A Little Bit More
All trees in northern climates must undergo hardening to some extent. Since water expands when it freezes, moisture-laden plant cells would suffer irreparable damage if they were not prepared in some way for below-freezing temperatures. But conifers or evergreens (pines, larches, spruces, firs, and hemlocks), become more thoroughly hardened than deciduous trees such as the maples, oaks, elms, and willows. Some live in climates where temperatures reach –58C (-70F)! Decreases in daylength and temperature together initiate hardening; a decrease in only one or the other produces only a slight effect. The process takes place gradually in the fall, but reverses itself rapidly in spring.
It may make intuitive sense that needles should fare better through the winter than broad, flat leaves could, even without a hardening process. Freezing is a problem, of course, but so is water loss. Cold air is dry, and the high surface-to-volume ratio of broad leaves causes them to lose moisture more readily. What little surface a pine or spruce needle exposes, is protected by a tough, waxy epidermis. Many tiny holes in the epidermis allow the needle to “breathe”, both for normal cell function and for photosynthesis – the process by which a green plant makes food from sunlight. But those holes are controlled by surrounding guard cells, which can close to conserve water.
Activities
Freeze and Thaw
Objectives: See what freezing does to plant tissue
Materials: Tomato, access to refrigerator and freezer
There is an easy way to demonstrate the damage that freezing can do to plant tissues.
1. Obtain a fresh tomato from the store and slice two ¼” thick slices from the middle.
2. Place the two slices on a similar plate (paper will do fine) and place them uncovered, one in the refrigerator and the other in the freezer. Leave both for approx. 24 hours.
3. Remove the two slices (make sure you know which is which but don’t let students know), allow them to stand at room temperature for an hour and then examine each.
4. Have students closely examine both slices. Allow them to use a pencil to gently press down upon each slice. It should be immediately obvious that the two slices are not the same.
5. Now quiz the students as to which one is the “freezer” slice and ask them what they think has happened to make the freezer slice “ooze” fluids.
Answer: As the fluids in the tomato’s cells froze, the water expanded and ruptured cell walls allowing the contents to spill out onto the plate. This process kills the ruptured cells and is therefore extremely dangerous. Hardening, as explained in the poster, prevents this process from taking place and allows plants with delicate tissues, like needles, to survive subfreezing winter weather.
Key Concepts
Adaptations and Diversity, Identification, Structures and Functions
Questions
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Posted by Robert Cooper on March 29, 2006 at 02:48 PM
Q: We have a Norway Spruce that was grown from a tiny plant and is now approximately 4-1/2ft. tall and about 3ft. in diameter. It's spent it's entire life indoors. My question: Can this tree be planted outdoors at this stage of it's life and survive?
A: If you don't plant your tree outside (or at least keep it outside) it will die. Norway Spruce are adapted for life in a temperate climate and therefore NEED to be dormant part of the year. So, may I make this recommendation? I'd wait a wee bit yet until there is little or not chance of a frost and then put your tree outside. Since your tree has never been exposed to the outside sun or wind, it may be "burned" with too much sun so you need to introduce it to the outside in steps. Place your tree (in its pot) in a shaded area. Allow it to stay there for a week or so then gradually bring it more and more into an open area. This will allow the tree to acclimate to the wind and sun and assure that it won't be killed. Once your tree is doing well in the sun, you can plant the tree in the ground (just remove from pot, lightly break up the outer dirt ball and pop in the ground). Best of luck with your tree.
Posted by sue on July 05, 2004 at 10:53 PM
Q: Is a Norfolk spruce the same as a Norfolk pine? Can a spruce survive a bitter winter?
A: I think what we have here is a bit of confusion with common names. There is a Norway Spruce, no Norfolk Spruce that I know of, that is native to Europe and commonly planted in North America. The Norfolk Island Pine is from an island about 1000 miles northeast of Australia and is not a pine at all, it just looks vaguely similar. Pines and spruces are related but are definitely different kinds of trees. Spruce are common in the colder areas of North America, growing well north in Canada and Alaska as far north as the tree line. So yes, spruce can and do survive bitter winters. If you can get a look at a good field guide to trees, you'll be able to see the differences between pines and spruce, as well as their ranges across North America.
Posted by anna on August 30, 2003 at 05:42 PM
Q: Do River Birch trees keep their leaves year-round? Thanks, Houston, Texas.
A: They do not. River Birch in Houston may hold on to their leaves longer than those in the north, but they do lose them and the tree passes through a domant period during the winter.
Posted by E. Atkins on January 24, 2003 at 10:50 AM
Q: Why are there lots of tips of fresh white pine branches on the ground now. We live in Mass. Are squirrels responsible?
A: I don't think the squirrels are doing this. White Pine branches and twigs can be very brittle in cold weather and I'm guessing they're breaking off in the wind. I live in central New York and have had freezing rain then cold and snow. We also had lots of pine twigs come down and the combination of ice, cold weather, wind and brittle branches is probably the reason.
Posted by Madeline on January 23, 2003 at 12:35 PM
Q: I received a small maple tree in a pot this summer and have left it outside for the winter. Should I have brought it in or not?
A: You didn't say where you lived, but I will assume somewhere where you receive winter weather. Your maple tree needs to spend a dormant period just as all of the trees do living in climates where they have winter. So, keeping the tree outside provides this, however, since it is in a pot, the roots will get much colder than they would in the ground.
I would suggest that you place your tree in a sheltered area like an unheated garage or unheated basement and if it doesn't get much below 26F, it should be fine. A little freezing is OK but the roots shouldn't get too cold.