A review of tree root conflicts with sidewalks,curbs, and roads

Literature relevant to tree root and urban infrastructure conflicts is reviewed. Although tree roots can conflict with many infrastructure elements, sidewalk and curb conflicts are the focus of this review. Construction protocols, urban soils, root growth, and causal factors (soil conditions, limited planting space, tree size, variation in root architecture, management practices, and construction materials) are discussed. Because costs related to sidewalk and curb damage are substantial, a review of research addressing repair, mitigation, prevention, and litigation costs is included. Finally, future research needs are discussed.

Potential for conflicts between trees and sidewalks/curbs is high when one or more of these factors are present: tree species that are large at maturity, fast growing trees, trees planted in restricted soil volumes, shallow top soil (hard-pan underneath top-soil), shallow foundations underneath the sidewalk (limited or no base materials), shallow irrigation, distances between the tree and sidewalk of less than 2.0–3.0 m., trees greater than 15 to 20 years old.

The results of this survey indicate that cities are spending substantial sums of money to address conflicts between street tree roots and infrastructure. It can be inferred that most of these expenditures are spent dealing with problems that already exist. However, this raises the question: How much is being spent now to ensure that conflicts are minimized in the future?

Future research should concentrate on plant factors, site design, and construction of sidewalks and curbs. Also, more knowledge is needed about interactions between root growth and management techniques, such as pruning and irrigation. Finally, there is need for studies that will assist policy-makers to efficiently allocate funds among repair, mitigation, prevention, and legal remedies.

     

Urban forest and environmental inequality in Campos dos Goytacazes,Rio de Janeiro,Brazil

Social and spatial inequality regarding environmental resources and services is one of most complex issues affecting contemporary urban life. The objective of this research is to study the spatial distribution of trees in public areas in Campos dos Goytacazes, Rio de Janeiro, Brazil. This research presents data gathered in ten neighborhoods in Campos. These neighborhoods were split into three distinct groups using wealth levels. Data obtained include the number of trees and private gardens and tree species diversity per neighborhood street. Our results demonstrate that the wealthier neighborhoods have both the highest tree biodiversity and number of trees. In contrast, the poorer neighborhoods present a low biodiversity level and fewer tree species. Our results also showed that age of the neighborhoods was not a factor in explaining the number of trees in public spaces. Socioeconomic and education levels of the population seem to play a more causal on tree quantity and species diversity. This inequality stresses a problem with environmental justice, a characteristic of Brazilian cities intrinsically connected to urban sustainability.     

Rainfall interception by Santa Monica's municipal urban forest

Effects of urban forests on rainfall interception and runoff reduction have been conceptualized, but not well quantified. In this study rainfall interception by street and park trees in Santa Monica, California is simulated. A mass and energy balance rainfall interception model is used to simulate rainfall interception processes (e.g., gross precipitation, free throughfall, canopy drip, stemflow, and evaporation). Annual rainfall interception by the 29,299 street and park trees was 193,168 m³ (6.6 m³/tree), or 1.6% of total precipitation. The annual value of avoided stormwater treatment and flood control costs associated with reduced runoff was $110,890 ($3.60/tree). Interception rate varied with tree species and sizes. Rainfall interception ranged from 15.3% (0.8 m³/tree) for a small Jacaranda mimosifolia (3.5 cm diameter at breast height) to 66.5% (20.8 m³/tree) for a mature Tristania conferta (38.1 cm). In a 25-year storm, interception by all street and park trees was 12,139.5 m³ (0.4%), each tree yielding $0.60 (0.4 m³/tree) in avoided flood control costs. Rainfall interception varied seasonally, averaging 14.8% during a 21.7 mm winter storm and 79.5% during a 20.3 mm summer storm for a large, deciduous Platanus acerifolia tree. Effects of differences in temporal precipitation patterns, tree population traits, and pruning practices on interception in Santa Monica, Modesto, and Sacramento, California are described.     

