In a previous post we saw some of the ways that cold-adapted plants are able to tolerate sub-freezing temperatures, or not.
But how do trees and other plants survive extreme cold, that is, temperatures well below zero degrees C.
“Different tree species differ in the depth of low temperatures each can survive. For example, live oak (Quercus virginiana) can survive down to about 20o F (-7oC); magnolia (Magnolia grandiflora) down to 5oF (-15oC); sweetgum (Liquidambar styraciflua) down to -15oF (-26oC); American elm (Ulmus americana) down to -40oF (-40oC); and, black willow (Salix nigra) down to -100oF (-73oC).” (from Ref. 1 below)
Exactly how trees survive extreme cold is complex, and much about the cellular mechanisms of extreme cold tolerance in trees is unknown. Also, much depends on how long the cold lasts and how cold it actually gets.
Basically, however, the chief problem faced by trees exposed to extreme cold is to try to keep living cells from freezing. They can do this in a number of ways, such as accumulating or producing solutes (sugars, e.g.) in order to decrease the freezing point of water and producing proteins that inhibit ice crystal formation. This allows deep supercooling of cells, to many degrees below the freezing point, without ice formation.
But at temperatures of -40o F and lower, these strategies may no longer work. Such temperatures typically cause ice crystal formation, intracellular freezing and cell death. In tree species that can tolerate temperatures below -40o F under natural conditions, living cells must be able to withstand gradual dehydration as the water freezes. How they do so is not well understood, though it likely involves the biosynthesis of specialized proteins and lipids that help protect cellular structures in conditions of extreme dehydration. (see Ref 2 below) It’s not surprising that some desert plants may use similar protective substances under extreme drought conditions.Evolution of Plant Cold Tolerance
In recent news, a group of researchers has “…found new clues to how plants evolved to withstand wintry weather. In a study to appear in the December 22 issue of the journal Nature, the team constructed an evolutionary tree of more than 32,000 species of flowering plants — the largest time-scaled evolutionary tree to date. By combining their tree with freezing exposure records and leaf and stem data for thousands of species, the researchers were able to reconstruct how plants evolved to cope with cold as they spread across the globe. The results suggest that many plants acquired characteristics that helped them thrive in colder climates — such as dying back to the roots in winter — long before they first encountered freezing.” (from: Study offers clues to how plants evolved to cope with cold.)
Zanne, et al. (see Ref. 3 below) identified three traits that help plants cope with the freezing and thawing that causes air bubbles to form in the plant’s internal water transport system.
1. Some trees avoid freezing damage by dropping their leaves before the winter. This effectively stops the flow of water between roots and leaves. When warmer weather returns, they grow new leaves and water transport (xylem) cells.
2. Other trees also protect themselves by having narrower xylem cells, which makes the parts of the plant that deliver water less susceptible to blockage during freezing and thawing.
3. Many plants avoid winter temperatures by dying back to the ground in winter and re-sprouting from their roots in the spring, by producing seeds that can survive the winter and then, later on, the seeds germinate under warmer conditions, or by doing both.
Here’s a YouTube video that nicely illustrates the above:
1. Coder, K. D. (2011) “Trees and cold temperatures”. Publication WSFNR11-12 from The Daniel B. Warnell School of Forestry and Natural Resources, University of Georgia, Athens, Georgia, United States. (PDF)
2. Gusta, L. V. and M. Wisniewskib (2013) “Understanding plant cold hardiness: an opinion.” Physiologia Plantarum, Vol. 147, pp. 4-14. (PDF)
3. Zanne, A. E., et al. (2013) “Three keys to the radiation of angiosperms into freezing environments.” Nature, 2013/12/22/ advance online publication, http://dx.doi.org/10.1038/nature12872. (Full Text)
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