How do they do it?

Stephen G. Saupe, Ph.D.
Biology Department
College of St. Benedict/St. John�s University

submitted Nov, 2003 for publication in Sagatagan Seasons

One of my favorite Christmas carols is �O Tannenbaum� because it acknowledges the amazing ability of conifers to tolerate our winter weather.  When we sing � �O Christmas tree, O Christmas tree!  Thy leaves are so unchanging.  Not only green when summer�s here, But also when �tis cold and drear� � it reminds us that these trees are easily able to cope with our short growing season and bucket-loads of snow.  And what�s even more impressive is that they do it all without liquid water!

Evergreen needles are the solution for a short growing season.  This allows conifers to carry out photosynthesis whenever the conditions are favorable.  Although there is some controversy about whether conifers can actually photosynthesize on warm mid-winter days, evergreen needles are clearly an advantage in the spring and fall; by not having to form new leaves in the spring conifers get a quick jump on photosynthesis and can continue into the autumn long after the deciduous trees have shed their leaves.  As an added bonus, by not discarding their leaves every year conifers are able to conserve precious soil nutrients which are often limited in boreal forests because cold temperatures slow the rate of decomposition and nutrient cycling.

One complication of having evergreen leaves that help extend the season for photosynthesis is that water can escape through them.  This is a serious problem since liquid water is a precious commodity in the winter.  To solve this problem, conifer needles are designed to reduce water loss.  The needles are coated with a waxy layer to reduce water loss and the pores through which they obtain carbon dioxide for photosynthesis (stomata) are recessed in chambers to protect them from the wind.  The leaves of broadleaf trees would loose too much water during the winter and hence is the primary reason they are shed in the fall. 

In order to deliver water to the leaves, there must be a continuous water column in the transport cells.  This is necessary because water is essentially sucked out of a plant by evaporation from the leaves in a manner that is analogous to using a straw.  In winter, ice formation in the water-transport tubes can block flow just like a chunk of strawberry will plug up your straw when slurping a milkshake.  More importantly, as water freezes in the transport cells gas bubbles will come out of solution and form a vapor lock in the column.  Similarly, you won�t be able to suck up that milkshake with an air bubble in the straw.  Conifers solved these problems by having narrow water transport cells, called tracheids.  These cells, which are much narrower than those in broadleaf trees, greatly decreases the chances that gas bubbles will develop.  In addition, there are special check valves (called a torus) between adjacent tracheids so that even if a bubble comes out of solution in one tracheid it will close the check value and isolate this cell from others.  When the temperature warms up the gas will dissolve and re-establish the water columns for transport.  When gas bubbles form in the transport tubes of broadleaf trees they quickly spread rendering large areas useless.  Thus, it is especially critical for broadleaf trees to add a new layer water transport cells every year.

Snow removal is a problem in my driveway but not for conifers.  They have evolved their classic �Christmas tree� shape with a single leader as a means to shed snow and minimize broken branches.  Further conifers have long fiber cells in their wood that help to make the branches more flexible and support greater loads.

When contemplating the many adaptations of conifers to winter weather I heartily agree with the carol, �O Christmas tree!  Much pleasure thou can�st give me; How often has the Christmas tree, Afforded me the greatest glee!