Passive restoration of areas affected by mining activity relies on natural plant community succession to return disturbed areas to native ecosystems. However, colonization by invasive plant species frequently places communities on successional trajectories that are slowed or progressing towards undesirable states in which native species are unable to thrive. In this study, we examined the progress of passive restoration of former surface topsoil and gravel mines, now managed by Cuyahoga Valley National Park, Ohio, USA, where mining had exposed alkaline glacial till parent material. We then examined the effects of autumn olive (Elaeagnus umbellata), a nitrogen-fixing invasive shrub, which is the dominant woody species that has colonized the sites. Although invasive, the nitrogen-fixing capability of autumn olive could also be enriching the soil with available nitrogen (N) and so encouraging more rapid succession in the immediate vicinity of individual shrubs. We therefore compared the effect of autumn olive to effects of other woody shrubs on the surrounding herbaceous vegetation and soil N availability.
Results/Conclusions
Succession towards a forest ecosystem under passive restoration appears to be far slower than suggested in site restoration plans developed in the 1980s. Using a 35-year chronosequence, we observed inconsistent increases in woody vegetation, and most of the woody plants present today are invasive species (autumn olive and glossy buckthorn, Rhamnus frangula) that are expected to be counterproductive to reestablishment of native forest. As predicted, we found soil N fixation potential was elevated underneath autumn olive shrubs, which also had higher leaf tissue N than other woody shrubs. However, mineralized soil N and herbaceous species leaf tissue N concentrations under autumn olive did not differ from levels under non-fixers, indicating that the autumn olives have not enriched their surroundings with any extra fixed N. Overall, this study raises doubts that the passive restoration approach can lead to the desired outcome of a native species forest when under colonization pressure by invasive species. Native woody plants struggle to establish, whereas invasive woody plants are thriving. Initial autumn olive colonization occurs through establishment of isolated shrubs, which appear to be able to take advantage of N-fixation without any N benefiting the surrounding community.