Contrasting natural regeneration and tree planting in fourteen North American cities

Field data from randomly located plots in 12 cities in the United States and Canada were used to estimate the proportion of the existing tree population that was planted or occurred via natural regeneration. In addition, two cities (Baltimore and Syracuse) were recently re-sampled to estimate the proportion of newly established trees that were planted. Results for the existing tree populations reveal that, on average, about 1 in 3 trees are planted in cities. Land uses and tree species with the highest proportion of trees planted were residential (74.8 percent of trees planted) and commercial/industrial (61.2 percent) lands, and Gleditsia triacanthos (95.1 percent) and Pinus nigra (91.8 percent). The percentage of the tree population planted is greater in cities developed in grassland areas as compared to cities developed in forests and tends to increase with increased population density and percent impervious cover in cities. New tree influx rates ranged from 4.0 trees/ha/yr in Baltimore to 8.6 trees/ha/yr in Syracuse. About 1 in 20 trees (Baltimore) and 1 in 12 trees (Syracuse) were planted in newly established tree populations. In Syracuse, the recent tree influx has been dominated by Rhamnus cathartica, an exotic invasive species. Without tree planting and management, the urban forest composition in some cities will likely shift to more pioneer or invasive tree species in the near term. As these species typically are smaller and have shorter life-spans, the ability of city systems to sustain more large, long-lived tree species may require human intervention through tree planting and maintenance. Data on tree regeneration and planting proportions and rates can be used to help determine tree planting rates necessary to attain desired tree cover and species composition goals.     

Characteristics of the urban forests in arctic and near-arctic cities

The urban forest of three arctic and near-arctic cities (Murmansk in Russia, Nuuk in Greenland, and Reykjavik in Iceland) were surveyed to determine tree species composition and structure. Interviews with local authorities were conducted to learn about the history of urban tree planting. The urban forests of all three cities were composed of a limited number of trees and tree species due to abiotic constraints of the arctic and near-arctic environment. These limitations include: low temperature, short growing season, high wind velocity and permafrost. A total of 28 species were observed in the three cities. Only three of these (Alnus incana (L.) Moench, Picea abies (L.) Karst., Betula pubescens Ehrh., and Salix glauca L.) were observed in all three of the cities. Planting designs that created windbreaks for pedestrians were common along streets in Murmansk and Reykjavik. Street trees have not yet been introduced in Nuuk, but trees were found in cemeteries, parks, and on private property. Older portions of cemeteries in all three cities were characterized by trees planted directly on graves. These grave trees were the first trees to be introduced in these cities. Subsequently, ‘trial and error’ was used by local residents to plant trees. Much of these plantings failed. In recent times arboreta were established near all three cities to identify species suited for planting under arctic and near-arctic conditions. Trees identified in these test gardens are now being planted in Murmansk and Reykjavik.     

Selecting trees for urban paved sites in Scandinavia – A review of information on stress tolerance and its relation to the requirements of tree planners

Urban paved sites are complex stress environments and when selecting trees for such sites, tree planners must prioritise stress tolerance above aesthetic appeal and functional aspects. This requires detailed information about the tolerance of tree species to environmental stresses, so as to support urban tree planners in selecting a wide range of trees. In Scandinavia there is currently no overview of the extent to which the character of information about stress tolerance and its dissemination in books and papers support urban tree planners in selecting a wide range of species for paved sites. Books on dendrology, literature on plant use in cities, tree nursery catalogues and scientific papers were therefore reviewed. For the review, eight tree species were selected along a gradient representing their intensity of use in Northern Europe. We examined the character of information and assessed it relative to tree planners’ requirements for information to be; contextual, local to Scandinavia, referring to existing plantings in paved sites and recommendations for use of the species in paved sites. The results showed that existing information is piecemeal and that most is either too general (dendrology literature) or too specific or contradictory (scientific literature) to meet the requirements of urban tree planners, while books intended for plant use in cities do not sufficiently integrate the local perspective. Moreover, contextual information local to the Scandinavian region is mainly provided for already much used species. These findings led to suggestions on how future urban forestry and arboriculture research and dissemination efforts in Scandinavia can encourage tree planners to use a greater variety of tree species in urban paved sites.     

