Atlanta households’ willingness to increase urban forests to mitigate climate change

Investments in urban forests have been increasing in many US cities. Urban forests have been shown to provide countless ecosystem benefits with many addressing climate change issues, such as sequestering carbon, reducing air pollution, and decreasing the heat island effect. Individual groups within the American public may not respond to the issue of climate change in the same way, thus engaging each group in climate change solutions will require different approaches. It is therefore important to understand how the public perceives climate change, their values and preferences, and barriers that might constrain their engagement to policy solutions. A mail survey was implemented, focused on households’ willingness to support and pay for urban forests as a climate change mitigation method. Atlanta, Georgia, USA was selected for this study given its environmental issues such as heat island effect and land cover changes, including conversion of forestland, that come with rapid population growth and urban sprawl. A Tobit model was used to model willingness-to-pay as a function of several variables derived from survey results; and a multivariate weighting strategy was used to address nonresponse issues. The analysis showed that Atlanta households are willing to pay $1.05 million to $1.22 million per year, or $5.24 to $6.11 million over a five-year period. The WTP amount was significantly related with the residents’ income, media source from where they received climate information, and the relative coverage of tree canopy around their residence. Results are relevant to city managers who are interested in understanding the public value of urban greening programs and developing strategies or policies to expand urban forests as part of a climate change strategy.     

Tree canopy change and neighborhood stability: A comparative analysis of Washington,D.C. and Baltimore,MD

Trees provide important health, ecosystem, and aesthetic services in urban areas, but they are unevenly distributed. Some neighborhoods have abundant tree canopy and others nearly none. We analyzed how neighborhood characteristics and changes in income over time related to the distribution of urban tree canopy in Washington, D.C. and Baltimore, MD. We used stepwise multiple regression analysis to identify strong predictors of UTC, from variables found in neighborhoods with different patterns of wealth-stability over time. We then built spatial lag models to predict variation in UTC cover, using the results of a Principal Component Analysis of the socioeconomic, demographic, and housing characteristics of the two cities. We found that: (1) stable-wealthy neighborhoods were more likely to have more, and more consistent, tree canopy cover than other neighborhood types; (2) decreases and increases in income were negatively associated with UTC in Washington, D.C. but not Baltimore, where income stability in both wealthy and impoverished neighborhoods was a significant predictor of UTC; and (3) the association of high socioeconomic status with UTC coverage varied between the two cities.     

The inequity of distribution of urban forest and ecosystem services in Cali,Colombia

Little is known about urban forest planning, management and its benefits in emerging countries. The uneven distribution of tree canopy cover and parks in urban area is related to environmental justice, especially with disadvantaged socio-economic and marginated communities. However, the inequity of urban forest in many cities of emerging countries where often found irregular and unregulated land use patterns and social and socio-economic inequities, is hardly highlighted. This study explores the inequity of distribution of tree canopy cover and public park in Cali, Colombia. Utilizing the traditional socio-economic indices, the stratification, linear regression analysis is conducted to describe relationship between total tree canopy cover, tree canopy cover of various land use types, number of parks and park area per capita. The result demonstrates that lower income communities have lower tree canopy cover, fewer parks and smaller park area than higher income communities. This paper discusses importance of accounting for urban forests and ecosystem service in city planning efforts and better strategies of reducing inequity in emerging countries. Addressing the inequity of urban forest could be a better strategy to create resilient, sustainable, safe and livable cities in emerging countries.     

Spatial and temporal variability of air temperature across urban neighborhoods with varying amounts of tree canopy

Recent studies have emphasized the presence of microclimates in urban settings, but most do not have the high resolution observations necessary to understand the interactions taking place at a neighborhood scale. This study used a network of 10 identical weather stations and high resolution land cover data in Knoxville, Tennessee, to analyze the microclimates of a medium-sized city with a temperate climate. Two stations were installed in each of four urban neighborhoods in locations with varying localized tree cover, and two additional stations were installed in the center of downtown and in a nearby urban nature center. The intra-neighborhood results suggested that there is significant temperature variability within a single neighborhood based on the tree canopy density immediately surrounding a given weather station. However, the inter-neighborhood variability (differences between neighborhoods) was similar in magnitude, which suggests that the overall differences in neighborhood characteristics also have an effect on climate. Land cover at the neighborhood scale (in particular tree canopy percentages at the 500-m radii) had the highest correlation with the minimum daily temperature (Tmin) during the summer season. Maximum daily temperature (Tmax) relied most on the distance of each station from Downtown and the amount of impervious area in the 50 m surrounding each station. Tmax was also most influenced by surrounding land cover during dry conditions (a Dry Moderate air mass). Overall, highly localized impervious land cover percentages and larger-scale forested canopy were important in explaining temperature fluctuation, pointing to the importance of scale in microclimate assessments. Dry air masses enhanced the relationship between land cover and temperature during the day, while moist air masses did the same overnight. These data can be used to better inform planning strategies to build resiliency to extreme heat into urban environments by considering the influence of tree canopy.     

