CHAPTER Climate change and urban forest soils 10 Richard V. Pouyata, Tara L.E. Trammellb aScientist Emeritus, US Forest Service, Northern Research Station, University of Delaware, Newark, DE, United States, bDepartment of Plant and Soil Sciences, University of Delaware, Newark, DE, United States ABSTRACT With the proportion of the global population living in urban areas expected to reach 67% by 2040, soils located in these areas, or urban soils, will become increasingly important, particularly with regards to how these soils respond to climate change and their ability to provide ecosystem services. Moreover, urban areas to a large degree have already experienced elevated air temperatures, carbon dioxide concentrations, and increased variability in precipitation, and thus can serve as analogs for how forest soils will respond to climate change. This chapter reviews the wide range of soil characteristics in urban landscapes, their response to both urban environments and changes in climate, their potential to serve as analogs for understanding the future effects of climate change on soils, and how they provide ecosystem services including those that assist in mitigating the effects of greenhouse gas emissions. Introduction Urban populations are increasing with the number of mega (10 million people or more) sized cities expected to increase from 10 in 1990 to 41 in 2030. Additionally, 67% of the global population is expected to reside in urban and peri-urban areas by 2040 (United Nations, 2015). In the United States, the human population is not growing as fast as the rest of the world, but proportionally the expansion rate of urban areas has kept pace, or exceeded, global estimates with land converted to urban uses growing by more than 34% between 1980 and 2000 (USDA Natural Resources Conservation Service, 2001). The current estimate of urban land area is almost 6% of the total conterminous United States (Homer et al., 2015). Additionally, in the latest national census almost 250 million people lived in urban and suburban areas, which roughly accounts for 81% of the total population of the United States (United States Census Bureau, 2010). The conversion of land from primarily agricultural and forest uses to urbanized landscapes has the potential to greatly modify the Nation’s soils especially with respect to their ability to store carbon (C) and mitigate the release of greenhouse gases (Trammell et al., 2018). Additionally, changes in climate, including higher temperature and more intense precipitation events, may in turn affect the potential of soils in urbanized areas, or urban soils, to provide various ecosystem services, some of which are vital for reduc- ing the vulnerability of densely populated areas to natural disasters, such as flooding (Anne et al., 2018). The impacts on soil in the conversion of native ecosystems to agricultural use, and recovery from agricultural use, have been relatively well studied, while conversions to urban land uses have received little attention, particularly with respect to climate and environmental changes. We know, for example, that as agricultural practices have been abandoned in previously forested areas in the United States, Global Change and Forest Soils. https://doi.org/10.1016/B978-0-444-63998-1.00010-0 189 Copyright Ó 2019 Elsevier B.V. All rights reserved. 190 CHAPTER 10 Climate change and urban forest soils forest regrowth in these soils has resulted in a gradual recovery of aboveground, and to a lesser extent belowground, C pools over many decades (Caspersen et al., 2000; Flinn and Marks, 2007). With urban land conversions, soil scientists are just beginning to understand how soils respond to, let alone recover, from the long-term effects of urban land use, especially with respect to a rapidly changing climate. The purpose of this chapter is to explore the array of modifications of soil from anthropogenic activities in urban areas, particularly within the context of global environmental change. Specifically, we: (1) define and describe urban soils and their physical, chemical and biological characteristics; (2) predict urban soil response to climate change, particularly in remnant forest patches; and (3) consider the potential of urban soil to provide ecosystem services and mitigate future climate change. What is an urban soil? The term “urban soil” was first used over 50 years ago by Zemlyanitskiy (1963) to describe the char- acteristics of soils located in urban areas. Urban soil was defined by Craul (1992) as “a soil material having a non-agricultural, man-made surface layer more than 50 cm thick that has been produced by mixing, filling, or by contamination of land surface in urban and suburban areas.” This definition was in part derived from earlier definitions by Bockheim (1974) and Craul and Klein (1980), which sim- ilarly focused on highly modified soils. As soil scientists took a broader view of urban landscapes and human impacts on soil in general, they used the term “anthropogenic soil”, which placed urban soils in a broader context of human altered soils rather than limiting the definition to urban and suburban areas alone (Evans et al., 2000; Capra et al., 2015). To recognize even a wider set of observed soil conditions in urban landscapes, Effland and Pouyat (1997), Lehmann and Stahr (2007), and Morel et al., (2017) more broadly defined urban soils to include soils that are relatively undisturbed yet altered by urban environmental changes, such as the deposition of atmospheric pollutants or warmer air temperatures. This broader view of urban soil results in a continuum of soil conditions occurring in an urban land- scape at any point in time, which is the definition used in this chapter. The Urban Soil Continuum Based on the progression of efforts seeking to understand and define urban soils, it has become obvious to scientists that the range of soil conditions that occur in urban and suburban landscapes varies widely e perhaps more widely than those encountered in rural landscapes (see Morel et al., 2017). Indeed, soils in urban areas have been found to range widely with respect to all variables used to characterize soils, which can be best described as a continuum of conditions, or the “Urban Soil Continuum” (USC). The USC includes the full range of urban soil types, from types with rela- tively low human impacts (e.g., remnant native forest or grassland soil) to those that are greatly impacted by human activities that typically occur in urban areas (Fig. 10.1). The least modified soil types have not been physically disturbed, but rather are primarily affected by environmental changes associated with urban land uses. The most highly modified soils are without structure (Short et al., 1986), made up of human-transported materials (Shaw and Isleib, 2017), sealed at the surface (Scalenghe and Marsan, 2009), highly managed as with public or residential lawns (Trammell et al. 2016), or engineered for specific purposes such as tree pits (Grabosky et al., 2002) and rain gardens Direct and indirect effects 191 FIG. 10.1 A conceptual characterization of the Urban Soil Continuum (USC). Soil conditions within the USC range from the least impacted soils that are associated with remnant patches of forest or grassland (left side of continuum) to highly disturbed and managed soils that are associated with developed areas (right side of continuum). The most modified soils include those that are engineered to provide various ecosystem services. It is hypothesized that soils being effected by urban environmental changes (indirect effects) will be the most sensitive to climate change, while the most managed or engineered soils have the most potential to mitigate the effects of climate change, if appropriately designed. (Ahiablame et al., 2012). Intermediate in modification are soil types that are associated with aban- doned land or are managed, but experience relatively low impacts from ongoing disturbance such as perennial gardens (Edmondson et al., 2014). Where soil types fall along this continuum should reflect how they will respond to long-term changes in climate and their ability to mitigate greenhouse gas emissions or provide ecosystem services. Direct and indirect effects Soils become modified in urban areas through direct or indirect effects and often times both (Fig. 10.2). Direct effects include those generally associated with soil modifications occurring on the more highly disturbed end of the USC (Fig. 10.1). For most urban areas, these types of impacts occur during, rather than after, the land development process including leveling and moving soils with heavy equipment, excavating and refilling, or contouring and creating urban landscape features. Soil management prac- tices such as fertilization and irrigation that are introduced post development are also considered direct effects. Management is generally an effort by humans to overcome some limitation frequently caused by a direct effect and often result in highly productive soils (Pouyat et al., 2007). Whereas direct effects largely occur from human disturbances, indirect effects involve changes in the abiotic and biotic environment that are caused by human activities (e.g., emission of pollutants), which occur in some form across the entire USC, but with a proportionately greater effect on undis- turbed soils associated with remnant parcels (Fig. 10.1). Urban environmental factors include the urban heat island (Mount et al., 1999; Imhoff et al., 2010; Savva et al., 2010; Hall et al., 2015), 192 CHAPTER 10 Climate change and urban forest