There are several question threads on forums that have a lot of answers of stories about people turning into weeds, people turning into trees, sentient trees that people combine with or join.
Although my favorite was and still is Katherine MacLean's about nonhuman peoples having a natural phase of adult life as a semi-aquatic plant:
It's surprising (to me) how many similar stories there were about one Earth human, or in one case a family of them, turning into trees or large plants.
On this forum there was
a question that was answered with Alan Dean Foster's "Midworld".
This question has a fair whack of answers. (The OP never confirmed any of them.)
I've bookmarked them and have read several. And several, including my favorite by MacLean, could be a possible answer to that question.
It also linked to an excellent
search asking for help finding a "science fiction short story where people grow roots and become...plant-like".
I kept this search as my cynosure when I was going nutz trying to rediscover MacLean's story. Ironically,
no one gave (or confirmed) the right answer there. Someone did give a precisely matching description matching MacLean's story, gave no title, and attributed it to Robert Sheckley.
I've since read several of the stories in the answers: "Gone Are the Lupo", "Whatever Became of the McGowans?", "Piper in the Woods", and they all had good stories people turning into plants.
One of the answers answer
this question too. And there are a lot of leads that might, you know, lead to story in question.
Since the elders become trees, it's probably one of the off-Earth stories. Maybe?