"Nature Guide Journal"

21 February 2002

The significant wind event we experienced two weeks ago was impossible to miss. The tree damage from that wind is still impossible to miss.

In the most basic terms, wind is caused by differences in atmospheric temperature. As anyone who has climbed a ladder to change a light bulb or paint a ceiling knows, warm air rises. In the broadest view, the air above the sun-warmed equatorial region of our planet heats up and rises; so, too, does the air above sun-warmed land masses at middle-latitudes. That rise begins a domino-effect, as cooler air moves in to replace the risen warm air. Wind is the atmosphere leveling out.

Add the effects of the planet's spin, friction with the surface and with other winds, and smaller-area differences in heating and cooling, and the result is an interactive, three-dimensional complex of air movement. Further, conflicts between moving air masses, and between those masses and the topography, result in various levels of turbulence in the air flow. Planet-wide, most air flow is horizontal, parallel to the surface.

The sudden storm on 7 February 2002 appeared to have caused damage more severe than expected for the average wind speed of that storm. While broken limbs and uprooted trees are fairly common effects of our strongest winter storms, this storm seemed to have a surprisingly high proportion of trees that were snapped off high above the ground.

According to an on-line report written by George Taylor, Oregon State Climatologist, Thursday's intense storm was apparently turbulent enough to generate "downbursts." Strong, local downdrafts, the downbursts flashing through that storm are estimated to have suddenly whipped the air at speeds up to 90 miles/hour. "Microbursts," what we probably experienced, are downbursts of short distance (less than 2.5 miles) and short duration (5 to 15 minutes) .

Most often associated with tornadoes and tornado-generating weather, downbursts are a common cause of bumpy plane landings and (the rare) plane crashes. Like tornadoes, downbursts may produce a roaring sound. Tornadoes tend to lay trees down in a rotation pattern along a narrow track, while downbursts tend to lay trees down without rotating them and along a broader track.

Although the trees splintered by the microbursts may be painful for us to observe—and may even have caused damage to our built property, they have some consequences we do appreciate.

The raw, twisted wood is now exposed for easy invasion by fungi and pioneering insects, which will begin to penetrate and break down the wood fiber. Those pioneers will release some of the nutrients stored in the wood, as well as soften it for further action by more plants and animals.

As the process of rot continues, woodpeckers, especially the chiseling pileated woodpecker, will excavate large nesting cavities that are used later by other birds and mammals—such as owls, flying squirrels, and bats.

The fallen tree tops and large limbs will be recycled by successive waves of pioneering insects, fungi, plants, followed by more animals. The later arrivals take advantage of the work of the early arriving animals and plants, with many later arrivals feeding on the pioneers. Finally, the eventual remains of the greenery and wood will condition the soil.

Large conifers that have been snapped off near the top but still have living branches are, in the right locations, favored nest-building sites for bald eagles. (Osprey prefer building nests on the top of completely dead snags.)

The wads of roots pulled out of the ground, and the holes left behind, will provide new homes for small mammals, amphibians, and reptiles.

In due course, the windfall from that storm's microbursts will provide noticeable nutrients, food, and habitat for our plants and wildlife in the years to come.

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Visit our page on the life of a dead tree for more information on this related topic.

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