Urban tree cover: an ecological perspective

Analysis of urban tree cover is generally limited to inventories of tree structure and composition on public lands. This approach provided valuable information for resource management. However, it does not account for all tree cover within an urban landscape, thus providing insufficient information on ecological patterns and processes. We propose evaluating tree cover for an entire urban area that is based on patch dynamics. Treed patches are classified by their origin, structure, and management intensity. A patch approach enables ecologists to evaluate ecological patterns and processes for the entire urban landscape and to examine how social patterns influence these ecological patterns and processes.     

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.     

Environmental and crown related factors affecting street tree transpiration in Helsinki,Finland

We investigated the drivers of street tree transpiration in boreal conditions, in order to better understand tree water use in the context of urban tree planning and stormwater management. Two streets built in Helsinki in 2002, hemiboreal zone that had been planted either with Tilia?×?vulgaris or Alnus glutinosa f. pyramidalis were used as the study sites. Tree water use was measured from sap flow over the 2008–2011 period by the heat dissipation method. Penman-Monteith based evapotranspiration models of increasing complexity were tested against the tree water use measurements to assess the role of environmental and tree related factors in tree transpiration. Alnus and Tilia respectively used 1.1 and 0.8 l of water per m² of leaf area per day under ample water conditions, but the annual variation was high. The Penman-Monteith evapotranspiration estimate and soil water status changes explained over 80 % of the variation in tree transpiration when the model was parameterized annually. The addition of tree crown surface area in the model improved its accuracy and diminished variation between years and sites. Using single parameterization over all four years instead of annually varying one did not produce reliable estimates of tree transpiration. Tree transpiration, scaled to different canopy cover percentages, implied that the columnar Alnus trees could transpire as much as all annual rainfall at or less than 50 % canopy cover.     

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.     

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.     

Ecological integrity in urban forests

Ecological integrity has been an umbrella concept guiding ecosystem management for several decades. Though plenty of definitions of ecological integrity exist, the concept is best understood through related concepts, chiefly, ecosystem health, biodiversity, native species, stressors, resilience and self-maintenance. Discussions on how ecological integrity may be relevant to complex human-nature ecosystems, besides those set aside for conservation, are growing in number. In the case of urban forests, no significant effort has yet been made to address the holistic concept of ecological integrity for the urban forest system. Preliminary connections between goals such as increasing tree health, maintaining canopy cover, and reducing anthropogenic stressors and the general notion of integrity exist. However, other related concepts, such as increasing biodiversity, the planting of native species, and the full meaning of ecosystem health beyond merely tree health have not been addressed profoundly as contributors to urban forest integrity. Meanwhile, other concepts such as resilience to change and self-maintenance are not addressed explicitly. In this paper we reveal two camps of interpretation of ecological integrity for urban forests that in turn rely on a particular definition of the urban forest ecosystem and a set of urban forest values. Convergence and integration of these values is necessary to bring a constructive frame of interpretation of ecological integrity to guide urban forest management into the future.     

The effects of landscape scale on greenery and traffic relationships with urban birds and butterflies

Actions and policies to enhance biodiversity in the urban landscape must match the spatial scale at which biodiversity responds to the management and target variables. To this end, we compare the importance and effect of different kinds of greenery cover and road-lane density on bird and butterfly species richness between two landscape scales: 50-m versus 126-m radii around point counts (equivalent to areas of 0.8 h and 5 ha, respectively). We also compared the results against those of an earlier study using 500-m walking transects with widths of 100 m (i.e., 5 ha). Road lane density was more important at the 126-m than 50-m radius for both birds and butterflies. For birds, natural vegetation or forest cover and cultivated shrub cover were also more important at 126-m radius whereas the cultivated tree canopy cover was more important at 50-m radius. Cultivated tree cover and natural vegetation or forest cover were positively associated with species richness while road lane density and cultivated shrub cover were negatively associated with species richness. The results from point counts generally corroborate the results from the transects-based study, except that the short-duration point counts performed poorly in sampling butterflies. Our results indicate that in designing urban greenery policy, the plot sizes of individual developments is an appropriate spatial scale for the stipulation of tree cover targets, while urban planners have more flexibility to allocate natural greenery at broader spatial scales.

