Leafless tree branches set against a snowy backdrop set a rather bleak scene. However, while it may appear lifeless, deciduous trees have several strategies to survive the cold and dark conditions of winter.
Preparations for winter survival begin in the late summer months when the amount of daylight begins to diminish. During this time period, a large number of physiological changes happen in the trees stem, roots, and leaves. Water loss is a major issue that trees must contend with at all times of the year. But as winter approaches, freezing becomes an even bigger issue.
One freezing-prevention method you are likely most familiar with is the dropping of leaves at the onset of autumn. One of the reasons deciduous trees drop their leaves is because they are a major source of water loss. The dramatic color change that occurs during the fall season is a result of the chemical breakdown of chlorophyll (the molecule that gives a leaf its characteristic green color) — as the chlorophyll breaks down, it reveals the yellow, orange, and red colors of other leaf compounds such ascarotenes, xanophylls, and anthocyanins.
Depending on where you live, you may have noticed that trees don’t all lose their leaves at the same time. Some trees even retain their brown and withered leaves long into the winter season, not dropping them until new buds burst in the spring.
Like other living organisms that inhabit cold climates, the main issue a tree must contend with in cold conditions is the prevention of the formation of ice crystals in cells.
There are three basic strategies that tree cells have to prevent their contents from freezing.
- Cell membranes change so that they become more elastic, which lets water move out of the cells and into the spaces between cells.
- The tree converts starch to sugars, which serve as a kind of antifreeze. The sugars become concentrated in the cellular fluid found within a cell, serving to lower the freezing point of the contents. This means that the contents inside the cell do not freeze, while the water that has moved into the spaces between cells does freeze. Because the cellular membrane becomes more elastic during the cold season, it bends rather than bursts when ice crystals form in between the cells. In addition, when the water surrounding the cells freezes, it releases small amounts of heat energy; this heat energy, though tiny, also helps to prevent the internal cellular fluid from freezing.
- Eventually the contents within the cell reach a glass phase in which the liquid contents becomes so thick they appear to almost be a solid. This third strategy is triggered by the dehydration that occurs within the cell as a result of the first and second strategies. The thickened nature of the cells contents also aids in the prevention of the formation of ice crystals.
Both deciduous and conifer trees use the above strategies to prevent their cells from freezing. Conifers have additional strategies which make them well-adapted to the cold wintry conditions of northern climates. Because conifers retain their needles year-round, the trees can begin to photosynthesize earlier in the spring and continue to photosynthesize later into the fall than deciduous trees. However, maintaining moisture to conduct early- and late-season photosynthesis means conifers must have certain adaptations to allow for such extreme conditions. Conifers have tracheids, which are the tubes that transport water through a tree. Compared to a deciduous plants water-conducting vessels, a conifer’s tracheids have a narrower diameter. The narrowness of the tracheids reduces the formation of air bubbles. Air bubbles form when water freezes and air comes out of solution. The small size of a conifers tracheids only allows for the formation of small air bubbles. These small air bubbles are more likely to be reabsorbed than to burst within the water column. Bursting air bubbles are a problem because when they break, the tracheid or vessel is punctured, and, like a leaky straw, prevents the movement of water throughout the tree.
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