Aspect is defined as the orientation of a slope in relation to cardinal points or compass directions. For instance, a south-facing slope would be said to have a southern exposure.
Effect on the Fire Environment
The effects of aspect on the fire environment and fire occurrence are many. Aspect captures different levels of solar irradiance based on the sun azimuth (i.e., how high the sun is in the sky) and latitude, as these two factors dictate the length of time a slope is exposed to the sun and thus the amount of energy it receives (Kumar et al. 1997). Other factors influencing the amount of solar irradiance and heat load received are slope steepness, where steeper slope gradients receive more energy (McCune and Keon 2002), as well as the timing of the day during which a slope is exposed to the sun. Hence, a slope aspect receiving the afternoon sun will benefit from a greater heat load than aspects receiving the morning sun (McCune and Keon 2002). Overall, reflected sun radiation is more important in the mountains as radiation is mainly reflected from accentuated terrain (Kumar et al. 1997). For instance, the differential effect of aspect will be more pronounced in rugged mountainous terrain, where tall mountains cast long shadows reducing the length of sun exposure in narrow valleys, than in low rolling foothills. For a thorough description of solar radiation variation and effect on the Earth’s surface and calculations with a Geographic Information System, please refer to Kumar and others (1997).
In mountainous areas, differential heating in relation to slope and aspect can create micro-meteorological processes causing wind turbulence based on the rate of adiabatic cooling or heating along a slope gradient (Whiteman 2000). Adiabatic cooling occurs when light, warm air moves up in altitude toward colder air masses that cool air molecules in the process. While this turbulence can potentially modify fire behavior, the chief role of aspect is on soil and plant moisture conditions, as well as vegetation types and assemblage (Bennie et al. 2008). Fuel moisture content and wind are the two most important components affecting wildland fire behavior (Sullivan 2017). The amount of water content in fine surface and aerial fuels affect the ease of fire ignition, while the overall fuel moisture content will dictate the rate of combustion and the amount of fuel consumed by a fire (Scott et al. 2014). In the northern hemisphere, the percent humidity of south aspects can be much lower (Sharples 2009) than cool aspects during the period of maximum heat load with warm aspects having been reported to be 3 °C warmer than cooler ones (Geiger et al. 1995) under calm wind conditions. Variations in the heat load received across all slope aspects have a direct impact on the vegetation fuel drying rate and availability for burning (i.e., time lag after a moisture event such as rain or morning dew). Xeric-adapted plant and tree species will be found on warm aspects.
Another impact of aspect is on the length of the fire season. In mountain regions where snow cover is persistent for several weeks or months during the winter, cool aspects will retain snow earlier in the fall and later in the winter and spring (Whiteman 2000). The differential snow accumulation and snow melt can result in a longer fire season, sometimes extended by several weeks, on warm aspects. Following a severe fire where canopy mortality occurred, snow depth on sun exposed aspect will be reduced as a result of ablation. This phenomenon reenforces the dry cycle condition of warm aspects and can make those slopes even more susceptible to burning (Stevens 2017).
The long-term role of aspect on fire occurrence has been captured by many fire history studies focusing on the effect of terrain on mean fire return intervals (Beaty and Taylor 2001; Rollins et al. 2002; Flatley et al. 2011; Rogeau and Armstrong 2017). The mean fire return interval (MFRI) is the average time, in years, between fire events in a defined area such as a point on the land, or a larger surface (i.e., watershed, region). The general consensus in the northern hemisphere is that warm aspects (west and south facing) have shorter MFRI that can be half as long as their cooler counterparts (Rogeau and Armstrong 2017). However, there is a documented synergy between aspect and elevation, or slope position. For instance, in dry fire-adapted ecosystems of the Canadian Rocky Mountains, MFRI from warm aspects tends to be similar to those at valley bottoms or at the lowest elevations, and regardless of aspect, MFRI will increase along an elevation gradient where cooler and moister conditions prevail near ridgetops (Rogeau and Armstrong 2017). The pattern of greater fire frequencies on south-facing slopes is well established and has been detected in charcoal studies back to 9000–12,000 BP (Gavin et al. 2003; Carcaillet et al. 2009).
To summarize, warm aspects are drier and naturally experience greater fire frequencies. Historically, fires occurring at short enough intervals (Van de Water and Safford 2011) could maintain low fire intensities because the fuel load and fuel structure did not have the opportunity to develop (Wright and Agee 2004; Rogeau et al. 2016). In contrast, northerly aspects, which harbor cooler and more humid conditions that are conducive to generating productive forests (greater wood volume, thicker understory, and thicker duff), do not burn as frequently. The longer fire-free intervals associated with cool aspects contribute to greater fuel development and thus higher fire intensities when fuels are available for burning during a sustained drought.
However, with decades-long fire management policies of fire exclusion near communities, the vegetation and forest cover on warm aspects have evolved into mature stands with a well-developed surface duff layer, similar to those of cool aspects. We are now seeing an increased threat as fuels on warm aspects will burn as high intensity fires with high severity consequences, which will delay the post-fire regeneration and can have lasting impact on the future fuel trajectory.