Soil metal contamination influences the vegetation assemblage development of an urban brownfield

Background/Question/Methods Recognizing the growing importance of both structure (maintenance of biodiversity) and function (fostering natural cycles) of urban ecologies, we examine fine and coarse scale vegetation (alpha and beta guild) trajectories in an urban brownfield that had been undisturbed for almost forty years.  Our previous work had demonstrated that assemblage distribution and primary productivity could be correlated with total soil metal load.  Specifically that hardwood assemblage distribution was positively correlated with increasing soil metal load.  Using current vegetative assemblage distribution maps and digitized historic aerial photographs; we document distinct assemblage development trajectories above and below a critical soil metal load. 

Results/Conclusions: These results indicate that hardwood assemblages, dominated by metal tolerant species, were early colonizers and became successfully established in areas of high total soil metal load.  Conversely shrub assemblages remain relatively absent from these areas for more than 25 years.  In addition, two scenarios for future assemblage trajectory, the first considering soil metal mitigation as the result of metal adsorption to or occlusion with organic matter, and a second considering a soil metal feedback loop via continuous cycling of metals in the leaf litter are proposed. These data indicate that assembly theory allowing alternate stable stages may provide a better model than traditional directed succession for the establishment of restoration objectives on degraded sites.