Some years ago, an arborist made a snag from one of our yard trees. Less than a decade has passed, but it is beginning to soften up, attracting he interest of some cavity-nesting birds. The nuthatch in this video spent several long days working on a couple of nest cavities, only to be run off by a northern flicker. After it made some test holes, the flicker decided it wasn’t the perfect place. Meanwhile, it is a favorite perch for the dawn chorus.
Equally interesting in the saga of this big snag, is the cascade of changes initiated by the sudden absence of a large tree. Resources – light is one of the most important – are limited in ecosystems. A large tree taking up air space and light has a big effect on surrounding vegetation. When it is gone, suddenly other trees and plants shift into high gear to take maximum advantage of the newly abundant resource. Birds and other wildlife are quick to make use of the changed environment too.
The tree-that-became-a-snag in our yard was suppressing nearby trees, and since it died, its neighbors have sprouted new growth from dormant buds on their trunks and branches (called epicormic branching). The thick new growth is fine for nest platforms and cover; Steller’s jays (Cyanocitta stelleri) are taking advantage. We watched a pair put together a nest in this limb thick with new growth (see video here). When trees snap off and lose branches naturally, the same thing occurs. This is why disturbances like wind and freeze damage create great habitat.
The tree pictured below was so close to its neighbor, that shaded lower branches died off. But new ones were able to re-sprout when exposed to sun. The tree has thick branching from dormant buds on the limbs, and epicormic buds on the trunk that exist for just this moment of newly available light.
Light gaps are important for regeneration in dense forests, and for diversity of species. This is where seedling trees, shrubs, annuals, and perennials get a start. Their blooms provide nectar for pollinators and food for animals.
During some restoration work here, we had three largish Douglas firs limbed and topped for wildlife snags. We didn’t kill them, because the thinking at the time was that live damaged trees remain standing longer to provide wildlife habitat. Ten years on, these guys do, in fact, still have a lot of life left in them. One study1 found that 23% of non-fatally topped Douglas fir (Pseudotsuga menziesii) were still alive 16-18 years later. Raptors can use them for perching, and the thick growth stimulated by cutting the leader makes a wide nesting area. The height and breadth of a tree provides the structural diversity of vertical space and occasionally cavities, but see below.
Dead trees equal higher biological diversity
I would argue that dead trees might be more useful than live snags. Once dead, biological resources in a live tree’s wood, sequestered during its lifetime, become available to microbes, fungi and arthropods. That is a gateway to creating living space for cavity nesting birds, mammals, bees, and other wildlife.
At first, even a dead tree is still hard and intact. Woodpeckers have to wait awhile before they can really tuck in and excavate nest cavities. Also their prey may not be able to get past a live tree’s defenses, which are many.
After defensive chemicals leach from the dead tree, wood-rotting fungi (ubiquitous in Pacific Northwest forests) move in to soften up the wood and begin the recycling process. One of these pioneers is a mushroom called veiled polypore (Cryptoporus volvatus). Fruiting bodies (mushrooms) appeared on the snag above, soon after it died, indicating that the mycelium had already invaded the sap wood.
This fungus is called a veiled polypore because it forms a pouch or envelope over the pore layer where spores are produced. The Latin name is more descriptive: hidden pores, covered by a sac-like membrane. At first look, one might mistake it for a puffball type mushroom (I did).
Bark beetles and many other insects take up residence inside the moist and nutritious interior. They may be eating or parasitizing each other, or just taking advantage of the warm, moist, and protected space and available mushroom food. Beetles carry spores into the bark when they bore into the sap wood of this or other dead and dying trees. Billions more spores are shed and dispersed via air currents.
This fungus colonizes recently dead or almost dead trees, causing sap rot that softens the wood under the bark. That’s the beginning of an explosion of diversity and nutrient recycling: microorganisms, invertebrates, birds, mammals, and others use the resources built by the tree over its lifetime. Some, like molluscs, newts, frogs, and reptiles, take advantage of the spongy, water-retentive rotting wood and physical shade during the dry season. Others forage for insects, eat algae, or feast on abundant carbon in the wood itself. A large log on the ground even attracts nitrogen, an essential plant nutrient in short supply in the soil.
