Children of the Comet
This book reminds me of "Orphans In The Sky," not so much because of the plot, although it features a colony ship that cannot exceed light speed. It is more the look and feel of mid-20th century conquer the galaxy-ness. It's a much more positive view of the future than in "Orphans," that dystopian treat. "Children of the Comet" has plenty of struggle and strife, but the good guys come out okay in the end. In other words, you could give this book to a 10-year-old, maybe an 8-year-old.
In another way the book is like Poul Anderson's classic "Tau Zero." In the near future we learn to build ships that can pretty rapidly approach speed of light and therefore take advantage of time dilation. We learn this from a couple of small expository lumps. Most of the action takes place billions of years in our future. Giant trees have seeded the Oort cloud, and humans living there too. We have also colonized other galaxies. Now, sometime dilated humans are coming back to solve system. The stars are billions of years older, but the humans have only lived a few decades. When the tree dwellers meet the space travelers things start to really get interesting.
The animals and plants are charming and well thought out. The times and speeds probably are completely impossible, but they seem plausible the way that Moffitt writes about them. The people and their interactions are excellently done. This is one of the strong points of the book.
I already cheated and told to that it has a happy ending, but there's plenty of excitement on the way.
And I did think of the integral trees; he borrowed the same concept that Niven used.