The onset of winter evokes different feelings in different people. Some folks look forward to fall foliage and cooler temperatures. For others it means the arrival of hunting or trapping seasons. For others still, holiday seasons spent with friends and family. Many of us look forward to idyllic snow-covered mountains and farm fields. But some dread the snowy commutes, high heating bills, and the end of blue crab season.

Colder temperatures, snowpack, shorter days, and reduced food sources create challenges for many organisms throughout the forests of the Bay watershed. While winters in the Chesapeake region are typically mild, the seasonal shift is significant enough that plants and animals here have had to develop adaptations that allow them to persist in an environment with ever-dwindling resources. As temperatures drop, we retreat to the coziness of our homes to wait for the warmth of spring to return. While some organisms employ a similar strategy, others have developed physiological or behavioral adaptations that allow them to endure the winter conditions. Deciduous plants shed their leaves and stop photosynthesizing, storing nutrients for spring regrowth. Many birds migrate south, while some mammal species retreat to their dens or burrows to wait it out. Most reptiles and amphibians spend the winter hibernating, and invertebrates utilize a diversity of strategies to survive the winter season.

Winter Adaptations of Trees

The forests of the Chesapeake are comprised primarily of deciduous tree species, including oaks (Quercus spp.), hickories (Carya spp.), maples (Acer spp.), cherries (Prunus spp.), tulip poplars (Liriodendron tulipifera), American beeches (Fagus grandifolia), and many others. As a result, the forest community in this region is generally described as a ‘temperate deciduous forest.’ The term ‘deciduous’ refers to the act of ‘shedding or losing leaves’ on an annual or seasonal basis (also known as abscission). Shorter winter days mean less sunlight for plants to use for photosynthesis. So as summer turns to fall, diminishing sunlight (and other factors) trigger the process of leaf abscission. Chlorophyll production decreases and the true coloration of leaves is revealed. While the mechanism that controls this process is somewhat complex, the end result is a plant that has shed its energy-collecting structures to safely store resources through the darker, colder winter season.

Deciduous leaves losing their chlorophyll in fall, revealing their true colors. Northern red oak (left), sumac (center), black gum (right) (Photos by Will Parson).

Deciduous trees are so dominant in our eastern forests because their deciduous strategy gives them a competitive advantage. Shedding leaves in winter allows tree and shrub species to exploit the seasonal changes that occur within temperate regions. Seasonal shifts in sunlight and temperature means that plants must be able to capture the abundant summer sunlight while developing mechanisms to cope with the coming cold, snow, and shorter days of winter. Growing large, plentiful leaves during summer allows these species to effectively capture the summer sunlight. Broad, flat leaves are effective at capturing sunlight during the summer months, but would create a liability for trees during winter. If broad-leaf trees were to retain their leaves year-round, freezing temperatures could cause the cells within thin delicate leaves to freeze and rupture. The weight of heavy snow on these leaves would cause damage to trees by breaking limbs. There are also a number of insect species that exploit the palatability of broad-leaved trees, damaging leaves throughout the growing season. Shedding these insect-damaged leaves at the end of summer allows deciduous plants to start fresh the following year. And leaves accumulating on the forest floor enrich the soil and provide nutrients that can be re-utilized during the next growing season.

In addition to deciduous tree species, some ‘evergreen’ species are prevalent within the forests of the Chesapeake, including eastern hemlocks (Tsuga canadensis), eastern white pines (Pinus strobus), mountain laurel (Kalmia latifolia), rhododendrons (Rhododendron maximum), and others. Relatively common in the Chesapeake region, coniferous and ‘evergreen’ species increase in abundance in northern latitudes. Thin, narrow needles do not accumulate snowfall and reduce potential damage to limbs as a result of heavy snow. And the shape of these needles means less surface area, reducing the capacity for water loss in winter. The needles and leaves of evergreens also have thicker, waxy cuticles that prevent further water loss and protect from exterior stresses. Rhododendron leaves will often ‘curl’ or ‘droop’ during winter months to further prevent dehydration and shed falling snow. These ‘evergreen’ species outcompete deciduous trees in northern latitudes, while deciduous trees are able to thrive and exclude many conifers and spruces from temperate regions. In equatorial regions, species adapted for tropical climates retain large broad leaves year-round, outcompeting deciduous species where winter leaf abscission would be a disadvantage.

