Street Tree Population Research Articles

The abundance and distributional (in)equalities of forageable street tree resources in Lagos Metropolis, Nigeria

AbstractForaging for wild resources links urban citizens to nature and biodiversity while providing resources important for local livelihoods and culture. However, the abundance and distributional (in)equity of forageable urban tree resources have rarely been examined. Consequently, this study assessed the abundance of forageable street trees and their distribution in Lagos metropolis, Nigeria. During a survey of 32 randomly selected wards across 16 local government areas (LGAs) in the metropolis, 4017 street trees from 46 species were enumerated. The LGA with the highest number of street trees was Ikeja, with 818 trees, while Lagos Island had the lowest count, with two trees. This disparity in tree numbers could be attributed to variations in human population density within each LGA. Ninety‐four percent of the street trees surveyed had at least one documented use and 76% had two, making them potentially forageable. However, the most common species had relatively low forageability scores. Only 5.6% of the total street tree population was rated as highly forageable, with a usability score of at least 11 out of 15. The most forageable street trees were fruit trees and non‐native species. The forageable street trees in the LGAs showed a significant disparity in their distribution, as evidenced by a Gini coefficient of 0.81. Overall, richer neighborhoods had a higher street tree abundance, richness, and forageability potential. To meet greening and foraging goals and address the current inequitable distribution, we suggest allocating more funds for greening, particularly in low‐income neighborhoods. Further research should evaluate forageable species from other sites to acquire a detailed understanding of the distribution and abundance of forageable resources in Lagos metropolis.

Street Tree Inventory: A Case Study Comparing Systematic Sampling vs. Stratified Systematic Sampling in Piracicaba City, Brazil

AbstractBackgroundThe inventory of street tree populations has acquired new importance due to interest in the provision of ecosystem services. That said, this paper aims to compare systematic sampling with stratified systematic sampling using different sizes of sampling units to estimate the variables of interest: number of trees per kilometer of sidewalk (DF), basal area per kilometer of sidewalk (Dg), mean total height, volume per kilometer of sidewalk (DV), and number of species per kilometer of sidewalk (DE). An innovative contribution here is testing new alternative density variables.MethodsIn the densely urbanized area of Piracicaba (Sao Paulo State, Brazil), 90 sets of 4 blocks were systematically sampled. They were used to compose sampling units of 1, 2, 3, and 4 blocks. Stratification was based on the percentage of street tree cover obtained with geoprocessing tools. Only public trees with a circumference at breast height greater than or equal to 12 cm and planted on sidewalks or avenue medians were included.ResultsThe effect of sampling unit size and stratification on estimate accuracy, sample size, and sampling intensity were analyzed. The results show that stratified systematic sampling was the more accurate process, especially forDF,Dg, andDV.ConclusionsReductions in sample size were more significant when stratified systematic sampling of 2-block sampling units were used.

The ecosystem disservices of trees on sidewalks: A study based on a municipality urban tree inventory in Central Italy

Urban green infrastructure, including street trees, plays a key role in providing ecosystem services to urban residents. However, to fully understand the effective role of trees in the urban context, it is also necessary to evaluate the disservices that they can produce in the development of their functions if not managed in an adequate and integrated way. This contribution aims to demonstrate an approach to assess three disservices (pavement damage, aesthetic damage, likelihood of tree failure) of street trees at the municipal level, starting from the existing municipal tree inventory. In this case study, from the street tree population, a sample of approximately 5% of the trees was drawn by stratified random sampling, where the strata were composed of groups of tree species. In particular, a sampling scheme is adapted in which the probability to select a tree in the sample is greater for bigger trees, under the assumption that the bigger the trees the greater are the disservices caused. In this way, a greater precision of the estimates of the considered disservices for the population of urban trees is expected. The results show a high variability of disservices provision among species groups. The results also confirmed a positive correlation between the considered disservices and tree diameter at breast height, while other tree attributes such as total height and crown diameter were found to be positively related only to pavement damages. Finally, severe pruning can lead to a high level of the aesthetic and functional disservices even for shorter and younger street trees.