A multiscale assessment of the diversity of New Zealand’s nursery trees

Nurseries play an important role providing trees for a variety of managed environments including urban forests. The diversity of urban forests and forest restoration projects are influenced by nursery species availability, and as such, there is a need to better understand tree species diversity at nurseries. We collected tree species lists from 75 nurseries throughout New Zealand, which were used to describe species richness (alpha diversity) and to examine similarity in the composition of native and non-native species assemblages among nurseries (beta diversity) at three spatial scales: island, region, city. Together, the nurseries grew 863 species, 174 of which were native to New Zealand, from 312 genera and 130 families. Nurseries grew significantly more non-native species (μ = 63.5, σ = 60.6) than native species on average (μ = 31.7, σ = 22.7) (t = 2.99, df = 48.45, p = 0.004). Beta diversity for native and non-native tree species were only significantly different at the scale of cities or regions, not at the larger scale of islands. Few species were grown in all cities or all regions and the majority of those that were common were native species. In contrast, non-native species dominated the unique species at all spatial scales, (i.e., species uniquely grown in one city, region, or island). By quantifying tree species diversity in New Zealand’s nurseries, this research provides a basis to better understand the influence that nurseries have on urban and peri-urban tree diversity, and ultimately how that diversity impacts resilience and the provision of ecosystem services.     

The impact of pruning and mortality on urban tree canopy volume

Urban trees provide a wide range of ecosystem services for city residents, with tall, mature trees with wide crowns generally regarded as preferable. The tree biomass which is responsible for shading, pollution removal, rain runoff retention etc. gets periodically reduced by the municipal tree management practice of pruning. This is a necessary activity, which reduces the risk of infrastructure damage and falling branches, but many estimates of ecosystem service provision in cities do not consider its impact explicitly. Tree mortality is also higher in cities, preventing trees from attaining and remaining at large sizes. This study used extensive field measurements of tree structure to estimate the impact of pruning on 8 tree species in two Italian cities: Taranto and Florence. Crown widths were reduced by 1.6 m on average, however there is large variation between species variation with branches more often being removed for thinning crowns resulting in larger gap fractions, which increased by 15% on average. No significant differences were observed for crown widths or gap fraction between trees pruned 3 and 4 years previously, suggesting that tree crowns structurally recover from pruning after 3 years. A deterministic model revealed that current urban forest pruning rates (every 6 years) and mortality (1%) may create a situation in which a city dominated by the species studied benefits from 93.5% of the maximum ecosystem services possible. This work will allow more nuanced estimates of urban forest services to be calculated.     

Assessing the structure and stability of street trees in Lhasa,China

Street trees can provide important environmental services to residents living in high-altitude cities. Nevertheless the performance of street trees in this unique environment is largely unknown. This article examines the impact of high-altitude environments on the growth of street trees through a case study in Lhasa, China. The structure, species composition, and health condition of street trees in Lhasa were surveyed using a representative sampling approach. The history of street tree programs and factors that affect the health of street trees was also analyzed. The results showed that there were 24 species and cultivars in 2032 sampled street trees. The street tree population in Lhasa contained a significant number of small trees, which was due to the large-scale planting program initiated in recent years. The street tree population in Lhasa was not very stable due to the uneven age distribution. The health conditions of street trees were affected by climatic factors as well as by management practices. We concluded that unfavorable environmental conditions in high-altitude cities may affect the sustainability of street tree populations to some degree but that human management of the street tree population is a more significant factor.     

