The wildland–urban interface in the United States based on 125 million building locations

The wildland–urban interface (WUI) is the focus of many important land management issues, such as wildfire, habitat fragmentation, invasive species, and human–wildlife conflicts. Wildfire is an especially critical issue, because housing growth in the WUI increases wildfire ignitions and the number of homes at risk. Identifying the WUI is important for assessing and mitigating impacts of development on wildlands and for protecting homes from natural hazards, but data on housing development for large areas are often coarse. We created new WUI maps for the conterminous United States based on 125 million individual building locations, offering higher spatial precision compared to existing maps based on U.S. census housing data. Building point locations were based on a building footprint data set from Microsoft. We classified WUI across the conterminous United States at 30-m resolution using a circular neighborhood mapping algorithm with a variable radius to determine thresholds of housing density and vegetation cover. We used our maps to (1) determine the total area of the WUI and number of buildings included, (2) assess the sensitivity of WUI area included and spatial pattern of WUI maps to choice of neighborhood size, (3) assess regional differences between building-based WUI maps and censusbased WUI maps, and (4) determine how building location accuracy affected WUI map accuracy. Our building-based WUI maps identified 5.6%–18.8% of the conterminous United States as being in the WUI, with larger neighborhoods increasing WUI area but excluding isolated building clusters. Building-based maps identified more WUI area relative to census-based maps for all but the smallest neighborhoods, particularly in the north-central states, and large differences were attributable to high numbers of non-housing structures in rural areas. Overall WUI classification accuracy was 98.0%. For wildfire risk mapping and for general purposes, WUI maps based on the 500-m neighborhood represent the original Federal Register definition of the WUI; these maps include clusters of buildings in and adjacent to wildlands and exclude remote, isolated buildings. Our approach for mapping the WUI offers flexibility and high spatial detail and can be widely applied to take advantage of the growing availability of high-resolution building footprint data sets and classification methods.

File: Ecological-Applications-2022-Carlson-The-wildland-urban-interface-in-the-United-States-based-on-125-million-building.pdf

The importance of small fires for wildfire hazard in urbanized landscapes of the northeastern U.S.

Frequent, small wildfires can pose dangers to homes in the wildland–urban interface, but are not often included in wildfire hazard models. We assessed patterns of small wildfire occurrence probability in the Northeast region of the United States, focusing on (1) spatial and seasonal variations; (2) differences between small and large fires (size threshold of 4 ha); and (3) how predicted probabilities are influenced by inconsistent wildfire definitions in urbanised landscapes. We analysed fire incident report data from 2005 to 2017 to parameterise maximum entropy (MaxEnt) models based on land cover, topography, climatic water deficit, soil moisture and road density. Overall, wildfire occurrence was highest in areas with lower agricultural cover and with more low-density urban development (explaining 53.5 and 28.6% of variance, respectively, in our region-wide model), while larger fires were concentrated in areas with intermediate levels of development, higher climatic water deficit and more rugged topography. These patterns were largely consistent when we assessed models for individual states, but differences in wildfire reporting patterns led to differences in the effect of urban development on fire probability. Our results provide novel understanding of small wildfire patterns in the Northeast and demonstrate the need to more reliably quantify these hazards.

File: nrs_2021_carlson_001.pdf

Growth of the wildland-urban interface within and around U.S. National Forests and Grasslands, 1990-2010

The wildland-urban interface (WUI), where housing is in close proximity to or intermingled with wildland vegetation, is widespread throughout the United States, but it is unclear how this type of housing development affects public lands. We used a national dataset to examine WUI distribution and growth (1990–2010) in proximity to National Forests and created a typology to characterize each National Forest’s combination of WUI area and housing growth. We found that National Forests are hotspots for WUI growth, with a 38% increase in WUI area and 46% growth in WUI houses from 1990 to 2010, in excess of WUI growth for the conterminous U.S. Growth within National Forests was higher than the surrounding area. Diffuse intermix WUI, where houses are intermingled with wildland vegetation, is common within National Forests, but WUI houses around National Forests were primarily in denser interface WUI areas, which lack substantial wildland vegetation. WUI was more prevalent within and around National Forests in the East, while National Forests in the West experienced higher rates of WUI growth. National Forests with the most challenging WUI issues—extensive WUI area and rapid growth in intermix and interface—were found primarily in the South and interior West. Given the diversity of WUI landscapes, effectively responding to current and future WUI challenges will require both engagement with individual homeowners dispersed throughout National Forests, as well as increased emphasis on mitigating denser interface development around National Forests. At a time when wildfire risks are expected to intensify due to climate change, and 75% of privately owned land within and around National Forests is not yet WUI, understanding WUI growth patterns in proximity to public lands is vital for land management and human wellbeing.

