Hazard tree liability in the United States: Uncertain risks for owners and professionals

The liability United States (U.S.) tree owners face from hazardous trees on their private properties has and continues to undergo a transformation. The traditional common law legal test focused upon whether a tree owner had acted to create a hazardous condition on the property. The modern trend is towards a test that imposes greater responsibility and greater uncertainty on tree owners and arborists. Abandoning the natural/artificial distinction, the modern test, adopted in half the U.S. jurisdictions, imposes general principles of negligence on tree owners. This paper discusses the U.S. legal context for hazardous trees, particularly the impacts on individual tree owners. It closes by addressing various practical means to minimize the risks associated with the modern legal standards, both for susceptible tree owners, and for professionals assessing the subject property.     

Residential building energy conservation and avoided power plant emissions by urban and community trees in the United States

Urban trees and forests alter building energy use and associated emissions from power plants by shading buildings, cooling air temperatures and altering wind speeds around buildings. Field data on urban trees were combined with local urban/community tree and land cover maps, modeling of tree effects on building energy use and pollutant emissions, and state energy and pollutant costs to estimate tree effects on building energy use and associated pollutant emissions at the state to national level in the conterminous United States. Results reveal that trees and forests in urban/community areas in the conterminous United States annually reduce electricity use by 38.8 million MWh ($4.7 billion), heating use by 246 million MMBtus ($3.1 billion) and avoid thousands of tonnes of emissions of several pollutants valued at $3.9 billion per year. Average reduction in national residential energy use due to trees is 7.2 percent. Specific designs to reduce energy use using urban trees could increase these values and further reduce energy use and improve air quality in the United States.     

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.     

Trees stocks in domestic gardens and willingness to participate in tree planting initiatives in low-cost housing areas of the Eastern Cape,South Africa

Increasing human populations and rapid urbanization in sub-Saharan Africa have prompted the development and maintenance of urban green infrastructure, including urban trees for sustainability, human wellbeing, liveability and climate resilience. However, there are still insufficient amounts and large inequities in the distribution of trees between and within towns and cities of the Global North and South. In South Africa, urban green space planning and planting are encoded in several policies at national level. However, these policies are rarely translated into specific guides, standards or actions, and consequently disparities in urban trees and green space distribution persist. This study assessed the prevalence of urban trees in domestic gardens in low-cost housing areas (LCHAs) of eight small to medium-sized towns in the Eastern Cape province of South Africa and examined residents’ perceptions in this regard. This was done via surveys with 800 households in old and recently developed LCHAs. The results revealed that most households (52 %) had at least one tree in their yard, with more households in the older neighbourhoods (60 %) reporting having trees than in the newer ones (44 %). Most of the trees (66 %) had been deliberately planted as opposed to natural regeneration. Experience of formal urban tree planting programs was low, but 75 % of residents expressed willingness to participate in the future, preferably in tree planting and maintenance. Urban green spaces and trees cannot be an afterthought in the development of sustainable human settlements, and municipal plans should reflect tangible commitments in this regard. Meeting goals for greener LCHAs requires the involvement of local residents, and for municipal authorities to be receptive to the wishes of residents and willingness to green their residential areas.     

Tree and impervious cover change in U.S. cities

Paired aerial photographs were interpreted to assess recent changes in tree, impervious and other cover types in 20 U.S. cities as well as urban land within the conterminous United States. National results indicate that tree cover in urban areas of the United States is on the decline at a rate of about 7900 ha/yr or 4.0 million trees per year. Tree cover in 17 of the 20 analyzed cities had statistically significant declines in tree cover, while 16 cities had statistically significant increases in impervious cover. Only one city (Syracuse, NY) had a statistically significant increase in tree cover. City tree cover was reduced, on average, by about 0.27 percent/yr, while impervious surfaces increased at an average rate of about 0.31 percent/yr. As tree cover provides a simple means to assess the magnitude of the overall urban forest resource, monitoring of tree cover changes is important to understand how tree cover and various environmental benefits derived from the trees may be changing. Photo-interpretation of digital aerial images can provide a simple and timely means to assess urban tree cover change to help cities monitor progress in sustaining desired urban tree cover levels.     

