SNAGS: PART II

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.

Thickened new growth makes better nest platforms

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.

A few Thoughts About Snags Part I

Biological richness of snags and logs

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.

Recent heat waves and droughts have hastened death for some trees.

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).

Up Next: Part II – Structural Diversity

Notes

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