By Lorelle Sherman, OSU Extension Forester

Oregon’s forests can look quiet in winter. Leaves are gone, wildflowers have faded, and many animals are harder to spot or have left on a migration to a warmer climate. This is because winter is a demanding season for wildlife. Food is scarcer, temperatures are lower, and shelter becomes essential. Life in forests does not stop when growth slows, though. It simply shifts, unfolding quietly in cavities, under bark, and within rotting wood.

Wildlife interact with dead wood at micro and macro scales, both in time and space. Microscopic fungal mycelium infiltrate stressed and dying trees, softening the wood just enough to become a nest site for the impressively large Pileated Woodpecker. Tiny insect larvae, or grubs, hibernate deep within rotting logs only to become a snack for the forest’s connoisseur of dead wood, black bears. These processes sometimes play out in days and sometimes in years.

So what is dead wood and why do we need it?
Dead wood describes both snags (standing dead trees) and downed wood (fallen logs and branches). Research from Oregon and across the Pacific Northwest shows that dead wood forms a complex, long-lasting ecological network that unfolds over decades or even centuries, depending on tree species, size, and environmental conditions. Dead wood is not static, but is a living, evolving habitat. Once a tree dies, decomposition is driven by fungi, insects, bacteria, and other organisms. Early stages of decomposition are dominated by insects and fungi that specialize in fresh wood, while later stages support mosses, plants, and microbes that contribute directly to soil formation.

Dead wood is sometimes seen as a nuisance – a wasted resource, tripping hazard, or even a fire hazard. Fortunately, the dead wood most important for wildlife is large, and large wood typically poses less fire risk than finer fuels. While dead wood can influence how fire moves through the landscape, it also plays a key role in shaping wildlife habitat.

Why do wildlife need dead wood?
Historically, undisturbed westside Oregon forests held ~10–18 snags and 50–140 down logs per acre (USDA DecAID). An abundance of deadwood in the environment led to numerous associations with wildlife. Over 40% of wildlife species in the Pacific Northwest forests interact with deadwood in some way. Mark Harmon, forest ecologist at Oregon State University, found that a single snag can have over 12,000 beetle galleries and hundreds of species of other associated organisms, including fungi, mites, nematodes, protozoa, and bacteria. Estimates say between 30-40 species of birds in the Pacific northwest depend on snags for nesting and feeding. Small rodents, like northern flying squirrels, and bats are also known to nest in cavities with bats sometimes also roosting under slabs of loose bark.

Pacific martens, fishers, black bears, and raccoons, too, use cavities (a hollow area or a hole in a natural structure). Cavities excavated by woodpeckers are too small for these creatures, so they rely on large cavities formed by decay or fire. These cavities are becoming increasingly more uncommon because old, large trees are now uncommon.

Why do wildlife need dead wood in the winter?
Reptiles and amphibians are ectothermic, meaning they rely on external sources to regulate their body temperature. So, utilizing the insulation of a hollow log, root hollow, rotting log, or slab of dead bark is essential to survival year-round, but especially during the winter months. During cold spells, snakes and lizards will slow their breathing and metabolic rates down (also known as brumation) during the dark days only leaving their insulated safe space on sunny days to bask on top of rocks or downed logs. Salamanders and frogs also brumate, but they don’t often bask as their permeable skin makes them highly susceptible to dehydration. Two salamanders that particularly depend on large downed wood are clouded salamanders and western red-backed salamanders.

Small-bodied wildlife that generate their own internal heat (endothermic) still rely on dead wood for insulation. Mice, shrews, chipmunks, and squirrels will all find cavities or burrow under dead wood to get through the coldest temperatures. There are differences in dependence on dead wood by species, though. Small mammals that eat insects benefit even more from large downed wood as rotting logs can serve as a buffet for food in addition to protective habitat. Insect-eating species like the Pacific jumping and shrew-mole show a strong preference for sites with an abundance of large downed wood when compared with other small mammals (Maguire 2002). On the other hand, red tree voles don’t appear to associate with dead wood because they are canopy-dwellers and rarely come to the ground.