Homogenization of trees in urban green spaces along the moisture gradient in China

The species composition of urban green spaces (UGSs) in different climate areas has been found to be homogenized compared with that of rural forests (RFs). However, the homogenization degree in response to moisture as a single ecological factor is still unknown. Here, we used 18 cities in the temperate zone of China to analyse the tree species composition and diversity index of UGSs and compare them with those of RFs. The results showed that (1) a total of 462 tree species were recorded in the UGSs in 18 cities, and 88 species were found in the UGSs of more than 9 cities. Among the 88 species, 47 species occur in UGSs in humid, semiarid and arid areas, including Ginkgo biloba, Salix babylonica, Acer mono, and Pinus tabuliformis. (2) The tree species richness and Simpson index of UGSs tended to converge (CV_UGS ≈ 1/2 CV_RF) along the moisture gradient (mean annual precipitation 200–800 mm). (3) The similarity of tree species composition among UGSs (J ∼ 0.30) was significantly higher than that among RFs (J ∼ 0.07), revealing homogenization among cities in areas with different humidity conditions. (4) The similarity increased when the difference in moisture and per capita GDP (gross domestic production) of cities decreased, indicating that ecological and economic factors jointly affected the homogenization of UGSs. This study also provides guidance for the management of UGSs.     

Tree establishment practice in towns and cities – Results from a European survey

A working group within the European Union funded COST Action E12, “Urban Forests and Trees”, carried out a survey between 1999 and 2001 to study current tree establishment practice in European towns and cities. An extensive questionnaire requesting information on the urban area, selection of tree species, establishment and aftercare practices and the main damaging factors was sent to tree professionals in urban areas in each of 17 countries.Indicators such as the relationship between the urban population and the number of street trees were used to compare urban areas. While most central European cities have a ratio of 50–80 street trees per 1000 inhabitants, the tree density was as low as 20 street trees per 1000 inhabitants for Nice. Often only a few species are planted and this may give cause for concern, since species diversity is considered an important factor in increasing the resilience of the urban tree population to abiotic and biotic stresses.The planting of larger street trees of 20–30 cm circumference, usually with hessian-wrapped rootballs, is becoming increasingly common practice. However, some countries report the use of mostly bare-rooted stock of much smaller trees (less than 12 cm circumference). Establishment costs for street trees range from less than 200 Euro to over 1500 Euro each. Poor site conditions, and impacts such as utility trenching are considered to be major restrictions to healthy tree life. Vandalism affects up to 30%of newly planted street trees in some towns and cities.Overall, the survey reveals large differences in tree establishment and management practices among European cities. There is a need to provide urban tree managers with guidance on good practice for tree selection and establishment based on empirical information. The survey was a first step towards collecting the necessary urban forest information from across Europe.     

Biodiversity in sacred urban spaces of Bengaluru,India

Urban green spaces provide critical social and ecological support for cities, but we know little about their diversity and composition in cities of the Global South. This is especially true of lesser known urban spaces such as sacred sites, which are of important cultural and biodiversity significance. We examine tree diversity and composition in sacred sites in Bengaluru, one of India’s fastest growing cities. We recorded 5504 trees from 93 species across 62 temples, churches, and Hindu, Christian and Muslim cemeteries in central areas of Bengaluru. Over half (52%) of the tree species were of native origin, a much higher proportion when compared to other green spaces in the city such as parks. Tree density in sacred sites was much higher than that in parks and informal settlements in Bengaluru. Temples and Hindu cemeteries contained the highest proportion of native species, with large numbers of Ficus benghalensis, a keystone sacred species. Trees in sacred spaces provide an important buffer against urban environmental stress in Indian cities, and serve as refuges for urban wildlife and biodiversity. We need greater information on these lesser known, but culturally significant alternate spaces. They play an important, though ignored role in the environmental sustainability of rapidly growing cities in the Global South.     

Species richness and ecosystem services of tree assemblages along an urbanisation gradient in a tropical mega-city: Consequences for urban design

