The winter life of trees

Mary Miller, Special to The County
2 months ago

Bears, as most schoolchildren know, hibernate during the winter. Some animals slow down, while others strategize eating and sleeping patterns. Many birds fly south; perennials hunker down, annuals wither and die. But trees embrace the winter with grace and strength, resilience and innovation. And with beauty, always beauty. 

Deciduous trees, the kind that lose their leaves, begin to power down in the fall. Daylight hours decrease, so the trees’ work of photosynthesis — converting the sun’s rays into energy — necessarily decreases as well. Chlorophyll, the leaves’ green pigment used in photosynthesis, breaks down, allowing the other colors to shine through and enchant us. 

Meanwhile, cells at the base of leaf stems are sacrificing themselves, dying in order to keep water and nutrients from getting to the leaves. Far better to keep those inside the tree, helping it stay alive during its winter dormancy. The leaves in turn also give themselves up, dropping and enriching the soil below. At the same time, up in the tree’s twigs, next spring’s tiny new leaves, even flowerets, are already forming, packaged into buds and covered with a hard waterproof casing. Such is the remarkable circle of life for deciduous trees. 

Conifers, on the other hand, employ a different seasonal strategy. Their needles, with a smaller surface, hold less water; the spills are also coated with a wax-like substance that helps prevent freezing. In addition, evergreen needles contain chemicals that keep ice from forming inside — and also give them a delightful scent. So the green remains, and photosynthesis continues, albeit at a slower pace with less sunligh. Energy production for the tree is carried on, even in the colder, darker season. 

So needles and leaves do their work of living and dying in winter. But what about inside the tree? How do those water-laden cells of the tree’s transportation system survive without destructive freezing? Through a very cool dehydration process, which springs into action when temperatures drop. 

First, cell walls become more pliable, allowing most of the water to move out before it can turn to ice, expanding and breaking the membranes. Next, starchy energy turns to sugar in the remaining cell water, thickening it into sap and acting as a natural antifreeze. Finally, the cells protect themselves by super-cooling to become glasslike, almost solid. Cells stave off freezing, while nutrients are sent to the roots. Trunks remain upright, branches survive and food is saved.

The gallant bark, it should be said, plays its vital role for the tree in winter. It protects the inner wood against the elements, and also acts as insulation, dispersing whatever heat is available. Sometimes, though, there is a cost. Water that lodges in the bark may freeze by night and thaw by day’s sunshine, eventually creating frost cracks. And very cold temperatures can cause the bundles of cells just inside the surface to freeze and pop. For the bark, it’s all in a winter’s work. 

Finally, below the ground, the tree’s roots are quietly but steadily working. They are storing and using nutrients, taking in water. In soil that is not frozen, insulated by snow and natural mulch, they are even growing. Feeder roots near the surface may be damaged or die, but the root system remains intact, a foundation of sure support. 

Just like bears, birds and humans, trees in winter survive and adapt, rest and prepare. Resilient, flexible and multi-skilled, they find ways to endure cold temperatures and diminished light. Even ice and snow, which may break and bend some branches, are met with fortitude and loveliness. And along the way, winter trees stir our souls, lighten our hearts, enliven our minds, and give us courage. Such a gift.

Mary Miller is a member of the Houlton Canopy Crew, a group committed to caring for trees and gardens.