Dumping of industrial waste is a common source of human disturbance in urban and suburban areas. Slag, waste from the steel-making process, contains significant amounts of calcium, magnesium, iron and other heavy metals. Because of its composition, low pH and water retention ability, slag is considered an inhospitable environment to plants. Nevertheless, the spontaneously generated plant communities on slag are surprisingly diverse, but the assembly and structure of such communities is poorly studied. Previous studies have identified a slow process of succession due to low growth rate and therefore slow accumulation of topsoil under environmental conditions similar to slag. Using two former industrial sites on the South Side of Chicago, IL, we investigated whether slag communities display similar patterns. We removed all vegetation from a total of 16 1 × 1m plots on both slag and non-slag soil to test whether recovery rates differ. We monitored species growth and diversity over a single growing season (June to October 2018). Plant surveys were conducted on both slag sites and within each experimental plot. To assess plant growth more directly, selected species were planted on both sites and harvested. Soil samples from all sites were collected and analyzed.
Results/Conclusions
We show that recovery rates from vegetation removal plots were different on slag and non-slag. Using percent coverage and species number over time, we show that the recovery process on slag was overall slower; however, the difference was not significant until 6-8 weeks of experiment, suggesting potential stage-dependent effect of slag on plant growth. Furthermore, the growth in biomass of planted species on slag were significantly lower. Linear regression showed that both recruitment rate and biomass were negatively correlated with major characteristics of slag soil: pH, calcium and sand content. Functional trait analysis suggested that graminoid and early successional species preferentially colonized slag plots, but failed to show strong signals of environmental filtering. Slag plots recovered more slowly from disturbance, suggesting a slow successional process that would hinder natural recovery. However, the quality of one slag site was comparable to natural habitats, according to Floristic Quality Index. Therefore, slag also has the potential of hosting flora of analogous habitats native to the area, serving as a plant refuge. Restoration efforts at such sites should be informed by the low possibility of natural recovery, and its potential as native plant refuge.