Hotspots of pest‐induced US urban tree death, 2020–2050

Abstract Urban trees are important nature‐based solutions for future well‐being and liveability but are at high risk of mortality from insect pests. In the United States (US), 82% of the population live in urban settings and this number is growing, making urban tree mortality a matter of concern for most of its population. Until now, the magnitudes and spatial distributions of risks were unknown. Here, we combine new models of street tree populations in ~30,000 US communities, species‐specific spread predictions for 57 invasive insect species and estimates of tree death due to insect exposure for 48 host tree genera. We estimate that 1.4 million street trees will be killed by invasive insects from 2020 through 2050, costing an annualized average of US$ 30 M. However, these estimates hide substantial variation: 23% of urban centres will experience 95% of all insect‐induced mortality. Furthermore, 90% of all mortality will be due to emerald ash borer (Agrilus planipennis, EAB), which is expected to kill virtually all ash trees (Fraxinus spp.) in >6,000 communities. We define an EAB high‐impact zone spanning 902,500 km2, largely within the southern and central US, within which we predict the death of 98.8% of all ash trees. ‘Mortality hotspot cities’ include Milwaukee, WI; Chicago, IL; and New York, NY. We identify Asian wood borers of maple and oak trees as the highest risk future invaders, where a new establishment could cost US$ 4.9B over 30 years. Policy implications. To plan effective mitigation, forest pest managers must know which tree species in which communities will be at the greatest risk, as well as the highest risk species. We provide the first country‐wide, spatial forecast of urban tree mortality due to invasive insect pests. This framework identifies dominant pest insects and spatial impact hotspots, which can provide the basis for spatial prioritization of spread control efforts such as quarantines and biological control release sites. Our results highlight the need for emerald ash borer (EAB) early‐detection efforts as far from current infestations as Seattle, WA. Furthermore, these findings produce a list of biotic and spatiotemporal risk factors for future high‐impact US urban forest insect pests.

Is Street Tree Diversity Increasing in New York State, USA?

Diversity in tree populations is viewed as essential for protecting the public investment in urban trees and for preserving the environmental, social, and economic benefits that these trees provide. It is therefore crucial for officials responsible for the management of municipal trees to know the diversity of their municipal tree populations and whether their efforts to increase diversity have been effective or should be modified. We assessed street tree diversity in New York State, USA by analyzing municipal street tree inventory data from two data sets, the first comprised of 75 inventories collated from municipalities, and the second comprised of 32 sets of inventories conducted at multiple points in time. This analysis builds on two previous papers containing similar assessments by analyzing more current data and by calculating diversity index statistics and relative abundance percentages for prevalent street tree species and genera. Findings indicate that there has been substantial progress to increase street tree diversity in New York State. This progress is correlated with reductions in the dominance of Norway maple (Acer platanoides), the state’s most prevalent street tree species (17% of street trees statewide), and in the dominance of maple (Acer), the state’s most prevalent street tree genus (35% of street trees statewide). Work remains to be done to further increase species and genus diversity so as to meet the challenges posed to municipal street tree populations by invasive pests and climate change. Strategies are proposed for accomplishing this.

Tracking Changes to Urban Trees over 100 Years in Ithaca, NY, USA

Municipally managed urban trees provide environmental, social, and economic benefits. Continued provision of these benefits depends on the health and sustainability of these trees, which depends in turn on tree managers having the type of information usually found in a tree inventory. The city of Ithaca, New York, USA possesses 7 inventories of its street and park trees dating back to 1902. This paper uses the data contained in these inventories to assess the health and sustainability of the city’s street and park tree populations. Attention is given to the structure of these populations with emphasis placed on species and genera diversity and DBH size class distributions. Prior to 1987, the city’s municipal tree population was dominated by a few species, such as Norway maple (Acer platanoides), and genera such as maples (Acer) and elms (Ulmus), and the DBH size class distribution was skewed unsustainably towards older trees. From 1987 onwards, new plantings have significantly increased species and genera diversity, and the DBH size class distribution suggests sufficient younger trees to account for tree mortality and removals. These changes did not occur quickly due to the persistent legacy effect of past planting preferences and practices, but required a consistent effort by municipal tree managers over many years. As a result, based on an analysis of the most recent tree inventory conducted in 2019, the city’s street and park trees and the benefits they provide look to be on a more sustainable footing, although challenges still remain.

