This time of year, buds, twigs and bark serve as the main identification feature of woody vegetation, in the absence of leaves. Winter identification forces many us to spend quite a bit of time contemplating the various characteristics of tree and shrub twigs.
Given the fact that somewhere around 50 percent of the mass of a mature tree is water, it always fascinates me to examine a twig in winter and not see some effect from freezing. As organisms that cannot produce their own heat to stay warm, how do trees and shrubs survive winter without freezing solid from all that water? Winter hardiness is a key evolutionary feature of perennial woody vegetation that has some quite interesting factors at play.
Freezing water inside plant parts has potentially catastrophic effects if plants are unable to manage this environmental condition. To avoid freeze damage of valuable tissues, plants have evolved several mechanisms that manipulate the behavior of water molecules within their cells to either avoid freezing altogether or regulate the way it happens.
One very simple way that trees avoid cell damage from freezing is by not allowing water within cells to freeze. In preparation for winter weather, trees move a lot of water out of their cells to avoid damage. This relocated water is then allowed to freeze, and as it does, a tiny bit of heat is produced that can help take some of the edge off of if any remaining water inside the cells freezes.
However, cells with less water are at risk of dehydration, especially with extracellular water frozen around them. This freeze-induced form of dehydration causes the most widespread damage in plant cells over winter.
To further resist freezing, and potentially dehydration, trees convert starches in their cells to sugars. These sugars help to preserve tree parts by effectively lowering their freezing temperature due to a higher concentration of sugar in the solution. However, concentrating sugars into water within their cells can only help so much. It varies by species but may only provide protection down to about 20 degrees F or so.
We all know that water expands when it freezes, which could certainly wreak havoc on microscopic cell walls if the inside of the cell becomes frozen. Trees have developed a special mechanism that exploits the basic physics involved in freezing water molecules to gain some serious freeze protection.
As liquid water approaches freezing, its molecules are arranged into a symmetrical crystal structure. Think of the shape of snowflakes as an example. Each snowflake needs something at its center (or nucleus) to begin its beautiful yet individual crystal pattern. In the case of a snowflake, the nucleus is some kind of very fine particulate matter floating in clouds, like a speck of dust. So, some material other than water must be present for ice crystals to form at 32 degrees.
As ice crystals form, their symmetrical pattern takes up more space, resulting in expansion, which explains why water expands and becomes less dense as it freezes. Many plants avoid damage within their cells by eliminating a nucleating source (like dust specks in clouds) for crystal formation in a process called supercooling. Plant cells containing liquid water can then remain unfrozen down to temperatures as low as minus-40 degrees.
This week’s snowfall was absolutely stunning the way it stuck to the twigs and stems of trees. As I spent some time in nature taking it all in, it was fascinating to think about all the remarkable processes at play to keep plant cells alive in this winter wonderland.