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Complex, hidden, mysterious systems help keep trees ever green

Believe it or not, a tough day for a conifer is a winter's day full of sunshine and warmth, notes outdoors columnist
20061122_Scotch Pine (Hawke)
Pine trees and their conifer cousins retain most of their leaves all winter long, quite unlike the deciduous trees that drop all their leaves in autumn. Why is that? What survival techniques do these evergreens employ to thrive through the upcoming bitter cold? David Hawke/OrilliaMatters

Winter tires on? Check. Storm windows installed? Check. Snow boots, scarf and toque dug out from back of closet? Check.

Just like you preparing for winter, Nature also has a few tricks to cope with the cold season. Last week this column looked at ways in which deciduous trees retain their water. Don’t laugh… water retention for a maple tree is a critical measure as to whether it will survive and thrive, or just barely survive, the winter months.

As you may recall (and you should, as it was only a week ago) deciduous trees (like maple, ash and beech) get rid of their leaves to minimize water loss. Those big broad leaves did wonderful service during the summer months to gather solar energy, convert nutrients to tree food and both take in and release moisture, but with freezing temperatures and darn little sunlight the leaves are now cut loose and the tree internalizes its moisture-moving process.

And so you are no doubt wondering: why do pine trees keep their leaves (better known as needles) all year long? Good question, glad you mentioned it. Conifers, which are often called evergreen trees, have a whole other system figured out to keep water flowing throughout the bitter cold of wintertime.

The first big difference is that conifer trees keep their needles for about three years, as opposed to a maple tree’s attitude of use ‘em and lose ‘em within a few months of creating them.

Although nicknamed “ever green”, conifers do indeed drop old needles every year, just not all of them which results in the one- and two-year old needles still hanging on and looking good and green.

Needles are meant to last and so are built more ruggedly than those thin and flimsy broad-leaf leaves. Whether it’s a white pine, balsam fir, cedar or hemlock, the leaves have a thick wax-like outer coating that slows water loss yet allows the leaf to go about its business.

Although the remaining green needles do allow for photosynthesis on winter thaws and early spring-like days, the keeping of these needles presents a few challenges in water management.

Actually, a tough day in winter for a conifer tree is a day full of sunshine and warmth. All that sunshine quickly heats up the dark green needles, and warm needles then generate demands on the tree, one of those being the need for more moisture.

By growing close together, conifer trees shade each other thereby reducing some of the winter warming effect. Cloudy days are great as the clouds block the warming sun and the wind removes the heat from the needles, thus reducing the need for water.

You may have noticed that many of our local conifers prefer to live in wet areas: swamps are full of cedar, balsam fir, hemlock and the occasional white pine. When the blanketing snow insulates the ground, all that water remains available in liquid form for conifer usage. Even the melting snow itself (the bottom layer) provides water to both rootlets and low laying branches.

Okay, so the conifer tree now has lots of water available at the roots, now it has to get that moisture up the trunk and out to the branches without freezing. You’ll no doubt recall from last week’s article that there are tiny tubes running up the tree called tracheids, the water transportation system. If ice should form in these tubes a bubble is created, and that bubble will not allow water to go past it. Not good news if you are a tree.

I’m going to quote and paraphrase a paragraph from the Michigan Forests Forever teacher’s guide, simply because they explain the complex situation in fairly easy to understand wording:

“Within the tracheids of conifer trees there are tiny ‘check valves’ between each cell. When the water freezes and expansion occurs, these little valves close the ends of the tracheid tubes. Expelled gas is held within these sealed off sections and can build to quite a pressure.” Your winter snow tires may be inflated to about 30 psi, but within that cedar tree at the end of the lane the internal pressure can build to 900 psi! Thankfully trees are built strong and tend not to blow up under pressure.

Later, when the ice melts, the gas is forced back into solution and the pressure is off; the little check valves then open and the moisture continues on its upward journey. Nothing to it.

While we’re talking about conifer trees there are few more interesting facts that maybe you’d like to know. Because so many needles are retained over winter there is quite a large surface for ice and wet snow to accumulate upon. Bad news for balsam fir trees.

However, since conifer trees have branches that only grow from one point (the tip), the branch tends to be long and limber (as opposed to a maple branch which has numerous small branches pointing in all directions) and a long slender branch tends to bend and droop rather than break. Snow load dealt with.

As you walk the trails these gloomy days of Autumn, pause and consider the complex, hidden, and not yet fully understood world and workings of trees. They really are pretty amazing structures.