Trees Ready for Spring During Winter 'Cold Sleep'

New botanical research suggests that the lengthening of daylight as winter turns into spring has less to do with how soon a plant beings to bud that previously believed. According to a study from the Technical University of Munich (TUM), plants will begin to bud in the spring after they've received adequate "cold sleep" in winter.

The German researchers found this to be true for native tree species such as oak and beech, which rely on the cold periods to fortify themselves against late spring freezes.

"Contrary to previous assumptions, the increasing length of the day in spring plays no big role in the timing of budding. An ample 'cold sleep' is what plants need in order to wake up on time in the spring," said lead study author Julia Laube.

Laube added that pioneer species such as hazel bushes as and birch trees, which have moved into Germany from warmer climates, behave differently.

"These trees take the risk of starting earlier in the spring, because they are less strongly dependent on the cold periods," Laube said, "and in addition they sprout more quickly as temperatures rise."

The research suggests that a mild winter could lead to unintended environmental consequences in the forest. After a mild winter, native species run a risk of developing their leaves too late. If that happens, more sunlight is allowed to hit the forest floor, providing the engine for growth of low-lying plants and some types of invasive species.

"Even under warmer conditions, we won't be seeing 'green Christmases' under freshly blooming trees," said Annette Menzel, TUM Chair for Ecoclimatology and a fellow of the TUM Institute for Advanced Study. "Nonetheless, the differing growth patterns will affect the entire plant and animal world. The native tree species in our forests have only a limited ability to adapt themselves to climate change."

For the researcher, the scientists used sapling specimens from 36 different trees and controlled the climate conditions in special chambers in the lab.

The benefits of a cold sleep were more apparent for beeches, hornbeams, and the North American sugar maple, while other species, including lilac, the hazel bush, and the birch proved to be less dependent on a cold season for spring growth.

"Overall, however, a chaotic picture emerges," Menzel said. "Through warmer winters, the usual sequence of leaf development can get completely mixed up. Many of the cultivated species that are at home today in central Europe come originally from warmer climate zones. In the absence of adequate protection against freezing, they could become victims of their own too-flexible adaptation -- and freeze to death in a late frost in the spring."

The research is published in the journal Global Change Biology.