We assess how tree species richness and ecosystem services vary along a tropical urbanisation gradient in a rapidly expanding mega-city (Bangkok, Thailand). We conduct tree surveys in 150 1 km cells selected by random stratification across an impervious surface cover gradient. In each cell, surveys were conducted at the centre (representing typical conditions) and in the largest patch of trees (assessing woodland retention impacts). We estimated trees’ contributions to i) carbon storage, ii) food production for people, iii) biodiversity support (production of food for frugivorous birds), and iv) economic value (assessed using regulations for using trees as collateral for financial loans). Surveys detected 162 species (99 natives) indicating substantial species loss relative to nearby natural forests. Despite this, and contrasting with typical patterns in temperate cities, tree species richness (including of natives) and ecosystem service provision is relatively stable across the urbanisation gradient. This finding has two important consequences. First, growing cities through high intensity developments that require less space may benefit regional biodiversity without compromising ecosystem services. Second, even the typically very small woodlands present in highly urbanised locations contribute to supporting biodiversity and providing ecosystem services; thus such woodlands require protection. Species richness is not strongly positively associated with most of our focal ecosystem services. Urban planners must therefore pay attention to both biodiversity and ecosystem services as these do not automatically accrue from each other, partly because non-native species contributed substantially to most ecosystem services except biodiversity support. Finally, trees provide substantial value as collateral for financial loans (averages of £643 ha at random locations and £2282 ha in wooded locations). Policies promoting such valuations may reduce tree removal and encourage tree planting, but the list of eligible species warrants revision to include additional species that enhance biodiversity support, ecosystem services, and resilience against future environmental instability.     

Building shade affects light environment and urban greenery in high-density residential estates in Singapore

Urban green spaces are an important component of the urban ecosystem of cities as they provide a range of ecosystem services that contribute to sustainability and livability of urban areas. The extent to which such services are provided is influenced by limitations on biological processes that underpin such ecosystem services. A poorly understood limitation in the urban environment is the effects of shade created by buildings on the adequacy of photosynthetically active radiation (PAR) for plant growth. We examined the effects of building shade in high-density, high-rise residential estates in Singapore on the level and distribution of PAR, and how PAR might in turn be correlated with growth of plants in community green spaces nested within these estates. Our estimates showed that high-rise and high-density buildings reduced daily PAR by almost 50% when compared to fully exposed conditions. The reduced PAR levels were correlated with lower vegetative and reproductive growth of several species of shrubs, and increased slenderness of two tree species. The shade environment created by buildings was differentiated from shade under vegetation canopies by longer periods of high instantaneous PAR during a diurnal cycle. There was also evidence of higher red to far-red ratio in the spectral composition of PAR. We suggest that an understanding of the spatial and temporal characteristics of PAR is necessary for appropriate selection of plants, particularly to match daily PAR received on site to daily light integral requirements of plants for improved delivery of ecosystem services.     

Drivers of diversity and tree cover in gardens,parks and streetscapes in an Australian city

While we know that urban vegetation is often distributed unequally, most studies have been undertaken in cities with relatively high levels of income inequality, using a single measure of distribution (usually tree cover) and in a single land use. This study explores predictors of both tree cover and species richness in gardens, streetscapes and parks in Ballarat, Australia. Spatial regression models found that education level was a more important predictor of tree cover than household income across all land uses in Ballarat which can be explained by some people with high incomes relative to education level choosing to live in new residential developments with disproportionately low levels of tree cover. Inequality in tree cover was greater in streetscapes than in residential gardens, suggesting that ‘top down’ political factors are more important than individual behaviours in determining tree cover in Ballarat. In contrast, physical rather than socioeconomic factors were better predictors of species richness across all land uses, highlighting that different measures of vegetation distribution are not necessarily correlated.     

Growth rates of common urban trees in five cities in Great Britain: A dendrochronological evaluation with an emphasis on the impact of climate

The knowledge of the rate at which trees grow in urban areas is an important aspect to consider as it can influence our quantification and valuation of the ecosystem services provided by an urban forest. This study investigates growth variations in diameter and height for four common urban tree species (Acer pseudoplatanus, Betula pendula, Fraxinus excelsior and Quercus robur) across five cities in Great Britain (GB) and how the typical radial growth of two of those species (F. excelsior and Q. robur) changes with climate. Dendrochronology was used to identify tree age and changes in ring width and diameter at breast height (DBH) and tree height were measured in-situ at the time of coring. Results indicate a substantial variation in the mean annual growth rates and the relationships between DBH and age or height and age of each species across different cities. However, the multiple factors affecting tree growth seem to influence different species in different ways, with for example A. pseudoplatanus trees showing overall the fastest growth in Peterborough but B. pendula ones showing the slowest. Precipitation and temperature had an effect on radial growth of F. excelsior and Q. robur trees in GB, but the strength and direction of influence varied with time of year, species and city. In particular, low precipitation at the start or during the growing season was found to be a significant factor limiting radial growth. A trend towards a reduction in ring width increment was therefore identified in hot and dry years, primarily in south-eastern cities but in other cities too. This highlights the risk that a changing climate may have on the growth and, consequently, on the ecosystem service provision of healthy urban trees.     

