Rise and shine: How do northwest trees know when winter is over?

Abstract

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. If a tree bursts bud prematurely, delicate tissue may be damaged by a late frost. Conversely, if a tree bursts bud too late in the spring, it will be unable to achieve substantial height growth before summer drought sets in. Although most Northwest tree species require a combination of chilling and forcing hours to promote budburst, the number of hours needed differs by species. 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. They tracked the timing of budburst and created possibility lines that describe the combination of chilling and forcing hours required by each species. As the climate changes, the timing of budburst is also expected to change, so the scientists developed landscape models to predict when a species' budburst would likely occur in 2080.

Citation

Watts, Andrea; Harrington, Connie; Gould, Peter. 2016. Rise and shine: How do northwest trees know when winter is over? Science Findings 183. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 5 p.