Defining the Wildland-Urban Interface

Federal wildland fire policy in the United States has been substantially revised over the past 10 years and new emphasis has been given to the wildland– urban interface (WUI), which creates a need for information about the WUI’s location and extent. We operationalized a policy definition published in the Federal Register (US Department of the Interior [USDI] and US Department of Agriculture [USDA]), 2001, Urban wildland interface communities within vicinity of federal lands that are at high risk from wildfire. Fed. Regist. 66(3):751–777) to create national maps and statistics of the WUI to guide strategic planning. Using geographic information system analysis, we evaluate the national WUI by altering the definition’s parameters to assess the influence of individual parameters (i.e., housing density, vegetation type and density, and interface buffer distance) and stability of outcomes. The most sensitive parameter was the housing density threshold. Changes in outputs (WUI homes and area) were much smaller than parameter variations suggesting the WUI definition generates stable results on most landscapes. Overall, modifying the WUI definition resulted in a similar amount of WUI area and number of homes and affected the precise location of the WUI.

File: Stewart-et-al-JOF-2007.pdf

Rapid growth of the US wildland-urban interface raises wildfire risk

The wildland-urban interface (WUI) is the area where houses and wildland vegetation meet or intermingle, and where wildfire problems are most pronounced. Here we report that the WUI in the United States grew rapidly from 1990 to 2010 in terms of both number of new houses (from 30.8 to 43.4 million; 41% growth) and land area (from 581,000 to 770,000 km2; 33% growth), making it the fastest-growing land use type in the conterminous United States. The vast majority of new WUI areas were the result of new housing (97%), not related to an increase in wildland vegetation. Within the perimeter of recent wildfires (1990–2015), there were 286,000 houses in 2010, compared with 177,000 in 1990. Furthermore, WUI growth often results in more wildfire ignitions, putting more lives and houses at risk. Wildfire problems will not abate if recent housing growth trends continue.

File: Radeloff_2018_PNAS_SI.pdf

Why do some houses burn and others do not when a wildfire hits?

Four Miles Fire in Boulder, CO. Picture taken in 2012, two years after the fire (Photograph by Patricia Alexander)

Wildfires are a major threat to houses and people in the US. About 2 billion dollars are spent every year in preventing and suppressing fires by the US Forest Service alone, and about 1,300 houses are burned each year on average. Housing is expanding every year and the number and frequency of wildfires in increasing as well, suggesting that this problem is likely to get worse in the future. Understanding the how wildfires affect houses, and what we can do to prevent those damages, is key to guide land-use planning and management efforts in fire prone places.

Cedar Fire in San Diego, CA in 2003, study area layout.
Cedar Fire in San Diego, CA in 2003, study area layout.
Four Mile Fire in Boulder, CO in 2010, study area layout.
Four Mile Fire in Boulder, CO in 2010, study area layout.

“People want nice views and to live out in the country, however, houses in such locations are under high risk of wildfires”, said Patricia Alexandre, a PhD student at Silvis. Understanding the factors that explain the likelihood of a house to burn during a wildfire is of major need for land-use planning in wildfire prone areas, and for agencies such as the US Forest Service. However, there is little knowledge on this topic according to Patricia. Patricia’s research focuses on identifying the key factors that explain the likelihood of a house to burn during a fire. For this, she is using two wildfires as case studies, including a wildfire that occurred in San Diego (California) and one in Boulder (Colorado). High resolution imagery before and after the fire is being used to map all the houses within those fires, that burned due to the fires. ‘In the Boulder fire for example, we mapped about 1100 houses, and we see that 10% of them were burned after the fire’, Patricia said.

The study explores about 40 environmental variables to predict the likelihood of a house to burn, such as vegetation conditions, topography (aspects, slope, elevation, topographic position), and the spatial arrangement of house (housing density, distance to near house). According to Patricia, predicting the likely of a house to burn is a complex task. “In Colorado, the houses most likely to burn were those on high slopes or on top of ridges, as well as those located at the edges of the neighborhoods. In San Diego, however, the results were more variable. What might explain things in one fire may not work in other.”