In the Pacific Northwest, most birds start nesting in the spring as warmer temperatures hint at the summer to come. Raptors (not the Jurassic Park kind… the owls, eagles and hawks kind) start courting their mate and selecting nest sites in the dead of winter. Great horned owls will even lay their eggs from January through March. Great horned owls will utilize broken or dead tops of large trees as well as large snag cavities. In fact, many of our owl species nest in snag cavities and will also utilize wooden boxes made by humans to replicate natural cavities.

How can I increase beneficial deadwood for wildlife on my property?
Incorporating dead wood into managed forests does not mean you have to sacrifice safety or productivity. Instead, it can involve thoughtful retention of a diversity of dead wood types—different sizes, tree species, and stages of decay. This structural diversity supports a wider range of species and ecological functions.

Landowners and managers can support wildlife and forest health by:

• Retaining snags where safe and feasible… the larger in diameter and height, the better!
• Where snags are not safe or feasible, like near your home or other structures, installing wooden nesting boxes to mimic cavities (different species require different sizes)
• Leaving large downed logs after harvest
• Creating habitat piles from logging slash

In winter especially, snags and fallen logs remind us that dead wood and decay is not a sign of neglect, but a fundamental part of how forests function. The next time you walk through a winter forest, pause at a standing snag or a large downed log and take a look around. What wildlife tracks do you see in the snow?

Read more about forest ecologist Mark Harmon and his work with dead wood.

By Lorelle Sherman, OSU Extension Forester

It’s mushroom season! If you’re like me, you’ve been out in the woods searching for edible wild mushrooms at every free moment. While mushrooms can fruit year-round, fall is the undisputed season of fungal abundance. And we aren’t the only ones who notice. Many forest creatures take advantage of this seasonal bounty. Mushroom hunters often see telltale signs of wildlife nibbling, scratching, or taking generous bites out of mushrooms.

The consumption of fungi is called mycophagy, and animals that eat fungi are known as fungivores. Though mushrooms are mostly water (70–90%), they also contain proteins, amino acids, vitamins, and minerals. For some wildlife, fungi are a year-round dietary staple; for others, they’re a seasonal snack that provides essential nutrients when other foods are scarce or low in quality.

Let’s take a look at some of the wildlife that share our taste for mushrooms.

Rodents
Rodents are gnawing animals and have large, chisel-like incisors which often leave teeth marks on mushroom caps. They don’t usually consume an entire mushroom fruiting body, but will target the gills where protein-dense spores increase the caloric value. Structural parts of a mushroom, like the stipe, are mostly made of chitin and water which lacks the same nutrition. Chipmunks and squirrels (family Sciuridae) consume a remarkable variety of fungi. For some species, fungi make up over 70% of their yearly diet by volume.

In northern latitudes, squirrels even harvest and dry mushrooms for later use. I was elated to witness this in the Yukon Territories this past summer. After noticing one odd mushroom hanging in a tree, I started seeing dozens of mushrooms decorating trees like ornaments. Nature’s dehydrator! Once the mushrooms are dried out, the squirrels collect them for storage through the winter. Drying concentrates food value; fresh mushrooms contain only 0.1–0.7 kcal per gram, compared to 4–6 kcal per gram when dried. (Curious about truffle-loving rodents? Check out this blog post.)

Bears
Black bears are omnivorous and opportunistic, meaning they will eat whatever is available. Black bears in the Pacific Northwest are known to eat truffles (hypogeous) and mushrooms that fruit above ground (epigeous). In fact, I’ve seen clear signs of black bears chowing down on Matsutake (Tricholoma murrillianum) right along the coast. Historical accounts from coastal Washington describe bears as heavily feeding on fungi (Poelker and Hartwell 1973).

Grizzly Bears in the Rockies obtain most of their energy from the seeds of whitebark pine (Pinus albicaulis), or from bison or elk carrion. In years when whitebark pine seed availability is low, grizzly bears will consume more mushrooms to fill the protein deficit. With whitebark pine now listed as endangered, fungi may become an increasingly important food source for these bears.