An Assessment of Street Tree Diversity: Findings and Implications in the United States

Integrating tree species diversity into urban forest management can help create resilient tree populations. In this study, the abundance of trees within families, genera, and species levels was determined through a system developed to evaluate the diversity of urban street trees at different scales. Municipal foresters were asked to report the six street tree species most commonly used in urban forests as an assessment of tree diversity. Through the use of this question in a series of urban forest management surveys, we were able to describe the diversity of urban street tree species in the 48 continental United States across time (1974 to 2014) and space (national, regional, and local levels). Throughout the United States the top six street tree species were distributed in 115 species, 71 genera, and 32 families. At the national scale, no one tree species or family dominated, but the Acer genus was 21.2% (0.8 standard error, or SE). At the regional level, 16.5% (3.3 SE) of all reported street trees were Acer platanoides in the Northeast region, and 8.1% (1.1 SE) of the South region were Quercus virginiana. At the local level, however, a lack of tree diversity becomes apparent. In any community, the top six reported tree species account for 61.5% (0.4 SE) of the total street tree population and the most common tree accounts for 23.7% (1.0 SE) of the total. When assessed over a 40-year period, changes in tree species and genera in each region were relatively minor. Among them, the Mideast and Northeast regions continued to be dominated by Acer and the South region maintained its preference for Quercus. This study provides a method to assess street tree populations when using surveys to quantify municipal forestry programs at state and national levels.

Street Tree Diversity in Massachusetts, USA

Pests, disease, and climate change pose major challenges to street tree survival, and diversity in tree species and genera is widely considered to promote the sustainability of municipal street tree populations. Conversely, the lack of sufficient diversity in street tree population was judged a contributing factor in the death and removal of thousands of street trees in Worcester, Massachusetts, that state’s second most populous city, due to an infestation of the Asian longhorned beetle (ALB, Anoplophora glabripennis). Therefore, reducing the dominance of prevalent street tree species and genera and increasing tree species and genera diversity are considered vital to sustainable street tree management and to the preservation of the ecosystem services and social benefits that street trees provide. This paper assesses street tree diversity in Massachusetts by analyzing a nonrandom sample of collated municipal street tree inventory data stratified by plant hardiness zones. Consistent with results previously found for Connecticut, New Jersey, New York, and Pennsylvania, results in Massachusetts indicate that a relatively small number of species and genera dominate the composition of most municipal street tree populations, including in particular Acer spp. (maple), one of the ALB’s favorite host genera. There is accordingly a need for greater species and genus diversity in municipal street tree populations statewide. While there may be a trend towards increased street tree diversity and reduction in the dominance of Acer spp., considerable work remains to be done.

How should we measure the DBH of multi-stemmed urban trees?