Another important finding from this study is that the spatial arrangement of the houses matters. According to Patricia, “previous efforts used only topography and spatial arrangement to predict the likelihood of a house to burn given a wildfire, but we decided to add vegetation to see how much in fact is vegetation contributing to this phenomenon. We see that vegetation alone cannot explain why a house burned alone, the way houses are arranged on the land is also important”. Currents efforts are focused towards refining the models so to have a better understanding of the local forces that result in burned houses within a single fire. However, the ultimate goal is to expand the study to the whole US.Patricia hopes that this study will help the government to allocate resources (e.g. fuel management) in a more efficient manner, and will provide land-use planners, urban planners, and home owners with useful information and recommendations about housing construction in wildfire-prone areas. This study is a step towards Patricia’s dissertation focused on understanding the factors that explain house loss to wildlife in the US. “My ultimate goal is to develop a risk map for the whole US that tells you how likely it is that your house burns if a fire occurs”, Patricia said.

Where should we burn? Prioritizing prescribed fire in Wisconsin

Imagine an expansive prairie, dotted with Bur Oak trees, leaves swaying in the afternoon breeze. Bison were common in this scene, along with other vestiges of a time’s past. When Euro-Americans first settled southern Wisconsin, over 150 years ago, this is more or less what they found. However, with settlement came changes in land use, resulting in changes in land cover. The prairies and oak savannas with their rich and deep soils were converted to agriculture, urban areas sprouted, and natural disturbances, most notably fire, which were necessary to maintain the structure of the vegetation communities, were suppressed. As a result, the expansive prairies and oak savannas of Southern Wisconsin were nearly wiped from the map.

Prescribed fires, like this one at Muralt Bluff Prairie State Natural Area, help biologists restore and maintain prairies and grasslands in key locations around Wisconsin. Photo by Tallgrass Prairie and Oak Savanna Fire Science Consortium.

Fast-forward 150 years, and resources managers throughout Wisconsin are challenged to restore these once common habitats. The most effective approach is prescribed fire. Prescribed fires re-create natural burns in a safe and controlled manner. Fire is critical for knocking back woody vegetation, replenishing the soil, allowing certain plants to sprout or reproduce, and shaping the structure and composition of the habitats, which directly impact wildlife communities. Because prairie and oak savanna used to be expansive, covering most of southern Wisconsin, there are many lands today that need fire to maintain the character of the native habitats. Yet, this poses a different challenge. Resource managers have finite funds and personnel for applying prescribed fire. With so many lands that would benefit from fire, and limited resources to conduct prescribed fires, there is a real need to better understand where priorities lie. To achieve this, Sarah and Dave, along with collaborators from the Tallgrass Prairie and Oak Savanna Fire Science Consortium, are using an approach that considers the potential ecological gains of applying prescribed fire along with the effort needed to burn a given area and the ease with which managers are likely to be able to apply prescribed fire in a particular landscape.

Prescribed fire is an important tool for managing fire-dependent natural communities, like these oak barrens found at Quincy Bluff and Wetland State Natural Area. Photo by Tallgrass Prairie and Oak Savanna Fire Science Consortium

The group is piloting this approach in Wisconsin, but is hoping that it may be more broadly applied across the region, and possibly in other regions, in the future. ‘Prescribed fire is a critical management tool for many landscapes in the US. Given limited resources, it is important to be able to use this tool strategically to achieve the greatest possible gains for our conservation dollars,’ says Sarah. To that end, the group is relying heavily on free, publically available datasets (e.g., Landfire vegetation data, Wildlife Action Plans, and Wildland-Urban Interface housing data), and common sense approaches to prioritization. ‘It is really important to us that others to be able to understand and apply this approach in their own landscapes. Many land management agencies and conservation organizations are working very hard to conserve, manage, and restore our native landscapes. The work is time consuming, complex and often expensive. We want to provide these managers with practical tools that can help them decide where they can get the greatest benefits from their efforts.'”

