Effects of a Spring Prescribed Burn on the Soil Seed Bank in Sagebrush Steppe Exhibiting Pinyon-juniper Expansion

—Pinyon-juniper (Pinus monophylla–Juniperus osteosperma) woodlands are expanding into shrubsteppe ecosystems in western portions of the Great Basin. Often, highly competitive trees displace the understory, and prescribed fire is increasingly used as a restoration tool. To inform management decisions about post-fire recovery, we examined immediate and long-term (i.e., 2 growing seasons) responses of the germinable seed bank to a spring prescribed fire. One week before and 1 week after a May 2002 prescribed burn, soil samples were taken under P. monophylla trees, under Artemisia tridentata ssp. vaseyana shrubs, and from interspace microsites. Two growing seasons after the prescribed burn, soil samples were taken from the same microsites and from similar microsites in an adjacent unburned area. Prior to the burn, the germinable seed bank under shrubs contained the highest seed density and the highest species richness followed by interspace and tree microsites. Shrub litter was consumed by the burn, causing complete loss of the seed bank from that microsite and depleting the A. tridentata seed bank. Interestingly, the density of germinable seeds in soil under P. monophylla trees increased immediately following the burn; 70% of that increase originated from the annual forb Descurainia pinnata. Two growing seasons following the prescribed burn, no overall differences in germinable seed density between burned and unburned plots were observed, but seed bank species composition differed, with seed banks in the prescribed burn having a greater abundance of Eriogonum elatum, E. microthecum, and Gayophytum diffusum compared to unburned plots, which had a greater abundance of A. tridentata, Lappula occidentalis, and Descurainia pinnata. Our results indicate that rapid restoration to prefire vegetation cannot rely upon the soil seed bank. To ensure rapid recovery, land managers should select sites for prescribed burns that have fire-tolerant perennial vegetation.