo-China was teak-hunting. It would be difficult enough back in this wild hinterland without antagonizing the tribes. These strangely dead-alive men, whatever drug or compulsion they were suffering from, could not be in danger if others were near.
And the setting is somewhere in Laos, in what was then French Indo-China:
"This is it—the path to the Government station," he said, in great relief. "We must have lost it back at the ravine. I have not been this far back in Laos, many times."
Not an alien planet, then, but the forest, experienced at its own slow tempo, is described as an "alien" world in the story (and its title):
Farris exclaimed, "Berreau, why do you do it? Why this unholy business of going hunati, of living a hundred times slower? What can you gain by it?"
The other man looked at him with haggard eyes. "By doing it, I've entered an alien world. A world that exists around us all our lives, but that we never live in or understand at all."
"The world of green leaf and root and branch," Berreau answered. "The world of plant life, which we can never comprehend because of the difference between its life-tempo and our life-tempo."
Farris began dimly to understand. "You mean, this hunati change makes you live at the same tempo as plants?"
Berreau nodded. "Yes. And that simple difference in life-tempo is the doorway into an unknown, incredible world."
The forest is not exactly "behaving as a sentient being", it's every plant for itself:
"But it was not peaceful or serene, that life of the forest. Before, it had seemed to Farris that the plants of the earth existed in a placid inertia utterly different from the beasts, who must constantly hunt or be hunted. Now he saw how mistaken he had been.
Close by, a tropical nettle crawled up beside a giant fern. Octopus-like, its tendrils flashed around and through the plant. The fern writhed. Its fronds tossed wildly, its stalks strove to be free. But the stinging death conquered it.
Lianas crawled like great serpents among the trees, encircling the trunks, twining themselves swiftly along the branches, striking their hungry parasitic roots into the living bark.
And the trees fought them. Farris could see how the branches lashed and struck against the killer vines. It was like watching a man struggle against the crushing coils of the python.
Very likely. Because the trees, the plants, knew. In their own strange, alien fashion, they were as sentient as their swifter brothers.
Hunter and hunted. The strangling lianas, the deadly, beautiful orchid that was like a cancer eating a healthy trunk, the leprous, crawling fungi—they were the wolves and jackals of this leafy world.
Even among the trees, Farris saw, existence was a grim and never-ending struggle. Silk-cotton and bamboo and ficus trees—they too knew pain and fear and the dread of death.