Initial soil amendments, but not seed applications, still affect plant community composition after nine years of succession in a heavy metal contaminated site

Background/Question/Methods

            Ecological restoration of polluted sites is important in reclaiming these sites for productive uses.  Monitoring restoration sites over time may also teach us about succession.  The factors influencing succession are of basic interest to ecologists, but also highly relevant in restoration, where succession may be at least as important as initial revegetation in restoring desired plant communities.  We examine whether initial soil amendments and seed treatments affect succession such that plant communities nine years later still bear lingering differences attributable to the initial treatments. 

            In 2012, we conducted a vegetation census, identifying and measuring percent cover of all herbaceous species in twenty-four 1 m² quadrats and all woody species in twelve 100 m² quadrats in each of 30 one-acre plots in the Palmerton Zinc Superfund Site, a >2000 acre mountainside contaminated and devegetated by >80 years of zinc smelting.  In 2003, each plot received one of three groups of grass seeds differing in species composition, and one of five compost types.  We asked whether current plant communities differ due to initial restoration treatments in order to determine whether compost or seed application may affect succession.  This information allows us to evaluate the most effective treatments in light of restoration goals.

Results/Conclusions

            We recorded 10 of the 13 species originally planted, along with approximately 92 species not planted at the site.  Constrained analysis of proximities (CAP) revealed that compost type significantly affected plant community composition, but seed treatments did not.  Differences between compost treatments were driven mainly by differing abundance of the few most common species, including two herbaceous species which are of special interest because of their suspected high metal uptake and/or tolerance. 

            Compost type, but not seed treatment, was also related to exotic species abundance.  Plots amended with sewage sludge supported the lowest relative frequency and cover of exotic species and the fewest species overall, but, surprisingly, the greatest total plant cover.  Plots amended with Lehigh County compost and mushroom compost had seven more species on average, but also greater proportions of exotics.  Although species invasions are often a problem after disturbance, exotic species are unusually rare in Palmerton: for instance, the average relative frequency and relative cover of exotic species per plot are 10% and 8%, respectively.  We conclude that compost types can have long-term effects on plant community composition and succession.  Soil treatments should thus be carefully considered when seeking to revegetate disturbed areas with native species.