Branching out to residential lands: Missions and strategies of five tree distribution programs in the U.S

Residential lands constitute a major component of existing and possible tree canopy in many cities in the United States. To expand the urban forest on these lands, some municipalities and nonprofit organizations have launched residential yard tree distribution programs, also known as tree giveaway programs. This paper describes the operations of five tree distribution programs affiliated with the Urban Ecology Collaborative, a regional network for urban forestry professionals. We analyzed the programs’ missions, strategies, and challenges as reported through surveys and interviews conducted with program staff. The programs were led by nonprofit organizations and municipal departments in New York City, NY; Baltimore, MD; Philadelphia, PA; Providence, RI; and Worcester, MA. These organizations focused their tree distribution efforts on private residential lands in response to ambitious tree canopy or planting campaign goals. We assessed these programs through the framework of urban forests as social-ecological systems and discuss the programs’ biophysical, social and institutional contexts. Programs face principle-agent problems related to reliance on individual tree recipients to meet goals; their institutional strategies meant to ameliorate these problems varied. Differing organizational and partner resources influenced the programs’ abilities to perform outreach and follow-up on tree performance. Programs attempted to connect with diverse neighborhoods through free trees, targeting areas with low existing canopy, and forging partnerships with local community groups. Given tree recipients’ demand for smaller flowering or fruiting trees, as well as lack of resources for tree survival monitoring on private lands, program leaders appeared to have turned to social measures of success − spreading a positive message about trees and urban greening − as opposed to biophysical performance metrics. We conclude with suggestions for outcomes monitoring, whether those outcomes are social or biophysical, because monitoring is critical to the sustainability and adaptive management of residential tree programs.     

Air pollution removal by urban trees and shrubs in the United States

A modeling study using hourly meteorological and pollution concentration data from across the coterminous United States demonstrates that urban trees remove large amounts of air pollution that consequently improve urban air quality. Pollution removal (O₃, PM₁₀, NO₂, SO₂, CO) varied among cities with total annual air pollution removal by US urban trees estimated at 711,000 metric tons ($3.8 billion value). Pollution removal is only one of various ways that urban trees affect air quality. Integrated studies of tree effects on air pollution reveal that management of urban tree canopy cover could be a viable strategy to improve air quality and help meet clean air standards.     

Zoning,land use,and urban tree canopy cover: The importance of scale

Declining urban tree canopy cover in the United States underscores the importance of elucidating factors that influence the distribution of urban trees. This is particularly relevant as most urban trees are located on private property while their canopies maintain ecosystem services that constitute public goods. Thus, municipalities establish institutions in the form of canopy cover goals and various policies to incentivize private actions to meet those goals. However, urban land use, as governed by municipal zoning policies, plays a role in the abundance, distribution, and potential future location of urban trees independent of policies meant specifically to manage canopy. For instance, previous research finds that lands zoned for residential and park development have the highest canopy cover relative to other land uses. Yet, little research has explored whether this conclusion holds across scales of analysis and how it might influence our understanding of potential canopy cover and relative canopy cover. Thus, we ask, does the nature of the relationship between zoning and canopy cover change between aggregated and disaggregated zoning scales and how might this knowledge improve the sustainability of urban forest management? To answer this question, we classified high resolution National Agriculture Imagery Program (NAIP) images of Bloomington, Indiana land cover and compared existing, potential and relative canopy cover across aggregated and disaggregated zones. Results demonstrate an important exception to the oft-cited theory that residential lands have higher canopy cover, a conclusion that our data supports only at the scale of an aggregated interpretation of zoning. At a disaggregated scale, residential high density zones are significantly different than all other residential zones and more akin to commercial zones in terms of all canopy metrics. For urban forest managers and urban planners, this suggests the relevance of fine-scale variation in land-use policies and related canopy cover policies.     

Spatiotemporal patterns of tree canopy cover and socioeconomics in Melbourne

This paper introduces a method to study the temporal relationship between the distribution of trees in cities and the residents’ income, rate of home ownership and level of education. Through photo-interpretation methods, it documents tree cover percentages in five inner city Local Government Areas in Melbourne. A 10-year time frame (2001–2011) is examined. Prior socioeconomic indicators are juxtaposed against future tree cover levels to investigate relationships. This study demonstrates that tree cover inequity is increasing over time in Melbourne. The study indicates that prior income level is a fair precursor to future canopy cover. By comparing different tree policy approaches of the five adjacent local government areas in Melbourne, it is identified that progressive policy helps generate positive outcomes for the urban forest.     