Attitudes towards tree protections,development, and urban forest incentives among Florida (United States) residents

Tree ordinances can be an effective means of preserving urban forests in the face of development pressures. Despite this, they also have the potential to be divisive among the public - especially when applied to privately-owned land. In this study we surveyed 1716 Florida urban residents to understand how they value regulation and management of the urban forest. Specifically, we asked about: tree protection ordinances, incentive programs to manage or plant trees, justification for tree removal, and development. Most respondents supported tree protections, even when applied to trees on their own property or when they had the potential to limit development activities. Additionally, there was limited support for removing healthy trees for development. Respondents supported the use of funds for urban forestry efforts – particularly at the local or state level.     

Factors influencing long-term street tree survival in Milwaukee,WI, USA

Street trees are exposed to a variety of site conditions, environmental factors, and physical disturbances which influence their survival in urban areas. This study draws on 25 years of urban forest monitoring data from the city of Milwaukee, WI (United States) to model the impacts of these factors on tree survival for a single cohort of trees. Tree condition, tree size, tree species, and site attributes were measured initially in 1979. These factors were measured again in 1989 and 2005 and compared to construction data for the same area during the study period. Multivariate logistic regression was used to identify factors associated with tree survival. Cross-validation show the final model could successfully predict tree survival nearly 85% of the time. Results indicate that tree survival varied by species. Additionally, trees were more likely to die as trunk diameter increased, planting space width decreased in the tree lawn, and tree condition decreased. Finally, trees adjacent to construction were nearly twice as likely to die as those not exposed to development and redevelopment activities.     

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.     

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.     

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.     

The extent and perceptions of vandalism as a cause of street tree damage in small towns in the Eastern Cape,South Africa

Street trees are important foundations of urban sustainability due to the ecosystem services that they provide society and the environment. However, street trees are vulnerable to vandalism and damage, especially when small, which constraints the flow of benefits they provide and also increases the costs of planting programmes. Despite being a common phenomenon, there is limited knowledge regarding the extent of vandalism and the reasons for it. Here we seek to understand the causes and extent of street tree damage in eleven small Eastern Cape (South Africa) towns and to assess the perceptions of residents and officials. The condition of newly planted street trees was assessed in each town and residents were interviewed in the two towns with the highest number of newly planted street trees. Almost half (42%) of recently planted street trees were totally snapped, ranging between 0% and 63% per town. There was no difference in the prevalence of trees being snapped between those with protective structures and those without. Each town used different structures around newly planted street trees, but in only two towns were all the structures intact. The prevalence of damage declined with increasing trunk thickness and increasing town size. According to residents, boredom, misbehaviour, lack of appreciation of trees and collection for wood were the main factors for tree vandalism by people along with damage by livestock. Ward councillors recognised the presence of vandalism, but indicated that it was not a priority topic in their ward meetings. Suggestions by residents to prevent vandalism included: planting in sensible areas, re-designing the protective structures, re-locating livestock and engendering community participation and ownership in all aspects of street tree planting.     

Relationships between urbanization,tree morphology,and carbon density: An integration of remote sensing,allometric models,and field survey

Urban trees store and sequester large amounts of carbon and are a vital component of natural climate solutions. Despite the well-recognized carbon benefits of urban trees, there is limited effort to examine how spatial distribution of carbon density varies across distinctive social, demographic, and built dimensions of urban landscapes. Moreover, it is unclear whether specific aspects of landscape structure and design could help increase carbon densities in urban trees. Here, we produced a fine-resolution carbon density map of urban trees in New York City (NYC) by integrating high-resolution land cover map, LiDAR-derived tree metrics, i-Tree Eco, and field survey data. We then explored spatial variations of carbon density across the gradients of urban development intensity, social deprivation index, and neighborhood age, and we examined the relationships between carbon density, and fragmentation, aggregation, size, and shape of tree canopy cover. We find that carbon stored in urban trees in NYC is estimated as 1078 Gg, with an average density of 13.8 Mg/ha. This large amount of carbon is unevenly distributed, with carbon densities being highest in Bronx and in open parks and street trees. Furthermore, carbon densities are negatively associated with urban development intensity and the social gradient of deprivation. Regarding the impacts of tree morphology on carbon density, our results show that while the amount of tree cover is the most influential factor in determining carbon density, small-sized forest patches and moderate levels of forest edges are also conductive to increasing carbon densities of urban trees. To incorporate urban forestry into developing innovative, effective, and equitable climate mitigation strategies, planners and decision makers need to identify the optimal spatial configuration of urban forests and invest in tree planting programs in marginalized communities.     