Massaging the U.S. Census to look at the WUI and at housing growth trends

Today I’m sitting down with Dave Helmers, a GIS/Research Specialist in the SILVIS lab, to talk about his magic fingers.DL: So when you’re massaging US Census workers, do you use the same technique for everyone? Or are these individual-specific? Swedish or Shiatsu?DH: Ha. I don’t actually massage the workers themselves. What I do massage, though, are the data collected by those workers. I would argue that both are equally in need of a backrub.DL: Oh… I see… so why massage the data?DH: Out of the box, US Census data was not really designed for ecological research, but does provide a wealth of information that can be manipulated to answer ecological questions. Several questions of particular interest to the SILVIS lab have been how to map the Wildland Urban Interface (WUI; pronounced ‘WoooWeee’) across the conterminous US, and whether housing density has changed adjacent to protected lands over time. The census data comes in a database comprised of census blocks, where each block is of a various size and whose boundaries are generally defined by political instead of natural boundaries. Housing information is reported per census block – an administrative boundary with no information on where housing may or may not be distributed in actual space.DL: I see, this is similar to the work I do with weather radar, where I’m using it to study bird migration, an application for which it was never intended. We do some serious data massaging to correct for all sorts of atmospheric issues with the data. Okay, you’ve got my attention, so what exactly do you do with the census data?DH: My first order of business is to identify those areas where houses ‘can’t’ exist. These are areas in the public domain; protected areas, national parks, reserves, etc. I call this the ‘public land adjustment’ (PLA). To do this I overlay a public lands spatial database with the census data to partition each census block into public and private areas. I then reallocate all of the housing numbers to just the private zones of blocks.

Map of a portion of Manistee National Forest (Michigan) showing the effects of the public land adjustment (PLA) on WUI mapping. Blue areas are block portions that fall within public/protected areas (no housing) and are removed from WUI. Red areas show newly concentrated housing block portions that now have enough density to qualify as WUI.

DL: Aha, so this correctly allocates density to the spatial region where houses are allowed, which is important when you’re trying to understand how housing pressures are changing over time, or when defining risks to fires, etc.DH: Exactly. For instance, in the western US, where population density is lower, census blocks are typically much larger and may encompass a large proportion of public lands. Add to this the prevalence of large fires and you start to see the importance of accurate housing density numbers to correctly designate WUI status and to understand the spatial arrangement of high-fire-danger zones. Since the WUI designation is based on housing density (at least 1 housing unit per 40 acres), if we’re going to use census data to map the WUI, we need to have a spatially accurate measure of housing.DL: So is that the only massaging you do to the census data?DH: No, not at all. The other big knot we have to work out, if you will, is the fact that census blocks may change in shape and size from one Census to the next (the US census takes place every 10 years) so even if we know that the raw numbers of houses have gone up or down between periods, we can’t actually say anything about housing density until we account for any spatial changes. To deal with this we overlay the perimeters of three time periods (spanning 30 years) for each census block and map those areas that are shared between them, and those which differ, and compute a time-corrected housing allocation for each resulting polygon. Effectively this allows us to correct for spatial variation in census blocks and make comparisons of housing density across time.

To look at the problem of mismatching 1990 and 2000 census boundaries, let’s take a look at a few counties in Oregon. On the left we see 2000 block boundaries in white overlain on black 1990 boundaries. There is some correspondence but many 1990 boundaries are evident within 2000 blocks, especially in the northern or upper parts of the counties. On the right we see black 1990 boundaries overlain on white 2000 boundaries. Here there are many 1990 blocks that contain several 2000 blocks.

DL: Wow! That sounds like a lot of work, but in the end you’ve got an ecologically relevant product.DH: Yeah, that’s the idea. So the two main questions we’ve been asking with this dataset in the SILVIS lab are ‘how is the WUI changing over time’ and this includes some modeling efforts to predict how it will expand or contract in the future. We’re also asking ‘how does housing density change with respect to protected areas’. Since we see proximity to protected areas as a desirable lifestyle choice, we predict that housing density should increase near these areas. By correctly appropriating the housing numbers to those areas where houses may be built, and by correcting for the changes in census blocks over the years, we have been able to demonstrate that indeed housing around protected areas has increased and that this is having an effect on some guilds of birds. That last bit is from work done by Eric Wood and currently in review.

Animation showing housing growth around Great Smoky Mountains National Park between 1940 and 2010.

DL: So are these datasets and products you’re creating for in-house use, or are they available to the public?DH: We serve this data to the public in several forms, ready-to-use maps of housing density and housing density change over time, as well as the geospatial (GIS) data for additional crunching should someone be interested in using it for an analysis. These data area available on our website at the following links: One last question, so what cool products should we be looking for coming out of the SILVIS massage parlor in the near future?DH: I think the next coolest product will be the WUI change map. Everyone wants to see this, especially the US Forest Service, but the impacts of this dataset will be far-reaching from basic science to management applications, to insurance adjusting. As the U.S. population increase and the old paradigm of having to live near your work degrades, we’re going to see more people living in fire-prone ecosystems. If the climate predictions are accurate, and extreme weather events are going to increase, this type of data is going to be super relevant.DL: It sounds like your masseuse skills are being put to good use. Thanks for taking the time to talk, now if you would, I’ve got this knot on the right side here…”

Why and where do fires start (in the Huron National Forest)?