Uncertainty in urban forest canopy assessment: Lessons from Seattle,WA, USA

Increasing urbanization around the globe is leading to concern over the loss of tree canopy within cities, but quantifying urban forest canopy cover can be difficult. We discuss methods of assessing canopy cover within cities, and then use a case study of Seattle, WA, USA to examine issues of uncertainty in canopy cover assessment. We find that uncertainty is often not reported, and when reported, may be biased. Based on these findings, we provide a list of recommendations for those undertaking canopy cover assessment in complex urban environments.     

Urban forests and social inequality in the Pacific Northwest

Research has shown there is a positive relationship between urban greenness and the well-being of city residents. But greenness is often unevenly distributed across a city, raising environmental justice issues. In 2011 and 2012 the USDA Forest Service, Forest Inventory and Analysis program installed ground plots in the urbanized areas of Oregon and Washington. We analyze these data for the urban areas west of the Cascade Mountains, linking it with demographic data from the U.S. Census to examine the relationship between greenness and socioeconomic status at a sub-regional scale. To explore some relations between urban forest measures and socioeconomic conditions and measures we developed four models: presence of tree canopy cover with a logistic mixed model, and on a subset of the data, percent tree canopy cover with a linear mixed model and tree count and tree species count with Poisson mixed models. We found that median household income, house value, land use, and years in the Tree City USA program contributed to explaining measures of greenness, such as canopy cover presence, percent canopy cover, tree counts, and tree species counts. This agrees with other studies, but does so at a broad scale covering the most densely populated areas in the Pacific Northwest.     

Individual households and their trees: Fine-scale characteristics shaping urban forests

In urban areas, the pattern of trees is often a result of municipal policy, built form, neighborhood socioeconomic conditions, and the actions of local actors. Recent research has focused on the role of neighborhood socioeconomics, and begun to explore the underlying causes of uneven distributions of urban forests associated with different socioeconomic groups. To date, little work has explored property-level tree conditions in relation to disaggregated household characteristics and actions, yet the household is the scale where most decisions about residential tree planting and care are made. This study examines the role of property-level built conditions, household socioeconomics, and residents’ actions and attitudes in relation to property-level canopy cover and tree density. The study area is four neighborhoods in the City of Mississauga (ON, Canada). Regression analyses were conducted to explore significant variables related to the two tree measures for all properties together and separately by neighborhood. The results indicate that property conditions and residents actions are more important in relation to tree variations than socioeconomic factors. Additionally, several significant factors have opposite relationships with percent canopy cover and tree density. These results highlight the need to consider property-level built conditions, residents’ actions, and multiple measures of the urban forest to better understand the patterns of trees in cities.     

Why don’t people plant trees? Uncovering barriers to participation in urban tree planting initiatives

Urban tree cover is inequitable in many American cities, with low-income and non-white neighborhoods typically having the least coverage. Some municipal and non-profit tree planting programs aim to address this inequity by targeting low-income neighborhoods; however, many programs face lack of participation or resistance from local residents. In this study, we aimed to uncover the economic, social, cultural, and physical barriers that community leaders face in planting trees and fostering engagement in a neighborhood with low tree canopy. In collaboration with an urban greening nonprofit in Philadelphia, Pennsylvania (US), twenty in-depth interviews were conducted with community leaders in a low canopy neighborhood, North Philadelphia. Half of these leaders were already involved with local tree planting programs, while the other half were not. Findings reveal that despite broad appreciation for trees and greenspaces, there are concerns about the risks and costs residents assume over the course of a tree’s life cycle, the threat of neighborhood development and gentrification associated with trees, limited plantable space, and limited time and capacity for community organizations. Additionally, these barriers to participation may be amplified among low-income and communities of color who face the legacies of historical tree disservices and municipal structural disinvestment. Addressing community concerns regarding the long-term care of trees beyond the initial tree planting would likely require further programmatic support. Overall, this research highlights the complexity of addressing inequities in tree canopy and the importance of integrating resident and community leader perspectives about disservices and management costs into tree planting initiatives.     

