COS 18-1 - A Chronosequence soil seed banks: Homogenization of the Mojave Desert species pools after fire.

Background/Question/Methods

Soil seed banks often act as ‘biodiversity reservoirs’, where species that occur in highly variable environments emerge under suitable conditions (storage effects). However, disturbances such as fire and associated invasions by non-native species can disrupt these reserves and fundamentally alter community dynamics. In the Mojave Desert of North America, wildfire occurrence over the last several decades has been increasingly driven by highly flammable, invasive annual grasses. A number of studies have investigated effects of fire on aboveground plant communities in the Mojave, but virtually no information exists regarding short and long-term effects of fire on seed banks. Therefore, we conducted assays of the soil seed bank along an elevation gradient with 111 unburned reference plots and 432 plots that had burned 1-3 times between 1972 and 2010. We assessed how three fire regime components (frequency, time since fire and burn severity) interacted with climate (precipitation and temperature) and abundance of invasive plants within the seedbank to influence overall seed bank diversity

Results/Conclusions

In general, plots that burned at mild to moderate severity within the past 10 years supported higher levels of diversity (N₀,N₁,N₂) than either high severity, or control plots. Seed banks from burned plots tended to be dominated by non-native invasive species, primarily one of three grasses, (Bromus rubens, Bromus tectorum, Schismus spp.), as well as an invasive forb (Erodium cicutarium). These invasives were distributed in overlapping distributions across the entire elevational gradient of our study. Invasive plants increased in their proportional abundance in mild to moderate burn severity, and high severity plots experienced similar levels of invasive dominance to moderate burn severity plots. The most striking pattern we observed was a collective sharp decline in N₁, N₂, evenness, as well as both local and regional scales of beta diversity with increases in invasive species dominance, indicating the homogenization of seed bank communities with the colonization of invasives species after fire. While this pattern was strongest in the decade after fire, seed bank communities tended to still be dominated by invasives species even at 35 years post-fire.