COS 41-4
Short and long term impacts of fire on soil nutrients in Larrea tridentata (creosote bush) shrublands in the Mojave Desert
Fire is an important disturbance in many ecosystems that can alter vegetation and soil properties. Soil nutrients in desert ecosystems are scarce and heterogeneously distributed, being mainly restricted to “fertility islands” created by perennial shrubs (e.g., Larrea tridentata). The fertility islands created by Larrea are important for maintaining the annual plant community under the shrub canopies. North American deserts have been invaded by grasses and forbs, however, and these exotic species have contributed to increased fine fuels that promote fire spread. Nevertheless, the temporal changes of soil properties after fire in deserts remain unclear. We examined the short- and long-term effects of fire on soil nutrients in Larrea dominated areas of the Mojave Desert. The short-term (i.e., 6 months after experimental burnings) effects of fire was determined by comparing soil nutrients nitrogen, phosphorus and potassium (N, P, K) under experimentally burned and unburned Larrea canopies and in the open-areas among shrubs. The long-term (i.e., 7 years after fire) effects of fire were evaluated by measuring soil nutrient availability under living Larrea and on soil mounds where Larrea were killed by natural fire. Plant Root SimulatorTM -probes were used to measure soil nutrients.
Results/Conclusions
In the short-term, we found higher amounts of N and K around burned Larrea shrubs and under their canopy, decreasing with distance from the shrub. Soil P availability was not affected by fire or microhabitat. On the long term, seven years after fire, we found a significant and negative effect of fire on both N and K, but not on P. N and K were lower around burned shrubs (e.g., dead Larrea) compared to unburned shrubs (e.g., living Larrea), suggesting that over the long term, the fertility island effect created by Larrea tends to diminish. These findings reinforce the key role of Larrea in structuring soil nutrients and, therefore, the annual plant community associated with these shrubs. Once the shrub is removed by fire, and soil nutrient availability has changed over time, favorable conditions for the establishment of exotic species (especially grasses) are created, as these species have been shown to have better competitive skills in acquiring limiting resources. Ultimately, the positive feedbacks between invasive grasses and fire could potentially lead to greater impacts on soil properties at broader spatial scales across the landscape.