Foresters use diameter at breast height (DBH) to estimate timber volumes, quantify ecosystem services, and predict other biometrics that would be difficult to directly measure. But DBH has numerous problems, including a range of “breast heights” and challenges with applying this standard to divergent tree forms. Our study focuses on street trees that fork between 30 and 137 cm of height (hereafter “multi-stemmed trees”), which researchers have identified as particularly challenging in the ongoing development of urban allometric models, as well as consistency in measurements across space and time. Using a mixed methods approach, we surveyed 25 urban forestry practitioners in twelve cities in the northeastern United States (US) about the measurement and management of multi-stemmed street trees, and intensively measured 569 trees of three frequently planted and commonly multi-stemmed genera (Malus, Prunus, and Zelkova) in Philadelphia, PA, US. Specifically, we measured stem diameter at several distances above the ground: at the root collar, at 30 cm, just below the fork (which occurred between 30 and 137 cm), and at 137 cm (up to six stems following established protocols). Survey responses indicated that current mensuration practices are burdensome, that practitioners employ alternatives to the current protocols for measuring at 137 cm, and that small-statured, frequently multi-stemmed trees are an increasing proportion of street tree populations. Analysis of field data did not find substantial differences between methods of measurement with regard to predictive power for total height and average crown width. Alternatives to the current protocols for measuring at 137 cm have other advantages, including time required, ease of measurement, simplicity, and capacity to compare measurements between trees and over time. For trees that fork between 30 and 137 cm, we recommend taking a single diameter measurement at a lower height—either just below the fork or at 30 cm. Diameter measurements at 30 cm better serve researchers seeking to consistently measure radial growth over time, whereas diameter below the fork may suit practitioners who do not need fine resolution in trunk measurements.

Demographic trends in Claremont California’s street tree population

The aim of this study was to quantify street tree population dynamics in the city of Claremont, CA. A repeated measures survey (2000 and 2014) based on a stratified random sampling approach across size classes and for the most abundant 21 species was analyzed to calculate removal, growth, and replacement planting rates. Demographic rates were estimated using a hierarchical Bayesian framework. The community-level (all species) median growth rate was 1.41% per year (95% CI: 1.21–1.65%) with Pinus brutia and Pistacia chinensis growing significantly faster than the community-level median. The community-level median removal rate was 1.03% per year (95% CI: 0.66–1.68%), with no significant differences between species and the community-level medium. Once removed, only 7.2% (95% CI: 4.4–12.9%) were replaced annually. Presence of overhead utility lines influenced tree removal rates while age, diameter-at-breast-height, and prior tree condition influenced tree growth. Overall live aboveground biomass in sampled sites was 713.29 Mg in 2000 and increased to 877.36 Mg by 2014. Biomass gain from growth outweighed loss from removals nearly three-fold; replacement contributed 0.5% of the total biomass gain. We conclude that to increase the resilience of the street tree population will require 1) an increase in percent of full stocking or biomass stock and 2) a shift in the species palette to favor species less vulnerable to pests and expected disturbance from climate change and 3) ongoing monitoring to detect departures from baseline demographic rates.

The influence of sidewalk replacement on urban street tree growth

Interactions between tree roots and sidewalks can result in damage to sidewalks and when sidewalk damage is repaired adjacent tree roots are often severed. The objective of this study was to quantify the growth response of urban trees in restricted planting spaces pre- and post-sidewalk construction. The research included four trees species commonly planted along streets in Minneapolis and Saint Paul, Minnesota, USA. Species included were: Acer platanoides, Celtis occidentalis, Gleditsia triacanthos, and Tilia spp. Two street tree populations were sampled: trees adjacent to replaced sidewalk panels (<1.75m) and trees on streets with sidewalk construction that were greater than 3m from replaced sidewalk sections. In total, increment core samples from 292 trees were analyzed. Annual rings from each tree were measured and converted to basal area increment (BAI) for analysis. Comparisons of BAI were conducted between the two sample populations to assess differences in tree growth patterns. Pre- and post-sidewalk construction BAI was also evaluated to determine the influence of construction on growth trajectory. Growth response was quantified using resistance, resilience, and recovery indices. Species were found to differ in their response to construction disturbance. Planting space width was also found to influence post-construction growth. Tilia spp. had the highest resilience and fastest overall growth recovery post-sidewalk construction and A. platanoides exhibited the lowest resistance, resilience, and recovery post-sidewalk construction.

