The Family Tree is my favourite novel by the prolific Sheri S. Tepper. In typical Tepper fashion, the book follows two separate storylines which eventually converge. One concerns a present day police officer who's made a rather unfortunate marriage but happily comes to her senses and leaves the jerk; in her time a new species of tree is aggressively taking over any space unused by humans and several prominent scientists who genetically engineer animals are being murdered. The second is set in the distant future, and involves a motley collection of our successors on a mystical quest to solve the Great Enigma and, incidentally, discover why a certain species of tree has turned disorderly, even deadly.
You've figured out that this is an sf novel - more speculative than science - into which Tepper weaves her usual feminist and environmentalist themes. It's less preachy than many of her books, though, and has a few plot twists so surprising and amusing that you'll shake your head in rueful amazement, even as you laugh with glee. I won't reveal what they are, though; you'll have to find that out for yourself. Highly recommended.