Mystery of the Month: February

Question: Every Minnesotan knows when the leaves change color it's time to start preparing yourself for another long winter, complete with snow, ice, and wind!  Thank goodness for warm houses, cars and winter clothing!

But what living thing withstands such brutal punishment without seeking shelter and survives until spring?

Answer: The question this month could have a few answers, but we were talking about trees and their amazing survival mechanisms!

Winter Survival of Trees: Whatever gets you through the ice
by Tracy Lawler, Natural Resources Intern

 

Our forests include mainly deciduous and coniferous trees. These trees have adapted different strategies to survive the change in seasons.

Deciduous trees lose their leaves every year; this includes species like maples, oaks, ash, and elm.  When nights become longer in the fall, the benefits of photosynthesizing decrease- the sun is further away and shines for shorter periods. The longer nights signal the trees it’s time to start slowing down photosynthesis and store energy for the long winter.  Chlorophyll, the chemical which gives leaves their green color, is the primary actor in photosynthesis.  As photosynthesis decreases the leaf is stressed and chlorophyll starts to break down. This allows the leaf to show other colors underneath the chlorophyll.  If the tree has carotenoid pigments under the chlorophyll, the leaf will turn yellow, orange, or brown. Some trees have anthocyanin pigments which appear red or purple.  Trees will then reabsorb nutrients stored in the leaves.  After, the leaf will start a process called abscission that will release it from the branch. In abscission, a hormone called auxin is released. This hormone tells the leaf to create a tear between the leaf and branch, and tells the branch to seal the hole to prevent the loss of sap.  The tree, which has now stored the nutrients in its protected trunk and deep roots, can fall into dormancy until spring when longer days mean photosynthesis can resume.  A lot of energy will have to be used to re-grow the leaves these trees drop every year, but there are advantages to this strategy.  No leaves mean less chance of limbs breaking in the winter or loss of water; two things that will help the tree grow in the spring. 

Coniferous trees (also called evergreens) hold on to their leaves (needles) from season to season.  Some examples of coniferous trees are cedars, spruces, pines, firs, and junipers.  In the winter evergreens will lose more water than coniferous trees and run a higher risk of predation when their needles are the only food source for animals.  So why do conifers keep their leaves year-round? Because conifers have an advantage of saving the energy deciduous trees use to regrow their leaves.  They have also adapted their leaves to be able to withstand the cold conditions and still be able to photosynthesize in the winter! The color of their dark green needles is also thought to better absorb energy from the sun. Their waxy, needle-like leaves help them to preserve water.  Even their trunk’s cone shape and drooping branches help them to more easily shed snow and ice so their needles are exposed to more sunlight. Conifers can process the smaller amount of sunlight in the winter and be ready to go in the spring, giving them a jump up on deciduous trees that have to re-grow all their leaves before they photosynthesize sunlight.

Gervais Mill Pond trees

Both conifers and deciduous trees change their biochemistry to keep the water inside them from freezing, using a process called “hardening”.  If the water inside their cells were to freeze and crystalize then it would damage the cells and the tree would die. Fortunately, the trees in Minnesota and in other regions with brutal winters have a few strategies to prevent that from happening.  In addition to moving nutrients into their well-protected root systems, they move water out of their cell walls and collect it in a way where it cannot form crystals that would rupture their cells. They also collect sugars inside their cells to lower the freezing point. This is like putting anti-freeze in your car’s cooling system so your car still drives in below freezing temperatures! The process of hardening takes a few months.  If temperatures were to fall quickly before the tree has completed this process it could be severely damaged or even die.  In the spring, when the threat of hard freezes have passed, the trees go through a process of “un-hardening” so their water and nutrients can come out of storage and be used for energy.

There are always exceptions, even in the tree world. There is actually one conifer in Minnesota that changes color and loses its needles each winter; the Tamarack.  Did you know that even without their leaves, some trees can still photosynthesize? Aspen, a deciduous tree, lose their leaves every fall, but have evolved so their bark contains chlorophyll.  That is why the aspen’s bark looks a little green. It is only able to absorb a small amount of energy during the winter but that still helps it survive and make new leaves in the spring!

And so there you have it, the different ways the coniferous and deciduous trees we love survive the freezing Minnesota winter we don’t always love.   It’s a good thing they are able to survive each winter so they can provide their benefits all year around. These benefits help us every day and include carbon sequestration, erosion control and soil stabilization, food and shelter for animals and insects, oxygen production, and much more. Thanks to their adaptations you can count on them to be there after every hard winter, still full of needles or growing back leaves, welcoming the new spring.