By the time late January hits the Upper Midwest, woody plants throughout the region are deep in their dormant period. Under sheets of snow and battered by icy winds, trees and other hard-stemmed plants weather the—well, weather in a beauty sleep of sorts; stems bare, roots covered in frozen earth.
Come spring, they’ll re-emerge and bloom again, seemingly none the worse after six-odd months of non-life. But what exactly happens during this dormant period? How does it help the forests and prairies of Minnesota and her surrounding states survive their harsh native climate? It’s actually pretty cool—and we’ll break it down below.
During spring and summer months, trees move huge volumes of water up and down their trunks. Big trees pull thousands of gallons of water a day through their root systems. This movement happens because of one key biological function: transpiration.
Transpiration might remind you of “respiration”—which is not a bad way to remember the term, since “transpiring” refers to the release of water vapor from a plant’s leaves. Think of it as a lot of tiny, watery exhales. As water leaves the tree’s system through the leaves’ stomata, more water gets drawn in through the many capillaries that run the length of the twigs, branches and trunk.
This capillary action is what draws in water from the soil, through the root systems, all the way up to the tree’s tips. During summer months, a tree is upwards of 50% water by volume. But in the winter months, all that water in the soil and the atmosphere starts to freeze—so how do trees cope with this slow decline in water supply?
The survival mechanism that cold-climate woody plants have adopted is essentially a full system shutdown during winter months. This is called a dormant period, during which the plant remains totally unresponsive to its usual growth stimuli.
A tree “going dormant” isn’t a sudden process. It all starts with the shorter, cooler days that accompany summer’s transition into fall. The dwindling sun hours are one of the signals for trees to slow their growth processes and start “phasing out” their energy generators and respiration centers—their leaves.
As leaves drop during fall, the trees continue their slow march towards true dormancy by forming winter buds; tough, scaly shields for the tree’s delicate twigs that will pop into leaves during the next spring.
The onset of a tree’s truly “dormant” period is marked by a series of physiological changes that happen within the tree. When a tree is in full dormancy, it’s totally incapable of growing—even if it were suddenly to be summer again, with abundant sun and water.
While a tree’s growth is stalled and water circulation has stopped, there are still biological processes occurring—though the activity is largely centered in the twigs. Within their buds, new leaves are preparing at a cellular and enzymatic level for the usual spring growth.
In these frigid months, water that’s stored within the tree’s trunk remains in a liquid state thanks to a solution of hormones and minerals that prevent freezing. By extension, this prevents the formation of ice within the tree’s structure, which can lead to cracks and structural damage.
In early spring, trees begin to move from deep, true dormancy into a post-dormant state. As environmental factors like sun hours and temperature signal to the tree that it’s time to “wake up,” structures within the tree begin to restart their normal processes. This emergence from dormancy is not an overnight transformation, but rather a gradual restoration of normal cellular function.
In order to trigger spring growth, many woody plant species require what’s called a “chilling period;” an amount of time during which temperatures are above freezing, but still cool. This acclimation period encourages buds to open up and prepare to grow once again.
We all have our strategies for getting through the cold winter months, and trees are no different! Happily, come springtime, trees and woody plants across the Upper Midwest have made it through another cold and snowy winter—and are just as ready to burst into warmth and light as we are.