When the sentient aliens reach adolescence they mentally block out the rest of the tribe and die crossing a desert in a futile attempt to reach — we later learn — a similarly isolated female tribe and tree. Humans provide a waterway or divert a river across the desert to enable the offspring of the male and female free to meet and mate. The story ends with the first of the new seeds produced germinating.
I read it in an anthology before 1976. I imagined the aliens as small and frog-like, but whether that was just my imagination, I am not sure. The tree itself is non-sentient. The story is initially told from the POV of the tribal leader, who is trying to understand the mystery of why his people are compelled to die crossing the desert when they mature. The humans are mostly interstellar scientists and explorers.