Rosebay rhododendron foliage curls up and clamps shut amid sub-freezing temperatures hitting the forest surrounding the Blue Ridge Parkway and the Appalachian Trail at Reeds Gap in George Washington and Jefferson National Forest in Nelson County, Va., on Jan. 14, 2019. The evergreen, cold-tolerant shrub is able to protect itself from the drying effects of extremely cold air. (Photo by Will Parson/Chesapeake Bay Program)

Herbaceous Plants in Winter

The term ‘herbaceous’ refers to vascular plants that do not retain annual above-ground woody structures. Common herbaceous plants within the Chesapeake region include many species of forbs (flowering herbaceous plants that are not grasses) and wildflowers. Recognizable species include goldenrod (Solidago spp.), milkweeds (Asclepias spp.), beebalms (Monarda spp.), hemp dogbane (Apocynum cannabium), and Virginia bluebells (Mertensia virginica), just to name a few. Like deciduous woody plants, changes in daylight, temperature, and the angle of the sun trigger hormonal and cellular changes within herbaceous plants. But unlike woody deciduous species, herbaceous plants do not retain any living structures above the ground through winter. As winter approaches, herbaceous plants slowly stop photosynthesizing and respiring and the above-ground portions begin to change color, desiccate, and senesce completely. Perennial herbaceous plants shift carbohydrates and other nutrients into their roots where they are stored until spring, keeping individual plants alive through winter. Annual plants do not store nutrients in their roots, instead dedicating all their resources toward annual seed production, thus utilizing a different strategy to cope with winter conditions.

To Migrate or Hibernate?

Plants are rooted to the ground, so moving in response to the changing seasons isn’t an option. Thus, they have had to develop physiological adaptations to survive the winter. But what about mobile organisms like birds and mammals? While many species of birds and mammals have developed physiological adaptations, they also have the ability to develop behavioral changes to survive the winter. Birds and mammals are ‘endothermic.’ This means they have the ability to maintain a constant, stable internal body temperature. This ability is quite advantageous, because it allows these animals to remain active regardless of fluctuations in ambient air temperature. As a result, most mammals and birds are able to stay active throughout the winter. However, maintaining this internal temperature is energetically costly. It means that birds and mammals must continue to acquire food throughout the winter or develop alternate strategies to reduce their metabolic rate and food intake. Many small mammal species (rabbits, squirrels, voles, moles, shrews, and some mice) are able to remain active throughout winter because the foods that they eat are still available. Some of these species will even ‘cache’ or store food they have collected in fall to feed on during winter months. In turn, many carnivore species (such as foxes, coyotes, and bobcats) also remain active throughout the winter, feeding on these small mammal species.

The development of fur in mammals is another adaptation that has allowed many species to maintain consistent internal body temperatures throughout winter, and remain active as long as food sources are available. Despite having hair and subcutaneous fat, some mammal species do not remain active through the winter, because they can’t find enough food to maintain homeostasis (a stable internal body temperature). Because of this, these mammals have evolved alternate behaviors and physiological adaptations that allow them to persist through the winter without obtaining food. For example, black bears (Ursus americanus) enter a state of dormancy during winter. Bears are omnivores, and have significant food requirements to maintain their large size. So as winter sets in and their food supplies dwindle, bears reduce their metabolic rates and enter a state of ‘torpor,’ or winter dormancy. Black bears prepare for this dormant state during the fall, during a period known as ‘hyperphagia’ which describes a frenzied period of food consumption. Acorns, beech nuts, berries, and other food sources help black bears to develop a thick layer of subcutaneous fat that keeps them warm and provides resources during their winter torpor. While bears are not true hibernators, they spent most the winter within their dens, where they rarely (if at all) eat, drink, defecate, or urinate. When early spring food sources like skunk cabbage (Symplocarpus foetidus) begin to emerge, bears arise from their winter slumber and begin to forage again.

A sedated female black bear and her ear-tagged cubs in their winter den (Photo by Jim Kauffman).

Organisms that fly have the ability to move longer distances to avoid winter’s grip. While some large mammals also undertake significant seasonal migrations, here in the Chesapeake region we only have one group of mammals that migrate annually, a group known as the ‘tree-roosting bats.’ All bat species within the Chesapeake region eat insects, and insects are mostly inactive during winter. As a result, our bat species must either hibernate or migrate. The tree roosting bats include species like the red bat (Lasiurus borealis), the silver-haired bat (Lasiuris noctivagans), and the hoary bat (Lasiuris cinereus). These ‘tree-roosting bats’ spend their summers in northern latitudes, migrating south for the winter to regions where insects remain active.

But not all bats migrate. Species like the little brown bat (Myotis lucifugus), northern long-eared bat (Myotis septentrionalis), and the big brown bat (Epitesicus fuscus) have evolved the ability to hibernate, and remain here throughout the winter by seeking shelter in caves and subterranean voids (known as ‘hibernacula’) where they roost through the winter. These hibernating bats have evolved the ability to reduce their metabolism, heart rate, and respiratory rate, surviving on stored fat reserves during this period of inactivity. Unfortunately, many species of our hibernating bats are seriously declining due to a disease known as white-nose syndrome (WNS). WNS is an affliction caused by a fungus called Pseudogymnoascus destructans. This fungus was first detected in North America in 2006 and has since spread rapidly throughout the eastern U.S. and Canada. Native to Eurasia, WNS was likely carried to this continent on equipment used by international ‘cavers.’ The fungus develops on the muzzle and wings of bats during hibernation, causing discomfort. This discomfort wakes them from their winter slumber, where they burn precious energy reserves needed to survive the winter. Since the emergence of this disease, some species have declined over 98%. Scientists and biologists are now taking steps to protect winter hibernacula and pursue ways to combat the fungus in an attempt to save these species.

