Photograph by Andrea Watts

U.S. Forest Service Pacific Northwest Research Station Science Findings
March 2016

Trees bursting forth with new leaves signal the arrival of spring. Budburst for most temperate tree species occurs after a tree has been exposed to a sufficient number of chilling and forcing hours over the winter. Waiting until these chilling and forcing hours have accumulated is a survival mechanism.

To identify the chilling and forcing requirements of 11 common Pacific Northwest tree species, scientists with the U.S. Forest Service, Pacific Northwest Research Station exposed seedlings to various combinations of chilling and forcing temperatures. Read more…