Field of Science

Why don't trees freeze?

The Phactor gets questions, and what could be more appropriate than asking why don't trees freeze and die on a morning where the temperature is -13C and my system is still trying to get from tropical to temperate, so arctic is still beyond me. In actual fact some trees do freeze, but they do not die. The biology of some woody plants, those whose ranges extend the farthest into the high latitudes, allows their cells' water to freeze without harming the cells. These are called "freezing tolerators" and once frozen they can withstand almost any temperature. One such shrub in the Phactor's collection is the Siberian cypress (Microbiota) and it is hardy to Zone 2, -45C. It doesn't have to worry about such severe cold here in Lincolnland, but it can suffer from the opposite, our summer heat.
Other trees don't freeze; they have the ability to deep super-cool the water in their cells and avoid freezing, although that gets more difficult as the temperatures get colder. The ability to deep super cool has its limits at right around -40 (F or C!), where the two temperature scales actually equal each other. This means that freeze avoiding species can disperse and grow further north but then when they finally encounter a really cold temperature, they will freeze and die. The first map shows the distribution of a freeze tolerating species, the red-osier dogwood, which ranges north right up to tundra. The second set of maps shows the distributions of 4 species of freezing avoiders (A-D, American beech, white ash, sugar maple, and northern red oak). The heavy line running just north of the Canadian border is either the border drawn by Google maps (not a funny joke in Costa Rica right now) or the -40 C minimum temperature isoterm, which by definition connects all the points where the minimum expected temperature is -40 C. Other freezing avoiders are less able to deal with colder temperatures. Here in Lincolnland the minimum expected temperature occurs about every 10-15 years (-29 C), so a less cold tolerant tree or shrub might dodge the bullet for quite awhile before the climatic extremes demonstrate that my sweet bay magnolia is growing too far to north. Obviously global warming will shift such climatic zones and the distributions of organisms along with it. 54 million years ago when the climate was much warmer tropical forests extended to 50 N latitude! Imagine southern magnolias growing in Green Bay, Wisconsin!


Stone Art said...

A very good question!

The Phytophactor said...

A very good question? It's a very good answer!