Wildfires worldwide annually burn an area of nearly 1.4 million square miles, and their number and intensity are likely to increase as the climate warms. By clearing out vegetation, fires increase the vulnerability of soils to erosion and flooding. The hotter the soil gets during a fire, the greater the potential damage to the ecosystem following the fire, and the more difficult it becomes for the burned area to recover. A recent study revealed a surprising relationship between aboveground fire intensity and soil temperatures—and consequently, soil damage.
A team of Dutch and Portuguese scientists led by Cathelijne R. Stoof, then at Wageningen University in The Netherlands, planned a controlled fire in a Portuguese shrubland watershed, a region at high risk of fire and land degradation. They mapped the vegetation in the area and collected data on the characteristics of the landscape, solar radiation, and fuel load—the density of vegetation. After deploying an extensive array of instruments to record the effects of the blaze, they set twenty-two acres of forest on fire, and monitored soil temperatures, flame temperatures, fire intensity, and fire spread. Although previous, smallscale experiments had shown that soil temperature during a fire increased with fuel load and fire intensity, Stoof’s team found contrasting results. Sparsely vegetated areas—those with the lowest fuel load—recorded significantly higher maximum soil temperatures (370 degrees Fahrenheit) than heavily vegetated areas (189 degrees Fahrenheit) where the flames were more intense. This reverses the current understanding of where the greatest fire damage likely occurs.
Stoof explained that in areas of dense vegetation, increased heat transfer upward and into the atmosphere left the soil cool. Because fire also spread faster in these areas, that soil was exposed to the heat for a shorter time. In addition, the moisture from tall, dense vegetation that hadn’t dried out completely before the fire helped protect the soil from overheating. Finally, because shrubland fires move primarily through the canopy, not at ground level, the taller the vegetation the less the soil was affected.
Land managers could make use of these findings to better plan prescribed burns, as well as to predict areas of greatest post-fire risk and to improve the efficiency of restorative measures. (Geophysical Research Letters)