By Brandon M. Gallagher Watson
Many years ago, before I got seriously interested in arboriculture, I was at an event at our local arboretum. It was in the middle of winter and, being in Minnesota, it was quite cold. The event had tables set up for all sorts of local “green” clubs trying to recruit fellow green geeks to joining their groups. It included master gardeners, the fanatic rose growers, the begonia society, and so on. The booth that caught my eye was the bonsai club; so I went over to check out the small, twisted trees in pots. One specimen was particularly striking, I can still picture it today; it looked like a perfectly scaled miniature tree with its roots growing over a rock. One problem though, it looked dead. I asked the guy at the booth what kind of tree it was, and he informed me it was a Japanese maple. I said, “It’s beautiful, is it dead?” He informed me it was quite alive. When I asked, “Why doesn’t it have any leaves then?” He paused for a moment, and being polite to my ignorance, he replied, “Because it’s winter.”
What I didn’t understand at the time was even though this tree was “domesticated” and being kept in a pot, it was still going through all the cycles of the seasons. In fact, it had to go through the seasons to stay alive. Many bonsai trees have been killed by loving owners who think they are helping their living sculpture by bringing them indoors when the weather changes in the fall. This is the same reason why the species we consider houseplants are all from tropical regions where it is 72 degrees Fahrenheit year-round, while you could not keep, say, a hosta in your living room for more than a year before it would die. Plants that are adapted to temperate climes must go through a cold rest period, known as vernalization, in order to complete an annual cycle.
As the trees in our urban forests prepare for cold, they go through several physiological changes. The most obvious is probably the turning of fall leaves on deciduous trees. The leaves have actually been preparing for their annual leaf drop since they budded out in the spring. Glucose has been moving out of the leaves to feed the tree, and waste products have been moving it. Along the base of the petiole, where the leaf attaches to the twig, is a line of cells known as the abscission layer. While actively growing, there are a series of tubes running through the abscission layer transporting water from the vascular system into the leaf. When triggered by a decreasing photoperiod, the cells in this layer swell and form a cork-like material, known as suberin. As the suberin swells and expands, the abscission layer begins to prevent water from entering the leaf. Glucose, pigments, and waste products remaining in the leaf become trapped there, and, without a water source, the green color begins to fade.
So what about the vibrant fall colors? During the growing season, leaves are actively taking in CO2 and using sunlight to combine it with water to make sugar and oxygen through photosynthesis. This makes chlorophyll, and chlorophyll, of course, gives leaves their green pigment. As fall approaches, and the days get shorter, the leaves cease producing chlorophyll and the green color fades. The bright colors that are revealed were actually there all season long but we couldn’t see them as the chlorophyll was blocking them. The yellow-orange colors result from carotenoids, the same pigments that color carrots, bananas, and daffodils. The reds and purples are caused by anthocyanins, the same molecules that give apples, plums, and grapes their iconic hues. The browns revealed in trees such as beech and oak are tannins, the bitter-tasting waste product that is left in the leaves (that also gives tea its color). All leaves have stored waste products, but the coloration is often blocked by other compounds so we don’t see them until the leaf as completely died — hence why all leaves will eventually turn brown.
Evergreens prepare for the change in seasons in other ways. Conifers have leaves already modified to resist drought and freezing temperatures in the form of needles; but that doesn’t mean they don’t shed their foliage in the fall like a deciduous tree does. Conifers will drop leaves every fall, just not all of them. Typically, conifers retain 3 to 5 years worth of needles, meaning every fall they will shed a layer of older needles. I used to work at a garden center where every fall we were bombarded with “My pine tree is dying!” phone calls. We would ask if the tree was dropping needles from the tips of the branch or from the inside of the tree. 100 percent of the time it was from the inside. After ensuring them this was perfectly normal, we would take the time to inform them their pine was actually a spruce. Of course, there are exceptions to the rule, such as the larch or the dawn redwood, which will drop all their needles each fall. Needles that are retained generally do not freeze in the winter, even in extreme temps, as their resin acts as a natural antifreeze.
Beyond the foliage, trees are prepping for the cold in other ways. Temps can drop pretty low in the winter (often to -20 degrees Fahrenheit where I live) and, remarkably, the trees survive all season just fine. However, if one were to move a cold-hardy tree, such as a paper birch, into a -20-degree Fahrenheit freezer in midsummer, the tree would be dead by morning. Just like you need a wardrobe change before going form the beach to a snow bank, trees need to get prepared for cold temps by “hardening off” before the cold sets in. The liquids within the cells of the tree, known as protoplasm, change their concentration of sugars and salts — essentially lowering their freezing point, and allowing them to withstand frigid temps. Different plants have differing abilities to do this, which is what gives rise to the USDA hardiness zone map. The seed source plays a role in hardiness as well for trees with a wide geographic range. Red maples ([ital>Acer rubrum<ital]) are native from Florida to Ontario, but transplanting one from the South to the North or vice versa would likely be its demise as there are adaptations they have made over time to thrive in their native conditions.
During the winter, trees are dormant, but that doesn’t mean lifeless. Quiescence is a term that roughly means “resting but ready,” and that’s what trees are doing during winter. There are metabolic processes going on even in extremely low temperatures. Evergreens continue to respire, moving liquid water from the soil up through their stomata in the leaves. Winter burn is a common aliment on conifers as the sun heats up foliage, causing desiccation (if no liquid water is available, the foliage dries out and dies). Winter burn is most common in years of low snowfall, as the snow acts as an insulator, preventing the soil from freezing solid too deep. Below ground, the soils are often warmer than the air and roots have adapted for this. In fact, the air can be 20 degrees Fahrenheit and the soil can be as warm as 35 degrees Fahrenheit. Utilizing stored energy, trees can send roots out to explore for resources and grow in the spring while the world still looks to be frozen.
Caring for trees going into winter can be done with some simple tips for tree owners. Continue to water trees, particularly evergreens, weekly until the ground freezes. Trees are often fertilized in fall. This puts the nutrients where they are needed for spring; and nutrients such as potassium can benefit evergreens throughout the winter by reducing winter burn. Many plant health care treatments, such as soil applications of insecticides or applications of injectable nutrients such as iron and manganese are also applied during this season. Protecting trees from rodent feeding during winter can be achieved by wrapping trunks of young trees with corrugated plastic tubing. Wrapping evergreen foliage, or even creating a screen with a fabric such as burlap, can prevent winter burning and reducing the browsing of hungry deer.
Winter may be hard on humans, but our trees are pretty tough. Given the proper care through the growing season and into fall can put them in a position to thrive when spring arrives again. This is true for young tree, mature trees, and even little trees in pots.
Brandon M. Gallagher Watson is creative director at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).