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Waiting For Spring

Posted: January 19, 2017

A Closer Look at Forest Ecology in Winter

A fascinating aspect of a woodland ecosystem in winter is the evidence of all the various ways by which animals and plants survive harsh winter conditions. In late fall, the growth rate and reproduction of both plants and animals begin to change, triggered by the length of day and nights. These physiological responses collectively are called photoperiodism. The lengthening nights and shortening days trigger winter survival strategies. The primary strategies for adapting to freezing temperatures and reduced food availability are: migration, hibernation, and toleration.

Migration is perhaps the most commonly considered of these strategies. It is fairly easy to notice the comings and goings of many bird species. Warblers and other songbirds have left for warmer southern climes, making more noticeable the dark-eyed juncos, tufted titmice, Carolina wrens, and other species that remain in the winter woodlands. Other birds, including pine siskins, common redpolls, evening grosbeaks, snowy owls, Northern harriers, and numerous other species of raptors and waterfowl migrate to and through Pennsylvania and other temperate zone states through the winter. Migration even occurs among mammalian species such as the woodland-dwelling silver-haired bat, and even insects like the monarch butterfly.

The second type of winter adaptation observed among woodland animals is hibernation. While other animals are more commonly thought of when considering hibernation, woodland insects also survive through winter in a type of hibernation called diapause. Diapause is simply an “intermission” in development prompted by adverse environmental conditions. Diapause occurs at different life stages for different types of insects. In many butterfly species, winter diapause happens during their pupal form, while the young insect undergoes complete metamorphosis while protected inside a case-like structure called a chrysalis. This pupal, or chrysalid, stage in development entails a breaking down of larval body tissue and reforming into the adult form of the insect. Wooly bear caterpillars, the larval form of the Isabella moth, get through the winter a different way. They overwinter in that larval (caterpillar) form, often sheltering in the leaf litter or under tall grasses.

The emerald ash borer—an insect that many woodland owners are on the lookout for—survives the winter in a form that is somewhat in between the larval and pupal development phases. These pre-pupae are protected from the freezing temperatures in the inner bark or the outer ½ inch of the sapwood of the ash tree or log. A study done in Ontario, Canada found that the larvae of the borer are cold-hardy to the point of -23°F, at which point even the adaptive strategy of diapause is not sufficient to survive the cold. Emerald ash borers are successful in surviving these extreme temperatures by accumulating high concentrations of glycerol, a sugary compound with a very low freezing point. They also are able to produce other compounds which act as antifreeze within their bodies.

In the woods during winter, one will also see some adult insects that remain active or semi-active. Smaller body size of some species that remain active helps to eliminate surface area exposed to cold and moisture. Like the emerald ash borer, the winter survival strategy used by many insects that overwinter as adults is the production of sugars, alcohols, and proteins, which have properties that act as antifreeze. These cryoprotective compounds replace most of the water in an insect, and prevent the bodily injury that would occur from water turning to ice crystals.

The third strategy, tolerance, is the primary adaptation woodland plants rely upon to survive winter’s freeze. All plants have a physiological response to freezing temperatures, shortening days and lengthening nights, though the types of responses vary. In annual plants, individual plants are killed by winter weather, but the plant community survives because of the mature seeds that will spread and produce new plants in the spring. Other plants, including almost all of our native perennial flowers and herbs, die back in winter except for their rhizomes (roots), which go dormant until warmer temperatures trigger new vegetative growth in the spring.

Trees, with their height, experience significant exposure to freezing temperature, strong winter winds, ice, and snow. They rely on both chemical and structural changes that help them tolerate the winter. Structurally, the shape of conifers helps to protect against limb breakage by allowing gravity to help clear snow-laden branches. The fallen snow also creates a protective “fence” around the tree. Some species, like oaks and beeches, retain their leaves well into winter as an added layer of protection against water loss through leaf scars. Bark also plays a significant role in preserving water resources during winter.

In fact, most of the winter tolerance adaptations of trees relate to water—preventing water loss or damage to the tree’s living cells by ice crystals. In late fall, a plant hormone called abscisic acid (AA) is triggered by the shorter days. One of the functions of AA is to increase the flexibility in cell walls within the tree. This helps in two ways. First, permeability of the cell walls allows water to move from within the cell to the space in between cells. The water can freeze there without causing damage to the cells. Secondly, because the cell walls are now more pliable, any water that does remain in the tree’s cells and freezes has less risk of the crystals puncturing the cell wall, which ultimately would stress the tree.

Along with other responses triggered by photoperiodic changes in late fall is the conversion of starches to sugar within tree cells, which lowers the freezing point. Another fascinating way that trees tolerate winter through managing water at a cellular level has been described by Paul Scharberg—a plant physiologist with the U.S. Forest Service Aiken Forestry Sciences Lab in Burlington, Vermont—as a “glass phase.” In this cellular adaptation to prevent freezing, water mimics the almost-solid characteristic of molten silica as it cools to form glass, which appears solid but actually continues to move ever so slightly. The cell water, like silica, retains its liquid properties, allowing the slightest degree of movement.

Though adaptations like these can’t be observed with the naked eye, there are plenty of other winter survival strategies to observe during a walk in the woods. In any case, getting to see our woodlands in different seasons is a big part of developing an understanding and appreciation for the complexity and interdependence of all the living parts of a forest. Throw on an extra layer and head out to see what you can see for yourself—at the very least, it will give you time to think about what forest stewardship activities you might want to add to this year’s to-do list.  

Contact Information

Leslie Horner
  • Forest Stewardship Program Associate
Phone: 814-867-5982