Integral trees.

The Integral Trees is a 1984 science fiction novel by American writer Larry Niven (first published as a serial in Analog in 1983). Like much of Niven's work, the story is heavily influenced by the setting: a gas torus, a ring of air around a neutron star. A sequel, The Smoke Ring, was published in 1987. It was nominated for the Nebula Award for Best Novel in 1984, and the Hugo Award for Best Novel in 1985.