Contrasting strategies for tree species to cope with heat and dry conditions at urban sites

Street trees planted at tree pits with impermeable pavements are exposed to a high level of both, atmospheric and edaphic stress. For many species tree physiology, optimal growth, and consequently long-term vitality may be hampered under these harsh conditions. Therefore, maintaining the vitality and promoting the ecosystem services provided by urban trees, for example aesthetic values and microclimatic benefits, it is important to establish trees with a high tolerance to stressful urban environments. Measurements of leaf gas exchange, leaf water potential, leaf surface temperature and chlorophyll-fluorescence were used for identification of specific response of Corylus corluna L., Ginkgo biloba L., Liriodendron tulipifera L., and Tilia cordata Mill. ‘Greenspire’. The summer months in 2013 with a period of heat and drought over eight weeks only interrupted by two rainy days were appropriate to access tree’s strategies to cope with low water availability. During the measurement period, the influence of high temperatures, high values of vapor pressure deficit (VPD) and low soil water availability on stomatal conductance, water-use efficiency (WUE) and leaf water potential (Ψmin, Ψmax ) was highest for Liriodendron followed by Ginkgo. These species showed high reductions in WUE in case of higher air temperatures and high values of VPD. Results indicate an avoidance strategy under increasing heat and VPD. In contrast, Corylus and Tilia followed a tolerance strategy indicated by decreasing values of Ψmin and lower reductions of WUE. In conclusion, wide species-specific variation in response to high temperatures and high VPD indicates that substantial potential exists in the selection for dry and hot urban places.     

Drivers of variability in water use of native and non-native urban trees in the greater Los Angeles area

Trees in urban ecosystems are valued for shade and cooling effects, reduction of CO₂ emissions and pollution, and aesthetics. However, in arid and semi-arid regions, urban trees must be maintained through supplemental irrigation, in competition with other water needs. Currently, a comprehensive understanding of the factors which influence water use of urban tree species is lacking. In order to study the drivers of whole tree water use of two common species in the Los Angeles Basin urban forest, four sites in Los Angeles and Orange County were instrumented with sap flow and meteorological sensors. These sites allowed comparisons of the water use of a native riparian (Platanus racemosa Nutt.; California sycamore) and non-native (Pinus canariensis C. Sm.; Canary Island pine) Mediterranean species, as well as the spatial variability in water use under different environmental and management conditions. We found higher rates of sapflux (J _O ) in native California sycamore as compared to non-native Canary Island pine. Within each species, we found considerable site-to-site variability in the magnitude and seasonality of J _O . For Canary Island pine, the majority of inter-site variability derived from differences in water availability: response to vapor pressure deficit was similar during a period without water limitations. In contrast, California sycamore did not appear to experience water limitation at any site; however, there was considerable spatial variability in water use, potentially linked to differences in nutrient availability. Whole tree transpiration (E) was similar for the two species when water was not limiting, but Canary Island pine was able to withstand unirrigated conditions with a very low E. These results add to the currently small pool of data on urban tree water use and ecophysiology, and contribute to establishing a more quantitative understanding of urban tree function.     

Food in a row: urban trees offer valuable floral resources to pollinating insects

Urbanization affects the availability and diversity of floral resources (pollen and/or nectar) for wild pollinating insects. For example, urban green areas are characterized by an abundance of ornamental plant species. Increasingly, trees are planted to improve the aesthetics of urban streets and parks. These urban trees might offer important floral resources to pollinating insects. To examine the suitability of urban trees as resources for pollinating insects, we investigated the chemical composition of pollen and nectar as well as the amount of nectar produced by the nine major insect-pollinated tree species planted in cities of Western Europe, namely Acer pseudoplatanus, Aesculus carnea, A. hippocastanum, Robinia pseudoacacia, Tilia cordata, T. x euchlora, T. x europaea, T. platyphyllos and T. tomentosa. The analyses revealed that globally the Tilia trees provide pollen with lower contents of polypeptides, amino acids and phytosterols compared with the other species. Urban tree flowers offer abundant nectar with relatively high sugar contents (0.16–1.28 mg/flower); sucrose was the predominant sugar in all nectars. The investigated tree species could therefore be considered in future city plantings.     