Diversity and distribution of the urban tree population in ten major Nordic cities

In order to have a healthy and sustainable urban tree population, a high diversity of species and genera is needed. This study examined (1) the diversity and distribution of genera and species of urban trees in the Nordic region; (2) the diversity in different sites of the city, distinguishing between street and park environments; and (3) the presence of native versus non-native tree species in urban environments in the Nordic region. The analysis of tree diversity was based on urban tree databases comprising a total of 190 682 trees in 10 Nordic cities – Aarhus and Copenhagen in Denmark; Espoo, Helsinki, Tampere and Turku in Finland; Gothenburg, Malmo and Stockholm in Sweden; and Oslo in Norway. The tree databases for Copenhagen, Espoo, Helsinki, Stockholm and Tampere only record street trees, while the remaining databases also include park trees. Tilia was the most dominant genus in Arhus, Copenhagen, Espoo, Gothenburg, Helsinki, Oslo and Stockholm, while Sorbus was the most dominant in Malmo and Betula in Tampere and Turku. Tilia × europaea was the most common species, comprising 16.0% of the total number of tree species. There was a higher proportion of species in parks than in street environments. The number of non-native species was higher than the number of native species in both street and park environments. However, the number of individuals belonging to native species was higher than the number of non-native individuals in all cities and environments except park environments in Arhus. The concluding recommendation from this study regarding greater diversity of genera and species is to exploit local experiences of rare species from local urban tree databases. After appropriate evaluation, urban tree planners can evaluate these rare species in larger numbers for e.g. street environments, where the need is greatest.     

Visual preference of trees: The effects of tree attributes and seasons

Trees in cities are often viewed as objects of attraction. Previous literature suggests that some tree attributes, such as height, canopy size and leaf color, are the driving factors for a tree’s aesthetic quality. However, the tree attributes used greatly vary among researchers who do not reach an agreement on which attributes affect people’s preferences. Also, differences of trees’ aesthetic quality in different seasons are poorly understood. To fill these gaps, twenty tree species were photographed in the exact same location in different seasonal stages, their aesthetic quality was judged by the general public using semi-quantitative response variable, and 11 tree attributes were abstracted and quantified to check their effect on the perception of trees’ beauty. Statistical analysis showed that the tree possessing the characteristics of higher branching trunk, dense canopy and moderate length of leaves (about 11 cm) was given a high preference rank. Although there is no significant difference among preference scores in four seasons, the tree ranked a high preference in spring or summer was more likely to indicate an intense fluctuation of preference among four seasons. The practical implication is that more tree species with the preferred attributes should be used not only for aesthetic appeal but also for ecological benefits. For the contrast of aesthetic quality in different seasons, the trees which rank as high preference in spring or summer should be planted.     

Tree composition and diversity in relation to urban park history in Hong Kong,China

