Trees do not just count cold – they use warm spells to track winter’s progress

In temperate and boreal regions, trees survive winter by entering dormancy, a protective state in which growth pauses and buds remain tightly closed. Only after a certain period of cold is dormancy released and the buds can open in spring. Getting this timing right is increasingly difficult, with winters becoming warmer and more unpredictable. If buds open too early and a hard frost returns, the damage can be irreversible, and if they open too late it shortens their growing season.

For decades, scientists believed that trees determined when it was safe to release dormancy simply by counting how long they had been exposed to cold temperatures. New research from the Rishikesh Bhalerao group challenges that view. Instead of just measuring cold, trees also pay attention to winter’s interruptions - warm spells that can temporarily interrupt the cold, especially in late autumn and early winter. 

“Trees are not just counting cold,” says Rishikesh Bhalerao, whose research team at Umeå Plant Science Centre led the research and collaborated with groups in France, the UK and the USA. “They are interpreting the rhythm of winter and use warm interruptions to decide when to start counting cold early in winter.”

Warm spells in winter are not just noise for trees

In the study, the researchers showed that trees respond very differently to constant cold temperatures, which is typically used in laboratory experiments, compared with cold that is repeatedly interrupted by a few warm hours. Such warm spells are common in natural winters and are becoming more frequent as the climate warms.

“What surprised us was how robust the plants’ responses are, even under these fluctuating conditions,” says Rishikesh Bhalerao, Professor at the Swedish University of Agricultural Sciences. “They are able to reliably integrate temperature signals to robustly time their developmental transitions, such as bud dormancy release.”

The researchers found that repeated warm spells in early winter act as a warning signal. Even if a tree experiences plenty of cold overall, these warm interruptions can delay the internal processes that allow buds to release dormancy. This delay reduces the risk that buds become active and start growth prematurely during brief thaws, only to be damaged when cold temperatures return.

Buds contain a microscopic safety system 

This ability relies on an unexpectedly sophisticated system inside the buds themselves. At a microscopic level, plant cells are connected by tiny channels that allow signals to pass from cell to cell. During deep winter dormancy, these channels are blocked. Prolonged cold gradually opens them, allowing growth signals to spread. Warm spells, however, delay the opening of the channels.

“It is a bit like a safety gate,” explains Bhalerao. “Only when winter has been consistently cold without long interruptions can the process of opening the gate be initiated, putting the bud in a ready state for spring.”

Not all buds respond in exactly the same way. Even buds on two genetically  identical trees, growing under the same conditions, can wake up at slightly different times. This variation is not a flaw, but a survival strategy known as bet hedging. If buds on one tree are damaged by a late frost, buds on the other tree, that starts growing later, may still survive. By spreading the risk, trees increase the chance that at least some buds escape damage if late frosts strike.

This built-in flexibility may become increasingly important in a changing climate. Warmer winters with frequent temperature swings already cause problems for forests and fruit trees around the world, including reduced yields and mistimed flowering. 

Understanding how trees process variable winter temperatures could help researchers and breeders develop more climate-resilient trees in the future. It also changes how scientists think about winter itself. These short warm spells are not just pauses in the cold, but part of the information trees use to survive winter – signals that tell buds to wait and remain safely dormant.