Wildland Interface Study Objectives

This analysis aims to provide objective and relevant data that can help inform the decisions of planners, communities, landowners and elected officials across the West. The objectives include:

1. Describing the current status of residential development in the wildland interface; and
2. Identifying counties with high existing risk and those with high potential future risk.

Methods

In order to identify where housing has occurred adjacent to fire prone wildlands in the West, maps of housing density were created at the scale of 2000 Census blocks. Forested areas where residential development (census blocks with mean lot sizes less than 40 ac) occured within 500 meters (0.31 miles) of public lands were then identified. The threshold of 40 ac lot sizes was used to identify residential development because at this home density, areas are generally considered to be more populated than working agricultural lands (Gude 2006), although some high value agricultural operations, including orchards, can be profitable at this lot size (Theobald 2005).

The maps of housing density were prepared similarly to those described by Theobald (2005). Geographic Information System (GIS) layers describing the Census block boundaries in 2000 were extracted from the TIGER/Line databases (US Census Bureau 2001a) for Arizona, California, Colorado, Idaho, Montana, New Mexico, Nevada, Oregon, Utah, Washington, and Wyoming. Tabular data describing the population and number of housing units in each block were extracted from Census Summary File 1 tables (US Census Bureau 2001b, US Census Bureau 2001c) and joined to the GIS layers. In order to calculate the mean lot size per Census block, the number of housing units was divided by the area of private land. Water, as identified in the National Hydrography Database (US Geological Survey 2001), and public lands, as identified in the Protected Areas Database (DellaSala 2001), were excluded from the area calculations. In Montana and Arizona, the Protected Areas Database was found to have substantial errors in the locations of public land boundaries, and other data sources (ALRIS 1998, MNHP 2007) were used instead.

A buffer of 500 meters surrounding forested public lands, including federal, state, and locally managed forests, was mapped, and residential blocks that fell within this buffer were identified. The Protected Areas Database (DellaSala 2001) was used to map public lands in California, Colorado, Idaho, New Mexico, Nevada, Oregon, Utah, Washington, and Wyoming, and state data sources were used to map public land boundaries in Montana (MNHP 2007) and Arizona (ALRIS 1998). The forested public lands were identified based on the following classes from the National Land Cover Dataset (Vogelmann 2001): evergreen needleleaf forest, evergreen broadleaf forest, deciduous needleleaf forest, deciduous broadleaf forest, mixed forests, closed shrublands. Although open shrublands and grasslands are also prone to wildfire, defending homes in these habitats tends to be less dangerous and less expensive from a firefighting perspective (Marcel Potvin, USFS firefighter, personal communication). Since guidelines for the amount of defensible space necessary to protect homes range from 40 to 500 meters around the home (Cohen 2000, Butler and Cohen 1998, Nowicki 2002), the threshold of 500 meters was used to identify where residential development has occurred adjacent to fire prone public lands. This is a conservative estimate of the WUI and the associated risk of fire, since it is unknown how many home owners within this zone have followed defensible space guidelines.

For each western state and for the West as a whole, the area of forested wildland interface containing homes,i.e., the WUI, was compared to the area of undeveloped forested wildland interface. Per state, the number of homes in the wildland interface was calculated, as well as the percent of these homes that are second homes. The number of second homes within the WUI was calculated by adding the number of "seasonally occupied" homes, as specified in by the Census SF1 H005005 field, to the number of "other vacant" homes, as specified in the Census SF1 H005007 field. These counts do not include homes that are vacant because they are for rent or sale (US Census Bureau 2001b, US Census Bureau 2001c). In addition to state metrics, two measures were used to identify counties with high existing and high potential risk of wildland fire to homes. Existing risk was measured in terms of the total area of WUI per county, and potential risk was represented by the area of undeveloped forested wildland interface, where homes construction could occur in the future.

Literature Cited

ALRIS (Arizona Land Resources Information System), 1998. Metadata for LAND. http://www.land.state.az.us/alris/metadata/ownership.htm (Accessed 6/4/07)

Butler, B.W., and J.D. Cohen. 1998. Firefighter safety zones: a theoretical model based on radiative heating. International Journal of Wildland Fire 8(2): 73-77.

Cohen, Jack D. 2000. Preventing disaster: home ignitability in the wildland-urban interface. Journal of Forestry 98(3): 15-21.

DellaSala, D. A., N. L. Staus, J. R. Strittholt, A. Hackman, and A. Iacobelli. 2001. An updated protected areas database for the United States and Canada. Natural Areas Journal 21:124-135.

Gude, P.H., Hansen, A.J., Rasker, R., Maxwell, B. 2006. "Rates and Drivers of Rural Residential Development in the Greater Yellowstone." Landscape and Urban Planning. 77: 131-151.

MNHP (Montana Natural Heritage Program), 2007. Metadata for Montana Public Land Ownership. http://nris.mt.gov/nsdi/nris/stew_owners.html (Accessed 6/4/07)

Nowicki, B., 2002. The Community Protection Zone: Defending Houses and Communities from the Threat of Forest Fire. URL: www.biologicaldiversity.org

Theobald, D. 2005. Landscape patterns of exurban growth in the USA from 1980 to 2020. Ecology and Society 10(1): 32.

US Census Bureau, 2001a. Tiger/Line Files Technical Documentation. U.S. Census Bureau, Washington, DC.

US Census Bureau, 2001b. Census 2000 Summary File 1. U.S. Census Bureau, Washington, DC.

US Census Bureau, 2001c. Census 2000 Summary File 1 Technical Documentation. U.S. Census Bureau, Washington, DC.

US Geological Survey, 2001. National Hydrography Dataset: Concepts and Contents. http://nhd.usgs.gov/techref.html (Accessed 8/6/07)

Vogelmann, J. E., S. M. Howard, L. Yang, C. R. Larson, B. K. Wylie, and N. van Driel. 2001. Completion of the 1990s National Land Cover Data Set for the conterminous United States from Landsat Thematic Mapper data and ancillary data sources. Photogrammetric Engineering and Remote Sensing 67:650-652.

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