In the Northern Hemisphere, people spend much of winter considering freezing weather, both out of practicality and for the fun of snow. In the Pacific Northwest, lowland rain may be more common than frost, but that doesn’t mean we never get freezing weather. Plants know this, otherwise our native forests would likely look very different. Have you ever considered how trees cope with frozen conditions?
When you are fifty percent water, and you can’t just eat food, go inside, or put on warmer clothes to keep from freezing, what do you do? Plants have methods of dormancy, similar to hibernation, which allow them to sit out the cold, dark days of the winter. Some produce new generations every year and die back, broadcasting a seed bank to overwinter and germinate when the time is right. Some die back to a rootstock, or have a low lying form that allows them to use the earth’s warmth to resist freezing. If a plant is deciduous, with soft, fleshy leaves, these are a weak point in freezing weather, full of moisture and unarmored against frost; these worth shedding. Evergreen plants have worked around this by developing thicker, waxy leaves (and needles), which allow them to take shelter against cold weather (as well as dry) and keep operating. Trees in general though, standing tall to catch all that sunlight, can’t escape freezing weather completely.
The overall goal for trees, is to keep live cells from bursting under the pressure of frozen water. The good news for a larger tree is that a good portion of the trunk is already dead, heartwood. This is the main structural element of a tree, which started as live sapwood and eventually became hardened heartwood. The rest, the phloem (or inner bark), cambium layer (just below the bark), and the sapwood are alive and contain moisture, and that’s what needs protection.
If you’ve ever pruned a tree in winter and then again in spring, you’ve noticed a significantly higher water content in the growing season. Reducing moisture content in the winter makes economic sense. Shorter, darker days mean less opportunity for chlorophyll to do its work creating energy with sunlight, water and carbon dioxide. There’s little point in pumping water from roots up to chlorophyll in leaves if there’s little sun to create food with. Dormancy, including reduced flow of sap, is an energy saving measure, but it also allows the tree’s live cells to avoid damage in freezing weather.
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You can see an expression of seasonal sapwood activity in many trees by looking at a round of the trunk. Each year has both early and late wood. Less dense, early wood is about quick transfer of water and nutrients and has large vessels to facilitate this. Latewood is less about transfer, and more about storing energy, and is correspondingly denser.
We live in the evergreen state, so it’s only right to take evergreen conifers into account. Deciduous, broad-leaved trees decrease water transfer and loose leaves, going into full dormancy. Evergreens, and conifers in particular, have created other ways to deal with freezing. Staying green year-round means growing year-round, and the cell walls of conifers are very strong, enduring much higher pressures from freezing water. Waxy leaves, as mentioned above are important, but for conifers, having condensed leaves like needles helps even further. Their sapwood can even contain special check valves that close in freezing temperatures and reopen if conditions allow for continued growing throughout the year.
There are thousands of species of trees in the world and those that encounter freezing weather have a variety of other adaptations that allow them to keep strong and tall year-round. Probably the most famous example in North America is that of maples, which increase sugar content in sap to decrease its freezing point (which we enjoy boiled down into deliciousness). Other plants are able to push moisture out of their cells in frozen weather, taking the pressure off their cell walls, as freezing temperatures create expansion. The constant race against challenging conditions has created a myriad of ways to deal with winter.
As you can probably guess, preparing for dormancy requires proper timing. While plants rely mainly on the photoperiod (both the length of night and day) for cellular regulation, they also read other environmental cues. Trees can have a hard time if they can’t rely on normal weather patterns to cue up for the year. The compounding effects of climate change can leave trees unprepared for winter weather, increased attacks by predatory insects and pathogens, and even regeneration in the spring. We rely on trees in the city to be happy and healthy ourselves, so it’s worth taking a look around and seeing if our neighbors are doing well or not.