It's not a book, but "Hybrid", a short story by Keith Laumer which was the answer to this old question, matches your description pretty well. It was first published in The Magazine of Fantasy and Science Fiction, November 1961, which you can read at the Internet Archive. You might have read it in one of these compilations.
"The Yanda have a very strange reproductive cycle. In an emergency, the spores released by the male tree can be implanted in almost any warm-blooded creature and carried in the body for an indefinite length of time. When the host animal mates, the dormant spores come into play. The offspring appears perfectly normal; in fact, the spore steps in and corrects any defects in the individual, repairs injuries, fights disease, and so on; and the life-span is extended; but eventually, the creature goes through the metamorphosis, roots, and becomes a regular male Yanda tree — instead of dying of old age."
[. . . .]
"We made a deal. The Yanda gave me this — " Pantelle pressed a thumb against the steel bulkhead. The metal yielded.
" — and a few other tricks. In return, I'm host to the Yanda spores."
[. . . .]
Gault considered Pantelle's remarks.
"What about these 'proper conditions' for the spores?" he asked
suddenly. "You wake up and find yourself sprouting some morning?"
"Well," Pantelle coughed. "That's where my part of the deal comes in. A host creature transmits the spores through the normal mating process. The offspring gets good health and a long life before the metamorphosis. That's not so bad — to live a hundred years, and then pick a nice spot to root and grow and watch the seasons turn . . ."
Gault considered. "A man does get tired," he said. "I know a spot, where you can look for miles out across the Pacific . . ."
"So I've promised to be very active," Pantelle said. "It will take a lot of my time, but I intend to discharge my obligation to the fullest."
Did you hear that, Yanda? Pantelle asked silently.
I did, came the reply from the unused corner he had assigned to the Yanda ego-pattern. Our next thousand years should be very interesting.