I remember the main character was on an alien planet where people lived in the tops of the trees. Almost all the plants were poisonous. There were levels, and the lower the level, the more deadly the level was. Also, I think you had to be introduced to the home tree or it would kill you. A corporation was exploiting the planet for new pharmaceuticals. In one scene, there were some ant-like animals that would go on a march and kill everything in their path. If you can help me identify this story, I'll be thrilled.
Midworld by Alan Dean Foster had some similarities to this. Very old though, so depends when you came across it.
Been a long time since I read it so this is really what jogged loose when looking it up again on Amazon and wikipedia.
The entire planet is arboreal and extremely hostile. The indigenes are descendants of a spaceship that crashlanded or something a long time ago and have adapted to the local environment developing psychic links with symbiotic animals (ish) that are vaguely ursine.
A hunter discovers a ship from 'civilization' that has come down looking for exploitable plants, and starts of helping them but then changes his mind when he discovers they are there with an exploitative agenda.
A very good book in its time, but definitely the more pulp side of the genre. Published in the 70's.