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Facing winter dead on and naked

Hanbidge on Horticulture
hanbidge
Through the wonders of nature trees are able to survive harsh prairie winter conditions while we huddle inside and try to stay warm.

I am always amazed by trees in the landscape but especially on the coldest days of winter, days that are so cold cars won’t run and the human factor hardly strays outside. Those are days of – 40 C with a wind chill that makes the temperatures outside deadly. 

For trees to survive the winter to again grow in spring it is really miraculous. However, they must have adaptations in order to survive the cold of winter, but also the severe drying conditions. Unlike the animals (and humans) they cannot change their geographical location and they certainly can’t put on, or grow, a winter coat. They meet winter head on without any clothes on at all. They, therefore, must rely on physiological and structural adaptations in order to survive.

In summer, the height of trees is advantageous as there is nothing in nature that competes for sunlight at that level. However, in the winter that same height is a liability as tissues become extremely exposed to the weather. In order to survive trees need to use a variety of strategies. 

Trees will either shed their leaves, or in the case of evergreens, will adapt strategies for leaf retention. The leaves are a major source of water loss and would be difficult to protect in winter conditions. Deciduous trees will therefore drop their leaves to aid in winter survival. This leaf drop is stimulated by changes in the photoperiod or day length that is followed by a radical change in the chemical and hormonal make up in the tree itself. 

There is a complex process where abscission layers between the leaves and stems are formed which essentially causes the leaf drop itself. You will notice not all trees drop their leaves at the same time. This is because the processes that occur within the tree are specific to the tree itself.  

Evergreens will also shed their leaves, but not all of them at the same time. The exception to this is the genus Larix, but the other evergreens benefit from the economy of using their leaves for more than one year. In addition, evergreens have a thick coating on the leaves (cutin) that assists in reducing the amount of water lost. They have a few other adaptations that include a tighter stomatal closure (pores that allow air and water to pass in and out of the leaf) which further reduces winter water loss.

Trees that are able to survive the winter have acclimatized. This cellular process helps to exploit the physical properties of water that aids in survival through the winter. However, each species and, in fact, each individual specimen will have a killing temperature. Within the cell itself there are changes that can slightly reduce the freezing point by changing the solute concentrations within the cell. This concentration change within the cell further helps with survival by causing the area outside the cell (which has a higher concentration of water) to freeze first, which in turn releases a small amount of heat energy, which also aids in individual cell survival.

For deciduous trees, the biomass above ground is relatively watertight as stems and buds lose very little moisture. However, for evergreens, even in winter there is water loss through the leaf tissues. On sunny days the sun shining on the leaves can increase the temperature significantly enough to be above the ambient air temperature. As a result, metabolic rates rise, the internal vapour pressure increases and even with all the protective measures will cause the net loss of water and desiccation of the leaf tissue. Hence, the importance of ensuring evergreens go into winter in a well hydrated state.

So the next bitterly cold day, take a moment to think about the trees that surround you and the wonders of nature and its adaptations that allows survival outside without any clothes on.

— Hanbidge is a horticulturist with the Saskatoon School of Horticulture and can be reached at 306-931-GROW(4769); by email at growyourfuture@gmail.com or check out our website at saskhort.com.