Management mitigates the impact of urbanization on meadow vegetation

Urban regions often contain remnants of ecologically valuable habitats. Whilst meadow habitats have been recognized as ecologically important and much studied, little attention has been given to meadow assemblages of urban locations. We studied the effects of meadow type, urbanization level, and management on vascular plant species richness, field layer diversity and soil chemistry in 18 grassland sites in the Helsinki Metropolitan Area (60°E, 25°N), on the southern coast of Finland during the summer of 2007. We recorded a total of 252 species, though the average number of species per m² was only 12.6. The negative effects of urbanization on forbs seemed to result in particular from increased soil nitrate (NO₃^- -N) concentration. The highest NO₃^- -N and Fe concentrations and ratios of total inorganic nitrogen (N_tot) to phosphorus (P) and potassium (K), were recorded from the soils of urban rocky meadows. Management by mowing decreased soil NO₃^- -N and Fe concentrations, ratios of N_tot:P and N_tot:K, and increased species richness and diversity. Elevated NO_x deposition is considered as a major driver of urbanization effects on vegetation, though changes in soil pH and metal concentrations, such as zinc (Zn), may also negatively affect the frequency of both forbs and grasses. This study shows that regular management by mowing and removal of hay mitigates these effects. We also recommend increasing the provision of dry meadows and maintaining more areas of supplementary semi-natural grassland habitats in urban green space as concrete measures for the conservation of dry meadow assemblages and urban biodiversity.     

Non-native plants are a seasonal pollen source for native honeybees in suburban ecosystems

In urban and suburban ecosystems, biodiversity can depend on various non-native plant species, including crop plants, garden plants and weeds. Non-native plants may help to maintain biodiversity by providing a source of forage for pollinators in these ecosystems. However, the contribution of plants in urban and agricultural areas to ecosystem services has often been underestimated in biodiversity assessments. In this study, we investigated the pollen sources of native honeybees (Apis cerana) in an arboretum containing native trees and urban and agricultural plants in a suburban landscape. We surveyed the flowering tree species planted inside the arboretum, which were potential pollen sources. The number of potential pollen-source species of native trees peaked in June and July and decreased after August. We collected A. cerana pollen balls every month and identified plant species of pollen in the collected pollen balls using DNA barcoding. In total, we identified 29 plant species from A. cerana pollen balls. The probability of A. cerana using pollen from urban and agricultural plants was higher in July and August than in June. A. cerana collected pollen forages from native tree species (53%), but also gathered pollen from crop plants (13%), garden trees (19%) and native and non-native weeds (14%); the predominant pollen sources in September and October were the garden tree Ulmus parvifolia and the non-native weed Solidago altissima. We found that native honeybees used plants from a variety of habitats including non-native plants to compensate for apparent seasonal shortages of native tree sources in suburban ecosystems. Our results highlight the importance of assessments of both positive and negative roles of non-native plants in urbanized ecosystems to improve biodiversity conservation.     

Provincial and cosmopolitan: floristic composition of a dryland urban river

High rates of intercontinental exchange of plant species have caused scientists to ask whether floristic areas with similar environments are undergoing global homogenization. We focused on riparian forests of the urban Salt River (Sonoran Desert, USA) to ask: (1) Is the forest dominated by cosmopolitan or provincial elements? (2) Which trees planted in the irrigated cityscape have established along the river? (3) Which types of restoration interventions have favored provincial species? We surveyed tree abundance, size and vigor in belt transects among five reaches that differed in degree of restoration, and obtained data on tree species composition of the urban landscape and pre-development riparian zone. Our results reveal the urban riparian forest to have many cosmopolitan elements, owing in part to spillover of trees from the cultivated cityscape (e.g., Acacia stenophylla, Vitex agnus-castus). Global spread of some regional (Neotropical) riparian taxa (e.g., Parkinsonia aculeata, Prosopis) also has contributed to the cosmopolitan status. Yet, the forests retain a distinct regional signature. Unintentional restoration of winter floods has allowed for regeneration of Salix gooddingii, a vernally-adapted provincial pioneer, although its long-term survivorship is restricted to limited micro-sites (storm drain outfalls). Urbanization-related changes in stream hydrogeomorphology explain increases in some regional species (e.g., Washingtonia spp.) that historically were excluded from the river.