Urban forest structure,ecosystem services and change in Syracuse,NY

The tree population within the City of Syracuse was assessed using a random sampling of plots in 1999, 2001 and 2009 to determine how the population and the ecosystem services these trees provide have changed over time. Ecosystem services and values for carbon sequestration, air pollution removal and changes in building energy use were derived using the i-Tree Eco model. In addition, photo interpretation of aerial images was used to determine changes in tree cover between the mid-1990s and 2009. Between the mid-1990s and 2003, tree cover in Syracuse exhibited a decline from 27.5 to 25.9 %, but subsequently increased to 26.9 % by 2009. The total tree population exhibited a similar pattern, dropping from 881,000 trees in 1999 to 862,000 in 2001, and then increasing to 1,087,000 trees in 2009. Most of this increase in the urban tree population is due to invasive or pioneer trees species, particularly Rhamnus cathartica, which has more than tripled in population between 2001 and 2009. Insects such as gypsy moth and emerald ash borer pose a substantial risk to altering future urban forest composition. The annual ecosystem services provided by the urban forest in relation to carbon sequestration, air pollution removal and reduction in building energy use are estimated at about $2.4 million per year. An improved understanding of urban forests and how they are changing can facilitate better management plans to sustain ecosystem services and desired forest structure for future generations.     

Modelling changes in dimensions,health status,and arboricultural implications for urban trees

Canberra, the capital of Australia since 1911, has been developed into a modern city from its original site on a nearly treeless plain. Today the city has about 300,000 inhabitants and 500,000 trees. The authors were requested by the managers of the urban public tree resource to survey their asset and to develop a computer-based system that would aid them in anticipating future maintenance requirements and its costs. This paper reports on our response. We have surveyed 3,000 streets and parks in the city, noting the species, number, and condition of every public space tree. We have also obtained the dimensions of sample trees, noting their total height, maximum crown width, height of maximum crown width, diameter at maximum crown width, and height at crown break. A management system has been developed using Microsoft Access^TM.Using standard regression techniques available on the package JMP^TM, we found that total tree height was related to age for all species and that all other parameters of interest were related to height or transformed values of height. We assumed a sigmoidal growth curve and calibrated 114 height/age curves to cover the 165,000 trees of the 340 species we have in our database. As well we used the data on tree condition to determine the rate at which populations change from healthy to stressed.By interviewing foremen and supervisors we were able to determine the maintenance treatments carried out in Canberra, the equipment used, and the number of trees that can be treated in a day, for each type of operation.The management system can be used to display the current inventory for each street or park, by suburb, in the database. It can also be used to model future increases in size or crown condition, to predict the operations that will be required as a consequence of tree growth or crown deterioration, and finally, by applying multipliers to equipment and personnel, to estimate the future costs of tree maintenance. Managers can use the system to anticipate problems such as uneven expenditure requirements in future years.     

Franklin Park: 150 years of changing design,disturbance, and impact on tree growth

Urban stresses are known to affect street trees growing in spatially restricted conditions, but even in relatively large parks the cumulative effects by construction, pedestrian uses, and impacts from temporary events may have deleterious long-term effects on plant health. In Columbus, Ohio, the 93 ac (42.3 ha) Franklin Park has been subjected to major construction events and site re-configuration about once per generation—at least six times in 150 years. The first recorded shade- and ornamental tree installations occurred between 1895 and 1910. Those trees, as well as those native to the park, were mapped in 1939, 1989, and again in 2000. Testing the hypothesis that cumulative disturbance reduces tree vigor and growth rate even in open-grown conditions, tree diameters were compared over the sixty year period between 1939 and 2000. Soil compaction and percolation were measured on a 50 m grid, and maps were overlain to determine where site disturbance,buildings, utility lines, paths and pavements had occurred since 1860. Only 8% of the park's surface escaped disturbance; undisturbed areas occurred only in isolated patches. For all trees, trunk diameter increase averaged 0.5 cm/year for 61 years. While late successional species (e.g., Quercus, Acer) had growth rates less than expected for open-grown specimens, many early successional species (Celtis, Populus) were less affected. Despite availability of soil nutrients and lack of canopy or understory competition, tree health was generally poor in all size classes and the majority of the trees present in 1939 were in decline by 2000.     

