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).
There are two birds whose presence, to me, would signify that we have attracted the holy grail of bird residents: the Acorn Woodpecker and White-breasted Nuthatch.
I looked out the window, and LO and BEHOLD! There was a White-Breasted Nuthatch!!
What these species have in common is their fidelity to, or requirement for, a vanishing Willamette Valley habitat – open oak woodland and savanna. The nuthatch will go for mixed conifer/hardwoods which is exactly what we have. They are not the only species of concern here, just a couple I particularly like. A brand new publication online in two parts [ here and here ] called the Land Manager’s Guide To Bird Habitat and Populationsin Oak Ecosystems of the Pacific Northwest provides a larger list:
Oak-associated bird species designated as being of conservation concern by the primary wildlife natural resource agencies in the Pacific Northwest…
Vesper Sparrow (Oregon)
As I noted, to my delight, a White-breasted Nuthatch arrived recently at our feeder and on the trees by our house. Because I don’t currently have the proper camera or skills, here is a nice photo swiped from the above publication.
White-breasted Nuthatch. Photo credit Tom Grey. Published in Landowner’s Guide to Bird Habitat and Populations in Oak Ecosystems of the PNW. 2012
Our species – the slender-billed subspecies – resides west of the Cascades. Fat white belly, an impression of upcurved bill because the lower bill curves toward the straight, sharp upper; used for hacking or ‘hatching’ open nuts and seeds it wedges in tree bark – a fun fact learned from my new Christmas book, the Sibley Guide to Bird Behavior. Oh, so cute and spunky. I hope they find lots of holes in our old trees to make nests.