Reaches restored by planting, weeding, watering, and geocountouring had the greatest abundance of provincial species and greatest floristic similarity to historic conditions.

     

Photosynthetic CO<Subscript>2</Subscript> uptake and carbon sequestration potential of deciduous and evergreen tree species in an urban environment

Urban tree planting programmes are increasingly promoted as a way to reduce atmospheric carbon dioxide (CO₂) mixing ratios. However, few studies have investigated the photosynthetic CO₂ uptake potential of different urban tree species across seasons. In particular little is known about photosynthetic CO₂ uptake in cities with a subtropical, oceanic climate where evergreen species are dominant. We addressed this shortcoming by measuring net photosynthetic rates of ten native and exotic tree species during different seasons and times of the day in Auckland, New Zealand. We also assessed the potential of leaf nitrogen (N) concentration as a proxy for net photosynthetic capacities of urban trees, which is of particular importance to upscale leaf-level photosynthetic CO₂ uptake to local and regional scales. In addition, we compared measured net photosynthetic capacities (light-saturated net photosynthetic rates) with carbon (C) sequestration rates estimated using tree growth measurements and allometric equations. Mean net photosynthetic capacities ranged between 2.37 and 10.48 μmol m^?2 s^?1 across all seasons and were closely related to tree C sequestration rates, suggesting that increased photosynthesis enhances growth rates and therefore tree C sequestration rates. Given that winter net photosynthetic capacities remained high in evergreen species (3.38–13.96 μmol m^?2 s^?1), with almost 50% higher mean net photosynthetic capacity compared to summer across all species, we suggest that tree planting programmes for CO₂ mitigation should favour long living evergreen tree species with high basal area increments (BAI). Leaf N explained 43% and 57% of the variability of photosynthetic capacities across species in summer and winter, respectively. These results indicate that leaf N may be used as a proxy for net photosynthetic capacities of commonly planted urban trees in Auckland. However, further research is required to provide robust models that may be used to estimate photosynthetic CO₂ uptake at a local and urban scale.     

Hadeda Ibis (<Emphasis Type="Italic">Bostrychia hagedash</Emphasis>) urban nesting and roosting sites

Hadeda Ibis (Bostrychia hagedash) have increased in population size and expanded in range in South Africa possibly as a result of increased use of exotic trees for nesting and roosting in urban areas. We investigated the urban nesting and roosting sites of the Hadeda Ibis by measuring tree height and species used by Hadeda Ibis for nesting and roosting in Pietermaritzburg, KwaZulu-Natal, South Africa. We expected Hadeda Ibis nests and roosts to have habitats, like wetlands, to be within 10 km of the roost or nest tree. Hadeda Ibis nest and roost locations were mapped using ArcGIS and available resources (grasslands, wetlands, plantations, other natural water sources) 10 km around each roost and nest tree were determined. Results showed that Hadeda Ibis use exotic trees for nesting and roosting in urban areas more than indigenous trees. This may be because exotic trees are more available in urban environments, particularly those trees that have ornamental value. Hadeda Ibis did not nest and roost closer to expected resources. Although previously associated with wetlands, Hadeda Ibis in urban environments were not close to natural water sources. This can be explained by swimming pools providing accessible drinking water and well watered lawns providing suitable foraging habitat therefore allowing them to roost and nest in this urban habitat.     