In densely populated cities that are dominated by concrete buildings, urban parks serve as major green infrastructures for ecological and environmental functions. Trees are one of the important components that support these green infrastructures. Despite plenty of urban parks established in Hong Kong in the last 20 years, knowledge of tree composition and diversity is outdated. There were also no studies that investigated the differences in tree diversity in relation to park history. Therefore, this study aims to identify the temporary changes in tree composition and diversity in Hong Kong, by conducting a plot-based tree survey in 32 urban parks of different ages. Overall, 2801 trees belonging to 181 species were recorded in 319 plots across all the parks. A ridit analysis was conducted and it indicated the mature size of trees were not significantly larger in old parks. However, the linear mixed-effects models and the post-hoc tests suggested that DBH, tree height and the proportion of crown dieback for each class of tree size were greater in the old parks. Moreover, the composition of top-ranking dominant tree species varied substantially, where more ornamental and exotic trees were adopted in new parks. For species richness, the sample-based species accumulation curves of different park age overlapped when the horizontal axis of the curve was scaled by the average number of combined individual trees. When the horizontal axis was scaled by the number of plots, the curve for the old parks was above the curve for the new parks. The differences derived from these two accumulation methods indicated a higher tree density in old parks. For species evenness, both the rank-abundance curves and Rényi diversity curves depicted a similar low species evenness in old and new parks. These results suggested that species diversity remained largely unchanged from old parks to new parks though the dominant tree species varied. Greater attention should be paid to increase the species evenness in all urban parks, increase tree density in new parks and improve tree maintenance in old parks.     

Participatory urban forestry in Nepal: Gaps and ways forward

Managing urban population and environment has been one of the most important challenges recently. This paper attempts to assess the existing practices, people's perception, gaps and ways forward for participatory urban forestry and aims to help promote UF through identifying the current status, future prospects for and peoples’ perception about UF in two cities of Nepal. The study was conducted in Pokhara sub-metropolitan city and Bharatpur municipality of Nepal. Total numbers of plants in the effective green spaces are estimated as 285,500 in Pokhara and 213,250 in Bharatpur, and the number of tree species was 62 and 52 respectively. The religious tree species – Ficus bengalensis and Ficus religiosa are the most common and hazardous in both cities. The urban people have preferred small, less branchy and attractive tree species. However, the choice of existing species is inappropriate and not according to the people's interest. Therefore, urban people are unsatisfied with the existing species and their frequencies. The municipal authorities and other related organizations do not have any specific plan, programs and activities nor is there any public involvement to promote urban forestry. Regular coordination, cooperation and monitoring are required for the involvement of various stakeholders such as municipality, government office, community organizations and urban people. Analysis further shows that people's perception on urban forestry is encouraging as most of them agreed that the urban forests are useful for addressing the challenging urban environment. Adequate institutions and financial resources coupled with effective cooperation and coordination among concerned organizations could ameliorate the urban environment as urban community forestry.     

Estimating the alignment of tree species composition with foraging practice in Philadelphia's urban forest: Toward a rapid assessment of provisioning services

Urban greenspaces (UGS) are increasingly recognized for their potential to provide provisioning services to residents foraging for food and other plant materials. The alignment of tree species composition with foraging practices in cities, and the provisioning services harvesters derive, in UGS remains less well studied. To address this gap, we draw on existing tree species data and forager practices in the City of Philadelphia to estimate the alignment between tree species composition and foraging in the city. Our approach uses an existing forest inventory, an international online database of useful species, online information for residents about useful species found in the city, and novel data about forager practices. By considering these datasets in tandem, we are able to identify useful tree species, species likely of interest to foragers, and species actively foraged, and estimate the relative abundance of species in each category. Our results suggest that managers may be able to analyze proxy data, through use of online quality ratings, to rapidly identify and assess forager interest in species found in their urban forests.     

Species diversity of urban forests in China

A good knowledge of species diversity is essential for urban forest planning and management. In this study, we analyzed species diversity of urban forests in China using data synthesized through a systematic review. Our analysis showed that 3740 taxa of woody plants at species level and below have been reported in urban forests in 257 cities. Merging to the species level, there were 2640 species, including 1671 trees, 743 shrubs, and 226 lianas. Salix babylonica L. was the most widely distributed urban tree species in China. Overall, native species accounted for 76.02% of the observed species while the rest were exotic species. Inside cities, parks contained more species than other types of land use. Among cities, composition similarity of urban forests decreased as spatial distances among them increased. Besides, there was a latitudinal pattern in compositional similarity of urban forests in China. The relatively low ratio of the number of woody plant species in urban forests to these naturally distributed in China indicates that there is plenty of room for increasing species diversity of urban forests in China. However, cautions must be taken to avoid increasing compositional similarity of urban forests in China at the same time.