This is likely The Genesis Quest by Donald Moffitt.
An alien race (The Nar) assemble humans from a stream of genetic information transmitted by radio from the Milky Way Galaxy. The resulting colony of humans spend some time integrated into the Nar society before growing restless, discovering the secret of human longevity, and embarking on the seemingly impossible millennia-long mission of a physical journey back to Earth. This epic journey is made in a gigantic space-grown semi-sentient Dyson tree known as Yggdrasil.
It was followed by Second Genesis
Many centuries ago, an alien race known as the Nar was able to recreate human beings from genetic code, broadcast from Earth into outer space by a beleaguered humanity. They allow a group of humans to travel millions of light years through the galaxy, in order to discover what has happened to the original occupants of planet Earth. However, none of the human participants of the expedition are prepared for what awaits them at the completion of their journey...
I found the reference on the TV Tropes page for Tree Vessel, requisite warnings that the site is a huge timesink.