A Resilient Tree: How Plants Adapt to Ice and Snow
A Corylus shrub’s male catkins froze over and received a thick layer of ice on the branches.
The view outside my living room window is white with ice as each tree branch glistens with an icy coating. Rains will come and melt the ice, but right now Eugene is a winter wonderland of frost and snow. It is especially beautiful from the view of my cozy living room with coffee piping hot by my side. For the trees I see outside this storm is not a cozy-cute event, rather it is a deadly extreme weather event that has brought on the death of hundreds of trees in Oregon. Trees in the surrounding areas do not typically have to withstand inches of ice buildup in their branches and needles, and the extra weight can be too much for weak trees to withstand. That, coupled with extreme winds and additional snowfall, can cause tree collapse, branch collapse, and bark damage.
I marvel at trees that do survive such an extreme weather event. They will remember this extreme weather, and they will pass on this knowledge to their future offspring. How do they do it, and what secrets are they hiding in their defense strategies?
To look deeper, I head out into the frozen landscape by the Willamette River armed with my camera and a lot of thick clothing. One way I start to think of trees and their protections is within the frame of my own cold defenses. Trees have bark as their protection, and like them, I have my winter coat to protect me. If my coat is open or damaged, the cold can get inside and I will begin to suffer. Similarly, tree bark protects the trees from cold temperatures that could cause their sap to freeze. If a weak tree has an opening, it is more susceptible to the ice creeping in to freeze its living cells.
As I approach the river and the surrounding forest, I immediately notice the conifers have way more ice buildup than the deciduous trees. Their green needles are white with snow and ice, and their branches are heavily weighed down. On some branches, ice is so heavily built up I can’t see their needles, only ice. In order to survive such an onslaught of cold, these conifers have developend an impressive defense strategy: antifreeze!
A Pinus tree is coated in ice after the January 2024 ice storm.
The tree’s antifreeze is made up of chemical compounds that prevent the sap within the foliage from freezing. In the same way that we manufacture chemical antifreeze for our cars’ engines and windshields, trees make their own chemicals for their own purposes. Look to trees in genuses such as Cryptomeria, Picea, and Abies to see the strength of tree antifreeze in the winter.
Coniferous trees also apply thin layers of wax to their needles, and they have tiny pores on the needless that make it difficult for water to escape. This is incredibly important as it is nearly impossible for trees to drink water during freezing temperatures, as water becomes ice and will not move throughout the trees’ vascular systems. These two mechanical defenses, along with the natural antifreeze, allow coniferous trees to retain their water and protect themselves from ice and snow storms.
Psuedotsuga menziesii survives the ice using antifreeze in its frozen needles.
Without antifreeze though, how do other trees like deciduous trees handle the cold? By the Willamette River, there are hundreds of deciduous trees, such as the Populus trichocarpa known commonly as the “Black Cottonwood Tree.” Trees like this one lose their leaves in the fall to avoid ice buildup of the kind conifers receive. Deciduous trees dump their leaves, only after they’ve sucked the nutrients from their leaves inside for safekeeping during the winter. Losing leaves reduces their tree cover, leading to less ice and snow buildup in the trees’ crowns. Ice on deciduous trees only occurs on their branches, and so less weight is added to these trees as compared to conifer trees.
Without leaves, deciduous trees do not have to build defenses on the leaves. You will notice too after handling a deciduous tree’s leaves that they are thin, easily torn, and not resistant to frost. Deciduous trees’ defense strategy is to lose these weak leaves entirely and to grow strong with less. Conifers, on the other hand, have highly impressive cold resistance with their antifreeze, waxy coatings, and deep pores on their needles.
But what happens when an evergreen coniferous tree has more ice on its foliage and crown than it can withstand? If a tree has more weight than it can bear, it will surely crack and splinter in places where ice buildup is heaviest. This is why during ice storms you can often hear the “crrrrack” sound of trees as their branches splinter and break off. Healthy trees may recover this crown loss within a few years and heal over, although many trees with severe ice damage will rot and die. Not only this, but when a branch cracks off of a tree it opens a wound into the tree’s heartwood that allows fungus, pests, and ice to invade.
A fallen deciduous tree blocks the way on the Willamette River Bike Path.
With each tree death, this ice storm is spurring a genetic evolutionary event called “bottlenecking,” in which only trees that survive the ice storm can pass on their genetic material to future generations. This reduces the amount of genetic diversity within the tree populations, although it also ensures that the trees will carry on more ice resistance than would their frozen-dead siblings.
Eventually, the warm spring weather will come and some damaged trees will sprout new foliage, seal off their wounds, and redevelop their crowns. For now though in the PNW, you can see many different trees that have split, broken up, or snapped in two. Only time will tell which trees survive to pass on their knowledge, and which ones will rot and feed the soil below.
Sources:
Liberty Hyde Bailey. Botany, an Elementary Text for Schools, by L.H. Bailey. 1901.
Wohlleben, Peter. The Hidden Life of Trees: What They Feel, How They Communicate. William Collins, 2018.
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