Birds
While mammals get most of the attention, some birds are also mycophagists. Turkeys and grouse scratch through leaf litter in search of food and often encounter fungi. They are opportunistic feeders which will consume whatever they uncover. Studies have found truffles in wild turkey (Meleagris gallopavo) stomachs and identified 31 fungal taxa in the digestive tracts of spruce grouse (Falcipennis canadensis). As any mushroom hunter will know, there are many species of mushrooms that readily become infested with insect larvae. Crows and jays tear apart mushrooms to reach the insect larvae inside, while gray jays (Perisoreus canadensis) sometimes eat fungi and slime molds directly.

Birds secondarily consume fungi by eating fungus-loving insects. There are also raptors that love to eat mushroom-loving rodents. Birds are likely acting as indirect fungal spore dispersers by consuming other creatures that eat fungi.

Some birds that that nest in cavities in standing dead trees, like woodpeckers, rely on wood decay fungi to soften the wood enough to facilitate cavity excavation. In turn, spores from the wood decay fungi get trapped in their feathers and are dispersed to new trees as they fly around. This act of fungal spore dispersion facilitates future cavity excavation for themselves or other woodpeckers.

Slugs & Snails
Slugs are true mycophagists, fungivores, mushroom lovers. Their feeding damage is easily recognized by their mining habits which sometimes create holes through the cap. You can often see the remnants of slime trails on or around the mushroom. Sometimes slugs eat the mushroom entirely and all that’s left are slime trails. The Pacific Northwest’s banana slug (Ariolimax columbianus) especially favors boletes, oyster mushrooms, and Agaricus species. Slugs play an ecological role, too. They act as small-distance dispersers of truffle spores and actually stimulate mycorrhizal growth between truffles and tree roots when fungal spores germinate in their guts.

Slugs aren’t the only Mollusk that feed on mushrooms. Snails are less studied, but community science platforms like iNaturalist have revealed many records of snail mycophagy.

Turtles
Box turtles are opportunistic feeders and will feed on whatever is available and abundant, including mushrooms. Instead of teeth, turtles have beaks made out of keratin. The beak shape varies by species depending on their diet. Box turtles, like the one in the photo, have triangular beaks that leave a very distinctive triangular bite mark in the mushroom. A series of these bites can leave a crown-like pattern—an unmistakable sign of turtle dining.

Ungulates (Deer & Elk)
Deer and elk often encounter mushrooms while foraging, and in some regions and seasons they actively seek them out. Fruiting bodies of mycorrhizal fungi such as Russula, Lactarius, Amanita, and Boletus species have been documented in their diets. These fungi are rich in protein, minerals, and moisture, which can be especially beneficial during dry summer months when forage quality declines. Fungal consumption also supplies micronutrients such as phosphorus, potassium, and trace elements that may be limited in plant tissues. Fungi generally have higher protein by volume than plant tissue and are easier to digest, making them a high-quality food source for ungulates. One study noted that elk always consumed mushrooms when encountered and sometimes had a taste for leafy lichens as well.

Despite its ecological importance, fungal foraging by wildlife remains under-studied. Seasonal availability, habitat type, and individual preference all influence how much fungi contribute to diet. But diet study methods often miss fungal remnants because soft mushroom tissue readily breaks down in the gut. Fungal spores are often missed when researchers use sieves to sort diet contents that are too large and allow the spores to pass through. Researchers are now using techniques such as DNA metabarcoding of scat samples to improve understanding of the complex relationships between fungi, wildlife, and forest ecosystems.

Next time you’re out mushroom hunting, look closely… those nibbled edges, bite marks, or slime trails might tell you who got there first. Who ate your mushroom?

By Lorelle Sherman, OSU Extension Forester

Imagine driving in the Cascades through a sea of orange butterfly wings. Hikers, backpackers, and folks living in Central Oregon have experienced an exceptionally large emergence – or irruption – of California tortoiseshell (Nymphalis californica) butterflies this summer. Although they look similar to the monarch (Danaus plexippus) butterfly, California tortoiseshells lack the famous ‘stained glass’ wing pattern and spotted body of monarchs.