Homestead greening is widespread amongst the urban poor in three medium-sized South African towns

Trees in urban areas are important sources of ecosystem services and benefits. In most towns the bulk of urban biodiversity, and trees specifically, are found in homestead gardens. But there is only limited understanding of the tree holdings in such gardens, and how they vary within and between towns, especially for developing countries where rapid urbanisation and high poverty influence the use of and reliance on land and local resources. We report on the nature of tree holdings in private gardens of poorer suburbs in three medium-sized towns along a gradient of decreasing mean annual rainfall in northern South Africa. A total of 3 217 trees were enumerated across 450 randomly selected homesteads. Most (90 %) households had at least one tree on their homestead, with an average of 7.7?±?6.1 trees. Most householders had planted the trees themselves. The density of trees declined along the moisture gradient. Within towns, tree density was positively related to garden size, which in turn was related to relative affluence and age of the suburb. Newer and poorer suburbs had the fewest trees per household. Sixty-two tree species were recorded, which were dominated by alien species, especially fruit trees. There was no relationship between the moisture gradient and tree species richness per household, but within towns there was a difference between suburbs, being lowest in the newest suburbs. Numbers of trees and species per household was positively related to age of the household head.     

Factors related to tree growth across urban-rural gradients in the Midwest,USA

Several environmental factors influence tree growth at any site. The objective of this study was to examine the relationship between biotic and abiotic factors and tree growth rate (mean ring width averaged over the last 10 years) in settings ranging from urban to rural. Six clusters, each with five communities and two rural parks, were sampled in Illinois, Iowa, Minnesota, Missouri, and Wisconsin, for a total sample of 320 trees. Within each community, trees in parks, and along residential and commercial streets were sampled. Five species were sampled: silver maple (Acer saccharinum L.), honeylocust, (Gleditisia triacanthos L.), hackberry (Celtis occidentalis L.), black maple (Acer nigrum Michx F.), and basswood (Tilia americana L.). Factors were investigated for three scenarios: (i) all trees sampled in all clusters, (ii) a single species, all clusters, and (iii) all species, a single cluster.Baseline variables (cluster, place population, site, species, and age) accounted for 49–71% of observed variation in growth rate. Combined biotic factors accounted for 5 to 6% of observed variation. For all species in a single cluster, combined abiotic factors accounted for 11% of observed variation. Biotic factors related to growth rate detected using multivariate analyses included number of other trees within 9 m, presence of disease and insects, and human-induced mechanical injury. Abiotic factors that were related to tree growth included presence of pavement and core bulk density. For trees in rural parks, number of other trees within 20 m, and for trees in both rural and community parks, number of other trees within 9 m of sample trees were associated with decreased growth rate.     

A time series of urban forestry in Los Angeles

There has been an increasing interest in the evolution of urban forests. This research uses historic and digital aerial photography to quantify changes in tree density in Los Angeles, California since the 1920’s. High-resolution geographic information system analysis (4 to 6 time periods) of three regions (San Fernando Valley, Hollywood, Los Angeles Basin) of Los Angeles reveals that there has not always been an increase in tree density with time. Tree densities on public and private land were highest in the 1940’s in Hollywood, while the San Fernando Valley and Los Angeles Basin experienced a near linear increase in tree density on both private and public land since the 1920’s. When historic tree density reconstructions were examined for the 15 Los Angeles city council districts from the 1920’s, 1950’s and 2006, most districts in Los Angeles have experienced a significant increase in tree density, however, there has been wide variation in tree densities among city council districts. Trees densities have generally been higher on private land since the 1920’s and currently tree densities on private land are significantly higher than on public land. Results suggest the evolution of urban forests in Los Angeles mirrors the dynamics of urban forests in desert and grassland cities. It is possible to reconstruct the development of urban forests in sections of cities using historic and contemporary aerial photography. We estimated that Los Angeles averages approximately 104 trees per hectare (82 private land, 22 public land) based on 2006 imagery at 0.3 m resolution, however, field validation suggests that we identified only 73% of trees. Although there is still space to plant trees on public land, private land owners will need to be heavily involved in order to achieve the goals of Los Angeles’ Million Tree Initiative.     