Street Tree Diversity in Three Northeastern U.S. States

Street tree diversity is widely viewed as a key component in the resilience of street tree populations to pests, diseases, and climate change. Assessment of street tree diversity is considered integral to sustainable street tree management and preservation of the ecosystem services and social benefits that street trees provide. This paper assesses street tree diversity in three northeastern U.S. states—New Jersey, New York, and Pennsylvania— by analyzing municipal street tree inventory data stratified by the 2012 USDA Plant Hardiness Zones. Despite the lesson learned from the historical devastation of overplanted American elms (Ulmus americana) by Dutch elm disease, and awareness of the contemporary threats posed to ashes (Fraxinus spp.) by the emerald ash borer (Agrilus planipennis) and to maples (Acer spp.), and other tree genera by the Asian longhorned beetle (Anoplophora glabripennis), results presented here indicate a current concentration of street trees among a relatively small number of species and genera, and in particular the dominance of maples as street trees. Results also show a positive relationship between street tree diversity and warmer average minimum winter temperatures. Consequently, there is a clear need in all three states for greater species and genus diversity in statewide and municipal street tree populations. However, meaningful impediments exist to increasing street tree diversity, especially in the short term.

Inventory of Street Tree Population and Diversity in the Kumasi Metropolis, Ghana

Inventory of Street Tree Population and Diversity in the Kumasi Metropolis, Ghana

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.

Street trees: paying their way in property value benefits

US research has demonstrated that property value benefits of street trees exceed the costs of their planting and maintenance, yet this has not been demonstrated in the Australian context. By unpacking the effect of street trees from other house, property and suburb features of 2,299 house sales between 2008 and 2010, this research derived a dollar value of Brisbane s street trees. Annual property value benefits of leafy streets, in 2010, were found to be worth more than twice the annual costs of street tree planting and management. For every $ 1 of Council s investment in Brisbane's street trees in 2010, leafy streets were returning $ 2.27 to property owners in higher house sale prices, an estimated $ 1.23 m in state stamp duties, and almost $ 1 m in Council rates revenue. When combined with the results of an analysis of the extent, structure and needs of Brisbane's street tree population in 2010, a business case and priorities for future investments emerged.

Diversity of street tree populations in larger Danish municipalities

Healthy and sustainable tree populations require a high diversity of genera and species. This study examined the occurrence and contents of tree inventories in Denmark's 30 largest municipalities. 59% of the municipalities had a tree inventory for street trees, but only about half of these were complete and updated. Only one municipality had a registration for trees other than street trees. Based on data from the tree inventories, the diversity of road side trees was analyzed at genus level and species level. A total of 82,072 street trees are part of the study. 11 different genera account for 92% of the total street tree stock, and 2–6 genera account for 40–80% of the street tree stock in the individual municipalities. Tilia was the most dominating genera (26%). 12 species account for 73% of the total street tree stock. The 6 most common species account for almost 50% of the total tree population. The species representing the largest numbers were Tilia×europaea (12%), Acer platanoides (10.9%), Platanus×acerifolia (7.2%), Tilia cordata (7.2%), Fraxinus excelsior (6.2%) and Sorbus intermedia (5.9%). The four most urbanized municipalities had a surplus of non-native species, but all municipalities apart from one had most street trees belonging to native species. The concluding recommendation of this study is that tree managers need to start working more strategic with their tree stock, in order to reduce the vulnerability, due to potential attacks from pests or diseases and climate change effects. A risk spreading system for the urban tree population is proposed, suggesting that no genera should account for more than 10% and no species for more than 5% of the tree population.

Simple Street Tree Sampling

Information on street trees is critical for management of this important resource. Sampling of street tree populations provides an efficient means to obtain street tree population information. Long-term repeat measures of street tree samples supply additional information on street tree changes and can be used to report damages from catastrophic events. Analyses of several street tree populations reveal that a 2%–3% sample of block segments with known length within a city will likely produce estimates on the total number of trees with a standard error around 10% of the total population estimate (relative standard error of 10%). Ratio estimates of number trees per length of street sampled reduced the number of block segments needed to attain a 10% relative standard error. Communities with a small tree population, or analyses of specific subsets of the population (e.g., individual species information), will likely need a higher proportion of block segments sampled to attain the same relative precision. This paper presents a simple means to sample street tree populations to aid in street tree management and presents information on how many block segments need to be sampled to achieve a desired sampling precision. Results can be used to develop simple, cost-efficient, and accurate means to sample street tree populations.

