Published on 03/18/96

Trees Make Antifreeze to Survive Cold

The chill of this spring hasn't been good for many trees. Several freezes separated by warm weather have caused major damage.

Your oak may produce no acorns this year because the cold has killed all the female flowers.

Trees can adjust to freezes, but these changes depend on the tree's health, how much food it has stored and how fast temperatures change.

For a tree, cold weather survival can be costly and disruptive, leading to stress and tissue death.

Many trees can survive extreme cold. A northern willow holds the record for survival at minus 320 degrees Fahrenheit in a laboratory.

Trees need time to prepare for freezes. Rapid change from warm to cold can damage the most hardy tree.

If given time to adjust, a live oak (Georgia's state tree) can survive to minus 18 degrees. Southern magnolia and longleaf pine can survive to minus 50, sweet gum to minus 20 and American elm to minus 55.

When tree tissues get this cold or colder, they're severely damaged. The parts a spring freeze will damage most are the growing areas at shoot tips and along twigs.

Flower buds are extremely sensitive to cold. On one tree, flower buds may be 20 to 30 degrees more sensitive than leaf buds.

Roots are tender, too. They can't adjust well to freezes once they're growing. Cold damages leaves, if present, along the main veins and connection to the tree.

The living tissue right behind a bud, connecting it with the twig, is highly susceptible, too. It's small, active, unprotected and exposed.

The water-carrying tissues, which are dead when functional, aren't affected. This leads to what is called false, or dead, blooming in some trees. The blooms come out but quickly wither because connections to the living tree have been killed.

Trees make a natural antifreeze to survive freezes. They do this in two stages.

The first requires a gentle cooling-down to below 55 but not to freezing. The tree starts to gather sugars and break down stored starches.

As the weather chills over a few days, trees start to change their living membranes to maintain a liquid, cold-temperature form.

Trees easily damaged by cold usually can't modify their membranes, which become impermeable solids when cold and suffocate the cells.

Stage two happens below freezing. Trees use collected sugars and proteins to bind water inside living cells and prevent ice crystals from forming.

Between the cells, ice can form and pull water from the cells. If too much ice forms, new or sensitive tissues are mangled.

Across the range of a tree species are usually many climatic races. Each has developed in areas with unique fall and spring temperature patterns.

Moving Southern races farther north can damage trees. A rule- of-thumb is to beware of moving native trees more than 100 miles south-to-north or 300 miles east-to-west.

Southern trees moved only a few hundred miles north can have many problems:

* They don't develop enough resistance to cold damage.

* They aren't fast enough in developing resistance in the fall.

* They come out too early in the spring.

* And they're killed by extreme freezes that happen every few years.

Northern tree races won't grow early enough in spring and will shut down too early in fall to stay healthy farther south.

Trees are tuned to their environment. They make all the materials and growth responses they need to survive and thrive in normal weather. But extreme and unseasonal cold can stress them badly.

Keeping trees healthy is one of the best things you can do to keep down the stress of hard freezes.

Kim Coder is a forester with the University of Georgia Warnell School of Forestry and Natural Resources.