Twelve-year change in tree diversity and spatial segregation in the Mediterranean city of Santiago,Chile

Tree diversity is one of the most important components of urban ecosystems, because it provides multiple ecological benefits and contributes to human well-being. However, the distribution of urban trees may be spatially segregated and change over time. To provide insights for a better distribution of tree diversity in a socially segregated city, we evaluated spatial segregation in the abundance and diversity of trees by socioeconomic group and their change over a 12-year period in Santiago, Chile. Two hundred vegetation plots were sampled across Santiago in 2002 and 2014. We found that overall abundance and diversity of urban trees for the entire city were stable over 12 years, whereas species richness and abundance of native tree species increased. There was segregation in tree species richness and abundance by socioeconomic group, with wealthier areas having more species and greater abundance of trees (for all tree species and native species) than poorer ones. Tree community composition and structure varied with socioeconomic group, but we found no evidence of increased homogenization of the urban forest in that 12 years. Our findings revealed that although tree diversity and abundance for the entire city did not change in our 12-year period, there were important inequities in abundance and diversity of urban trees by socioeconomic group. Given that 43% of homes in Santiago are in the lower socioeconomic areas, our study highlights the importance of targeting tree planting, maintenance and educational programs in these areas to reduce inequalities in the distribution of trees.     

Public satisfaction with urban trees and their management in Australia: The roles of values,beliefs, knowledge,and trust

The success of urban forest management strategies is dependent on public support for and engagement with urban trees. Satisfaction with urban trees and their management, and the level of trust people have in urban tree managers, are useful for understanding public opinions. Yet these concepts, and the mechanisms leading to the formation of public opinions remain poorly explored in the literature. Here we explore how satisfaction with urban trees and with urban tree management, and trust in the agencies responsible for urban tree management, are explained by cognitive factors (values, beliefs, and knowledge) and socio-ecological contextual factors (tree presence/canopy cover, cultural diversity, and socioeconomic status) using an online survey of 16 local government areas in south-eastern Australia. Analyses of 2367 responses revealed that people’s opinions about trees in general (values and beliefs) were overwhelmingly positive, while their opinions about more contextualised measures such as satisfaction and trust were more mixed. Two distinct pathways that influence satisfaction were identified: one linked to beliefs about having trees in cities, and another one linked to trust in urban tree management. At the local government level, satisfaction was negatively associated with a measure of cultural diversity and very low levels of tree canopy cover, but not with socioeconomic disadvantage. Satisfaction with local trees could be improved by increasing the quality of ecological function of trees, such as habitat provision and tree diversity. Community engagement could also improve satisfaction and trust, particularly perceived procedural fairness of decision-making, reinforce positive beliefs about the outcomes of having trees in cities, and dispel negative beliefs. Engagement processes should recognise that people hold complex and diverse opinions about urban trees, and by incorporating these opinions into decision-making we can meet the increasingly complex and diverse expectations being placed on urban forests.     

The Urban Tree Arboretum in Hørsholm,Denmark: A new tool towards an improved education of arborists and tree managers

The establishment and management of urban trees require expert knowledge and experience. The Urban Tree Arboretum (UTA) was established in Hørsholm, Denmark, to improve basic and advanced education for students and professionals working with tree establishment and management. The UTA presents 120 tree species and cultivars suitable for urban plantings. The trees were established in 2001 in replicates of three or six. Different pruning treatments allow for a comparison of their effect on the trees: formative pruning, shape pruning (topiary), pollarding and non-pruned. Tree dimensions are measured yearly and made available on the website www.bytraearboretet.dk. The website also offers photographic documentation of the trees and their tree features as well as a reference list open for entries by the public. The UTA is integrated part of several courses at the University of Copenhagen and has in 2007 and 2008 been visited by 570 external visitors on 27 guided tours.     

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.     

