Effect of Wildfires on Physical, Chemical, and Biological Properties of Soil Resource Islands in Joshua Tree National Park

Before 1965, most lightning-strike fires in and near Joshua Tree National Park burned <1/4 acre. Since 1965, fires have become larger (>5,000 acres) and more frequent. Recent fire spread has been facilitated by non-native annual grasses red brome and cheatgrass. El Niño conditions strengthened by global warming has resulted in higher desert winter rainfalls which stimulates the growth and increases the fuel continuity of these invasive annuals, allowing fire to spread between the Joshua trees and soil resource islands (mounds). Under similar conditions, the Pushwalla Complex Fire burned >2,000 acres of shrub and Joshua tree habitat in July 2006. Our objective was to determine post-fire changes in soil physical, chemical, and biological properties in the soil/shrub resource islands; and to observe how these changes might contribute to erosional processes and redistribution of nutrients between the mound and intermound areas. We measured soil bulk density, infiltration rate, and aggregate stability; soil pH and nitrate nitrogen; and soil respiration and soil organic matter (SOM) in both soil mound and intermound areas within the fire boundary and control area adjacent to the fire. For soil mounds, bulk density increased from 1.10 pre-fire to 1.23 g cm-3 post-fire, infiltration decreased by ~30 %, and aggregate stability was reduced. Soil pH increased from 6.7 pre-fire to 8.0 post- fire. Soil mound nitrate N decreased post-fire, but exhibited a slight increase in the intermound areas. Percent SOM dropped from 1.6 pre-fire to <1.0 post-fire, with soil respiration dropping >50%. Our results suggest that fire consumes the organic glues and fungal hyphae that bind soil particles together, and also reduces porosity by plugging macropores and increasing bulk density. With reduction in aggregate stability and infiltration, it is expected that wind and water erosion will be accelerated, leading to redistribution of soil nutrients into the intermound areas.