File: 1-s2.0-S0169204621002462-main.pdf

Post-wildfire rebuilding and new development in California indicates minimal adaptation to fire risk

Every year, wildfires destroy thousands of buildings in the United States, especially in the rapidly growing wildland-urban interface, where homes and wildland vegetation meet or intermingle. After a wildfire there is a window of opportunity for residents and public agencies to re-shape patterns of development, and avoid development in locations that are inherently at higher risk of wildfire destruction. We examined 28 of the most destructive wildfires in California, the state where most buildings are destroyed by wildfires, to evaluate whether locations of rebuilt and newly constructed buildings were adaptive (i.e., if building occurred in lower risk areas). In total, these fires burned 7,075 buildings from 1970 to 2009. We found minimal evidence for adaptation both in the number and placement of buildings post-fire. Rebuilding was common: 58% of the destroyed buildings were rebuilt within three to six years, and 94% within thirteen to twenty-five years after the fire. Similarly, we found minimal trends toward lower risk areas in the placement of 2,793 rebuilt and 23,404 newly constructed buildings over the course of 13–25 yr. In fact, long-term data revealed that relative risk of new construction either did not change significantly over time or increased. A destructive wildfire could provide an opportunity to assess and change building practices, yet our results show that such change is largely not occurring. As wildfires increasingly threaten communities, this lack of change could result in growing rates of destruction and loss of life.

File: Kramer_etal_2021_LUP.pdf

50 years of housing growth in the WUI in California

Southern California wildfires captured from the International Space Station. Image Credit: NASA Johnson
Southern California wildfires captured from the International Space Station. Image Credit: NASA Johnson
Southern California wildfires captured from the International Space Station. Image Credit: NASA Johnson

The wildland-urban interface (WUI), the area where houses and wildland vegetation mix or intermingle, is rapidly growing. Housing expansion into the WUI has increased wildfire frequency and risk. This is in part because human-caused wildfires are responsible for most of the wildfires in the WUI. The rising intensity and frequency of wildfires have led to increased damage and loss of life in recent years. Because wildfire risk increases when people live near or in the WUI, it is beneficial to study historical housing trends and urbanization in these areas.

Globally, more frequent wildfires have become the new normal, and it is important to understand if and how housing development has worsened the damage. To find answers, Neda is using historic and current high-resolution satellite imagery to map housing units in California over the past 50 years. Neda plans to evaluate housing growth from the 1970s to the present-day to evaluate if there has been a proportional increase or decrease of houses in California in the WUI.

Anu Kramer (former SILVIS lab member) digitized total buildings destroyed by 89 wildfires in California from 1985-2014 and the Tubbs Fire in 2017. Figure from H.A. Kramer et al. (2019)
Anu Kramer (former SILVIS lab member) digitized total buildings destroyed by 89 wildfires in California from 1985-2014 and the Tubbs Fire in 2017. Figure from H.A. Kramer et al. (2019)

Neda’s main objective is to identify the areas of increased housing in the WUI and, ultimately, find out whether these areas have adapted to potential risks caused by wildfires. The main idea is to provide insight into how humans have adapted to wildfires over the past 50 years. Researching whether policy and communities adjust where houses are built based on wildfire risk could provide insight into the extent of WUI growth.

With thoughtful housing choices, there could be substantial benefits to the environment, as well as saving human lives, money, houses, and wild animals. Since it is always wise to “learn from our own mistakes,” it is important to understand which choices in the 20th century have caused additional wildfire risk. Stay tuned and follow Neda’s work for insight into thoughtful housing choices in California.

Examples of intermix (left) and interface (right) wildland-urban interface areas located near Redding, California. Photos from Google Earth.
Examples of intermix (left) and interface (right) wildland-urban interface areas located near Redding, California. Photos from Google Earth.

Assessing vulnerability and threat from housing development to Conservation Opportunity Areas in State Wildlife Action Plans across the United States

Targeting conservation actions efficiently requires information on vulnerability of and threats to conservation
targets, but such information is rarely included in conservation plans. In the U.S., recently updated State Wildlife
Action Plans identify Conservation Opportunity Areas (COAs) selected by each state as priority areas for future
action to conserve wildlife and habitats. The question is how threatened these COAs are by habitat loss and
degradation, major threats to wildlife in the U.S. that are often caused by housing development. We compiled
spatial data on COAs across the conterminous U.S. We estimated COA vulnerability using current land protection
status and COA threat using projected housing growth derived from U.S. census data. COAs comprise 1–46% of
each region. Across regions, 28–82% of the area within COAs is vulnerable to future housing development, and
5–55% and 7–23% of that vulnerable COA area is threatened by projected dense housing and rapid housing
growth, respectively. COA vulnerability is greatest in the East. Threat from dense housing and rapid housing
growth is highest in the Northeast and Pacific Southwest, respectively. Results highlight that many areas
identified as important for reducing wildlife listings under the U.S. Endangered Species Act may need further
protection to fulfill their conservation goals because they are both vulnerable to and threatened by future
housing development. Our analyses can help practitioners target local government outreach, land protection
efforts, and landscape-scale mitigation programs to decrease future COA loss from housing development, and
could be expanded to address additional COA threats (e.g., wildfire, invasive species).

