Sand prairie seed banks, existing vegetation, and responses to disturbance history

Knowledge of seed banks on degraded land allows managers to assess whether they serve as a refuge for introduced species, and/or provide positive functions, such as to increase native species diversity, buffer species against extinction, or maintain local genetic integrity.  This study provided such an assessment for three degraded sand prairies with different disturbance histories in northwestern Illinois.  It also determined the relationship between seed banks and extant vegetation, and how both are affected by historical disturbances of grazing and introduced plant species.  In each site, 30 replicate 6 cm depth soil samples were extracted and the germinable seed bank was identified and quantified in a greenhouse study.  Concurrently, extant vegetation composition and cover were surveyed from 30 permanent plots adjacent to seed bank plots.  Abundance and frequency data were combined to obtain importance values (IV).  Species richness and importance values were indicative of past land use, but seed banks were not representative of extant vegetation composition.  Extant vegetation represented in the seed bank was 46, 41, and 28 percent in Sites 1, 2, and 3 respectively.  Seed banks of two recently grazed sites had similar species richness (Site 1: 25/9, Site 2: 28/10 (native/introduced richness)).  However, their importance values differed greatly (Site 1: 77/113, Site 2: 153/34 (native/introduced IV)).  Site 1 had an introduced species that dominated the extant vegetation and did not contribute to the seed bank.  The seed bank of Site 3, which had the longest time since grazing, had lower introduced species richness (25/3 (native/introduced richness)) and importance values (181/6 (native/introduced IV)).  These results confirm studies from other ecosystems where seed banks are not representative of extant vegetation.  This study underlies the importance of assessing the seed bank when considering management options and whether or not soil disturbance is precluded or can be tolerated.