Positive effects of afforestation efforts on the health of urban soils
Introduction
Afforestation has increased in pace and extent in recent years, as policies for greenhouse gas mitigation drive the conversion of other land uses into forests (Berthrong et al., 2009). Studies on the impacts of afforestation have focused primarily on the ability of newly created forests to sequester carbon in tree biomass and soils (Guo and Gifford, 2002, Vesterdal et al., 2002, Berthrong et al., 2012). Though much of this work has focused on the establishment of plantation forests in natural areas, afforestation projects are also increasingly common in cities. There, as in natural lands, projects are intended to capture carbon as well as improve air quality, lower air temperatures, increase storm-water infiltration and create wildlife habitat (Oldfield et al., 2013). These benefits rely on healthy urban soils to facilitate vigorous tree growth and to improve the environment for soil microbes whose activities cycle nutrients through decomposition and store carbon through the production and aggregation of microbial-derived compounds, the primary constituents of stabilized soil organic matter (Schmidt et al., 2011). Urban afforestation efforts have traditionally relied on street-tree plantings, but more recently cities such as Auckland, London, Los Angeles and New York have implemented large-scale, tree-planting campaigns to establish contiguous stands of urban forest composed predominantly of native species (Oldfield et al., 2013).
Assessments of how urban forests benefit people living in cities unanimously conclude that tree cover improves the urban environment (Brack, 2002, Nowak et al., 2002, Davies et al., 2011). These assessments, however, are based on established mature trees primarily planted along streets. Assessments of how urban forest stands affect ecosystem properties related to the health of the environment have instead been made across urban-to-rural gradients. Gradient studies have focused primarily on remnant forest patches that compare in age and composition to rural forests (Pouyat et al., 2002, Golubiewski, 2006, Pouyat et al., 2009). Largely unanswered is what happens to the properties of urban soils as they undergo afforestation. Data assessing the efficacy of urban afforestation projects at improving soil health are, as for urban afforestation effects on ecosystem properties and processes in general, necessary but lacking (Pataki et al., 2011). Cities are then investing in urban afforestation projects without knowing whether these new forests will provide the expected benefits to the urban environment.
Of critical concern to urban afforestation is whether or not urban soils can effectively support forest vegetation. To support a growing forest, soils need to provide physical, chemical and biological conditions that provide adequate physical support, oxygen concentrations, and nutrient and water availability. To this end, many urban soils require remediation and/or improvement as urban afforestation projects are often implemented on filled wetlands or land converted from urban or industrial land uses. Urban soils are typically anthropogenically altered or created, and commonly are compacted with high percentages of human-made artifacts (>10%), including concrete, asphalt, brick and coal slag (NRCS, 2010). It is then an open question as to whether such soils can be remediated sufficiently to facilitate the establishment and growth of stands of healthy trees.
Given that successful forest growth relies on creating healthy soils, soil ecological knowledge can increase our understanding of how ecosystems respond to restoration (Ruiz-Jaen and Aide, 2005, Heneghan et al., 2008, Pavao-Zuckerman, 2008). Yet soils receive little attention in restoration projects compared to vegetation performance metrics such as growth, survival and diversity (Callaham et al., 2008, Heneghan et al., 2008). Our project helps redress this imbalance by assessing the effects of site preparation and different land managements (e.g., compost amendment and tree species diversity) on soil health at an afforestation site located in New York City (Fig. 1). Our project is a research component of the City’s MillionTreesNYC Initiative. We assess how soil restoration and site managements affect key physical (e.g., bulk density), chemical (e.g., carbon concentrations) and biological (e.g., microbial biomass) properties of soils necessary for vigorous tree performance because of their influence on soil nutrient supply, aeration, moisture retention and hence root growth.
Section snippets
Site description and experimental design
Our experiment is dubbed the New York City Afforestation Project (NY-CAP). It is situated in Kissena Corridor Park (40°44′N, 73°49′W; 114 cm MAP, 13 °C MAT), a 40-ha urban park in eastern Queens, New York that includes recreational fields and facilities, a community garden and parkland. Situated in the interior of the park are 56 afforestation research plots (Fig. 1). Urban afforestation at our site, as in much of the MillionTreesNYC Initiative, focuses on restoring public parkland and so our
Site preparation impacts, 2009–2011
Across the 56 research plots, all of the surface soil properties that we measured changed significantly (P < 0.001) from 2009 to 2011. Linear changes were observed for microbial available carbon, microbial biomass, pH and bulk density, indicating changes resulting from site preparation and compost amendment (Table 1). Microbial available carbon showed the greatest change of all variables, being about 4-times greater in 2011 than 2009. Microbial biomass also increased, but to a lesser degree
Discussion
The starting conditions of urban soils and how they respond to restoration will affect the likelihood that afforested sites support healthy stands of trees and hence provide the intended benefits of afforestation initiatives. Our project assessed the effects of site preparation (measured as changes in soil parameters from 2009 to 2011) and management strategies (measured as the impacts of plot treatments on soil parameters in 2011) on soil health. Soil health is a catch-all term and we measured
Acknowledgements
We thank the New York City Department of Parks and Recreation for implementing the project and facilitating the research; the Soil Science class of 2010 at the Yale School of Forestry and Environmental Studies and Yoshiki Harada for soil sampling and processing; and Robert Warren for guidance with statistical models.
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These authors contributed equally to this work.