What is Happening Outside during your Asheville Winter Hiking
by Jenny Gruhn, PhD, Biologist and Owner of Asheville Hiking Tours
As you are hiking this upcoming winter season in the Blue Ridge Mountains, we hope you take a moment to stop and appreciate the complexity of the cold weather processes happening all around you – even inside of you! There are remarkable changes occurring in our climate, to the land, and within the living things around us. The extreme conditions characterizing the winter season act as a tremendous force in nature, resulting in physiological and behavioral responses in plants and animals which scientists are still trying to understand.
The following is a summary of many seasonal changes you may notice while you are on the trails this winter!
Blue Ridge Mountains Winter Climate
Winter is characterized by a combination of short days and cold temperatures, with the shortest day of the year occurring on the first day of winter. Long nights allow the environment to keep cool throughout the season. It is this change in day length that is the primary trigger for animals and plants to prepare for the winter season. Mammals and birds use their eyes and pineal gland to determine day length and produce melatonin to slow growth. Similarly, plant growth is slowed by the release of hormones from photosynthetic surfaces, such as leaves and stems, in response to day length.
Temperature impacts the growth and development of organisms, which is why metabolic processes slow down for most animals and plants in the winter. Temperature can also trigger cyclical processes such as the leafing out of trees and flower bloom at the start of spring. Since a single warm and spring-like day can occur in the middle of winter, several days of warm weather are often required for these events.
The Southern Appalachians are well-known for their abundant rainfall, but much of the precipitation occurs in the form of snow and ice during winter. The North American region with the most precipitation (east of the Pacific Northwest) is in Transylvania County, North Carolina, which averages 86 inches of rain each year. Clouds move up from the Gulf of Mexico and across the Atlantic Ocean and are trapped by the highest mountain peaks surrounding Asheville. These tall mountains create a sort of fortress, putting Asheville in a “rain shadow,” so that the city does not experience nearly as much rain, snow, or ice each year!
The only substance on Earth that expands when it freezes is water – a property that majorly impacts Appalachian ecology. When water freezes, its volume increases by 9 percent, creating a force of 400 pounds per square inch. This is the pressure equivalent of a great white shark bite! Water is constantly becoming trapped between the cracks in even hard ground (you can imagine these large cracks when you envision a roadcut). Then, when groundwater freezes, rocks and boulders are pushed off the mountainside, causing massive erosion. During our winters, and over millions of years, the freezing and thawing process of water has been bursting boulders out of the ground, a phenomenon known as the freeze-thaw cycle.
FREEZING WATER BODIES
Since ice is less dense and lighter than water, it also turns into a floating ice cube on our ponds and sometimes rivers. The ice on a frozen pond insulates the underlying water, protecting animal and plant life from subfreezing temperatures.
As you are hiking this winter, particularly on wetter mountainsides, you may find short shards of ice seeming to poke out of the ground – and these crystalline structures can become rather long! As groundwater seeps out of the ground, hairlike fibers of water ooze out and freeze, creating remarkable shapes resembling ribbons or flowers.
Perhaps the most important impact of water freezing is the one on the cellular tissues of animals and plants. Frostbite is the process of cells bursting and dying when the water within them freezes. Many winter adaptations are to prevent frostbite from occurring! For example, deciduous trees lose their leaves each winter because the tissues inside them would otherwise die. The reason evergreen leaves can persist on the pines, firs, and spruces of our mountains is due to resin. Resin is the smelly, sugary substance that acts as antifreeze for pine needles!
While liquid water supports life, frozen water does not! Cold winters are a primary reason why the Blue Ridge Mountains are not as diverse as tropical ecosystems. In the Appalachian Mountains, we relinquish some biodiversity in exchange for the beautiful snow and ice-laden trees of winter.
Where do the animals go?
With changes in day length, many animals are triggered to either migrate or hibernate!
The only mammal in the Appalachians that truly hibernates is the groundhog, otherwise known as the woodchuck. Its heartbeat drops from 100 beats per minute to just 4, it takes a single breath every 3 to 4 minutes, and its body temperature decreases from 98° to 40° F. Hibernation in this species occurs from early November to early March. Whole families may live inside the winter den, which can be composed of several chambers with several exits for emergency use.
Black bears do not truly hibernate, since their metabolism and body temperature do not drop as dramatically, and since they can be easily awakened. Nonetheless, black bears do not defecate or urinate during the slow winter season, and have the surprising capacity to raise and nurse young during their sleepy state (called torpor). Researchers are still trying to understand the many physiological phenomena of bears during the winter!
One might think that bats would migrate to survive winter, but few bat species actually do! The flutter of bat flight isn’t nearly as strong as that in birds, and echolocation would function poorly over long migratory distances. Most bats reach a hibernation-like state for 5 or 6 months in caves, where their temperatures drop to that of the cave and their heart rates drop to a few beats per minute. Bats emerge from their caves in the spring, when insects are available as food once more.
Reptiles and amphibians have bodies that passively track their environment, adapting their internal body temperatures to match, so they do not truly hibernate. Snakes spend the winter in dens, turtles bury themselves in pond bottoms, and most frogs and salamanders spend the winter underground. Not many insects can stay active through winter – some survive winter in a juvenile form, such as nymphs or larvae, and many hibernate underground or in ponds.