Relation between green spaces and bird community structure in an urban area in Southeast Brazil

Increased urbanization typically leads to an increase in abundance of a few species and a reduction in bird species richness. Understanding the structure of biotic communities in urban areas will allow us to propose management techniques and to decrease conflicts between wild species and human beings. The objective of this study was to describe the structure of the bird community in an urban ecosystem. The study was carried out in the city of Taubaté in southeastern Brazil. Point-counts were established in areas with different levels of tree density ranging from urban green spaces to predominantly built-up areas. We looked for a correlation between the richness/abundance of birds and the size of the area surveyed, the number of houses, the number of tree species and the number of individual trees. The results of multiple regression showed that bird richness had a direct relationship with vegetation complexity. The abundance and diversity of tree species were better predictors of bird species than the number of houses and size of the area surveyed. We discuss implications of this study for conservation and management of bird diversity in urban areas, such as the need to increase green areas containing a large diversity of native plant species.     

Urban channel incision and stream flow subsidies have contrasting effects on the water status of riparian trees

The water relations of riparian trees are often closely tied to stream hydrology and channel morphology, which can be altered by urban development. In regions with limited precipitation, urban water use can generate or augment dry-season stream flows, potentially providing a water subsidy to riparian trees. However, urbanization is also associated with high storm flows that cause stream channel incision, or downcutting, which could limit the effect of flow subsidies by lowering riparian water tables. We investigated the effects of urban dry-season flow subsidies and stream channel incision on the water status of streamside trees in Sacramento, California, which has a Mediterranean climate with a distinct and lengthy dry season. For two common facultative riparian species, Quercus lobata (valley oak) and Fraxinus latifolia (Oregon ash), we analyzed both midday stem water potential (Ψ_stem) measurements and leaf carbon isotope signatures (?¹³C) to determine whether channel incision and dry-season flow had interacting effects on seasonal midday water stress and longer-term water use efficiency. We found that stream flow could substantially reduce dry-season water stress in both tree species, but only at low levels of channel incision. Leaf ?¹³C signatures for Q. lobata suggested increased water use efficiency in trees growing along incised streams and those lacking dry-season flow. Urban management decisions that affect both dry-season base flows and channel incision can thus influence the growth and health of riparian trees, potentially leading to broader changes in riparian ecosystems.

     

Trees for urban environments in northern parts of Central Europe – a dendroecological study in north-east Romania and Republic of Moldavia

A limited number of species and genera currently dominate the tree stock in streets and urban sites. There has been considerable and persistent argumentation for the necessity of using a more varied and stress-tolerant selection of tree species. This paper reports results from a dendroecological study of six steppe forest reserves in north-east Romania and in the adjacent part of the Republic of Moldavia, where water stress regimes during the growing season and winter temperatures are comparable to those of inner city environments in northern parts of Central Europe and adjoining milder parts of Northern Europe (CNE-region). In each forest reserve, tree growth patterns were studied in five 20 m × 20 m plots, resulting in a total of 30 plots with an allocated area of 1.2 hectares. For all trees, height and diameter were measured and related to tree age by coring in order to detect the species growth and performance in these sites. In total 23 tree species were found, 13 of which were represented by 25 or more individuals with documented good growth in the study plots. The majority of these 13 species have very limited use in urban greenery in the CNE-region today and thus have the potential to increase the species diversity of the current urban tree population through further selection work.     

Habitat richness and tree species richness of roundabouts: effects on site selection and the prevalence of arboreal caterpillars

This study examined the relationship between the habitat and tree species richness of roundabouts and the abundance and species richness of tree-dwelling caterpillars, and thus the potential of urban roundabouts to support breeding populations. Total tree species number on sites was related to an increase in caterpillar abundance and a greater number of habitats was associated with a higher number of caterpillar species. An increase in the total number of trees was not correlated with a greater abundance of tree-feeding Lepidoptera; nor were native trees found to support proportionally more caterpillar species than all tree species, including introduced. This could be due to differences in species accumulation, the prevalence of generalist species or a statistical sampling effect. The occupancy and average abundance of caterpillar species was generally low, which does not support the theory that Lepidoptera in urban areas tend towards mono-dominance. It may, however, represent the presence of ‘accidental’ species on roundabouts. Habitat management and planting to increase tree species diversity and density on roundabouts has the potential to increase the species diversity and abundance of arboreal Lepidoptera, and increase abundance of butterfly and moth urban populations.     