     

Canopy precipitation interception in urban forests in relation to stand structure

Urban forests provide important ecosystem services. In terms of hydrological benefits, forest ecosystems in urban environments represent qualitative and quantitative filter for rainwater. We quantified the canopy interception in relation to urban forest stand structure and rainfall intensity in an urban transect of the mixed (upland) forest in the city centre, towards a riparian pine forest and a floodplain hardwood forest in the City of Ljubljana, Slovenia. Bulk precipitation in open areas and throughfall were measured with fixed rainfall collectors in each forest. Stemflow was estimated from a review of relevant literature. We found that canopy interception in selected urban forests was mainly affected by tree species composition and other stand structure variables, such as canopy cover and tree dimensions. Average annual canopy interception was highest in the mixed forest (18.0% of bulk precipitation), while the riparian pine forest had the lowest level (3.9% of bulk precipitation) and the floodplain hardwood forest had the intermediate level for interception (7.1% of bulk precipitation). The mixed forest exhibited the stand structure factors that contributed to the highest canopy interception among the studied forests: high assemblage of dominant coniferous trees, denser canopy cover and the highest growing stock. Furthermore, rainfall intensity has proven to be an important factor for the seasonal partitioning (comparing the leafed and leafless period) of canopy interception. A better understanding of precipitation interception processes in urban forests is needed to assist urban forest managing and planning, aiming at maximizing canopy interception for the mitigation of stormwater runoff and flooding in urbanized watershed.     

Comparing the relative abilities of tree species to cool the urban environment

Increasing urbanisation poses numerous challenges to human quality of life. Cities are particularly vulnerable to the urban heat-island effect, which will be amplified by climate change. Increasing tree cover may be one of the most cost-effective ways of moderating urban temperatures. Trees cool their surroundings by casting shade, reflecting solar radiation, transpiring, and intercepting rainfall that subsequently evaporates. However, the potential of trees to reduce the urban heat-island effect is underutilised. The aim of this study was to synthesise understanding of the relative abilities of different tree species to provide urban cooling in temperate regions of the world and thereby develop a pragmatic approach for choosing those trees that have greatest potential in that regard. Based on a literature review and semi-structured interviews with leading experts, we developed a series of scenarios to illustrate the impacts of a tree’s cooling mechanisms and tree species’ attributes on components of the surface-energy balance equation. This enabled us to select parameters and propose simple equations that can be used to compare the relative abilities of tree species in relation to each of the cooling mechanisms. The parameters selected were for: transpiration – crown diameter, Leaf Area Index (LAI), canopy aspect ratio, and stomatal conductance or growth rate; reflection – albedo, crown diameter and LAI; shading – canopy aspect ratio, crown diameter, LAI and tree height. The approach is intended for use by urban planners and managers who wish to make informed decisions about which tree species to select for planting to counter the urban heat-island effect.     

Getting closure: The role of urban forest canopy density in moderating summer surface temperatures in a large city

With a growing number of people living in urban areas, city residents are more frequently exposed to stresses related to the built environment including elevated summer surface temperatures contributing to the urban heat island (UHI) effect. This study examines the role of urban tree density in moderating surface temperature variation in a densely populated city (Toronto, Canada). By applying parallel aspatial (OLS) and spatial regression (GWR) approaches with satellite derived data describing tree canopy density and surface temperature, one dimension of tree canopy quality (i.e., canopy density) is shown to contribute directly to a reduction in the surface urban heat island (SUHI) effect at a local scale. Though the model fit differed between the two regression approaches, both exhibited moderately strong explanatory power and demonstrated that observed surface temperature decreases in locations with increasing high-density and closed tree canopy. Study findings provide an empirical basis to recommend that municipal urban forest strategies embrace canopy quality metrics, such as increasing the density of existing urban forest stands, as a part of tree planting objectives seeking to effectively deliver temperature moderation as a valuable ecosystem service.     