Modeling above-ground carbon storage: a remote sensing approach to derive individual tree species information in urban settings

Vegetation has gained importance in respective debates about climate change mitigation and adaptation in cities. Although recently developed remote sensing techniques provide necessary city-wide information, a sufficient and consistent city-wide information of relevant urban ecosystem services, such as carbon emissions offset, does not exist. This study uses city-wide, high-resolution, and remotely sensed data to derive individual tree species information and to estimate the above-ground carbon storage of urban forests in Berlin, Germany. The variance of tree biomass was estimated using allometric equations that contained different levels of detail regarding the tree species found in this study of 700 km², which had a tree canopy of 213 km². The average tree density was 65 trees/ha per unit of tree cover and a range from 10 to 40 trees/ha for densely urban land cover. City-wide estimates of the above-ground carbon storage ranged between 6.34 and 7.69 tC/ha per unit of land cover, depending on the level of tree species information used. Equations that did not use individually localized tree species information undervalued the total amount of urban forest carbon storage by up to 15 %. Equations using a generalized estimate of dominant tree species information provided rather precise city-wide carbon estimates. Concerning differences within a densely built area per unit of land cover approaches using individually localized tree species information prevented underestimation of mid-range carbon density areas (10–20 tC/ha), which were actually up to 8.4 % higher, and prevented overestimation of very low carbon density areas (0–5 tC/ha), which were actually up to 11.4 % lower. Park-like areas showed 10 to 30 tC/ha, whereas land cover of very high carbon density (40–80 tC/ha) mostly consisted of mixed peri-urban forest stands. Thus, this approach, which uses widely accessible and remotely sensed data, can help to improve the consistency of forest carbon estimates in cities.     

Tree diversity,distribution, history and change in urban parks: studies in Bangalore,India

Urban parks constitute critical biodiversity hotspots in crowded, concrete-dominated city environments. Despite the importance, they remain little researched. This paper assesses the biodiversity and distribution of trees in urban parks in the southern Indian city of Bangalore. 127 plots were used to survey tree distribution in parks across the city. The distribution is largely dominated by a few common species. The proportion of exotic species was very high, with 77% of trees belonging to introduced species. Park history had an impact on distribution. Old parks had fewer but larger trees, and greater species diversity compared to recently established parks. Old parks also differed in species composition, having a greater proportion of large canopy trees compared to young parks. Examination of size distributions revealed that large canopied species were gradually being phased out, and replaced by narrow and medium sized tree species which are easier to maintain, but which may not provide the same environmental and ecological benefits. Greater attention requires to be paid to the selection of trees in cities, not just with a view to easy maintenance as is currently the case, but to select an appropriate mix of trees that supports biodiversity and maximizes environmental and ecosystem services.     

Tree health mapping with multispectral remote sensing data at UC Davis, California

Tree health is a critical parameter for evaluating urban ecosystem health and sustainability. Traditionally, this parameter has been derived from field surveys. We used multispectral remote sensing data and GIS techniques to determine tree health at the University of California, Davis. The study area (363 ha) contained 8,962 trees of 215 species. Tree health conditions were mapped for each physiognomic type at two scales: pixel and whole tree. At the pixel scale, each tree pixel within the tree crown was classified as either healthy or unhealthy based on vegetation index values. At the whole tree scale, raster based statistical analysis was used to calculate tree health index which is the ratio of healthy pixels to entire tree pixels within the tree crown. The tree was classified as healthy if the index was greater than 70%. Accuracy was checked against a random sample of 1,186 trees. At the whole tree level, 86% of campus trees were classified as healthy with 88% mapping accuracy. At the pixel level, 86% of the campus tree cover was classified as healthy. This tree health evaluation approach allows managers to identify the location of unhealthy trees for further diagnosis and treatment. It can be used to track the spread of disease and monitor seasonal or annual changes in tree health. Also, it provides tree health information that is fundamental to modeling and analysis of the environmental, social, and economic services produced by urban forests.     

Effects of ground surface permeability on the growth of urban linden trees

Street trees are an important part of urban vegetation due to their provisioning of different types of ecosystem services such as local climate regulation and contribution to aesthetical and recreational values. In order to provide these services, urban trees need to endure many stress factors not present in natural environments, such as the widespread use of impervious surfaces in the vicinity of street trees. However, few studies have evaluated the effect of this potential stress factor on urban tree growth. The aim of this study was therefore to investigate how ground surface permeability affects stem and current-year shoot growth of linden (Tilia europaea) street trees in Gothenburg, Sweden. We found that a small fraction of permeable ground surface in the vertically projected tree crown area caused lower stem growth and strongly suppressed current-year shoot growth. This finding can guide future city planning, demonstrating that the vitality of street trees is compromised when the permeable surface area in the vicinity of the tree is small.     

