Occurrence of non-native species deep in natural areas of the Shawnee National Forest, Southern Illinois, U.S.A.
Biological invasion is a leading threat to the ecological integrity of forest ecosystems around the world. The objective of our study was to examine the relationship between the abundance of non-native species in the vegetation and the soil seed bank to abiotic factors and distance from the nearest forest edge. We identified and estimated the abundance of plant species in sixty-two 0.4 ha permanent plots in natural areas across the Shawnee National Forest in southern Illinois in late spring and summer 2002. The distance of each plot to the closest forest edge was determined using GIS. A 550 cm3 soil core was extracted from five random points within each of the plots in August 2002 to allow measurement of soil nutrient levels and estimates of the abundance of exotic and native species in the soil seed bank. Soils from these cores were placed in trays in a greenhouse on 10 January 2003. Seedlings emerging from the soil cores were identified and tallied every two weeks until no further seed germination was observed. The plots were located deep in the forest interior (up to 730 m), yet forest edge effects predominate. Distance from forest edges was negatively related to density of both native and non-native species in the seed bank. The largest numbers of non-natives in the seed bank were present in those plots closest to the forest edge. None of the nine important abiotic factors (K, Ca, P, Mg, pH, CEC, and OM, % silt, canopy opening) measured in this study was related to the seed bank density of the native and non-native species. Potassium was negatively and calcium was positively related to richness of understory native species and distance from forest edge. Phosphorus, Mg, pH, CEC, % silt, OM, and canopy opening were not related to the richness of native species and distance. Commelina communis, Lonicera japonica, Microstegium vimineum, and Rosa multiflora were non-native species present in the understory vegetation while Barbarea vulgaris, Cardamine hirsuta, Lactuca serriola, Lespedeza cuneata, M. vimineum, Mollugo verticillata, and Stellaria media were non-native species present in the seed bank. The discordance between non-native species richness in the seed bank and vegetation community indicates that management of invasive species must extend beyond eradication of non-natives in the aboveground vegetation. Land and forest managers should eradicate seedlings of non-native species before they reach seed production stage to decrease their accumulation in the seed bank even at sites located at a great distance from the forest edge.