File: SCarter_etal_LUP_2019.pdf

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High wildfire damage in Interface communities in California

Globally, and in the US, wildfires pose increasing risk to people and their homes. Wildfire management
assumes that buildings burn primarily in the wildland–urban interface (WUI), where homes are either ignited directly
(especially in intermix WUI areas, where houses and wildland fuels intermingle), or via firebrands, the main threat to
buildings in the interface WUI (areas with minimal wildland fuel, yet close to dense wildland vegetation). However, even
urban areas can succumb to wildfires. We examined where wildfire damages occur among urban, rural and WUI (intermix
and interface) areas for approximately three decades in California (1985–2013). We found that interface WUI contained
50% of buildings destroyed by wildfire, whereas intermix WUI contained only 32%. The proportion of buildings destroyed
by fires among classes was similar, though highest in interface WUI areas (15.6%). Our results demonstrate that the
interface WUI is where most buildings were destroyed in California, despite less wildland fuel. Continued advancement of
models, mitigation and regulations tailored for the interface WUI, both for California and elsewhere, will complement the
prior focus on the intermix WUI.

File: Kramer_etal_IJWF_2019.pdf

Globally, and in the US, wildfires pose increasing risk to people and their homes. Wildfire management
assumes that buildings burn primarily in the wildland–urban interface (WUI), where homes are either ignited directly
(especially in intermix WUI areas, where houses and wildland fuels intermingle), or via firebrands, the main threat to
buildings in the interface WUI (areas with minimal wildland fuel, yet close to dense wildland vegetation). However, even
urban areas can succumb to wildfires. We examined where wildfire damages occur among urban, rural and WUI (intermix
and interface) areas for approximately three decades in California (1985–2013). We found that interface WUI contained
50% of buildings destroyed by wildfire, whereas intermix WUI contained only 32%. The proportion of buildings destroyed
by fires among classes was similar, though highest in interface WUI areas (15.6%). Our results demonstrate that the
interface WUI is where most buildings were destroyed in California, despite less wildland fuel. Continued advancement of
models, mitigation and regulations tailored for the interface WUI, both for California and elsewhere, will complement the
prior focus on the intermix WUI.

Forests, houses, or both? Relationships between land cover, housing characteristics, and resident socioeconomic status across ecoregions

Residential development is one of the most intensive and widespread land uses in the United States, with
substantial environmental impacts, including changes in forest cover. However, the relationships between forest
cover and residential development are complex. Contemporary forest cover reflects multiple factors, including
housing density, time since development, historical land cover, and land management since development. We
investigated how forest cover varies with housing density, housing age, and household income over a range of
development intensities, in six ecoregions within New York State, Wisconsin, and Colorado. We find areas with
residential development do retain important forest resources: across landscapes they are typically more forested
than areas that remain undeveloped. However, forest cover consistently had a negative, inverse relationship with
housing density, across study areas. Relationships between forest cover and housing age and household income
were less common and often restricted to only portions of a given region, according to geographically weighted
regression analyses. A better understanding of how forest cover varies with residential development, outside of
the typically studied urban areas, will be essential to maintaining ecosystem function and services in residential
landscapes.

File: Mockrin-et-al_2019_Forests_JEM.pdf

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Where wildfires destroy buildings in the US relative to the wildland-urban interface and national fire outreach programs

Over the past 30 years, the cost of wildfire suppression and homes lost to wildfire in the U.S. have increased dramatically, driven in part by the expansion of the wildland-urban interface (WUI), where buildings and wildland vegetation meet. In response, the wildfire management community has devoted substantial effort to better understand where buildings and vegetation co-occur, and to establish outreach programs to reduce wildfire damage to homes. However, the extent to which the location of buildings affected by wildfire overlaps the WUI, and where and when outreach programs were established relative to wildfire, is unclear. We found that most threatened and destroyed buildings in the conterminous U.S. were within the WUI (59% and 69%, respectively), but this varied considerably among states. Fires with the greatest building loss were close to outreach programs (such as Firewise), but for 76% of destroyed buildings, the nearest outreach program was established post-wildfire. In these locations, as well as places new to the WUI or in areas where the fire regime is predicted to change, pre-emptive outreach could improve the likelihood of building survival and reduce the human and financial costs of structure loss.

File: Kramer2018_Fire_Buildings_IJWF.pdf

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Long-term changes of the Wildland-Urban Interface in the Polish Carpathians

The Wildland–Urban Interface (WUI) is the area where houses and wildland vegetation
meet or intermingle, which causes many environmental problems. The current WUI is widespread in
many regions, but it is unclear how the WUI evolved, especially in regions where both houses and
forest cover have increased. Here we compared WUI change in the Polish Carpathians for 1860 and
2013 in two study areas with different land use history. Our western study area experienced gradual
forest increase and housing growth over time, while the eastern study area was subject to a shock
due to post-war resettlements, which triggered rapid reforestation. We found that in both study
areas WUI extent increased from 1860 to 2013 (41.3 to 54.6%, and 12.2 to 33.3%, in the west and east,
respectively). However the causes of WUI growth were very different. In the western study area new
houses were the main cause for new WUI, while in the eastern study area forest cover increase was
more important. Our results highlight that regions with similar current WUI cover have evolved very
differently, and that the WUI has grown rapidly and is widespread in the Polish Carpathians.

File: Kaim2018_WUI_Carpathians_IJGI.pdf

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