Seed Bank Responses to Grazing History by Invasive and Native Plant Species in a Semi-desert Shrub-steppe Environment

Invasive alien plants frequently possess greater seed banks than native species in disturbed areas, but native disturbance-adapted species may also possess large seed banks. We analyzed seed banks of ten sites with varying grazing histories in a shrub-steppe ecosystem dominated by antelope bitterbrush (Purshia tridentata) in the Okanagan valley in British Columbia, Canada. Overall effects of grazing on seed banks were overshadowed by site-specific differences, but canonical correspondence analysis (CCA) showed that the composition of the seed bank was significantly affected by bare soil which was positively correlated with grazing. Other factors affecting seed bank composition included litter, cover of spikemosses (Selaginella), shrub cover and microbiotic crust cover. The seed bank of the invasive herb diffuse knapweed (Centaurea diffusa) was largest at two of the three most heavily grazed sites. Cheatgrass (Bromus tectorum), another invasive species, exhibited large seed banks at all sites regardless of grazing history. Native species such as sand dropseed (Sporobolus cryptandrus) and Douglas' knotweed (Polygonum douglasii) had substantial seed banks (> 1500 seeds m−2). Planting sand dropseed, Douglas' knotweed, needle-and-thread grass (Hesperostipa comata) or their ecological equivalents could aid in the restoration of arid ecosystems because of their adaptation to disturbance and persistence of seeds in the soil.