The term ‘wildland-urban interface’ (WUI) is used to describe a certain type of transition zone, where an urban settlement meets and intermingles with a natural environment that is loaded with wild vegetation.
The prevalence of WUIs has grown rapidly over the past few decades, which is largely the result of population shifts not only toward south-eastern and western regions of the United States, but in favor of suburban residential areas in a natural wooded environment.
The WUI has negative consequences on the environment of wildland landscapes, and poses a significant wildfire risk, due to both the large availability of woodland fuel and because urban areas are placed close to this hazard. Climate change-driven increases in drought frequency also put WUIs at a greater risk from wildfire.
What is the Wildland-Urban Interface?
A WUI is virtually any area where people live in houses close to areas with wild-growing vegetation, which includes forests, grassland, and shrubland, but not agricultural cropland. Although this transition zone can include vacation homes and cabins, a typical WUI is an area of suburban residential housing that is built on a large previously undeveloped area.
There are two types WUI: intermix and interface. The interface WUI is characterized by a clear demarcation between human settlements and wildlands. This is where residential areas, with their homes and infrastructure, directly abut undeveloped areas such as forests or grasslands. Conversely, the intermix WUI presents a more blended scenario where houses, roads, and other human developments are interspersed within natural vegetation. In these intermixed areas, the distinction between urban and wildland areas is less defined, creating a mosaic of man-made structures and natural landscapes.
Many WUIs are bordered on at least one of their sides by areas of woodland that remain rather intact, and are often part of a much larger forest.
The Wildland Urban Interface Prevalence in the United States
Around a third of all housing units in the United States are located within the WUI, which covers 9% of the total land area within the 48 contiguous states. This type of transition zone can be found in every state, especially around cities and urban centers that are surrounded by large natural woodlands.
Wildland-urban interfaces are most frequently found in the eastern coastal states – for example, 72% of Connecticut is made up of WUIs, which are commonplace from Maine down to Florida.
They are also a common feature around large cities in California, Oregon and Washington. However, WUIs are not exclusive to the United States – large areas of Europe’s Mediterranean region, sections that border the tropical rainforest near the Equator, and parts of Australia and South Africa have also seen large WUI growth over recent years.
The Impact of Wildland-urban Interfaces on Natural Habitats
Within this type of transition zone, the urban area intermingles with forested areas; thus, human activity in these once rural and quiet areas is much more commonplace than before. This has negative consequences on ecosystems located close to the new urban areas, with a reduction in biodiversity as animal habitats are destroyed, and many animals being scared away by the presence of humans and pets in and around their environment.
Native species of trees are often fragmented by infrastructure such as footpaths or roads, or by the introduction of non-native plant species to the wildland.
Wildfire Risk in Wildland-urban Interfaces
Wildfires are significantly more commonplace in WUIs than in other wildlands. The intermixing of human activity with a natural forested environment means that many activities can start a wildfire – bonfires, barbeques and firework displays, even in the back yard of a house backing on to a wildland, have been responsible for vast forest fires.
Additionally, if a fire spreads throughout a woodland close to a WUI, it puts the buildings within the transition zone under threat from being burnt down.
Wildlands can be a great source of fuel; as vegetation dies and leaves fall off the trees once a year, a large amount of fuel is built up over time, and if this is not cleared, the risk of a wildfire increases with time.
A notable lack of fire-related infrastructure in WUIs, such as firebreaks between houses and trees, and hazard fuel reduction projects, also increases the fire risk massively.
The Population Shift Toward Wildland-urban Interfaces
Between the years of 1990 and 2010, the U.S. wildland urban interface grew rapidly in terms of land area, according to research sponsored by the U.S. Fish and Wildlife Service. During this 20-year period, the WUI was the fastest growing land use type throughout the 48 contiguous states, with 13 million new homes constructed in the transition zone during this time frame.
This shows an interesting 20-year shift in the demographics within the United States, partly fueled by an increase in population from 250 million through to 310 million, with new areas of land being developed in order to house more people.
The shift not only highlights the movement of people from densely urbanized areas into much more suburban areas, but also of a specific desire for living in and around natural woodland landscapes, which are renowned for their solitude and privacy.
There has been a population shift over the past three decades across the entire United States, with people moving to western and south-eastern areas of the country. These regions are covered in large areas of woodland, meaning that WUIs have rapidly expanded in these areas as a response to nationwide population shifts.
Climate Change and Wildfires
Many climate change projections indicate that the conditions favorable for forest fires will occur more frequently in the future. These include longer droughts and warmer temperatures, which dry out the ground – along with any dead leaves, plants and tree branches resting on the surface – even more so than the weather would in a non-warming world.
Therefore, wildfires are a large threat to WUIs – along with the natural landscape and the people living within them – more so than fires have done in the past.
In 2021, many parts of the Pacific Northwest had a record-breaking hot and dry summer, with a drought lasting for months in some areas. Many catastrophic wildfires burnt through parts of the region as a result of these conditions; the state of Oregon saw more wildfires than in 2020, with similar trends were also noted in parts of Washington and south-west Canada. (Related: 2021 Wildfires in the U.S. and Canada)
Forest fires destroyed entire towns and suburban areas within the WUI during this season, such as the town of Lytton in British Columbia. The 2021 drought is strongly linked to climate change, with statistical modelling indicating that at least one of its events – a severe heatwave in late June – would have been ‘virtually impossible’ without climate change.
This trend is only expected to continue as global temperatures rise even further.
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