Expansion and Wildfire: How WUI Growth in California Changes Fire’s Interactions with Humans

California wildfires are constantly in the nation’s spotlight. The media spreads jarring images of burning houses and wildland, planes dropping fire retardant on blazing forests, and smoky skies. At the end of these destructive events, affected communities rebuild and attempt to continue their lives as normal. However, as fires become more common in the changing climate and as people better understand fire regimes and the land’s reliance on fire as an ecological force, human impact on these fires and their interaction with them comes into question.

The Wild-Urban Interface

The wild-urban interface (WUI) is when human development intersects with wildlands. Often, this refers to residential zones, with houses being built into the native vegetation. There are two forms of interaction between these neighborhoods and the wildland: intermix and interface. Intermix WUI refers to when houses are built into the wildlife, with the natural flora being integrated into the residential areas. Interface WUI is where there are clear boundaries between the residencies and the wildland (See Figure 1). Both forms of development lead to major consequences for both the humans living there and the natural landscape and native ecosystem that the construction is encroaching on. As humans disturb the wildlands, they disturb many of the ecological processes necessary to sustain the ecosystem as well as introduce non-native species and threats to the landscape. In turn, the ecology has to change, leaving the neighborhoods with an unstable and potentially unpredictable environment (Radeloff et al.).

Figure 1: The interaction between housing and wildland occurs in 4 different ways. WUI is differentiated by intermix (houses mixed with wild plants) and interface (houses and wildlife intersecting but staying separate). “Northern Research Station News Releases.” Most California Fires Occur in Area of Wildland-Urban Interface with Less Fuel and More People, US Department of Agriculture Forest Service, 24 Sept. 2019, https://www.nrs.fs.fed.us/news/release/wui-interface-intermix.

WUI in California

Across the United States, WUI is the “fastest growing land use type” (Radeloff et al.). In California, the WUI is one of the major zones of residential development, with 4.5 million homes in these regions as of 2017 (Schoennagel et al.). Figure 2 uses GIS data collected by the Silvis lab through the University of Wisconsin. QGIS mapped the geospatial data and created layers for each year that could then be cycled through to animate the growth of WUI zones over three decades (1990, 2000, and 2010). Significant growth of primarily intermix development occurred in the Central Valley (especially near Fresno) up past Sacramento. In addition, the region above San Francisco into North California shows significant growth of both interface and intermix WUI regions. Interestingly, Southern California did not seem to show much development into wildlands. A simple bar graph created by Tableau (Figure 3), a data visualization software, shows the areas of each type of wild-urban interface in comparison to each other in each decade. Here, the increase of WUI area over time is also clear.

Figure 2: Map of residential zones from 1990 to 2000 to 2010 to show growth of WUI zones. (Note: image can be enlarged by clicking on it). “Wildland-Urban Interface (WUI) Change 1990-2010.” SILVIS LAB, 2005, http://silvis.forest.wisc.edu/data/wui-change/.

Figure 3: Graph comparing acres of each type of WUI zone in each decade made by tableau and the same data from Silvis lab that was mapped in Figure 2. “Wildland-Urban Interface (WUI) Change 1990-2010.” SILVIS LAB, 2005, http://silvis.forest.wisc.edu/data/wui-change/.

Wildfire

One of the most observable impacts of California’s unrestrained growth into wildlands is wildfire. In California, the largest percent of wildfires burned in WUI zones (35%) (Schoennagel et al.). This is especially concerning due to the increasing numbers of fires starting each year. CalFire, along with the California state government collect an abundance of data related to many of the wildfires above a minimum area. From how the fire started to the time it was reported to how many acres it burned, CalFire collects as much as information as possible and then publicly release the files with the information. These files were put into a program like ArcGIS, which mapped the fires and could be queried to create artificial layers for each decade of interest. Figures 4-6 shows these maps for each of the decades where WUI data were mapped above.

Figure 4: Map of fire perimeters of wildfires restricted to date reported between 1990-1999 or year to 1990s. “Welcome to GIS Data.” Cal Fire Department of Forestry and Fire Protection, CA.gov, 2021, https://frap.fire.ca.gov/mapping/gis-data/.

Figure 5: Map of fire perimeters of wildfires restricted to date reported between 2000-2009 or year to 2000s. “Welcome to GIS Data.” Cal Fire Department of Forestry and Fire Protection, CA.gov, 2021, https://frap.fire.ca.gov/mapping/gis-data/.

