The Dragon Tree (Hall Family Chronicles #8) by Jane Langton
Strange and magical things continually occur at the Hall family's home at 40 Walden Street. Now there's a terrible sound throughout the town of Concord—the buzzing of a chain saw. Only one thing is worse for Eddy and Georgie Hall than that noise: the man who causes it, Mortimer Moon. When all the trees in town are falling to his hand and he threatens the mysterious tree sprouting in the Halls' backyard, Georgie and Eddy will do anything to stop him. In the eighth installment of the Hall Family Chronicles, secrets—all caused by the growth of a miraculous tree—will be unlocked.