Life Cycle on Ceanothus
All butterflies share similar life cycles where the young (also called larva or caterpillar) look very different from the adults. Butterflies go through complete metamorphosis through four stages starting with an egg, then larva, pupa, and adult. The illustration below starts with the very small eggs laid by the adult female butterfly. These eggs hatch into larva, which are called caterpillars when pertaining to butterflies and moths. Number one on the to do list of a caterpillar is to eat, eat, eat! Caterpillars grow rapidly, shedding their skin as they fill out their exoskeleton. Once they are fully grown, they attach themselves to and hang from the underside of a leaf or branch, eventually turning into a pupa. Inside the pupa is where the magic happens. Special cells in the larva allow it to transform into an adult butterfly, whose job is to mate and lay eggs, restarting the cycle.

The California tortoiseshell is strongly associated with plants in the Ceanothus genus, also known by many common names including: buckbrush, snowbrush, California lilac, and blue blossom. The adult lays its eggs on Ceanothus species, because this is the preferred food source of its caterpillars. Once hatched, the caterpillars can and will entirely defoliate large swaths of Ceanothus, especially in irruption years. If you’ve ever hiked in the Central Oregon Cascades, you know there is plenty of Ceanothus to go around. In the case of the California tortoiseshell, caterpillars will attach themselves under small branches of surrounding conifers or really anything that shelters them (Nymphalis chrysalis 2021.jpg).

Parasitoid Pressure
Parasitoid wasps are predatory insects who sometimes lay their eggs inside the chrysalid of a still forming butterfly. When the wasp eggs hatch, they feed on the pupa and eventually hatch out of the chrysalid. While this may seem gruesome, these native wasps are an integral part of the ecosystem. Parasitoid wasps are usually very small, but I happened to catch a larger species injecting its eggs into a California tortoiseshell chrysalid in Central Oregon. Irruption years likely help butterflies overcome predatory pressures like this one.

A Mountain Migration
While Monarch butterflies migrate thousands of miles each year to and from Mexico, the California tortoiseshell remains in the California and Pacific Northwest mountain ranges year round. Some individuals expand northward from California and Southwest Oregon in the spring, returning to warmer climates in the fall. Other individuals only move up in elevation in the spring and down in elevation in the fall to hibernate in the Oregon Cascades. The size and success of migration depends on environmental conditions and can range from a trickle of butterflies throughout the summer to a flood of butterflies which has, in past years, temporarily shut down mountain roads.

Summer adults can emerge in massive numbers during irruption years. Researchers don’t fully understand why irruption years happen when they do. Sometimes irruptions happen several years in a row and sometimes they happen with many years in between. What we do know is that several irruption years in a row leads to complete defoliation of the butterfly’s food source subsequently resulting in a population collapse of the California tortoiseshell. The boom and bust cycle may seem risky, but it’s worked well for this species which continues to be our most common irruption species in the Pacific Northwest.

Butterflies in Decline (and what you can do)
A recent study (Edwards et al. 2025) found an overall decrease in butterfly abundance across the contiguous United States at a rate of 1.3% annually. When they looked at the Pacific Northwest alone they found stability in butterfly populations overall. However, more species are declining than species increasing and measured population increases were likely driven by the irruption years of species like the California tortoiseshell.

Why the decline? The global decline of all flying insects is likely attributed to pesticide use, habitat loss, and climate change. Insecticides are a leading cause of butterfly decline in many regions of the U.S. If you’re looking to create or enhance habitat for butterflies in your backyard, skip the insecticides and use a little elbow grease instead. Plant native plants which serve as the food plants for native butterfly larvae. Butterflies are one of few taxa that respond well to small-scale changes to the landscape… even one backyard can make a difference!

So, the next time you’re driving mountain highways, and the air seems alive with orange wings, slow down and take a moment to witness a dramatic and beautiful butterfly story.

