Planning the urban forest to enhance urban resilience

Petri, Aaron C.

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https://hdl.handle.net/2142/105028 Copy

Description

Title

Planning the urban forest to enhance urban resilience

Author(s)

Petri, Aaron C.

Issue Date

2019-04-18

Director of Research (if dissertation) or Advisor (if thesis)

Wilson, Bev

Doctoral Committee Chair(s)

Wilson, Bev

Committee Member(s)

• Taylor-Lovell, Sarah
• Koeser, Andrew
• Olshansky, Robert

Department of Study

Urban & Regional Planning

Discipline

Regional Planning

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Urban Forest
• Resilience
• Microclimate

Abstract

The Intergovernmental Panel on Climate Change (IPCC) and the U.S. Global Change Research Program (USGCRP) both identify extreme heat as a significant threat to urban areas. This is largely due to the impact of heat on human health, their livelihoods, and access to resources (USGCRP, 2018; IPCC, 2014). Both the IPCC (2014) and the USGCRP (2018) identify afforestation as a key strategy for moderating the effects of extreme heat in urban environments because of trees’ ability to moderate urban temperatures. It is because of the ecosystem services trees provide (e.g., temperature moderation [Dorer et al. 2013], stormwater mitigation [(Xiao et al., 1998], reduction in energy demand [McPherson et al. 1997], and carbon sequestration [McPherson and Simpson, 2002]) that cities around the world are expanding their tree planting programs. Despite these afforestation efforts, studies show that urban tree canopy is declining overall (Nowak and Greenfield, 2011). This is partly due to urbanization, but also to the fact that these planting programs have not been as successful as cities would like (Miller and Miller, 1991; Nowak 1990, Jack-Scott 2013, Roman and Scatena, 2011). Recognizing the need for and value of trees and the ecosystem services they provide, the American Planning Association has released its own guide for afforestation initiatives entitled Planning the Urban Forest: Ecology, Economy, and Community Development (Schwab, 2009) that suggests ways in which planning agencies, whom often manage forest planning efforts, might approach the development and implementation of urban forest plans. Within this guide, the author notes that there is a need for better planning tools that help reduce tree loss and facilitate better matching of trees to sites (Schwab, 2009) for improved delivery of ecosystem services and to ensure trees survive and thrive in the landscape. The research presented in this dissertation aims to address each of these research gaps by answering two research questions: (1) Can microclimate simulation improve delivery of ecosystem services where they are needed and, (2) once need has been assessed, are there species-specific site preferences that might improve the chances of long-term planting success within the urban landscape? Chapter 1 provides a further introduction to the dissertation research and the structure of this document. Chapter 2 identifies and reviews two key strands of literature that serve as the theoretical framing for the empirical research conducted. Urban resilience is used as the broader theoretical framework for discussing the role of ecosystem services in reducing vulnerability to extreme heat. Ecosystem services provided by trees, such as shading and transpiration, enhance resilience and reduce vulnerability by moderating urban temperatures, and this is the central focus of the literature review presented in Chapter 2. As an integral part of urban heat moderation, I also discuss the institutional context within which urban forests are planned. One common goal that emerged from a review I conducted of forest plans from around the United States is to leverage urban trees to moderate local microclimates. To further explore this fundamental aim, I include in Chapter 2 a review of relevant literature surrounding urban microclimates and the software used to study the impact of urban trees on temperature moderation. However, because trees only provide heat moderating services when they survive in the landscape, I also review literature related to urban tree mortality. The literature related to microclimate and urban tree mortality set the stage for the two empirical studies presented in Chapters 3-5 and Chapter 3 provides an overview of the research design and methodology for each of the two empirical studies. Chapter 4 presents results from the microclimate simulation study. While cities across the United States are developing urban forest plans to capitalize on a myriad of ecosystem services provided by trees, the tools that are used to support this planning do not necessarily meet the needs and achieve the desired results of these municipal initiatives. This chapter highlights the disconnect between how urban forest planning is typically done and where the need for heat moderating ecosystem services is greatest within the urban environment. Although forest plans tend to focus on maximizing canopy coverage, microclimate simulation offers a means of evaluating need and the impact of different interventions on moderating local urban temperatures. I argue that while the i-Tree suite is the most commonly used tool and is immensely helpful for urban forest planners, cities and communities concerned with heat island mitigation should also leverage microclimate simulation tools to better understand the likely impacts of a plan. This study provides a general critique of the i-Tree tool for planning and illustrates how, when used with a microclimate simulation tool, the resultant siting of trees can have a greater impact on heat island mitigation. The focus here is on the community-scale impact of trees and a microclimate simulation model of a community in Chicago where tree canopy coverage is less than 10% and consistently declining, making it a prime target for urban forest planning. Results show that when microclimate is considered during the tree siting process, trees have a greater impact on surface and air temperatures during the summer. Reduced ground surface temperatures of around 3 ℃ to 7.5 ℃ were achieved around 13:00 (1:00 pm), whereas the impact of trees from shading had the greatest impact (0.75 ℃ to 7.75 ℃) on nearby buildings in the mid-afternoon (15:00 to 17:00 hours). Chapter 5 presents results from a statistical analysis of factors that influence the success of urban forest planning efforts. Climate change is expected to significantly impact cities where high mass and low albedo surfaces contribute to the urban heat island effect (UHI), and this is especially true for urban areas experiencing massive population growth (Taha, 1997). Afforestation, the process of establishing trees where none existed previously, is a widely accepted strategy for urban cooling being adopted by cities around the world. Although the ecosystem service and heat island mitigation potential of urban trees has been well documented (Norton et al., 2015; McPherson et al., 1997; Stewart and Oke, 2012), it is less well-understood precisely how variation in species, urban tree location, spatial configuration, and density might be manipulated to maximize ecosystem service provisioning. In summary, there is desperate need for a better understanding of what conditions improve tree survival and what species are most suitable for different landscape conditions. Further, species diversity is important for avoiding widespread canopy loss due to insect and disease outbreak as has been the case with Elm and Ash. Therefore, holistic and integrative planning for tree survival, diversity, and ecosystem services is essential to future urban landscape health. This chapter demonstrates an approach that municipalities with tree censuses can employ to better understand how trees interface with urban environments. Although the case study presented here relies on data for the Chicagoland area, I discuss how these results can be used to inform planning decisions regarding tree placement within other cities. This research is of value to cities seeking to improve the resilience of metabolic activity under changing climate conditions.

Graduation Semester

2019-05

Type of Resource

text

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Copyright and License Information

Copyright 2019 Aaron Petri

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