A little brown bat clinging to the cave wall of its winter hibernaculum (Photo courtesy of Penn State under a CC BY-NC-ND 2.0 license).

When one thinks of winter migration, birds are probably the most familiar example. We have all observed flocks of geese and other waterfowl species flying in V-shaped formations overhead as they head south during the fall. Waterfowl species that rely on open water must head south each winter to find unfrozen habitats. Ducks, geese, and tundra swans head toward the Chesapeake Bay itself to feed on marsh grasses, aquatic vegetation, and shellfish during the winter. The snow goose (Anser caerulescens) migration is an annual event that many of us within the Chesapeake region have observed first-hand. Each year, millions of snow geese touch down in the Chesapeake region as they travel from their nesting grounds in the arctic tundra to southerly open waters. Historically, snow geese utilized tidal marshes, wetlands, and rivers around the Bay. In addition to these historic wintering grounds, snow geese now also exploit human-altered landscapes in the Bay region, where open water and agricultural fields provide habitat through winter. As a result, snow goose populations have exploded and are negatively impacting arctic habitats through overgrazing. Birdwatchers and hunters ‘flock’ to the Bay region each spring and fall to experience the snow goose migration. Common snow goose viewing destinations include Middlecreek Wildlife Refuge in Lebanon and Lancaster Counties of Pennsylvania, and Blackwater National Wildlife Refuge in Dorchester County, Maryland.

Migrating snow geese land on a Pennsylvania farm pond (Photo by Jim Kauffman).

Many of our native songbird species also make the trek south for winter. Songbirds classified as ‘neotropical migrants’ only travel to the forests of the Chesapeake to breed and raise young during the spring and summer months. Common neotropical migrants include warblers, tanagers, orioles, and hummingbirds. As these birds’ insect or nectar food sources disappear in fall and winter, they must migrate south to find food in habitats where insects are active year-round. This is a strategy known as ‘resource partitioning.’ Migrant songbirds are able to exploit resources by dividing their time between two ecosystems, ensuring abundant seasonal food supplies. Many of these species migrate thousands of miles, spending their winters in Mexico, the Caribbean, or Central and South America. While these species must migrate, some birds utilize food sources that allow them to remain through the winter. Seed-eating birds like cardinals, blue jays, and chickadees are a common sight at backyard bird feeders during winter. Even some insect-eating birds are winter residents, having developed adaptations to obtain dormant insects through the winter. Woodpeckers, nuthatches, and brown creepers are able to forage for insects by peeling bark, drilling holes, or gleaning dormant insects from trees and snags.

A male northern cardinal perches on a budding maple during the last months of winter (Photo by Steve Droter/Chesapeake Bay Program).

Winter on the Bay

Along with the watershed, the Chesapeake Bay itself also undergoes changes during winter. The water becomes clearer due to a decrease in the abundance of microorganisms like phytoplankton and zooplankton. Tides are often lower, a result of changes in winds and lunar cycles. Salinity levels and temperatures fluctuate, which influences the amount of ice that is formed in the Bay and surrounding tidal habitats. If conditions are right, the Bay can actually freeze over. The last time this occurred was in the winter of 1976-1977, when nearly 85% of the Bay and its surrounding waters were covered by ice.

Plants and animals that reside within the Chesapeake Bay must modify their behavior or develop physiological adaptations to cope with these changes. Rockfish (striped bass) may leave the Bay entirely, migrating south to spend the winter in warmer waters. Oysters, like black bears, utilize nutrient reserves that they have stored in preparation for winter. Their plumpness and nutrient stores make them highly sought-after table fare during this time. Blue crabs head for deeper water where they burrow into the sandy substrate and remain dormant throughout the winter. During this period, fisheries managers conduct dredge surveys to assess blue crab populations throughout the Chesapeake. These surveys help biologists to estimate total population numbers, number of female crabs, and number of number of new young crabs. The results guide managers to make decisions on blue crab seasons, harvest numbers, and size limits.

Fisheries managers measuring a blue crab collected during a winter dredge survey (Photo by Will Parson/Chesapeake Bay Program).

The forests, fields, and estuaries of the Chesapeake Watershed look and feel starkly different in winter. The lush, green deciduous forests of summer have transitioned to drab brown, icy, and snow-covered landscapes. At first glance, it may appear that these landscapes are inhospitable and devoid of life. But the organisms living here have faced the challenge of winter for millennia, developing a diversity of adaptations to persist. As you travel across the bay region this winter, keep your eyes open for winter residents, and try to identify the adaptations they have developed to survive winter in the forests of the Chesapeake.