Socioeconomic and ecological perceptions and barriers to urban tree distribution and reforestation programs

Tree planting and reforestation initiatives in urban and peri-urban areas often use tree distribution or “giveaway” programs as a strategy to increase tree cover and subsequent benefits. However, the effectiveness of these programs in terms of increasing overall tree cover and providing benefits to low-income and disadvantaged communities has been little studied. We assess these programs by exploring community participation in, and barriers to, an urban tree distribution program in Fort Lauderdale, United States and the role socioeconomic background and tree functional types have on participation. We use a mixed-methods approach, panel data, choice experiments, and econometrics to quantitatively analyze respondent’s ranking of program options. High income, White respondents had the highest level of awareness and participation while low income, African Americans (AA) had the lowest level. Monetary rebates were perceived as positive and significant as the compensation value increased to US$8.00 - $12.00. Fruit-bearing and native tree functional types were more preferred than flowering or shade trees. Latinos, AA, and high income respondents preferred fruit trees, while White, high income preferred native trees. Overall, low income respondents perceived the greatest barriers towards participation. 20% of Broward County residents who participated in the survey were aware of the tree giveaway programs and 13% had previously participated. Findings indicate an adaptive governance mismatch between program objectives to equitably increase city tree cover via planting shade trees versus individual’s knowledge and preference for other tree types and functions. Results can be used for developing and evaluating reforestation initiatives to equitably increase tree cover and improve the governance of urban ecosystems.     

The influence of urbanization on forest stand dynamics in Northwestern Oregon

Urbanization has been shown to affect forest stand characteristics in nearby natural areas. The purpose of this study was to examine forest structure in a naturally forested area, Forest Park in Portland, OR and adjacent forested land. Tree community structure was examined at 25 sites distributed along an urban-rural land use gradient. All trees in three quadrats per site were identified to species and the diameter at breast height was measured. ANOVA was used to examine differences in species richness and diversity, and tree density and importance value among four categorical areas. Tree species richness and diversity, and the density, diameter and importance values of shade-tolerant (later successional) species of trees decreased with urbanization. Sites nearer the city of Portland had significantly fewer shade tolerant saplings and young trees and were dominated by earlier successional species of trees as compared with sites at the far end of the study area. The forest structure in the city section of the park was very similar to that in the significantly younger middle section. The lack of young shade tolerant saplings and young trees appears to be the result of urbanization, although the mechanisms for such a loss are unknown at this time. Such a lack of recruitment may interfere with normal successional processes at more urban areas of the park.     

Mortality and recruitment in an urban forest (Forest Park in Portland,Oregon) between 1993 and 2003

I examined tree recruitment and mortality over a ten-year period at permanent plots in an urban forest, Forest Park, in Portland, Oregon. The density and diameter at breast height (dbh) for all trees living and dead were measured in 1993 and again in 2003. Data were analyzed using paired Student t-tests. I found significantly fewer live and significantly more dead trees in 2003 than in 1993. The increase in mortality was significant for all species of trees and for all sizes except large diameter trees. Mortality rates ranged from 0% to 67% at my sites. Recruitment was lower at all sites in 2003 with significantly fewer seedlings and saplings. The high mortality and low recruitment resulted in a net loss of trees at all sites. Loss of trees was not offset by increasing tree diameter, which suggests self-thinning is not the cause. No strong correlation with an urban to rural land use gradient was observed. The results may be related to global climate change or pollution. The high mortality of trees of all species in many diameter classes without a concomitant increase in recruitment could lead to dramatic changes in forest structure.     

Determinants of urban tree canopy in residential neighborhoods: Household characteristics, urban form, and the geophysical landscape

The aesthetic, economic, and environmental benefits of urban trees are well recognized. Previous research has focused on understanding how a variety of social and environmental factors are related to urban vegetation. The aim is often to provide planners with information that will improve residential neighborhood design, or guide tree planting campaigns encouraging the cultivation of urban trees. In this paper we examine a broad range of factors we hypothesize are correlated to urban tree canopy heterogeneity in Salt Lake County, Utah. We use a multi-model inference approach to evaluate the relative contribution of these factors to observed heterogeneity in urban tree canopy cover, and discuss the implications of our analysis. An important contribution of this work is an explicit attempt to account for the confounding effect of neighborhood age in understanding the relationship between human and environmental factors, and urban tree canopy. We use regression analysis with interaction terms to assess the effects of 15 human and environmental variables on tree canopy abundance while holding neighborhood age constant. We demonstrate that neighborhood age is an influential covariate that affects how the human and environmental factors relate to the abundance of neighborhood tree canopy. For example, we demonstrate that in new neighborhoods a positive relationship exists between street density and residential tree canopy, but the relationship diminishes as the neighborhood ages. We conclude that to better understand the determinants of urban tree canopy in residential areas it is important to consider both human and environmental factors while accounting for neighborhood age.     