Resources:
Butterfly host plants western Oregon:
Native Plants for Pollinators & Beneficial Insects – Martime Northwest
Native Butterfly Host Plants for the Willamette Valley (a blog post)
Butterfly host plants central/eastern Oregon:
https://www.deschuteslandtrust.org/news/blog/2022-blog-posts/butterflies-and-host-plants
Native Plants for Pollinators & Beneficial Insects – Inland Northwest

Cited Source:
Rapid butterfly declines across the United States during the 21st century

By Lorelle Sherman, OSU Extension Forester

If you’ve noticed dead twigs and small branches scattered throughout the canopy of your Oregon white oak, especially near the tips of branches, you’re seeing what’s known as branch flagging. This phenomenon is noticeably more severe in certain years and in certain locations and is currently causing alarm throughout the mid-Willamette Valley.

This story starts with an insect smaller than a grain of rice and a squirrel with a taste for larval protein.

Gall wasps and their galls

Gall wasps (family Cynipidae) are highly specialized insects associated with gall formation on oak trees. The adult gall wasp lays its eggs into the tissue of an oak twig, leaf, or bud, where the egg will hatch into a hungry larva. Larval feeding on the plant tissue triggers swelling of the plant tissue. The result is a gall — a swelling or growth made from the tree’s own tissue, which forms a protective chamber for the developing larvae. Eventually the larva will pupate into an adult which exits through the gall to start the cycle over again.

Galls can be variable in size, structurally complex, and species-specific, often serving as microhabitats for other insects. Oaks, including our Oregon white oak, are common hosts to gall wasps and other gall-forming organisms. While most galls are harmless, repeated or clustered twig galls can girdle small branches, effectively cutting off the flow of water and nutrients. Over time, this causes localized dieback — branch flagging — that’s particularly noticeable in the summer.

The species at play in our Oregon white oaks is the oak twig gall wasp (Bassettia ligni). The Oregon white oak has evolved with gall-making insects like the oak twig gall wasp and is quite adapted to this cycle of crown thinning. Branch flagging is cosmetic and oaks will rebound from it in subsequent years. However, it is unclear how changing environmental factors like hotter and more frequent droughts will interact with the oak twig gall wasp.

Western gray squirrel and Oregon white oak

Our native Western gray squirrel (Sciurus griseus) is strongly associated with oak woodlands in western Oregon. Western gray squirrel diet includes acorns, catkins, nuts, berries, aphids, fungi, insect larvae, and other protein-rich plant materials. Oregon white oak is the only acorn-producing species across much of the Willamette Valley and it has highly variable acorn production from year to year. Oregon white oaks will produce an incredibly abundant acorn crop in one year – known as a mast year – and hardly produce any for several years after. This is a major limitation on oak-associated wildlife who depend on acorns as a food source. During mast years, squirrel populations will increase due to the abundant protein-rich food source. Failure of Oregon white oak to produce acorns can have serious impacts on squirrel populations.

If you have Oregon white oak on your property, you may have noticed an incredible abundance of acorns two years ago, which means we had a mast year that may have supported an increase in squirrel populations.

All these squirrels gotta eat!

Western gray squirrels discovered long ago that certain galls house protein-rich wasp larvae. In the Willamette Valley, this is the oak twig gall wasp! To access this hidden food source, they strip bark from infested twigs, removing cambium tissue in the process. This debarking behavior can compound damage caused by the galls themselves. It increases branch mortality and may contribute to more severe branch flagging in some trees.

Should we be concerned?

Despite the visible damage, Oregon white oaks (Quercus garryana) are remarkably resilient. Branch flagging due to gall formation and squirrel foraging is a natural and cyclical process that these trees have evolved with over millennia. Whether it’s an increase in squirrel populations, or an increase in gall wasps due to a mild La Nina event last year, we can be sure of two things. First, branch flagging of Oregon White Oaks in the mid-Willamette Valley is more severe than usual this year. Second, branch flagging is an eye sore (not a sign of mortality) that can be dealt with by calling an arborist to remove dead branches if desired.

ODF resource on oak pests: https://www.oregon.gov/odf/documents/forestbenefits/oak-pests.pdf

Sources

• Sciurus griseus
• Oak Gall Wasp – PNW Pest Management Handbook
• Oregon White Oak in the Willamette Valley… some in your face issues