By David Hawke -- Thank goodness for the rains of autumn! Nothing beats a good ground-soaker to help the trees prepare for winter. While most of us mobile creatures often bemoan the cold dampness of pre-winter, the stuck-in-one-place trees are, I’m sure, quite grateful for the additional moisture that’s dropped within their reach.
The air in winter is, generally, much drier than other seasons, and this lack of moisture can cause some real problems for trees. Actually, trees have four areas of concern when it comes to surviving winter: they have to get rid of their moisture-wicking leaves; there may be some physiological changes required to acclimatize to winter; they have figure out how to keep water moving throughout their system; and somehow they have to avoid damage from wind, snow load and being chewed on by rodents. It isn’t easy being green.
Over the few months of summer each tree must accumulate enough water to not only produce leaves, flowers and fruit, but also retain internal reservoirs to get through winter. As soon as the days start getting shorter the trees increase their levels of abscisic acid, a chemical that helps to weaken the grip between leaf and twig. Once the abscission layer has formed, water is no longer entering the leaf and is now held within the branch itself, and the leaf withers and falls off.
Conifer trees (pines, firs, spruces and such) keep most of their leaves, known as needles, all winter long. More about conifers later on -- for now we’ll keep looking at deciduous trees.
Just as some animals hibernate, and do so by altering their internal systems, trees also have to re-think how water is handled within the trunk and roots during the cold and dry season.
All tree species have what is known as their “killing temperature”, the slightly lower than lowest temperature that they can endure and still survive. Northern range limits are set by these temperatures, with species such as red oak, beech, sugar maple, white ash and others being restricted where they can grow by how cold it gets in wintertime. Other species, mainly conifers, can get by even in much lower temperatures, which is why northern Ontario is populated by spruce, fir and cedar while the hardwoods tend to stop about where the Canadian Shield begins showing itself.
The abscisic acid (AA) again plays a role in altering the chemical processes within a tree, so that each tree cell now has an increase in AA production; lipids (which are soluble fats) begin to unsaturate; lipid concentration increases within each cell; proteins de-polymerize; and cell membrane permeability increases. Really, I’m not making this up, I read it on the Internet!
The above process allows the internal cell to lower its freezing point… which means that any water that surrounds the cell freezes first. During the freezing process a small amount of heat is released, which is now captured by the internal part of the cell which isn’t freezing. Apparently twig temperatures actually rise several degrees during this freeze/capture process, which in turn allows the twig to survive and retain water. Whew, that’s some complicated!
This now leads to the burning question that I know all of you are just bursting to ask, “How does water move through a frozen tree?” Good question. First of all, where does this winter water come from? The soil which surrounds the roots has lots of water within it, and it does not always freeze solid; perhaps the insulating effect of snow or a south facing exposure will let the water remain liquid and be available for absorption by the roots.
Another source of water is under that same insulating layer of snow, as the snowflakes melt and turn to vapour some trees can take this in at the root crown, or the tiny condensing droplets fall into the soil and slowly saturate downwards. Particle by particle, that mighty oak or maple sucks in any and all water that becomes available.
Frozen water (a.k.a. ice) cannot be pushed through the tree’s transport system, so keeping the cells from freezing is very important. Water travels through a series of connected tubes called tracheids, which is found in the xylem (bet you didn’t know that!). The strong cohesive properties of water pull a continuous column of water up the trunk and to the branches (think about when you hang a piece of paper towel over a glass of water and just touch the bottom edge of the towel to the water… water ‘climbs’ up the rest of the towel).
If these columns are disturbed by ice blockage or physical damage, the water flow cannot be reconnected. Each spring, hardwood trees in particular, produce many new tracheids within their xylem to ensure successful water transport.
Conifer trees are a whole other kettle of fish, so to speak, in the ways they deal with winter. Let’s discuss them next week (you will probably want some time anyway to re-read the above to try and figure out what the heck is going on with these hardwoods.)
David’s Notebook: It's late November... already! Guess that means spring is just around the corner? Gotta have hope.
© 2019 David J. Hawke