Changes in Soil Functions Following Invasions of Exotic Understory Plants in Deciduous Forests

Eastern deciduous forests have been invaded by two exotic species that often dominate the understory vegetation. Berberis thunbergii, a woody shrub, forms dense thickets, while Microstegium vimineum, a C4grass, forms continuous lawns; the two species often co-occur. We hypothesized that a variety of characteristics of the exotic species may cause soil-based ecosystem processes to change following invasion, and that such changes could establish positive feedback processes that enhance the spread of the exotic. We examined changes in soil pH, nitrogen cycling, and litter dynamics in stands infested with both species in three locations in northern New Jersey (USA). Soils directly beneath each of the two exotics had higher pH values and higher nitrification rates and often had higher net N mineralization rates than did soils beneath adjacent patches of the most common native understory shrub, Vaccinium pallidum. These differences could be replicated in the greenhouse by growing the exotics in previously uninvaded soils. Berberis litter was much higher in N concentration than was litter of the native species; it decomposed more rapidly than litter of native dominant tree species, with little or no immobilization of N. In contrast, Microstegium litter decomposed more slowly than the native species' litters, and it immobilized N. We suggest that the same change in soil-increased pH and nitrification-result from different combinations of mechanisms in the two exotic species. Both species evidently favor uptake of nitrate, which may elevate pH. However, Berberis combines large biomasses of N-rich roots with N-rich leaf litter, while Microstegium populations combines small biomasses of N-rich roots with small biomasses of N-poor litter that leave much of the surface soil with few roots. The ability to change soil functions may be an important and previously unrecognized characteristic that renders species capable of invading intact communities, while the likelihood that changes in soil functions can alter the growth of the exotics may be an important component of invasibility.