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WSU/Pend Oreille Extension introduced the Sense of Place series in 1999, with a focus on local landscape and natural history of Pend Oreille County, Washington. A partnership with the Kalispel Tribe of Indians Natural Resources Department has allowed us to expand this program through EPA funding to include more classes, a newsletter and this website.

Brush and Wildfire Hazards

by Eric Trimble, U. S. Forest Service

Looking at brush from a fire control standpoint, I am most concerned with rate of spread and potential flame lengths. Flame length is a function of wind, slope, and live and dead fuel moistures. For purposes of comparing fuel moisture threshholds of live vs. dead fuels I'll use 4 feet as the critical flame length threshold since a 4 foot flame is the upper limit that a fire can be effectively controlled by hand crews. Depending on the height of the overstory and amount of ladder fuels (conifer seedlings, saplings and poles and crown ratios of the overstory) this is also the point at which many fires tend to transition from the surface to the crown.

Local weather data and fire predictive computer modeling runs (which compare live and dead fuels moisture as shown in the accompanying graph) have led to the following observations:

In normal precip years with 2 mph mid-flame wind speed: Potential critical flame lengths of 4' or greater may not be exceeded on these relatively windless days until mid October when live woody fuels (brush) drops below 80% and 10-hour dead fuels remain at or below 6%. This trend may continue until early November when fine fuels (on average) exceed 10% fuel moisture (two weeks of potential fire hazard).

Normal precip years with 10 mph mid-flame wind speed: Potential critical flame lengths of 4' or greater may be exceeded as early as August 1st when live woody fuels (brush) drops below 120% and 10-hour dead fuels are between 4% and 9%. This trend may continue until early to mid-November when fine fuels (on average) exceed 14% fuel moisture (three months of potential fire hazard).

Drought with 2 mph mid-flame wind speed: Potential critical flame lengths of 4' or greater may be exceeded on these relatively windless days as early as July 1st when live woody fuels (brush) drops below 80% and 10-hour dead fuels remain at or below 6%. This trend may continue until early November when fine fuels (on average) exceed 10% fuel moisture (four months of potential fire hazard).

Drought with 10 mph mid-flame wind speed: Potential critical flame lengths of 4' or greater may be exceeded as early as June 1st when live woody fuels (brush) drops below 120% and 10-hour dead fuels are between 4% and 9%. This trend may continue until early to mid-November when fine fuels (on average) exceed 14% fuel moisture (five-six months of potential fire hazard).

(Additional graphs illustrating weather data from the Tacoma Creek RAWS station and BEHAVE fire predictive modeling runs are available at the Extension office, S.418 Scott, in Newport.)

Recommended Homeowner Treatment Zones:

Zone 1: Your home. Decrease and/or eliminate the ignition potential of your home. Install nonflammable roofing materials, and enclose decks, soffits and overhangs. Remove debris from roofs and gutters.

Zone 2: Your landscape. Within 100 feet of your home, install a greenbelt of well-watered and maintained plants and trees. Use water wisely and concentrate irrigation efforts in the areas immediately surrounding your home. Plant perennials and annuals in clumps with individual trees and shrubs. Install rock, brick retaining walls and/or well-watered turf around these islands of vegetation. This zone requires pruning and removing dead, dry debris every 3-5 years. Extend this zone 50% on steep slopes leading up to the house.

Zone 3: Beyond 100 feet. This area is composed of native plants and trees that are thinned. Best to retain relatively large fire tolerant trees (i.e. western larch, ponderosa pine and Douglas-fir) and remove the fire intolerant ladder fuels (i.e. relatively small grand fir and Douglas-fir seedlings, saplings and poles). Slash is either removed or piled and burned.

Literature cited:
Barkley, YC., Schnepf, C., Cohen, J. 2005. Protecting and Landscaping Homes in the Wildland/Urban Interface, University of Idaho Extension. Station Bulletin No 67, Moscow, ID.

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