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.     

Long-term urbanization effects on tree canopy cover along an urban–rural gradient

Urban forestry can benefit from improved knowledge of urbanization??s effects on tree canopy cover (TCC), a prominent urban forest indicator. This study examined changes in TCC over a long time frame, with respect to land cover (LC) changes, and across municipal boundaries. Specifically, I used air photos at 14 dates from 1937 to 2009 to develop an exceptionally long record of TCC change in Minnesota??s Twin Cities Metropolitan Area. During the study period overall TCC nearly doubled from 17% to 33% while the proportion urban land cover rose by 47%, highlighting the opportunity for substantial TCC gains following urbanization in previously agricultural landscapes, even in regions that were forested prior to European settlement. Results demonstrate that more intensely developed sites generally had lower TCC, and older urban sites had higher TCC. Modern TCC was not adequately characterized by linear distance along the urban?Crural gradient, but instead peaked near the center of the gradient where mature residential neighborhoods are prevalent. Compared to other land cover changes, urbanization events caused the highest rate of immediate TCC loss (9.6% of events), yet urban areas had the second highest TCC (>35%) in 2009, indicating that urban land gained TCC relatively efficiently following development. The results of this study provide new historical context for urban forest management across an urban?Crural gradient, and emphasize the need to consider ecological legacies and temporal lags following land cover changes when considering TCC goals in urban settings.     

Detecting the scales at which birds respond to structure in urban landscapes

Little empirical information exists about how birds respond to urban landscape structure across multiple scales. We explored how the variation in percent tree canopy cover, at four different scales, affected the abundance of bird species across various urban sites in North America. Bird counts were derived from previous studies, and tree patches were measured from aerial photographs that represented areas of 0.2 km², 1.5 km², 25.0 km², and 85.0 km². At each of the four areas, we conducted regressions between bird counts and percent cover of various tree patch sizes. From these analyses, we determined the area (called the best prediction area—BPA) and the patch size (called the best patch size—BPS) that accounted for a significant amount of the variation in bird counts, beyond the variation accounted for by these parameters measured at other scales. BPA and BPS were calculated primarily to take into account the high degree of collinearity that existed among the amount of tree canopy cover measured across the four scales.We calculated BPA and BPS values for a variety of bird species and ascertained whether larger species had relatively larger BPS and BPA values. In the spring, middle-sized to large birds (16.6 g–184.0 g) had relatively larger BPS values than did smaller birds (3.2 g–16.5 g), but in the summer, the largest birds (61.7 g–576.0 g) had small BPS values. Spring BPA values showed a similar result but summer BPA values did not. A majority of birds of all sizes had summer BPA values at the finer scales of 0.2 km² and 1.5 km². Overall, body size was an approximate predictor of the area and patch size at which a species responds to trees in a landscape, but many exceptions did occur. These exceptions could be related to a variety of factors, one being the difficulty in relating human-biased measurements to avian measurements of a landscape. The method described in this study will help researchers design multi-scale studies to address the effect of landscape pattern on different animal species.     

Are plant communities mainly determined by anthropogenic land cover along urban riparian corridors?

