Picture yourself walking through a deep, dark coniferous forest just before nightfall. Trees tower overhead, their branches draped with lichen. Moss covers toppled trunks. An owl calls in the distance, a twig snaps somewhere nearby, and the faint sound of paws skittering across the forest floor sends a chill down your spine. You start to get the feeling you’re not alone.
Mature and old-growth forests replete with large, old trees, snags, and logs aren’t just magical places filled with creepy creatures and forest friends alike. Older forests play an important role in the cycling of water. They also absorb and store huge amounts of carbon that help mitigate impacts from climate change. It’s no surprise that wildlife loves them, too. These forests can host many species – often in greater numbers than other forest types – in part because tall trees with thick, furrowed bark, multi-layered canopies, and an abundance of fallen logs and leaf litter create more niches for organisms to live in close proximity.
As trees age, damage from insects, diseases, and windstorms creates cavities. Creatures of all types use these hideaways to evade predators, seek shelter from rain or snow, or build nests to raise their young. Standing dead trees, or snags, also provide valuable habitat for cavity-nesting birds, mammals, and insects.
Creepy tree cavities like this provide vital wildlife habitat. Photo credit: Portia Bharath
Older forests can also provide key habitat for species that feed on dead or decaying materials (known as “detritivores” and “saprotrophs”). These can include arthropods, fungi, and mollusks. (That’s right – mollusks live in forests, too: this group of organisms includes terrestrial snails and slugs!)
To some, snags and fallen logs might seem like “waste,” but we know that many species rely on these forest features. Increasingly, foresters and land managers will intentionally leave behind large snags or downed trees to provide habitat and to encourage other species – including seed-dispersing birds and mammals – to visit areas after timber harvest or extreme forest fires.
The U.S. Forest Service is currently considering a framework to ensure that mature and old-growth forests can survive and thrive even in the midst of our changing climate. That’s important not only for these forest creatures but for all of us humans who depend on the clean air, water, and climate benefits that forests provide.
Some of the organisms that live under logs or hide in holes in trees are misunderstood. Click on the photos below to learn more about six species who thrive in mature and old-growth forests across the country – and learn why they’re our friends, not foes. Each of them plays an important role in their ecosystem.
Giant American millipede (Narceus americanus) – Though found in a variety of habitats, the giant American millipede thrives under old logs and among leaf litter. When hatched, the millipede has three pairs of legs. As they molt, they add new pairs of legs, so adults may have dozens or even hundreds of legs. They don’t bite, but they do use their strong mouthparts to break decaying matter such as wood and roots into bite-sized pieces. As detritivores, they play an important role in moving nutrients through ecosystems. Photo credit: Micha L. Rieser
Northern saw-whet owl (Aegolius acadicus) – These small owls often forage in the open understory of mature forests. They also nest in cavities in trees or snags, which makes them vulnerable to the impacts of deforestation and logging. Northern saw-whet owls feed on small mammals and arthropods, and can play a role in keeping pest populations in check. Perched on branches, these nocturnal birds use their sharp eyesight and sensitive ears to locate prey. Photo credit: Andy Witchger
Pacific banana slug (Ariolimax columbianus) – One of the largest slugs in the world, growing up to 10 inches in length and vibrant yellow in color, the Pacific banana slug’s name is no mystery. This species is an iconic inhabitant of misty temperate rainforests from California north to British Columbia. Their slimy skin helps them to breathe and move, but it also repels predators. These organisms serve a key role in ecosystems, supporting nutrient cycling by eating decaying matter. They’re also a tasty snack for other organisms, including snakes, raccoons, and salamanders. Photo credit: Trougnouf
Wolf spiders (Family Lycosidae, such as the Hogna genus in the U.S.) – Although by no means limited to wooded areas, coming upon one of these sizable spiders on the forest floor might startle you. Unlike many spiders, wolf spiders don’t spin webs to catch their prey. Instead, using their keen eyesight, these agile hunters skillfully stalk and pursue their prey. (Don’t worry: wolf spiders won’t bite unless unprovoked, and their bites aren’t dangerous to humans.) They have another unique habit: after eggs hatch, a female wolf spider will carry her young on her back. The dozens of spiderlings will stay with her for days or weeks until they’re ready to head out on their own. Photo credit: Thomas Shahan
Old man’s beard lichen (Usnea longissima) – Sometimes mistaken for moss, a lichen is not a single organism, but rather a partnership (or symbiosis, from Greek, “together” + “living”) between algae or cyanobacteria and fungi. While it may add to the spooky feeling of a forest as it blows in the breeze, the presence of lichen can be an indicator of good air quality, as they are very sensitive to pollutants. Old man’s beard used to be common in the U.S., but it is now much more scarce, due to air pollution, land conversion, and forest degradation.
