Wildfires have made national headlines in recent years, with major fires in the West and South killing firefighters, burning homes, and threatening communities. With the population and size of the wildland-urban interface (WUI) expanding, more money is being spent on fire management in these locales. Federal funding for fire protection has increased over the past decade, and administration and congressional leaders have urged additional wildfire protection. In FY2014, approximately $3.9 billion was appropriated to protect federal lands and approximately $91.0 million to assist in protecting nonfederal lands. The U.S. Forest Service (USFS) in the Department of Agriculture, along with the Department of the Interior (DOI), are responsible for protecting the majority of federal lands from wildfire, including any federal areas that comprise a WUI. Wildfire funding accounts for approximately 56% of the USFS budget and roughly 8% of the DOI budget in 2014. While attention has focused on protecting life, property, and communities in the WUI, opinions vary over how to provide protection. This report describes the growth of the wildland-urban interface, wildfire suppression efforts, post-fire responses, and especially the programs and options for protecting the interface before the next wildfire strikes. The term wildland-urban interface (WUI) has been used for more than two decades to suggest an area where structures (usually homes) are in or near wildlands (forests or rangelands). There is no standard WUI definition. However, the definition listed in a 2001 Federal Register notice is commonly referenced—the urban-wildland interface community exists where humans and their development meet or intermix with wildland fuel. In this same notice, the USFS and DOI identified three types of WUI communities: interface, intermix, and occluded. Based on state data, they listed nearly 4,500 interface communities (with 11 states not providing data). The agencies defined an interface community as one where wildlands abut structures with a clear line of demarcation between residential, business, and public structures and wildland fuels, while an intermix community is where structures are scattered and intermingled with wildlands and fuels. An occluded community, generally existing within a city, is where structures abut an island of wildland fuels (e.g., park or open space). One source indicates that all states within the contiguous United States contain land classified as WUI. See Figure 1 . Furthermore, approximately 10% of all land within the lower 48 states is classified as WUI, with a significant concentration along the East Coast, although western states have the highest proportions of their homes in the WUI. The area of intermix communities is large and is growing faster than the area of interface communities, based on data from 1990 to 2000. Nationwide, in 2010, the interface area was equal to approximately 39 million acres, and the intermix area equal to approximately 152 million acres. In 2010, intermix communities in the three Pacific Coast states totaled 9.3 million acres, almost three times the 3.6 million acres in interface communities in those states. From 1990 to 2000, the 10-year growth in area of intermix communities was 14.1%, compared to only 2.5% for interface communities. However, the study acknowledged that determining the area of WUI communities was imprecise: "Mapping [the Federal Register ] definition of the WUI using data and operational definitions we developed, we arrived at one possible representation of the WUI." The intermingled nature of intermix communities poses significant challenges for fire protection efforts. In most of the United States, wildfires are inevitable. Biomass plus dry conditions equals fuel to burn. Add an ignition source (e.g., lightning or a match) and a wildfire may ignite. Fire is a self-sustaining chemical reaction that perpetuates itself as long as all three elements of the fire triangle—fuel, heat, and oxygen—remain available. Fire control focuses on removing one of those elements. There are two principal kinds of wildfire, although an individual wildfire may contain areas of both kinds. A surface fire burns the needles or leaves, grass, and other small biomass within a foot or so of the ground and quickly moves on. Such fires are relatively easy to control by removing fuel with a fireline, essentially a dirt path wide enough to eliminate the continuous fuels needed to sustain the fire, or by cooling or smothering the flames with water or dirt. A crown fire burns biomass at all levels, from the surface through the tops of the trees. Crown fires do not consume all the biomass; rather, a crown fire quickly burns the needles or leaves and small twigs and limbs on the surface and throughout the crown of the trees. Because the needles and leaves in the crown are green, they require more energy to burn than dry fuels on the surface. Furthermore, because of the green fuels and the often discontinuous biomass of the