Forest regeneration after the Meridian Boundary Fire

In 2010, the Meridian Boundary Fire burned over 8,500 acres in and near the Huron District of the Huron-Manistee National Forest. Like 99% of the other 340 fires that have occurred on the Huron since 1994, the Meridian Boundary Fire was human-caused. The fire destroyed 13 homes, damaged 2 others, and destroyed or damaged 46 outbuildings. There was nothing unusual or sinister about the fire. The ignition was accidental. Things simply went wrong as a resident executed a permit to burn brush.One reason why this area burns so often is that forests are dominated by Jack Pine, a native species that is extremely flammable. A second reason is that the soils are very sandy leading to rapid drying of fuels even after heavy rains. ‘Fires in this system can go crazy’, says Avi Bar Massada, a post-doctoral researcher in the SILVIS lab. ‘Burning jack pine stands can end up as raging crown fires’.

Bar Massada is part of a team of researchers seeking to predict where fires are likely to occur and which anthropogenic variables are most often associated with fire ignition.Bar Massada, Professor Volker Radeloff, and their collaborators from the Conservation Biology Institute and the U.S. Forest Service have compared multiple modeling approaches to determine which variables are most strongly correlated with fire ignition. Interestingly, Bar Massada says, ‘Fire ignitions behave pretty much the same as wildlife species occurrences.’ In fact, one of the modeling approaches he used for predicting wildfire ignitions is MaxEnt, a program that has been used to model distribution of wildlife species. MaxEnt uses known occurrences of a species (or, as in this case, a wildfire ignition event) to determine which factors best predict those occurrences. Those factors can then be used to predict future occurrence of the phenomenon of interest, such as fire ignition.

Wildfire ignitions and Land cover in the study area, Huron National Forest in central Michigan, USA.

The research has shown that the best predictor of fire occurrence is distance from the nearest road, a consistently good predictor of fire occurrence in the 48 contiguous United States. Fires on the Huron NF rarely occur far from a road, because humans – rather than lightning – are causing the fires. The other three major factors that predict where a fire will ignite include distance from the nearest house, housing density in the surrounding area, and elevation.The ultimate goal is to be able to direct limited resources to preventing fires before they start, saving life and property. Accurate prediction of ignition hazard is critical to achieving it. style=”text-align:”>

Predictive maps of ignition probability for the three model types. Values denote the suitability of each pixel for having an ignition occurrence.

How are fires in western Mexico affecting hummingbirds?

Cuchilla de la Tuna – a site without fire for more than 30 years

Sarahy Contreras has been studying hummingbirds in western Mexico for nearly 20 years. Her current project tackles the question of how fires, and the anticipated future increase in fires in western Mexico, may affect hummingbirds. Surprisingly, relatively little is known about hummingbirds as a group. Certainly they are hard to study and catch – a tiny bird in constant motion from flower to flower, species to species, and often through continent over the course of a year. Yet this is exactly why. Contreras wants to study them. How are they distributed across the landscape? How closely are their movements linked to the phenology of different flower species? And how do fires of varying intensities and frequencies affect those patterns?The biosphere reserve where Professor Contreras works has long been recognized as a hummingbird hotspot. Of the 24 hummingbird species that occur in the state of Jalisco, 23 of them are found within the Sierra de Manantlan Biosphere Reserve. The reserve sits in the mountainous area of western Mexico, which during the winter harbors the highest diversity and abundance of terrestrial migratory birds throughout the Neotropical region. But the reserve is not just for birds and the pine-oak forests which dominate its landscape. Nearly 400,000 people live within and around the reserve as well.Fire has traditionally been an important component of the landscape in western Mexico and an important tool for its people. The majority of fires are set by people to clear land for agriculture. On occasion, these fires escape and can burn quite large areas. Some of these wildfires are of relatively low intensity, primarily burning the shrub layer and rarely reaching tree crowns. Other fires are severe – burning everything up to and including the tree crowns. A large effort is currently underway in the Sierra de Manantlan Biosphere Reserve to change local attitudes and practices related to fire. Part of this project is an increase in capacity to put out fires once they are started. Another important component of the project is education for the communities inside the biosphere on the natural resources in the reserve and how those resources and species are affected by fires.

