Winter brings cold winds, icy precipitation, long periods of below freezing temperatures – winter in the temperate zones takes a toll on all living things. Humans seek shelter, many animals hibernate, some insects may overwinter as eggs or life stages that are dormant. A walk in the forest in February clearly indicates that most plants become dormant and nearly inconspicuous during this time. For many plants, above ground portions die back so that the living parts, the root system, protected buds, and seeds persist below ground.
Slowing down physiologically offers a way to protect against the harshness of winter. But how do trees survive and persist in the often-harsh conditions? Trees can’t go below ground, as some smaller plants may; rather, trees encounter the winter, directly. Some trees can live hundreds (even thousands) of years making it difficult to imagine the extreme winter conditions that affect the tree over its lifetime. Trees persist, however, through the bitterness of deep winter. Most trees do ‘slow down’ during winter, and deciduous trees that lose their leaves shut down photosynthesis entirely. Trees with needles (evergreen trees) that are retained over winter can actually photosynthesize during the winter. Such productivity is not without risk: In addition to exposure to cold temperatures, trees can also experience water deficits because of frozen ground. Conifer needles might actually dry out and turn brown during the winter if the trees are photosynthesizing and water is not available.
Ice and heavy snow certainly can damage trees, especially deciduous trees. Severe ice storms can cause branch breakage and may ultimately stress the tree sufficiently to cause death. A winter with heavy snow, ice and wind may mean that spring produces fewer leaves than expected. Conifers tend to have flexibility of sorts and pines, spruces, firs, and hemlock can tolerate heavy snow load and ice damage.
All trees have bark, and this protective covering serves as an insulator, protecting the vulnerable, living tissue just beneath the bark from cold and freezing. Bark also protects the tree from damage such as antler-rubbing, insects, or other physical and biological damage. However, bark can be vulnerable, too. Water (sap) inside the tree may freeze just beneath the bark, then rapidly thaw in bright winter sunshine. When repeated freezing, thawing, and re-freezing occurs, tissues beneath the bark will be damaged, dry out and crack. This process results in a very common looking tree damage known as frost cracking. The damage frequently occurs with thin barked trees, such young maple, birch, beech, or many ornamental trees or fruit trees.
While the freezing-thawing cycle may cause problems for many trees, the same cycle in mid- to late-winter results in benefits for pancake lovers. At this time of year sap movement and production greatly increase in maples trees – and other trees that produce sap. Once temperatures start rising above freezing during the day, but remain frozen during the night, the sap may start to run and tapping the maple trees may begin.
Rose-Marie Muzika is the Director of Science and Research at Carnegie Museum of Natural History. Museum staff, volunteers, and interns are encouraged to blog about their unique experiences and knowledge gained from working at the museum.