Frozen forests

Winter has become my favorite season to be out in the woods. I find the winter landscape to be shockingly beautiful and peaceful. I developed this affinity for forests during the winter as a graduate student at the University of Minnesota, where I worked in the woods all year round, including every week during the winter. As a result, I was able to see the gradual changes in the forest as fall turned to winter, and as winter turned to spring. 

It’s easy to admire the beauty of the winter landscape, from the bright blue sky on bitterly cold days to trees covered in hoar frost. But what about everything that we can’t see? What happens to the forest during the winter?

Aboveground

Minnesota trees and forests are adapted to the extreme cold. The most obvious way that deciduous trees survive the winter is by dropping their leaves. Deciduous trees, such as oaks, maples, and aspen, drop their leaves in the fall to conserve water and energy during the winter.

Coniferous trees, such as pines and spruces, retain their leaves (needles) in the winter. The needles on conifers are protected by a waxy cuticle and can grow over several years, in contrast to new leaf growth each year on deciduous trees. Tamarack, a common boreal species, is the one conifer in Minnesota that is an exception to this pattern. Tamarack has needles that turn bright yellow and drop each fall during the tree’s attempt to store nutrients and improve its efficiency as it approaches the winter season. 

As daylight gradually decreases with the approaching winter, trees enter their dormant period. During dormancy, trees are not actively growing and have slowed down many cellular processes to conserve resources over the winter. When the leaves on deciduous trees start to change color, this is a sign that trees are preparing to settle down for the winter. 

Trees hold onto a lot of water, and they are very good at transporting water and nutrients between their roots and leaves. But this means that the cells inside of trees are at risk of freezing during winter, and the crystallization of water within cells that transport water and nutrients (the xylem and phloem) can be harmful to trees. 

Trees have adapted to prevent cell freezing in a few ways. One of these ways to prevent intracellular freezing (freezing inside cells) is by creating the tree’s version of antifreeze to thicken the fluid within the cells. Combined with more flexible cell membranes and supercooling, trees can limit the extent of freezing within cells. 

Belowground

Many processes occur aboveground in a forest during the winter - but what happens belowground in the soil? Forest soils are an entire ecosystem and form the foundation of forests. Forest soils change during the winter, just like the trees, vegetation, and wildlife aboveground. 

As the aboveground air temperature starts to drop, this cold air penetrates the soil and causes the water held in the soil pore space to freeze. The surface of the soil will freeze first, and then the frost depth extends deeper into the soil as the winter continues. 

Several factors influence the extent of soil frost in forests. First, snow insulates the soil surface from freezing. A thicker snowpack will result in shallow frost because the snow blocks the cold air from freezing the soil. In contrast, a thin snowpack (or lack thereof) will result in deeper soil frost because the soil surface lacks the insulating layer of snow. 

Secondly, dry soils freeze to a greater depth and take less time to freeze and thaw than wet soils. So sandy soils freeze to a greater depth than clay-heavy soils since sandy soils hold onto less water and are inherently drier than clay-heavy soils. In forests, this means that a lowland forest, like a black ash swamp, will take more time to freeze and will not freeze to the same depth as a drier site, such as a sandy red pine forest. 

In Minnesota, the majority of timber harvesting takes place during winter when forest soils are frozen. Operating on frozen soils greatly reduces the risk of negative impacts from heavy equipment, such as compaction and rutting. But what happens when we have a warm winter, like what Minnesota has experienced so far this year? 

Warm winter temperatures result in delayed soil frost development - which means that as a woodland steward, loggers may not be able to access your property to harvest timber when they originally planned. Operating on frozen soils is essential to protecting the health of sensitive soils, such as wet and clay-heavy soils. 

Timber sale contracts can specify a winter-only harvest if there are sensitive soils on site. Understanding the types of soils in your woodland is a critical part of woodland stewardship. You can learn more about the soils in your woodland with tools like Web Soil Survey or get your hands dirty by performing a ribbon test. A natural resource professional will also be able to give you more insight into the types of soils in your woodland and the sensitivities associated with those soil types.

Implications of a changing climate

Minnesota forests are adapted to cold and snowy winters. Trees can slow down their biological processes in the winter months, and frozen soils provide the foundation for winter timber harvesting in the state.

As climate change continues to impact our winters, these seasonal dynamics in Minnesota’s forests will also change. Changes in soil freeze-thaw cycles will affect our ability to harvest timber in the winter with limited impacts on soil productivity. If you are a woodland steward looking to harvest timber in the winter, be aware that warm winters may delay loggers’ harvesting periods.

In general, understanding the year-round dynamics of your forest is a key part of being a woodland steward. By learning the intricacies of your woodland throughout the seasons and working with a natural resource professional, you can work to proactively steward your woodland under the impacts of climate change.