Primary Succession on Slag Sites and Uncontaminated Soil: A Comparison Draft Manuscript

Slag, waste from the steel-making process, contains large 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 slow process of succession due to low growth rate and slow accumulation of topsoil. Using two former industrial sites on the South Side of Chicago, IL, I investigated whether slag communities display similar patterns. I removed all vegetation from plots on both slag and non-slag soil to test whether primary succession differed over one growing season (4 months). To directly assess plant growth, selected species were planted on both sites and harvested. I show that primary succession differed on slag and non-slag soils. The recruitment process on slag, measured by percent cover and number of recolonized species, was overall slower; however, the difference was not significant until 6-8 weeks of experiment, suggesting potential stagedependent effect of slag on plant growth. Functional trait analysis found that graminoid and early successional species preferentially colonized slag plots. Slag plots recovered slower from disturbance, suggesting a slow succession process that would hinder natural recovery. However, slag also has the potential of hosting flora of analogous habitats native to the area, serving as plant refuge. Restoration effort should be informed by the low possibility of natural recovery, and its potential as native plant refuge. Introduction Human activities have drastically modified natural landscapes, creating many unique artificial systems. One such example is the urban ecosystem, which encompasses the biological activities and ecological processes that results from a built environment. With high disturbance, largely patchy habitat and highly variable soil composition, urban ecosystems generally have unique community dynamics (Swan et al. 2011). Among many urban habitats, industrial dumps have gained attention from both ecologists and public because of its unique environmental and public health concerns (Bayless et al. 1998, Lundholm and Richardson 2010). Slag, one common type of industrial dump, is the byproduct of steel production that contains a large amount of heavy metals and alkaline earth metal compounds (Bayless and Schulz 2003). This industrial waste is estimated to cover over 52 km of land surface in northeastern Illinois and northwestern Indiana in the greater Calumet region (Bayless and Schulz 2003), with sites usually covered in a layer of slag material that varies in size from small granules to big chunks to large contiguous surfaces (Kay et al. 1997). The chemical composition, as well as thin topsoil, low water retention and low organic matter makes slag site a generally inhospitable place for plant life (Bayless and Schulz 2003, Reddy and Amaya-Santos 2017). Although variable in specific contents, steel slag is generally comprised of calcium and magnesium silicate, compounds of iron, manganese and other heavy metals (Bayless and Schulz 2003). Some slag contains organic pollutants such as polycyclic aromatic hydrocarbons (Reddy and Amaya-Santos 2017). Because of the high calcium and magnesium content, the pH of slag is generally basic. Depending on specific composition of slag, some contents may leach into nearby water bodies, causing significant pollution (Bayless et al. 1998); even more strikingly, the Big Marsh site in the Calumet region hosts a pond with pH higher than 12 (Reddy and Amaya-Santos 2017). Due to its history as a center for steel production, the Calumet region has experiences extensive dumping of slag. Typically, slag and other waste was dumped on land adjacent to industrial plants, into pits, lakes or wetlands, or used as fill (Kay et al. 1997). Remediation of industrial dumps as sources of contamination has always been a major public health concern (Reddy and Amaya-Santos 2017); therefore, multiple slag sites in the region have undergone different degrees of restoration. Some, such as the Big Marsh Park, have been sites for active ecological restoration and have been transformed into vibrant urban parks (Chicago Park District 2014), while others such as U.S. Steel South Works have remained relatively untouched for decades. Common methods to prepare sites for ecosystem construction include phytoremediation or capping with topsoil. Nevertheless, it has been suggested that contaminants in slag might leach into the soil and affect plant growth even when capped with topsoil (Daniel Spencer and Lauren Umek, personal communication), and experiments with different plants in order for phytoremediation resulted in low survival (Reddy and AmayaSantos 2017; Lauren Umek, personal communication). Furthermore, in some cases, other debris such as construction and demolition waste has also been dumped at slag sites, complicating the land use history and making restoration more difficult. Slag sites, however, harbor diverse vegetation, and plant communities are comparable to those in urban disrupted land or vacant lots, with many non-native and weedy grasses, forbs and shrubs; native forbs and grasses are also common. The only trees successfully established on slag are cottonwood (Populus deltoides) and mulberry (Morus alba). Typical wetland species, such as sedges and common reed (Phragmites australis), are found near depressions on slag. Overall, given the harsh environment of slag sites, such spontaneous plant communities are amazingly diverse. Naturally, such spontaneous vegetative communities raise the question of how they have assembled. According to classical successional theory, soil formation is enhanced by primary successional species that can facilitate succession by other species (Clements 1916). Those species are generally droughtand heat-resistant, fast growing and produce larger amount of litter that integrate into the soil, increasing organic material content (Cowles 1899, Clements 1916, Kazakou et al. 2006). Therefore, the ability of the primary successional community to provide organic material for soil is important for the survival of later successional species, and the recovery of an ecosystem from disturbance (facilitation model; see Connell and Slatyer 1977). Without soil formation, many natural habitats display extremely slow or arrested primary succession. For instance, alvar habitats, comprised of thin topsoil on limestone bedrock, similarly have high pH, shallow topsoil and low nutrient availability; due to the low growth rate of early successional plants, the community is “arrested” at a primary successional stage and does not succeed to later stages (Stark et al. 2004, Tomlinson et al. 2008). A slag site can be considered as a habitat after disturbance: most plants were killed by dumping. As disturbance ends (no further dumping) and erosion breaks the contiguous slag surface, primary succession takes place and the present community assembles. Previous studies have estimated successional trajectories of other industrial sites, predicting recovery to the original climax community (Smith et al. 1997, Řehounková and Prach 2008). However, it is possible that slag site plant communities, though they have been relatively undisturbed for long periods of time, may have arrested primary succession. Because slag sites are usually characterized by a thin layer of topsoil with high pH, they might be comparable to alvars in displaying “arrested” soil and plant community development. The environmental stress on slag may lower growth rates of early successional plants, resulting in less accumulated plant litter and decreased soil formation (Chapin 1991). This hypothesis is supported by the fact that some sites, including the Big Marsh and the Van Vlissingen parks, have been undergoing succession for decades but could not support survival of later-successional species such as most trees and shrubs without addition of compost (Reddy and Amaya-Santos 2017). Nevertheless, more concrete evidence is still necessary to determine whether such slag plant communities are arrested in primary successional stage by the lack of nutrients. I tested whether primary succession was slowed down in slag soils, including whether 1) slag sites differed from non-slag sites by soil composition and vegetation, 2) the primary succession process differed for slag and non-slag sites, and 3) the plant growth rate were reduced in slag versus non-slag sites, 4) plants on slag differed in their functional traits. Over the course of one growing season (June to October 2018), a combination of plant surveys, soil testing and controlled experiments on two types of site, Slag and non-slag soils (hereafter Reference), were used to test the three hypotheses. I evaluated functional traits to assess if plant communities varied between Slag and Reference sites.