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Just Chill: How Cold Hardiness Works

By Brandon M. Gallagher Watson


In the upper regions of Siberia, nearly 450 miles above the Artic Circle, resides one of the world’s toughest tree species. Larix gmelinii, known as the Dahurian larch, holds the distinct title of “World’s Northernmost Tree Species.” Living in this area, just above the permafrost layer, is not for the faint of sap. Air temperatures have been recorded at an astonishing negative 94 degrees Fahrenheit (-70°C) during the winter, and summer temps climb above the freezing point for just a few short weeks. The growing season is less than 100 days long each year with polar night lasting from September to February. This species has adapted to low seed germination rates with the ability to sprout new trees off its root system, forming forest colonies of “creeping larch.” This growth habit is common among hardwoods, such as aspen, but uncommon amongst temperate conifers. Their tough wood and extreme cold keep insect and fungal pests to a minimum and, of the 268 other organisms that live on this tundra, there are no other tree species competing for sunlight. They can survive here for a long time. One individual was found to be 919 years old, while the root system may be as old as several millennia. Despite these inhospitable conditions, the Dahurian larch thrives here.

The Dahurian larch endures the local climate with great success thanks to a fairly complex series of adaptations we generally refer to “cold hardiness.” Hardiness is a measure of how well a plant can withstand adverse conditions, and can include cold, heat, elevation, drought, flooding and even wind. The lowest temperature that it can survive before freezing to death determines a plant’s cold hardiness The USDA has made this fairly simple by evaluating and categorizing landscape plants into Plant Hardiness Zones. Starting at the Canadian border, Zones are assigned in 10-degree increments down to the Mexican border. Zone 1a plants need to be hardy down to negative 60 degrees Fahrenheit, while, on the other end of the spectrum, Zone 13b plants are uncomfortably chilly at 65 to 70 degrees Fahrenheit. This helps provide some guidelines on what temps a plant may be able to tolerate, but it does not mean that they can tolerate it all the time. A paper birch may be able to survive through weeks of negative 40 degrees Fahrenheit in midwinter, but would likely die in 24 hours if exposed to those temps in midsummer. Plants must first acclimate to the cold before they can withstand it.

The process of getting ready for the coming winter begins with the shorter days and cooling temps of autumn. Chlorophyll production ceases, revealing the bright reds, yellows and orange pigments we all love so much. As this is going on, trees are actively moving carbohydrates into storage tissues in their trunks and roots and reallocating their moisture reserves as well. Water management is the biggest key to surviving subfreezing temperatures, as trees are comprised primarily of water. Water, of course, expands when it freezes; and this expansion will rupture the walls of plant cells causing the death of the cell, and, if widespread enough, the death of the tree.

Plants all around the temperate world have evolved different approaches for surviving winter. Annuals survive winter by not surviving at all. They have developed a way to complete their lifecycle in a single season, leaving only their more storage-friendly seeds to endure the off-season. Herbaceous perennials often have specialized belowground structures that can store water and carbohydrates for the winter, and can reactivate them in the spring. This includes familiar structures like the bulbs of a tulip, the tubers of a potato, and the taproot of a carrot. Hardy woody plants, such as trees and shrubs, do not have luxury of moving all their tissues into the soil, so they have developed a few different strategies for subzero temperature endurance, and it all starts with water management.

Although trees appear solid to us, their cells are comprised primarily of water, and what they do with that water in the winter determines their survival. One mechanism involves pumping sucrose and the amino acid proline into the cell. This acts essentially like salting your sidewalks in winter by lowering the freezing point of the solution inside the cell to remain liquid. Just like salting the sidewalk, however, this is only effective for combating “warmer” freezing temps of about 20 to 32 degrees Fahrenheit. This is due partly to the effects of osmotic pressure; you could keep increasing the quantity of dissolved solutes to continually lower a cell’s potential freezing point, but at a certain concentration the increased volume of “stuff” inside the cell begins to build pressure that increases its likelihood to rupture. Other plants take it one step further and undergo a process referred to as “deep supercooling.” Deep supercooling is a way to keep a liquid in a liquid state well below its freezing point. To do this, these plants produce a special protein to prevent freezing. These are conveniently known as “antifreeze proteins” or AFPs, and they are pumped into the spaces between the cells during the fall acclimation period. AFPs work not by lowering the freezing point, but by inhibiting the re-formation of ice crystals. They also function at very low concentrations, which means they do not have the same issues related to increasing the osmotic pressure as other methods. Deep supercooling also has its range limits, but will allow trees such as oak, elm, maple, beech, ash, walnut, hickory, rose, rhododendron, apple, pear and stone fruits to survive temps down around negative 40 degrees Fahrenheit. Although that number seems ridiculously cold to our tropical-loving bodies, that is still only about half as cold as the Dahurian larch can endure.

Trees such as paper birch, redtwig dogwood, willow, quaking aspen, and, of course, the Duhurian larch survive the damaging crystallization effects of freezing water by trying to rid themselves of as much water as possible. Rather than pumping more items into their cells, these plants spend the autumn pumping the water out of their cells. The water still freezes, but the crystallization occurs in the cytoplasm within the intercellular spaces, which is has much less damage potential than freezing inside the cell membranes. Additionally, removing water from the cell increases the concentration gradient of solutes within the cell, which, in turn, lowers the freezing point of the cell, similar to the mechanisms mentioned above. The species that can best perform these tasks are those that are considered the most cold hardy and come with the reward of being able to thrive in areas unsuitable for most other living things.

Even the most cold-hardy species will suffer damage from the cold if they do not acclimate to it properly. Even though water is the most damaging thing plants confront once the temps drop, it is vital to their preparation. Keeping trees well watered up until the ground freezes is one of the best ways to help them survive the coming winter. Given the proper care in autumn, and if the temps do not drop below a tree’s minimum comfort range, they should be able to resume activity in the spring. If only surviving the winter was as easy for us humans.



Brandon M. Gallagher Watson is creative director at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).






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