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| Trees in cold climates have evolved protections against ice and snow. Photo: Richard Primack CC BY-ND |
Tolerating cold
On winter days when temperatures fall below freezing, animals will hibernate underground or huddle together in protected spots. But trees and shrubs have to sit there and take it - the tissues in their trunks, branches and roots are alive. How do they survive the freezing cold?
In autumn, woody plants in many parts of North America start preparing for winter when their leaves change color and fall, and their twigs, branches and trunks start to lose water. As a result, their cells will contain higher concentrations of sugars, salts and organic compounds.
This lowers the freezing point of the cells and tissues, and allows them to survive temperatures that fall below the normal freezing point of water. The trick has its limits – so, extreme cold events can still kill certain plants.
Tree and shrub roots remain largely unchanged and inactive during winter, relying on the insulation from snow and soil for protection. For the most part, the temperature of the soil around roots stays at or above freezing. Soil, fallen leaves and persistent snow layers help insulate the ground above the roots and prevent it from losing heat.
The surprising danger of spring frosts
After plants withstand the cold winters, early spring can bring new dangers. Plants need to leaf out as early as they can in spring to take advantage of the growing season. This involves pumping water into their developing leaves, which reduces the concentration of sugars, salts and organic compounds in their tissues and removes their winter protection from cold.
Each species has a characteristic leaf-out time. For any species, leafing out too early is a risk because late frosts can damage or kill young leaves.
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| The leaves on this cherry tree have suffered damage from a late frost. Photo: Richard Primack CC BY-ND |
To guard against frost and still take advantage of the full growing season, trees and shrubs have developed three ways to know when it is time to start growing in spring:
1. First, plants have winter chilling requirements: They hold on to winter dormancy until they have been exposed to a certain number of cold winter days. This trait helps them avoid leafing or flowering during abnormally warm periods in midwinter.
2. Second, plants also have spring warming requirements that promote growth after they experience a certain number of warm days each spring. This feature helps them start to grow as soon as it is warm enough.
3. Third, some plants also have a photoperiod response, which means they react to the length of time they are exposed to light in a 24-hour period. This prepares them to leaf out as days get longer and warmer in the spring.
Author: Richard B. Primack, Professor of Biology, Boston University
Complete article: How Plants Manage