Houghton Mifflin Harcourt, 2010
The story takes place in the last decade of the 21st century. Someone has unleashed a plague that is destroying all the trees on the planet. Raven (as in The Raven from native American folklore) has recruited Kelsa, a 15 year old girl to be the one to put the cure in place. Mankind created the plague and mankind must be the one to cure it. Raven cannot do it by himself.
When the story opens, Kelsa's father, a botanist has died from cancer and Kelsa, resists Raven's plea to help. But eventually she agrees, and their quest takes them from Washington State into Canada and eventually Alaska.