I'm looking for a short story about a very old, giant tree that contains an extremely valuable substance at its heart. Don't recall if in a magazine or an anthology; perhaps 10 pages? In English. I guess 40-50 years ago.
The tree's planet is visited by a spaceship with three people: two not-nice male scavengers who intend to harvest the substance and a weaker man bullied by the other two. They find the tree, fallen over (I think maybe a giant storm toppled it?), and the two rip out its heart for the substance. Somehow the weaker person is left unconscious at the dying tree. The tree sends tendrils into him, observes that his makeup is inferior, and does various things to his body to strengthen him greatly. He then goes back to the ship and the two thug-types try to bully him further, but he's no longer the weak person he was, and demonstrates this--I think maybe (really weak recollection) by pressing his thumb into a metal bulkhead and deforming it.
I don't recall much else. It’s stuck with me for decades. Would love to find it again.