canopy, wind is usually needed to sustain a crown fire. Once burning vigorously, a crown fire can create its own wind—the strong upward convection of the heated air can draw in cooler air from surrounding areas, thus creating a wind that feeds the fire. The strong upward convection can also lift burning biomass ( firebrands ) and send it soaring ahead of the fire, creating spot fires and accelerating the spread of the wildfire. Thus, crown fires are difficult, if not impossible, to control. Firelines are often ineffective, especially if winds are causing spot fires. Water or fire retardant ( slurry ) dropped from helicopters or airplanes can sometimes knock a crown fire down (back to a surface fire) if the area burning and the winds are not too great. Often, however, crown fires burn until they run out of fuel or the weather changes (the wind dies or it rains or snows). Fires burn structures in one of three ways: through direct contact with fire (the fire burning right up to the structure); through radiation (heating from exposure to flames); and through firebrands landing on a flammable roof. Surface fires generally only burn houses through direct contact, and protection is a relatively simple matter of a break in the continuous burnable material. In observing houses that burned in Los Alamos in 2000, one researcher stated "in several cases, a scratch line that removed [pine] needles from the base of a wood wall kept the house from igniting." Crown fires, however, can burn houses in any of the three ways. The opportunity and ability to prevent structures from burning during a crown fire is small. Occasionally, water or some other wetting agent sprayed on walls or roofs can prevent ignition or extinguish firebrands from an advancing wildfire, but the firefighters could die of heat exposure or smoke inhalation from the approaching fire. Recovery and efforts to support recovery after a severe wildfire vary, depending on the nature of the damages. For burned structures, insurance payment is the standard means for homeowners to pay for recovery—repair, if that is possible, or replacement, depending on the insurance policy. In a severe event, a presidential declaration of an emergency (in response to a request from a governor) initiates a process for federal assistance to state and local governments and to families and individuals to help with recovery. The nature and extent of the assistance depends on several factors, such as the nature and severity of damages and the insurance coverage of the affected parties. For burned areas, site rehabilitation is sometimes warranted. In many temperate ecosystems, wildfires (including crown fires) are natural events, and the ecosystems are adapted to recover from the fire. Often, in severely burned areas, grass seed is spread to try to accelerate growth of ground cover and slow erosion, but grass often inhibits tree seed germination and growth, and thus may slow forest recovery. Rehabilitation efforts commonly focus on the firelines created to try to control the fire, since firelines consist of exposed bare earth and often run uphill, and thus can readily erode into gullies if left untreated. Some severely burned areas, particularly in coastal southern California, are susceptible to landslides during the subsequent rainy season. Monitoring can provide a warning to homeowners to evacuate an area prior to a landslide, but little can be done to prevent landslides in such situations. Various efforts can protect structures and wildlands from some of the damages of wildfires. A structure's characteristics and landscaping significantly affect its chance of surviving a wildfire. Evidence from models, experiments, and case studies demonstrates that structural characteristics, especially the roofing materials, largely determine whether a home burns in a wildfire. Homes of brick or adobe with non-flammable roofs (e.g., tile, slate, metal) are far less likely to burn than homes with wood siding and flammable roofs (e.g., wood shingles). Burnable materials (such as trees, shrubs, grass, pine needles, woodpiles, wood decks, and wooden deck furniture) within 40 meters (131 feet) of the structure also strongly influence whether the structure burns in a wildfire. Furthermore, the structure and landscape characteristics are more important than the intensity of the fire in determining whether a house burns. The Hayman Fire, in Colorado in June 2002, burned 132 houses—70 houses (53%) were surrounded by crown fire, while 62 houses (47%) were surrounded by surface fire. In addition, 662 homes (83% of all homes within the fire perimeter) survived the fire, even though 35% of the area was severely burned and 16% was moderately burned. This suggests that at least some of the structures survived despite a crown fire around them. The impact of wildfires on wildlands depends largely on the nature of the ecosystem. Some ecosystems are adapted to and recover from periodic crown fires—perennial grasslands, chaparral, lodgepole and jack pines, and more.