Tolerating winter

Winter may be the "quiet" season but it's far from dormant. Animal and plant adaptations to the stressful environmental conditions are complex and diverse.

A northern winter is a time of stress for living things. Michigan State University Extension says that the forest is far from “dead” although it might appear that way to some people.

Winter presents three major challenges:

  1. Cold temperatures
  2. Dry conditions
  3. Reduction in food sources

Animal and plant species approach these challenges in a variety of ways.

Certain species of wildlife are “chinophiles” or “snow lovers”, such as ruffed grouse. Other species get the heck out of Dodge and migrate. Spectacular movements occur every fall and spring. The champion migrator is the arctic tern which can log over 40,000 miles per year. There are also species that migrate from the arctic latitudes to the Great Lakes latitudes, such as chickadees, snowbirds, certain raptors and woodpeckers, and many others.

Migrations are not always over long distances. In the Upper Peninsula, deer move away from the deep snows in the Lake Superior snowshed. These twice-annual range movements average about 9 miles. Out west, mountain elk move up and down in elevation with the seasons. Many wildlife species have different ranges for summer and winter, even in geographically small areas.

Some animals enter one form of dormancy, including true hibernation. Reptiles and amphibians find relatively warmer microhabitats and undergo phenomenal chemical and metabolic changes. Some species, such as wood frogs, actually freeze, although the process is far more complex than making ice cubes.

Hibernation is practiced by species that have the ability to regulate their metabolism. For the most part, this is a mammalian feature, but there are bird examples as well. Over a period of time, a fantastic cascade of physiological change occurs, stimulated by the photoperiod.

Ground squirrels and chipmunks are true hibernators. The poster child of hibernation, the black bear, is not a true hibernator. On one hand, a winterized ground squirrel will not respond to handling, but bothering a bear in its winter den is risky business for humans and may cause abandonment of the den by the bear. And then there are species that demonstrate degrees of dormancy between true hibernation and normal activity. Bats, skunks, woodchucks and raccoons are common examples, often becoming active on warmer late winter days.

Animals that are active throughout the winter undergo preparations too, such as warmer fur or feathers, fat build-up, denning and behavioral changes. A few species change colors, such as the short-tailed weasel and snowshoe hare. The hare, in particular, has become more vulnerable to predation as their molt grows out of synch with shortened snow season lengths.

Bergmann’s Rule illustrates the geometrical relationship between body size and surface area. Large bodies have proportionately less surface area and will, therefore, lose less heat. That’s why the largest members of any particular species are usually found in more northern portions of their range.

The insect world displays a range of over-wintering examples. Many survive as eggs, others as larvae, pupae or adults. The mourning cloak butterfly is among the first to be seen in the spring. And many people wonder at the tiny moving black spots on snow surfaces. Those jumping flecks are called springtails. Some insects, among other taxa, are active all winter under the snow. This “subnivean” habitat is unknown and unnoticed by most people, but there is an intriguing and ephemeral ecology under deep snow.

Plants also have numerous techniques to survive winter. Many overwinter as seeds, rhizomes, corms, and other root forms using specialized chemical and structural adaptations. Perennials such as trees need additional adaptations to survive.

All trees must deal with cold temperatures and dry conditions. The process begins in early August, including preparation of buds for next year’s leaves and flowers:

Hardwoods (broad-leaved trees) and softwoods (needle-bearing trees) have differing strategies.

  • Hardwoods lose all of their leaves as they become a liability in the fall. Flushing new leaves in the spring is physiologically expensive. Therefore, a sufficient quantity of reserves must be stored. Some species of hardwood cast seeds during mid-winter.
  • Softwoods (needle-bearing trees) also lose leaves in the fall, but only the older, less efficient ones. Needles are better designed to withstand water loss. The advantage is the ability to photosynthesize all year, when environmental conditions permit. The risk of winter photosynthesis is the lack of liquid water transport.

Warmer winter days, with deeply frozen root systems, can lead to unmet water demands and subsequent needle death. This “winter burn” does not become visible until the spring, sometimes leading to misdiagnoses.

The capillary action within the xylem cells of trees is maintained by softwoods throughout the winter. In hardwoods, these water columns collapse and hardwoods cannot move water upward until they regrow these tissues in the spring. Softwoods have special structures at the ends of xylem cells that can prevent the rupture of these essential water columns.

For both animals and plants, frozen water within cell membranes will result in cell death. However, water in between cells can freeze without causing damage. Winter chemistry moves water outside cell walls. When that water freezes, small amounts of heat are released which can help maintain conditions within cells. As temperatures drop, there comes a point where a species can no longer prevent cell freezing. This “lethal” temperature varies with the genetic capacity of a species and sometimes within geographical populations of the same species.

Extremely cold winters with little snow will result in the death of many individuals, both animals and plants. As changing climates result in shorter snow seasons and shallower snow depths, more individuals and populations will become increasingly vulnerable. This may be compensated by fewer extremely cold winter temperatures.

Winter may be the quieter season in many ways. However, the notion of a “quiet season” is a bit misleading. The winter forest is robust with preparations, adaptations and diversity. It’s a wonderland in more than one way. 

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