When the wood is soft enough, primary cavity nesters2 begin to chip off bark and make holes. Secondary cavity nesters follow: squirrels, owls, and others that don’t excavate but need the holes for nesting and protection. Cavities are in short supply in modern landscapes and birdhouses do not replace the complexity and richness of large dead trees.
Whether as a standing snag or a log on the ground, dead and partially dead trees provide long lasting ecosystem benefits. Snags and large downed logs rule, obviously! Let’s keep more of them (looking at you Oregon Dept of Forestry).
Here in western Oregon we are gifted with many habitats and a variety of native plants for most any growing conditions. Wet soil? Try plants that love the ditches like camas, showy milkweed, or goldenrod. Shade? No problem – forests are full of shade loving shrubs and perennial plants. Gaps in the tree canopy and edges along roads or meadows are sunnier and packed with resources that increase habitat resilience and biodiversity.
Marginal land like ditches, roadsides, and rights-of-way might sound useless, but they are anything but that. Just check them and see how mosses, plants and animals take advantage of them. “Marginal” can mean not the greatest, oralongside. Land alongside roads, tracks, fields and power lines make up a tremendous amount of acreage.
Pollinators require nectar and pollen resources, but equally important is the availability of protected overwintering sites like leaf litter, large ferns, and tree cavities. Undisturbed forests, and even timber farms provide these in abundance when managed properly by leaving or adding plants that are often classed as non-timber resources. You might not be able to sell them, but they will enrich your forest in many hidden ways.
Agriculture and development daily gobble up land that was previously uncultivated or unnoticed, and that reduces refugia for native plants and animals. An example: unsprayed roadsides are one of the places milkweed can flourish unmolested to give monarch butterflies a chance to lay eggs and fuel up on nectar. Milkweeds, in fact, are some of the most prolific producers of nectar for lots of other pollinators too.
Oregon grape, vine maple, cascara, Nootka rose, Pacific ninebark, salal, spiraea and ocean spray are all common forest shrubs that provide superior habitat for pollinators. If your forest is lacking in diversity, you can plant these species either in the understory or along forest edges. If you have open spaces, you may even want to consider planting “pollinator patches” of native flowers, such as lupine, meadowfoam, clarkia, selfheal, goldenrod and aster. Many of our common forest plants, and indeed our food supply, may depend on this kind of proactive conservation work!
Kirk Hanson, Forestry Director Northwest Natural Resource Group
One advantage to growing forest-adapted shrubs, wildflowers and trees is early season nectar. The ethereal white flowers of Osoberry (Oemleria cerasiformis) appear against the green forest background as early as January. Tall Oregon grape (Mahonia aquifolium) may accompany red flowering currant (Ribes sanguineum) to dramatic effect. The dense creamy sprays of goatsbeard (Aruncus dioicus) on shady edges attract multiple species of native bees.
Late season nectar is often scarce because our hot, dry summers force most plants to get their reproducing done early. Summer dormancy is a survival strategy that produces yellow leaves on perfectly normal osoberry late in the season. However, goldenrod (Solidago spp. and Euthamia occidentalis), Aster spp, and rabbitbrush (Ericameria sp.) are used extensively by insects late in the season. With a little moisture, some perennials will even bloom a second time.
Pollinators aren’t the only beneficiaries of wise planting. Birds, mammals, and predators all use the fruits, structures, and protection that a diverse forest provides. Lower levels of pests and higher soil moisture and fertility are a few of the by-products for landowners and farmers.
Of course an equally powerful way to protect wildlife and pollinators is to reduce or eliminate agricultural chemicals on your property. Properly using those you need, in small quantities, and with great care with respect to timing of application is essential. If you live in Yamhill County, join the Yamhill Butterfly Gardeners, the Native Plant Society, or get in touch with Taylor Gardens for a consult to explore your options.