The effectiveness of schools-based National Arbor Week activities in greening of urban homesteads: A case study of Grahamstown, South Africa

Arbor Day, or Week, is a well-established greening initiative worldwide, focusing on tree planting and the benefits thereof. Frequently most effort is targeted at institutions such as schools and community groups. Yet there has been limited assessment of the success of Arbor Day, or Week, activities both at the schools, or the wider dissemination in neighbouring communities and suburbs. The objective of this study was to determine the influence of school-based Arbor Week activities on learners’ home-based practices regarding tree planting and urban forestry. Learners from three schools in Grahamstown South Africa, with good Arbor Week participation, were assessed on their tree planting knowledge. The learners’ parents were also interviewed to determine whether the information obtained by the learners at school was taken home. A control group consisting of people with no children or very young children was also assessed. This study found that Arbor Week activities were, for the most part, successfully taught in the case study schools, and most of the learners stated that their knowledge of trees and their benefits came from their school activities. However, many learners could not remember the activities in which their schools partook the previous year. Whilst most learners were aware of the importance of trees, few had encouraged tree planting at home. However, over one-fifth of adults claimed that their knowledge of the benefits of trees was as a consequence of Arbor Day activities when they were young. Numerous constraints to tree planting were provided by learners and both the adult groups, the two most frequent being limited space and falling leaves making their yards untidy. External organisations should become more actively involved and provide much needed support systems if greening initiatives are to reach the wider community.     

Statewide assessment of street trees in New York State,USA

In the United States, street tree management and planning occurs at regional, state, and local levels. However, state and federal officials charged with managing streets trees at the regional and state levels typically lack the comprehensive, detailed information available to local officials in a street tree inventory such as species composition and tree size distribution. Statewide street tree assessments employing a variety of methodologies have been conducted to fill this knowledge gap. This paper examines these past assessments and builds upon them in conducting a street tree assessment for New York State in which geographic variability in statewide street tree inventory data is accounted for through weighted averaging of estimates.     

Detecting,quantifying, and mapping urban trees' structural defects using infrared thermography: Implications for tree risk assessment and management

Urban trees are a fundamental and key component of urban green areas, however, they are subject to several stresses which can compromise their mechanical integrity through the development of defects such as wood decay. In this study, we evaluated the structural health state of trees in four urban parks in the city of Mytilene, Greece, using structural traits of the trees, their trunk surface temperature distribution as recorded using infrared thermography, and spatial statistics both at single tree and at park level. We developed thermal indices by analyzing data from 334 trees belonging to three main species (Robinia pseudoacacia, Morus alba and Melia azedarach). We estimated temperature spatial dependence across each tree trunk using Moran's I index, while statistically significant spatial clusters were assessed using local spatial autocorrelation statistics. Relationships between tree traits, thermal, and spatial indices were established using linear and logistic regression models. Finally, we used the Getis-Ord Gi* statistic for the identification of risky tree hotspots and we applied the kriging geostatistical procedure for mapping of such hotspots. Our results have shown that the thermal and spatial indices can sufficiently predict different types of structural defect, and to identify hotspots of risky trees and their spatial extent. This approach can successfully contribute to tree risk assessment for a more effective urban park management.     

Willing partners? Residential support for municipal urban forestry policies

Cities across North America are adopting ambitious goals to grow their urban forests. As existing trees and new planting opportunities are often located on private property, residents’ support and participation is needed in order to meet these goals. However, little research has examined support for municipal urban forestry efforts, including policies specifically targeting residential areas. The objectives of this research are to (1) assess resident’ level of support for common urban forestry policies and (2) determine if there are specific household characteristics associated with different levels of policy support. The objectives are addressed through a statistical analysis of survey responses and a qualitative examination of follow-up interviews with residents in four neighborhoods located in Mississauga (Ontario, Canada). The survey participants and their properties vary in their socioeconomic characteristics, age of development, and urban forest conditions. Our results found that the majority of residents had neutral to very positive attitudes toward common municipal policies encouraging planting and restricting removal of trees, but support levels were lower for the policies than for general statements about desired presence and size of urban trees. Several characteristics are significantly related to level of policy support, including age of household members, education-level, property-level tree density, recent tree planting activity and age of house. Interviews also highlighted residents’ apprehensions about living among tall trees and older resident's concerns with tree maintenance. The results suggest that most residents would be willing partners in urban forestry efforts, with many of these residents already actively planting and maintain trees. However, to increase support and participation rates, different types of trees – including those smaller in stature and ones that require relatively little maintenance – should be part of any planting program to meet the varying needs of households.