Urban density does not impact tree growth and canopy cover in native species in Melbourne,Australia

Trees provide multiple ecosystem services in urban centers and increases in tree canopy cover is a key strategy for many municipalities. However, urban trees also experience multiple stresses and tree growth can be impacted by urban density and impervious surfaces. We investigated the impact of differences in urban form on tree growth in the City of Merri-bek, a local government area in metropolitan Melbourne, which is the temperate climate zone. Merri-bek has a gradient in population density and urban greenness from north to south, and we hypothesized that tree growth in the southern areas would be lower because trees were more likely to have less access to water with high levels of impervious surfaces. We selected three common native evergreen species, Eucalyptus leucoxylon, Melaleuca linariifolia, and Lophostemon confertus that exhibit differences in climate vulnerability and assessed the tree canopy expansion in four urban density zones in Merri-bek between 2009 and 2020 using aerial image analysis. The differences in urban form did not significantly influence tree canopy growth and all species showed similar canopy expansion rates. However, smaller trees showed a greater relative canopy increase in the ten years, whereas larger trees had a greater absolute canopy growth. Thus, older and larger trees should be protected and maintained to achieve the canopy expansion. Our study indicated that differences in urban form are unlikely to have major impacts on the growth and canopy expansion of well adapted native tree species in open, suburban centers.     

Adapting and applying evidence gathering techniques for planning and investment in street trees: A case study from Brisbane,Australia

Trees along footpath zones (or verges) grow on the “front-line” of urban forest ecosystems, increasingly recognised as essential to the quality of human life in cities. Growing so close to where residents live, work and travel, these street trees require careful planning and active management in order to balance their benefits against risks, liabilities, impacts and costs. Securing support and investment for urban trees is tough and robust business cases begin with accurate information about the resource. Few studies have accounted for spatial heterogeneity within a single land-use type in analyses of structure and composition of street tree populations. Remotely sensed footpath tree canopy cover data was used as a basis for stratification of random sampling across residential suburbs in the study area of Brisbane, Australia. Analysis of field survey data collected in 2010 from 80 representative sample sites in 52 suburbs revealed street tree population (432,445 ± 26,293) and stocking level (78%) estimates with low (6.08%) sampling error. Results also suggest that this population was transitioning to low risk, small-medium sized species with unproven longevity that could limit the capacity of the Brisbane’s Neighbourhood Shadeways planting program to expand from 35% footpath tree canopy cover in 2010, to a target of a 50% by 2031. This study advances the use of contemporary techniques for sampling extensive, unevenly distributed urban tree populations and the value of accurate resource knowledge to inform evidence-based planning and investment for urban forests.     

Equally green? Understanding the distribution of urban green infrastructure across student demographics in four public school districts in North Carolina,USA

Green infrastructure (GI) provides a suite of ecosystem services that are widely recognized as critical to health, well-being, and sustainability on an urbanizing planet. However, the distribution of GI across urban landscapes is frequently uneven, resulting in unequal delivery of these services to low-income residents or those belonging to underserved racial/ethnic identities. While GI distribution has been identified as unequal across municipalities, we investigated whether this was true in public schoolyards within and among urban school districts. We examined schoolyards in four metropolitan areas of diverse socio-economic and demographic compositions in North Carolina, USA to determine if they provided equal exposure to GI, then compared whether this was true of the broader urban landscape. We first classified the land cover of elementary schoolyards and their neighborhoods, then used bivariate and multivariate approaches to analyze the relationships between GI (i.e. tree canopy cover and total GI) and the socioeconomic status and race/ethnicity of the schools and surrounding neighborhoods, respectively. We found that the extent of tree canopy cover and total GI in schoolyards was unrelated to the socioeconomic status and the race/ethnicity of students across the four school districts. In contrast, neighborhoods with lower socioeconomic status and larger populations of underserved race/ethnicity residents had less tree canopy cover and total GI. Although total GI was more evenly distributed in schoolyards, the extent of tree canopy cover and total GI in schoolyards was lower than that in the neighborhoods. This suggests opportunities for school districts to expand GI in schoolyards, leveraging their potential to increase ecosystem services to all children, from increased educational opportunities to improved mental, physical, and environmental well-being.     

Redeveloping the urban forest: The effect of redevelopment and property-scale variables on tree removal and retention

The effects of urbanization on urban forest canopy cover has received significant consideration at broad scales, but little research has explored redevelopment-related influences on individual tree removal at a property scale. This study explores the effect of residential property redevelopment on individual trees in Christchurch, New Zealand. The study monitored 6966 trees on 450 residential properties between 2011 and 2015/16. Of the 450 properties, 321 underwent complete redevelopment during that time, while 129 were not redeveloped. The percentage of trees removed on redeveloped and non-redeveloped properties differed markedly, being 44% and 13.5%, respectively. A classification tree (CT) analysis was used to examine the effects of different combinations of 27 explanatory variables, describing land cover, spatial relationships, economic, and resident and household variables, on tree removal or retention on the properties. The best model included land cover, spatial, and economic variables (accuracy = 73.4%). The CT of the corresponding model shows that trees were most likely to be removed if they were within 1.4 m of a redeveloped building on a property with a capital value less than $1,060,000 NZ. The strongest predictor of tree retention was that the property was not redeveloped. The model predicted that trees were over three times as likely to be removed from a redeveloped property relative to a property that was not redeveloped. None of the seven resident and household variables were selected by the CT as important explanatory variables for tree removal or retention. These results provide insights into the factors that influence tree removal during redevelopment on residential properties, and highlight the need for effective tree protection during redevelopment.     

