Warmer Winters Cause Trees to Sleep Longer: Impact of Seasons on Plants

In temperate zones, trees and plants follow the change of seasons, but what happens when temperatures are hotter than usual? Warmer winters could be impacting vegetation far more than we realize. Now, scientists have discovered that shrubs and invasive trees that don't depend on the cold could be given a leg up as our climate changes.

In order to study how warmer winters could impact plants, researchers examined 36 trees and shrub species. Native tree species such as beech and oak rely on resting in the cold to protect themselves from freezing by late spring frosts. Yet pioneer species such as hazel bushes start budding earlier in the spring.

In this particular experiment, the researchers exposed twigs around 30 centimeters long to different temperature and light conditions in climate chambers. Each chamber experiment lasted six weeks. This allowed them to assess how species adapted to the cold.

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

In fact, the researchers found that there may be consequences for the forest ecosystem. After mild winters, the native species run a higher risk of developing their leaves too late. In that case, more daylight reaches the forest floor, which benefits low-growing shrubs and other plants. This, in turn, means the shrubs sprout earlier and actually can hurt young trees that are still close to the ground.

"Even under warmer conditions, we won't be seeing 'green Christmases' under freshly blooming trees," said Annette Menzel, one of the researchers, in a news release. "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."

In the end, the researchers found that the cold affected beeches, hornbeams and the North American sugar maple most strongly. With shortened cold periods, bud burst occurred significantly later.

"Overall, however, a chaotic picture emerges," said Menzel in a news release. "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 findings are published in the journal Global Change Biology.