Park pruning prompts a competitive reversal of an exotic tree, <Emphasis Type="Italic">Ligustrum lucidum</Emphasis>, in urban forests of Japan

Ligustrum lucidum, a tree native to China has naturalized in Japan where it is often planted in parks and along roads. Naturalized individuals in urban woodlands have uncertain competitiveness against native species, particular in light of the frequent pruning undertaken in these woodlands. We compared fitness-related traits of Ligustrum against two co-occurring native species: a pioneer tree (Mallotus japonicus) and an understory tree (Eurya japonica) under pruned and un-pruned settings. A growth experiments where we assessed the leaf and whole plant characteristics of the 3 species under two environments simulating forest edge and forest interior, and two pruning treatments (with or without) was conducted. Under simulated forest edge without pruning, Ligustrum had lower stem growth and A_max (maximum photosynthetic rate) than Mallotus, suggesting that native species can competitively suppress invasive expansion. With pruning, however, Ligustrum resprouted vigorously in both spring and summer with significantly higher apical growth and total shoots than Mallotus and Eurya in forest edge and interior, respectively. Also, with similar A_max between resprouts of Ligustrum and Mallotus, we see a reversal of competitive advantage in favor of Ligustrum in forest edge. In the forest interior, Ligustrum exhibited shade tolerant properties (i.e., A_max, chlorophyll a/b, and leaf density) similar to Eurya but had more resprout shoots, suggesting that the invader also maintains an advantage under low light. The competitiveness of Ligustrum clearly increased with human disturbance in the form of park pruning. Such management practice can inadvertently enhance the invasiveness of exotic species in urban settings.     

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

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

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

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

Exotic trees can sustain native birds in urban woodlands

Native landscaping has been proposed as a means of increasing native bird diversity and abundance in urban landscapes. However residents’ preferences for vegetation are such that exotic plants are often preferred over natives. We investigated the extent to which native birds foraged in three common native and three exotic tree species in mixed urban woodland during four seasons. We predicted that native birds would spend more time foraging in native trees, and that food resources provided by deciduous exotic trees would be more seasonal than those provided by non-deciduous natives. Native birds spent a lot of time foraging in two of the native tree species, but very little time in native red beech (Nothofagus fusca). They used exotic oak (Quercus robur) throughout the year, and sycamore (Acer pseudoplatanus) seasonally. Oak and European beech (Fagus sylvatica) were used by the largest number of species overall, because they attracted both native and exotic birds. With the exception of tree fuchsia (Fuschia excorticata), which produces large volumes of nectar followed by fruits, all tree species were sources of invertebrates for insectivorous feeding. Seasonality of use was high only in sycamore, indicating limited support for our second prediction. We show that being native doesn’t necessarily entail being a good food source for native birds, and popular landscaping exotic species, such as oak, provide foraging opportunities across all seasons.     

Bird diversity in a subtropical South-American City: effects of noise levels,arborisation and human population density

Cities are highly modified environments in which the only areas that resemble natural landscapes are urban parks with low human population density. Attempts are frequently made to maintain high bird diversity in cities for aesthetic or educational reasons. However, it remains unclear whether local site characteristics are important in determining bird assemblage composition or whether simplification of the assemblage is an inevitable consequence of the changes associated with human population density. From May 1998 to December 1999, we undertook bird counts at 521 points in Porto Alegre, Rio Grande do Sul, southern Brazil. Our main goal was to understand the pattern of distribution of the bird species richness and density within the city and determine which variables most affect species assemblages. We recorded 132 species belonging to 43 families that are common in Rio Grande do Sul and obtained quantitative data on 121 species in survey sites. The two most abundant species (House Sparrow, Passer domesticus and Rock dove, Columba livia) were exotics. Analysis based on a reduced subset of 134 points surveyed in spring/early summer suggested that there was a North–south gradient in assemblage structure. Variation in assemblage structure was also affected by the number of trees, urban noise and human population density. However, human population density had a much smaller effect on richness and assemblage structure than variables subject to management, such as tree density and noise levels. These results suggest that complex communities may be maintained in densely populated urban areas of sub-tropical South-America given adequate urban planning.     

Rainfall interception by Santa Monica's municipal urban forest

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

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

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