Altered vegetative assemblage trajectories within an urban brownfield

Recognizing the growing importance of both structure (maintenance of biodiversity) and function (fostering natural cycles) of urban ecologies, we examine coarse scale (herbaceous, shrub and forest) beta guild trajectory in an urban brownfield. The distribution of the pioneer forest assemblage dominated by Betula populifolia Marsh. and Populus spp. could be correlated positively with total soil metal load (arsenic, cadmium, chromium, copper, lead, zinc, lead and vanadium),whereas herbaceous and shrub guilds were negatively correlated. Distinct assemblage development trajectories above and below a critical soil metal threshold are demonstrated. In addition, we postulate that the translocation of metals into the plant tissue of several dominant species may provide a positive feedback loop, maintaining relatively high concentrations of metals in the litter and soil. Therefore assembly theory, which allows for the development of alternate stable states, may provide a better model for the establishment of restoration objectives on degraded urban sites.

► Forest distribution and total soil metal load yield strong positive correlations.
► Shrub and herbaceous guild distribution and TML are negative and weaker.
► Below a critical TML threshold transition between guilds exhibit a standard trajectory.
► Above the critical TML threshold the shrub guild is virtually absent.
► Metal cycling has the potential to lead to an alternative steady state.