Urban forest biomass estimates: is it important to use allometric relationships developed specifically for urban trees?

Many studies have analyzed the benefits, costs, and carbon storage capacity associated with urban trees. These studies have been limited by a lack of research on urban tree biomass, such that estimates of carbon storage in urban systems have relied upon allometric relationships developed in traditional forests. As urbanization increases globally, it is becoming important to more accurately evaluate carbon dynamics in these systems. Our goal was to understand the variability and range of potential error associated with using allometric relationships developed outside of urban environments. We compared biomass predictions from allometric relationships developed for urban trees in Fort Collins, Colorado to predictions from allometric equations from traditional forests, at both the individual species level and entire communities. A few of the equations from the literature predicted similar biomass to the urban-based predictions, but the range in variability for individual trees was over 300%. This variability declined at increasingly coarse scales, reaching as low as 60% for a street tree community containing 11 tree species and 10, 551 trees. When comparing biomass estimates between cities that implement various allometric relationships, we found that differences could be a function of variability rather than urban forest structure and function. Standardizing the methodology and implementing averaged equations across cities could be one potential solution to reducing variability; however, more accurate quantification of biomass and carbon storage in urban forests may depend on development of allometric relationships specifically for urban trees.     

Potential impacts of emerald ash borer invasion on biogeochemical and water cycling in residential landscapes across a metropolitan region

Trees provide important ecological services in cities, yet the vulnerability of the urban forest to massive tree losses from pest outbreaks could threaten those services, with unknown environmental consequences. The outbreak of emerald ash borer is an imminent threat to the ash population in North America. In the Minneapolis?CSaint Paul, Minnesota, metropolitan area, ash trees are present in 50?% of residential landscapes in Ramsey and Anoka Counties. We used a large survey of household activities, a tree inventory, a Household Flux Calculator accounting tool, and a set of annual evapotranspiration measurements, to quantify the current carbon, nitrogen, and phosphorus storage in ash trees, the cycling of these elements, and the total evapotranspiration from ash trees in residential areas in the metropolitan region. Ash represented 6?% of the trees in residential areas and the removal of the entire ash population would correspondingly reduce net primary production and carbon sequestration by only a few percent and would have negligible effects on losses of nitrogen and phosphorus from residential landscapes. Similarly, the effects of ash loss on the hydrologic cycle would be minimal and would depend largely on management choices for the ground currently underneath ash tree canopies. Overall, the percentage change in biogeochemical and hydrological fluxes corresponded closely with the percent of the total urban tree population that was represented by ash, suggesting that areas with higher densities of ash would experience correspondingly larger effects. A hypothetical tree replacement scenario with similar broadleaf species was determined to be likely to re-establish the original biogeochemical and hydrological conditions once the replacement trees reach maturity.     

Investigations of the urban street tree forest of Mendoza, Argentina

The investigation of the urban street trees was undertaken in the oasis city Mendoza, Argentina. The analysis included 1,680 urban street trees in their structure (species, age, and spatial structure), vitality and irrigation conditions. A questioning of 120 residents analysed the perception and acceptance of the urban trees by urban dwellers. Different urban structures, residential, commercial and residential mixed structures and the city centre were investigated. The results show clearly the insufficient maintenance of the trees (composition, aging, irrigation, pruning etc.) and damaged irrigation system with leads to a loss of vitality. The questioning showed that easily recognisable problems (insufficient pruning etc.) are more reflected by people than long term problems (irrigation). There is a different reflection of urban street tree problems in the different urban areas, related to status, information level and management. The maintenance and relation to urban street trees was best in the residential area. Not only better maintenance of the trees but also a clearer recognition of street tree problems by residents and by the responsible municipal institutions is urgently needed. A public-private partnership and more environmental education could help to overcome the problems.