Often used as a mitigation tool to landscape fragmentation, urban riparian corridors also suffered from the effects of the urban expansion. This study explored the relationships between plant riparian communities and two major environmental variables (land cover, soil characteristics) and analyzed the floristic change along an urbanization gradient. Fifteen sites were surveyed on both riverbanks of two riparian corridors characterized by contrasting water regimes in Strasbourg, North Eastern France. Data of spontaneous species abundance was collected from 180 quadrats using (i) all plant species, (ii) herbaceous stratum and (iii) ligneous stratum (bush and tree). The diversity and compositional patterns of riparian plant species were analyzed within each corridor according to three levels of urbanization (urban, suburban, peri-urban). Relationships between riparian communities, land cover and soil chemical properties (pH, nitrogen and carbon content, moisture) were established by between-class co-inertia analyses. Land cover emerged as the main factor explaining changes in riparian communities along the rural–urban gradient while soil chemical properties discriminate water stress and fluvial dynamics of the two corridors. Similar compositional patterns were found within the most urbanized sites with the establishment of ubiquitous species. The herbaceous stratum is best linked to the level of urbanization, whereas the tree stratum is primarily correlated with corridor attributes (hydrological regime, soil properties). Although riparian species and communities are mainly determined by land cover, urban riparian corridors maintain native biodiversity up to the urban center.     

Trampling tolerance of understorey vegetation in different hemiboreal urban forest site types in Finland

The effects of trampling on the understorey vegetation were studied in boreal urban forests of different fertility in the greater Helsinki area, Finland. The three studied forest types in decreasing order of fertility were: 1) herb-rich heath forest, 2) mesic heath forest, and 3) sub-xeric heath forest. We inventoried the cover percentages of understorey vegetation in 40 herb-rich, 75 mesic and 40 sub-xeric biotopes located in 51 urban forests varying in size (0.6–502 ha). Cover percentages were compared to those of untrampled reference areas. In our study, trampling tolerance increased with increasing fertility of the forest type. Wear of understorey vegetation correlated positively with the number of residents (i.e. recreational pressure) around the forest patch. In general, understorey vegetation cover in all three forest types was lower than in the same forest types in untrampled reference areas. Ground layer cover in urban forests was less than half of that in reference areas. Mosses, lichens, and dwarf shrubs, especially Vaccinium vitis-idaea, proved to be sensitive to trampling and consequently decreased in cover. The cover of tree saplings, mainly Sorbus aucuparia, and some resilient herbs increased.     

Woody vegetation and canopy fragmentation along a forest-to-urban gradient

To identify patterns that can be used to predict vegetation and landscape characteristics in urban environments, we surveyed the species composition and size of woody plants, as well as the landscape structure of forest canopies, along a forest-to-urban gradient near Oxford, Ohio, USA. The gradient included six sites of increasingly urban land-use: a preserve, a recreational area, a golf course, a residential subdivision, apartment complexes, and a business district. We recorded species identity and stem diameter for all woody plants greater than 3 cm diameter at breast height (DBH) to examine the distribution of individual species as well as overall community composition. We used digitized aerial photographs to compare the spatial characteristics of the forest canopy at each site. We found predictable patterns in species diversity (Shannon index), spatial heterogeneity in species composition (mean percent dissimilarity), and all measures of patch fragmentation (canopy cover and patch number and size). There were clear differences in tree density and total basal area between forested sites and developed sites, but there was little resolution among developed sites. Species richness and average DBH showed no clear pattern, suggesting that landscaping preference largely determined these values. We present a modified version of an intermediate heterogeneity model that can be used to predict diversity patterns in urban areas. We discuss probable mechanisms that led to these patterns and the potential implications for animal communities in urban environments.     

Urban cover mapping using digital, high-spatial resolution aerial imagery

High-spatial resolution digital color-infrared aerial imagery of Syracuse, NY was analyzed to test methods for developing land cover classifications for an urban area. Five cover types were mapped: tree/shrub, grass/herbaceous, bare soil, water and impervious surface. Challenges in high-spatial resolution imagery such as shadow effect and similarity in spectral response between classes were found. Classification confusion among objects with similar spectral responses occurred between water and dark impervious surfaces, concrete and bare-soil, and grass/herbaceous and trees/shrub. Methods of incorporating texture, band ratios, masking of water objects, sieve functions, and majority filters were evaluated for their potential to improve the classification accuracy. After combining these various techniques, overall cover accuracy for the study area was 81.75%. Highest accuracies occurred for water (100%), tree/shrub (86.2%) and impervious surfaces (82.6%); lowest accuracy were for grass/herbaceous (69.3%) and bare soil (40.0%). Methods of improving cover map accuracy are discussed.     

