These trees have offspring that undertake a pilgrimage over water and land to a mountain, then return to their home, take root, and become part of their family forest. There are several, sometimes warring, tree settlements. Humans are invited to guard and assist the journey. Some humans established a colony centuries before the current earth visit and have adapted, evolved to survive the planet's different conditions and in relationship with the trees. It was written by a biologist, I believe. The trees communicate via "signals" (pollen/molecules?) in the air and through the root system. They can produce edible and potable matter for the humans' survival. humans & trees have a symbiotic, if strange, relationship. Thank you for your help!
It's called The Remarkables! It hit me like a sack of bricks. Phew!
Here's a link to Robert Reed's page on his own book. It contains this summary:
In the Realm, with its terraformed worlds, designed for the comfort of their genetically tailored human populations, the planet Pitcairn is legendary. Its inhabitants, descendents of Earth explorers, live in isolation from the Realm, in symbiosis with the Remarkables, the only other intelligent race discovered in thousands of years of galactic exploration.
Ranier Lu is one of six humans chosen to undertake a grand adventure -- one no others in the Realm have ever known. He and the others will accompany a group of Pitcairns and juvenile Remarkables on a passion, a ritual wilderness journey that marks the Remarkables' passage to adulthood. Ranier has his own reasons for leaving the comforts of the Realm for the long, difficult trek, and little by little he begins to discover those of his companions. But one of them as an agenda so unthinkable, so cruel, that Ranier comes to fear not only for his own safety but for that of the Remarkables, the Pitcairns, and even for the Realm itself.