The urban flower field: Testing links between biodiversity and phytoremediation in a large-scale public art project
The current rate of biodiversity loss far exceeds the estimated “safe-operating” threshold for humanity. However, the role of biodiversity in many systems is still poorly understood, and the importance of biodiversity loss is probably the environmental challenge that is most underappreciated by non-scientists. What is urgently needed is ecological research that quantifies how biodiversity affects ecosystem function coupled with outreach efforts that effectively communicate the value of biodiversity to the general public.
Our project, the Urban Flower Field project, tested whether biodiversity enhances plant uptake of soil heavy metals (phytoremediation) using an experiment embedded in a public art project. Phytoremediation has emerged as a potential low-cost alternative for improving urban soils. However, little is known about whether polycultures can increase phytoremediation rates in mildly contaminated urban soils. The experiment consisted of 96 1.6-m diameter circular plots. We used eight species of wildflowers: two legumes (Lupinus perennis, Baptisia australis), two C4 plants (Rudbeckia hirta, Phlox drummondii) and four known hyperaccumulators (Helianthus annuus, Helianthus annuus var., Brassica napus, Pelargonium hortorum). We planted wildflowers in 1, 2, 4 or 8 species plots, with 24 plots per treatment. We added 16g of total seed, divided proportionally based on the number of species present, to each plot. Heavy metal concentrations in soil and plant samples were measured using X-ray fluorescence and aboveground biomass was quantified for each species in each plot at the end of the growing season. A unique feature of the study is that it is embedded in a temporary art installation established on a previously vacant lot in a densely populated urban area (in St. Paul, MN).
Initial soil samples contained measurable concentrations of multiple heavy metals including lead, arsenic, cadmium, and cesium. These levels generally declined over the growing season, particularly in plots with high biomass production. Biodiversity generally enhanced primary productivity, likely due to positive allelopathic relationships or niche complementarity. Increases in hyperaccumulator biomass resulting from greater biodiversity may therefore augment overall heavy metal extraction from soil.
The project also had a significant outreach component. The public art was designed and implemented by St. Paul’s artist-in-residence, and served as a site for numerous gatherings focused on community building, urban renewal, and environmental science.