Figure 6: Map of fire perimeters of wildfires restricted to date reported between 2010-2019 or year to 2010s. “Welcome to GIS Data.” Cal Fire Department of Forestry and Fire Protection, CA.gov, 2021, https://frap.fire.ca.gov/mapping/gis-data/.

WUI and Fire

Figure 7 takes both sets of data and layers them on top of each other. Here, the WUI zones and the wildfires that spread in them can be compared. The increase in fires overall is clear between 1990 and 2010. When specifically focusing on the zones with significant WUI growth, a similar trend emerges. The region above San Francisco shows an increase in fires over the three decades at the same time as the development of the intermix WUI zone in particular. A similar conclusion can be drawn about the region in the Central Valley from Fresno up to Sacramento.

Figure 7: Both layers transposed over each other for all three decades.

The Effects of WUI on Fire and How to Move Forward

The observable trends of wildfires increasing, especially in areas with significant WUI growth, is concerning, especially as this growth is increasing over time. This brings up the question of why this trend exists and how California can prevent it. The answer to the first question must account for a multitude of ecological and societal factors. For example, the act of putting humans closer to the fuel that is naturally present in these wildlands automatically leads to an increase in “human ignitions”. This could be from sources such as campfires to the electrical lines necessary to support new neighborhoods (Radeloff et al.). Additionally, the increase in fire events is changing the composition of the wildlands, leading to even more unpredictable fire patterns (Syphard et al.). To help address the growing problem and dangers to human lives and infrastructure, the first step is risk assessment. Companies working on modeling and assessing the risk of wildfire to newly developed residencies in the WUI help homeowners address the dangerousness of their living situation (Mahmoud, Akshat). In addition, observing trends in higher risk environments can lead to the development of solutions. A 2017 paper studying how the country can face fire as frequency and severity rises found that a more targeted approach to fuel reduction (including through prescribed burns) and designing residencies to withstand wildfires (what they call specified resilience, or the ability for a community to remain stable after a destructive event) should be implemented. The authors argue that the state and local governments as welll as private jurisdications should be financially responsible for fire prevention through fire-resistant building materials and protecting watersheds. Overall, they call for the redistribution of resources from WUI fire suppression to overall prevention and adapting to withstand fires (Schoennagel et al.). Fire is an unavoidable force, and as humans infringe on natural lands that rely on it, they should expect to interact with it more often. By decreasing the risk of human-started fires and increasing a neighborhood’s ability to withstand it, communities can create a greater sense of coexistence with the nature that they are living with.

Sources

Kramer, Heather Anu, et al. “High Wildfire Damage in Interface Communities in California.” International Journal of Wildland Fire, vol. 28, no. 9, 30 July 2019, p. 641., https://doi.org/10.1071/wf18108.

Mahmoud, Hussam, and Akshat Chulahwat. “Assessing Wildland–Urban Interface Fire Risk.” Royal Society Open Science, vol. 7, no. 8, 26 Aug. 2020, p. 201183., https://doi.org/10.1098/rsos.201183.

“Northern Research Station News Releases.” Most California Fires Occur in Area of Wildland-Urban Interface with Less Fuel and More People, US Department of Agriculture Forest Service, 24 Sept. 2019, https://www.nrs.fs.fed.us/news/release/wui-interface-intermix.

Radeloff, Volker C., et al. “Rapid Growth of the US Wildland-Urban Interface Raises Wildfire Risk.” Proceedings of the National Academy of Sciences, vol. 115, no. 13, 12 Mar. 2018, pp. 3314–3319., https://doi.org/10.1073/pnas.1718850115.

Schoennagel, Tania, et al. “Adapt to More Wildfire in Western North American Forests as Climate Changes.” Proceedings of the National Academy of Sciences, vol. 114, no. 18, 2017, pp. 4582–4590., https://doi.org/10.1073/pnas.1617464114.

Syphard, Alexandra D., et al. “Human Influence on California Fire Regimes.” Ecological Applications, vol. 17, no. 5, 4 Jan. 2007, pp. 1388–1402., https://doi.org/10.1890/06-1128.1.

“Welcome to GIS Data.” Cal Fire Department of Forestry and Fire Protection, CA.gov, 2021, https://frap.fire.ca.gov/mapping/gis-data/.

“Wildland-Urban Interface (WUI) Change 1990-2010.” SILVIS LAB, 2005, http://silvis.forest.wisc.edu/data/wui-change/.