People or place? An exploration of social and ecological drivers of urban forest species composition

Urban forests have garnered increasing attention as providers of an array of beneficial ecosystem services. However, urban forest ecosystems are highly complex and heterogeneous systems whose structure are shaped by interacting social and ecological processes. Approaches to reliably identify and differentiate these processes could be valuable for addressing complexity and reducing uncertainty in decision-making in urban forestry. The purpose of this study is to identify and quantify a range of social and ecological drivers of urban forest species composition, distribution, and diversity. This was done using hierarchical cluster analysis and discriminant analysis with empirical plot data describing the tree species composition in Toronto, Canada. Tree density and imperviousness were by far the most influential drivers of species composition. Increasing imperviousness saw not just reduced tree density but a decline in native species abundance. Additionally, single-detached housing, homeownership, and income were closely associated and explained higher tree densities and abundances of native species. However, income had a lower than expected influence on urban forest species composition given its importance in canopy cover research. Continuous forest patches were highly distinct compared to the remainder of the urban landscape, which highlights the ecological distinctiveness of residual forests within cities and lends support to their conservation. Increasing the understanding of social and ecological drivers of tree species composition, distribution, and diversity within cities is an integral part of urban forest ecosystem classification, which can be a valuable decision-support tool for ecosystem-based management in urban forestry.     

The influence of small urban parks characteristics on bird diversity: A case study of Petaling Jaya,Malaysia

The capacity of small urban park to serve as urban habitats are rarely explored. This study analyses the characteristics of small urban parks and their potential to support urban biodiversity and ecological functions. Nine small urban parks were studied in Malaysia in August and September 2014 using the combined field survey method of structured observation and field measurements. The measured variables were divided into three broad categories of physical characteristics, species richness and human factors. Bird species richness and abundance were used as the indicators for assessing biodiversity. Pearson correlations and multiple regressions were conducted to analyse the relationships between variables and to identify which variables had a significant effect on bird species richness and abundance. The results demonstrated that park area and vegetation variables ( e.g. the percentage of tree canopy cover, open grass/ground, native-exotic plants) are the important predictors of bird species richness and abundance. The percentage of canopy covers (negative relation) and park area (positive relation) are the best predictors of bird species richness in small urban parks. Meanwhile, the best predictors for bird abundance are the percentage of canopy covers (negative relation) and native vegetation species (positive relation). Human activities and park surroundings have a marginal effect on the presence of bird species in small parks. Based on the findings, we provide two general recommendations that could probably increase bird diversity in small urban parks: (1) the park development and management plan should incorporate a social-ecological approach that can benefit both city-dwellers and bird species, and (2) findings from the study should be used to rethink the planting design and composition of especially newly established small urban parks.     

The influence of land use type and municipal context on urban tree species diversity

Recent research has focused on the ways urban forest patterns vary in relation to level of urbanization and socioeconomic characteristics, with most studies limited to one urban land use type or multiple non-differentiate land uses. Additionally, the majority of studies examining urban forest patterns focus on canopy cover extent, with less attention given to patterns of species diversity. This study explores how tree species diversity varies across different urban land uses and municipal boundaries to better understand the role of land use types in shaping urban forest patterns. The goal is addressed through an exploration of plot-level tree data in the urban municipalities of Peel Region located in the Greater Toronto Area (Ontario, Canada). Species composition and standard diversity metrics are calculated for eight land use types and four municipalities. Our results show that differences in diversity metrics and species composition are greater between urban land uses than municipalities. Moreover, Peel’s urban forest has relatively high alpha diversity but many species are present on only one land use type. The results suggest that different causal processes are associated with each land use type, and that urban forest managers should adopt land use-specific strategies to meet species composition goals within the urban forest.