ANIMALS TOUGHING IT OUT
Most mammals are simply too small to hibernate. High metabolisms and little room for fat storage mean that animals such as shrews, moles, mice, and flying squirrels must eat rather constantly through the winter. Eastern chipmunks spend short amounts of time in a deep sleep, awakening to raid their winter caches of nuts. Other creatures have learned to survive winter by raiding human-related sources of food, such as trash and bird feeders! These mammals that are active year-round include raccoons, skunks, and opossums.
Some animals travel seasonally across long distances and to different habitats – a process called migration. In the Blue Ridge Mountains, many bird species and a few bat and insect species migrate south to experience a warmer winter with a more abundant food supply.
Many birds follow food sources south, including hummingbirds, thrushes, warblers, and nightjars. Other birds, known as “generalists,” eat a variety of foods, such as both insects and seeds, and have an adequate supply to overwinter in the Blue Ridge. These common generalists, often called backyard birds, include chickadees, woodpeckers, sparrows, tufted titmice, crows, and northern cardinals. Many of these species occur in flocks over winter, in order to form greater protection from predators. Feeding on mostly smaller active rodents, owl species are also year-round residents. Some hawk species migrate while others stay as winter residents. In yet another case, many hawk species as well as turkey vultures have northern individuals who migrate south, and southern individuals who remain in the Blue Ridge through the winter!
While almost all insects have evolved complex life cycles or adaptations to winter, the monarch butterfly is one remarkable example of long-range insect migration. These fragile and colorful creatures travel thousands of miles from the Appalachians to the mountains of Mexico, in the eastern part of their range. After a journey that requires a few weeks of travel, monarchs will land in Mexico and huddle with hundreds to thousands of others, covering trees and slowing their metabolisms for 6 months. On the return journey to the Appalachians, several generations of monarchs are required to reach the northern limits of their range. Each generation stops to feed strictly on milkweed plants, which provide poisonous compounds that ward off predators.
The Human Response to Winter
People’s behaviors have been shaped by winter so much that we have lost the physiological responses to winter that other animals still require today. Just imagine: humans have used fire for at least 300,000 years to regulate temperature. Now, as autumn temperatures blend into the wintry cold, humans make use of many modern conveniences, such as putting on a wool coat, turning up the thermostat, or sipping on hot chocolate.
Our behavioral modifications mean that we have lost physical adaptations – such as thick, wooly body hair! Nonetheless, we humans still have the capacity for some physiological changes when clothing or heat sources aren’t adequate. In order to prevent heat loss, our blood vessels that run close to the skin narrow causing blood pressure to rise, a phenomenon more regularly seen in those people living in cold-weather climates. To replace heat loss, our metabolisms increase primarily through muscle contractions, otherwise known as shivering.
Living Things Experience Cycles
Of course, our explanations here on the changes of winter are just some of the many natural cycles evident in biological processes. Living creatures undergo numerous overlapping cycles that impact their physiology. To name a few:
Circadian rhythms, which change as we grow from juveniles to adults
Reproductive cycles, which in plants include springtime pollination and fertilization
Growth cycles, which can be days or years long depending on the event and the species
Communication cycles, which can be noted in birdsong each morning
Conservation of Winter
Our current anthropogenic warming trend is already impacting the winter season, and research studies are just beginning to scratch the surface of what might ensue with climate change. With a future climate defined by warmer temperatures and less precipitation, we can imagine that the iconic characteristics of winter may disappear in most if not all places across Earth.
We may find some strange solace in knowing that Earth’s climate has changed drastically many times over the past 100 million years. We know that tropical climates occurred towards the North Pole a hundred million years ago. We are still finding fossils of crocodiles and palm trees in the northernmost parts of our planet. The average lifespan of a given species is 5-10 million years. If you take into account that multicellular organisms have been on Earth for 600 million years, then the current composition of Earth’s species makes up 1 to 2% of all that ever lived!
Nevertheless, this warming trend is faster than any the Earth has experienced before, meaning animal and plant life has significantly less time to move to a cooler location (the movement of immobile species of plants occurs in the succeeding generation) and less time to adapt (change within one lifetime is called phenotypic plasticity, but in the next generation is adaptation). Even humans will have to move or adapt, and are already doing so! If an entire species cannot move or adapt in time, it becomes extinct.
We are at a critical point in the trajectory of animal and plant life on our planet. Will we move to a carbon-neutral Earth? Can we stop our detrimental impact on climate? Will we do it fast enough to protect winter as we know it?
Looking to Spring
We can all relate to the late winter changes as we anticipate spring and enjoy the reemerging life; the black bears crawling out of their dens and ephemeral flowers surfacing from the forest floor. Our spring series of Wildflower and Birding Walks are some of our favorite tours all year! Stay tuned for a related blog post as we anticipate these lovely springtime changes.
Are you excited to learn more, or want to experience and witness nature’s beauty for yourself? We are a group of skilled naturalists who guide scenic tours every day in the Blue Ridge Mountains, with both group or private tour options. Book a tour below! We hope to see you on the trails!
Frick-Ruppert, Jennifer. Mountain Nature: A Seasonal Natural History of the Southern Appalachians. 2010.
Lovei, Gabor L. Modern Examples of Extinctions. Encyclopedia of Biodiversity. 2007.
Marriott, B.M., Carlson, S.J., editors. Institute of Medicine (US) Committee on Military Nutrition Research. Washington, DC National Academies Press (US); 1996.
Natural Notes is Asheville Hiking Tours’ blog about nature, history and travel in the Appalachians. Asheville Hiking Tours offers day hikes, waterfall tours, and firefly tours, guided by naturalists, in the mountains around Asheville, NC. For more info visit www.AshevilleHikingTours.com.