Gatos bola – a site with a recent fire high severity fire

Professor Contreras is studying the effects of these fires on hummingbird populations in the reserve. In particular, she is examining how hummingbirds fare over time in sites that were burned by fires of different intensities. Professor Contreras is also interested in how hummingbird populations change over time in previously burned sites. Her study sites include a continuum of sites burned relatively recently (less than 10 years ago) to sites burned more than 25 years ago. As these sites become revegetated and move through various stages of forest succession, Professor Contreras is tracking not only which species are using the sites but also the health of the populations of the individual species. Metrics including abundance, age and sex ratios, and survivorship provide an indication of the health of these populations over time. Luckily Professor Contreras started collecting bird data in the reserve nearly 20 years ago. She initiated and now coordinates a bird banding program that is active both in the biosphere reserve and throughout Mexico. The resulting comprehensive database of bird observations across sites and over time is allowing her to look at population response to burned sites over a 25 year time period – a rare opportunity to understand long term responses of hummingbird populations to fire.

Team for bird surveys, PHD Sarahy at the Sierra de Manantlan Biosphere Reserve

Professor Contreras is just beginning to analyze her data. Initial results suggest that the response of hummingbirds to fire depends on the species in question. For example, rufous hummingbirds, a common visitor to the US and one of the hummingbird species with the longest migrations, do quite well in recently burned areas. However, as burned sites succeed back toward a closed canopy pine-oak forest, their use of sites decreases. In contrast, Amethyst hummingbirds, a resident forest specialist with a very restricted range a preference for pine-oak and cloud forest habitats, are essentially absent from recently burned sites and only return once the forest structure returns.The results of the study will be important for the conservation of hummingbirds, identifying the particular habitats that different species depend on, and how populations of those species respond to the presence of different intensities and frequencies of fire on the landscape. Results will be translated into recommendations for conservation and forest management within the region that may benefit both resident hummingbirds and migratory species which overwinter in the reserve. The study will also provide insight into how hummingbird populations may be affected an increasing presence of fire in western Mexico that current models of global climate change predict.

Site location
Rufus hummingbird

The Collapse of Socialism and the Rise of Fire in Kalmykia

Grassland donimated by Stipa capillata in spring

Nowadays, wildfire is a regular occurrence in arid grasslands of Kalmykia, Russia. However, landscape analysis of a 23 year period showed significant changes in the fire regime that might be directly linked to socioeconomic changes in Russia during that period. With the fall of the Iron Curtain in 1991, there were socio-economic changes that impacted all of society – from day-to-day life in the big cities to rural farming operations.When a forest burns, the change in the vegetation is dramatic and the recovery is slow. In contrast, effects of wildfires in grasslands are not as conspicuous and recovery happens quickly, often the same year of the fire. These characteristics require novel methods to map wildfires in grasslands accurately. (photo-stipa-green-nonburned and photo-burned-area)

Figure 1: Change in burned area

PhD candidate, Maxim Dubinin, has mapped the patterns of fire in this Russian region west of the Caspian Sea. Maxim wondered just what was happening in these arid grasslands and how the collapse of socialism affected landcover. His goal was to quantify how (and whether) fire changed after 1991. To accomplish this goal, he analyzed a series of satellite images between 1985 and 2007, two images per year, one prior to the fire season begins (early spring) and one image after (late summer / early fall). For each year in that time period, he mapped where grassland fires occurred.After 1996 there was a dramatic increase in the area of land burned each year (Figure 1). Maxim and his colleagues propose this is a direct result of the socioeconomic changes. You might wonder how a political shift might result in such a change and why there was a five year time lag? With the socio-economic change, livestock subsidies were eliminated, and farmers could no longer afford to keep their animals (Figure 2). The result was a dramatic decrease in grazing pressure and an abundance of grass that was ready to burn.

Figure 2: Livestock dynamics

Towards the end of the study period, the economy started to grow again and farmers were once again able to support more livestock, resulting in an increase of grazing, and possibly, a decrease in fire as evidenced by Figure 1. It is too soon to tell whether there is a direct link, but with a few more years of data and additional analysis, a trend of further reductions in wildfire activity might become more apparent.This is just one part of the story of fires in Russia. Next steps of Maxim’s project involve a more robust statistical study of causes of such change (including various human and environmental factors) and Maxim studying the changes in vegetation types, especially a shift from Artemisia shrublands to Stipa dominated grasslands, that might have been caused by the more frequent fires.”