Declining urban and community tree cover in the United States

Paired aerial photographs were interpreted to assess recent changes (c. 2009–2014) in tree, impervious and other cover types within urban/community and urban land in all 50 United States and the District of Columbia. National results indicate that tree cover in urban/community areas of the United States is on the decline at a rate of about 175,000 acres per year, which corresponds to approximately 36 million trees per year. Estimated loss of benefits from trees in urban areas is conservatively valued at $96 million per year. Overall, for both urban and the broader urban/community areas, 23 states/districts had statistically significant declines in tree cover, 25 states had non-significant decreases or no change in tree cover, and three states showed a non-significant increase in tree cover. The most intensive change occurred within urban areas, with tree cover in these areas dropping one percent over the 5-year period, compared to a 0.7 percent drop in urban/community areas. States/districts with the greatest statistically significant annual decline in percent urban tree cover were: Oklahoma (−0.92%/yr), District of Columbia (−0.44%/yr), Rhode Island (−0.40%/yr), Oregon (−0.38%/yr) and Georgia (−0.37%/yr). Coinciding with the loss of tree cover was a gain in impervious cover, with impervious cover increasing 0.6 percent in urban/community areas and 1.0 percent in urban areas over the 5-year period. Such changes in cover types affect the benefits derived from urban forests and consequently the health and well-being of urban residents.     

Average height of surrounding buildings and district age are the main predictors of tree failure on the streets of São Paulo/Brazil

Tree failure is an increasingly frequent issue in cities worldwide leading to the risk of property damage, financial loss, citizen injury, and death. Assessing tree failure is a challenging task since early signs are often not visible and require a detailed evaluation of each tree, which is limiting considering the management of trees across the whole city. We used Regression Trees and Bagging to assess tree failure on the streets of São Paulo / Brazil using parameters from the gray and green infrastructure that could be easily estimated in the field to support the proper preventive maintenance of street trees. We characterized the districts’ age, average building height, tree height, canopy cover, sidewalk width, sidewalk slope, and terrain slope of 26,616 fallen trees. The Regression Tree shows 82% accuracy and reveals that building height is the main predictor of tree failure, followed by district age, sidewalk width, and tree height. The proportion of tree failure in the most verticalized areas, with on average five stories buildings or taller, is twice that observed in the entire city. Tree failure also increases in districts older than 42 years. The proportion of tree failure is 37% lower than the city’s average in relatively newer districts with low building height, where trees taller than 9.58 m are more prone to failure. These results point to possible roles of wind tunneling, shading, pollution, canopy conflicts with service cables in the urban canyons, and the natural senescence of trees in the oldest districts. The present study establishes comprehensive guidelines for effective preventive maintenance of the street trees in São Paulo.     

The effects of land tenure and land use on the urban forest structure and composition of Melbourne

The urban forest provides valuable ecosystem services for enhancing human well-being. Its structure and composition determine the quantity and quality of these services. There has been little research on the heterogeneity in structure and composition of urban forests in the Australasian region, especially in the centre of a highly dynamic and rapidly urbanizing city. This paper quantifies the structure and the composition of the urban forest of Melbourne, Australia's city centre. The effects of land tenure and land use on the heterogeneity of canopy cover, tree density and canopy size were explored. Species and family composition by land use, land ownership and street type were also analysed using the Shannon–Wiener and Jaccard similarity indices. Most of the canopy cover in the city centre is located on public land and is unevenly distributed across the municipality. The mean canopy cover (12.3%) is similar to that found for whole city studies around the world, which often include peri-urban forests. Similarly to other cities, structure varied across different land uses, and tree size, density and cover varied with land tenure and street type. The diversity index shows that the urban forest is rich in species (H′ = 2.9) and is dominated by native species. Improving the distribution, and increasing tree cover and variety of species will result in a more resilient urban centre, able to provide multiple ecosystem services to their residents and its large population of visitors and workers. The study of the urban centre provides further understanding of compact city morphologies, and allows inter-city comparison independent of the size.