Comparing Species Composition and Planting Trends: Exploring Pest Vulnerability in Toronto’s Urban Forest

Urban forests represent a valuable resource for cities but are not without costs. These costs can include time, money, and the loss of beneficial services as results of pest infestations. Knowledge of an urban forest’s tree species composition and vulnerability to pests is needed to help managers enhance services delivered, while minimizing expenses over the long-term. Recent research has explored the impacts of individual pests on urban forests, but less attention has been given to the overall pest vulnerability. In this research, tree genera currently prevalent and commonly planted in Toronto, Ontario, Canada, were analyzed using a pest vulnerability matrix to explore how the city’s urban forest species composition and pest vulnerability may be changing. Current tree species composition was derived from existing inventory data, while the planting trends of a variety of local actors were determined through surveys and interviews. Results indicate there is somewhat limited diversity in current street and non-street tree populations, as well as a number of common tree species that have severe pest vulnerabilities. While new plantings replicate some current composition and pest vulnerability issues, several less common species are also being planted. As a result, overall pest vulnerability should decrease in the future, while some ongoing management concerns remain.

The balance of planting and mortality in a street tree population

Street trees have aesthetic, environmental, human health, and economic benefits in urban ecosystems. Street tree populations are constructed by cycles of planting, growth, death, removal and replacement. The goals of this study were to understand how tree mortality and planting rates affect net population growth, evaluate the shape of the mortality curve, and assess selected risk factors for survival. We monitored a street tree population in West Oakland, CA for 5 years after an initial inventory (2006). We adapted the classic demographic balancing equation to quantify annual inputs and outputs to the system, tracking pools of live and standing dead trees. There was a 17.2 % net increase in live tree counts during the study period (995 in 2006, 1166 in 2011), with population growth observed each year. Of the live trees in 2006, 822 survived to 2011, for an annual mortality rate of 3.7 %. However, population growth was constrained by high mortality of young/small trees. Annual mortality was highest for small trees, and lower for mid-size and large trees; this represents a Type III mortality curve. We used multivariate logistic regression to evaluate the relationship between 2011 survival outcomes and inventory data from 2006. In the final model, significant associations were found for size class, foliage condition, planting location, and a multiplicative interaction term for size and foliage condition. Street tree populations are complex cultivated systems whose dynamics can be understood by a combination of longitudinal data and demographic analysis. Urban forest monitoring is important to understand the impact of tree planting programs.

Assessing street tree diversity in four Ohio communities using the weighted Simpson index

Diversity is a key factor which should be considered during planning street replacements and involves assessment of existing street population for various factors especially resistance of trees to pests. Street tree replacement histories in the United States reveal extensive replacement of American elms killed by Dutch elm disease (DED) with maples, or ashes. In turn, these trees are now under attack by Asian longhorn beetle (ALB) and emerald ash borer (EAB). Two commonly used evaluation measure of diversity namely the 10:20:30 rule and the Simpson index takes into account only the number and evenness of the taxa in a street tree population. However these indexes fail to consider the differential functional values of trees which are critical in building a sustainable urban forest while attempting to increase diversity. The proposed weighted Simpson index which weights the absolute abundance of tree taxa based on some functional variable could be used as a guide to identify areas of focus for a community. A case study involving four communities in Ohio namely Toledo, Westerville, Dublin and Yellow Springs was conducted to examine the usefulness of the weighted Simpson Index. Three functional variables including environmental benefits, pest vulnerability and taxon adaptability at three different taxonomic levels, family, genus and species, were used to arrive at the weighted Simpson indexes for the four communities. Results revealed the usefulness of weighted Simpson Index in exposing areas of concern for each community. Generally all the four communities were lacking larger statured trees with greater environmental benefits.