Diversification of the urban forest—Can we afford to exclude exotic tree species?

Introduced tree species represent a substantial component of urban forests in cities all over the world. Yet there is controversy about the further use of introduced tree species. Many practice orientated publications,research papers and governmental websites in the fields of urban planning, urban forestry, and urban ecology argue for planting native species and avoiding introduced species. Such arguments for native-only species selection are also touted by environmental groups and the media. Consequently the debate has sometimes spiralled away from a sensible and rational platform where invasion risks and biodiversity loss are discussed, to a groundless and unreasonable argument where exotic species are generally considered incapable of providing ecosystem services. From a European perspective, we here aim to curate a set of necessary considerations for current and future discussions on native and non-native plant material in sustainable urban development. Using examples from Northern and Central Europe we illustrate that in some regions the catalogue of native tree species may be too limited to fulfil ecosystem services and resilience in harsh urban environments. A main message from our line of arguments is that we cannot afford to generally exclude non-native tree species from urban greening. If “native-only” approaches become incorporated in regional, national or international policy documents or legislation there is a risk that urban ecosystem resilience will be compromised, particularly in regions with extreme environmental conditions. Since both invasion risks and sizes of native species pools vary conspicuously at regional to continental scales we also argue to adapt urban policies on using non-native trees to regional contexts.     

Gradual or abrupt? An algorithm to monitor urban vegetation dynamics in support of greening policies

While greening becomes a more and more popular strategy to address multiple urban challenges and to enhance wellbeing and human-nature connectedness, there is an increasing need for usable methods and indicators to monitor its implementation. Earth observations produce a wealth of data on vegetation dynamics, but their use for monitoring urban greening policies is still limited. In this article, we develop and test an algorithm for the analysis of urban vegetation dynamics based on NDVI time series. Specifically, we focus on yearly greenest pixel composites that illustrate the maximum value of NDVI during the year (“greenness”): a key structural attribute to monitor urban ecosystems in the European Union. The algorithm is inspired by earlier examples of segmentation algorithms but fits the specific requirements of the targeted use in urban areas. It takes the series of NDVI values associated to each pixel, detects existing (multiple) break points, and quantifies related abrupt changes, as well as significant gradual changes that occurred during a selected period. We tested the algorithm on a 30-year Landsat series in Berlin and partially validated the output through a comparison with infrared ortophotos. The results reveal a net increase in NDVI between 1988 and 2017 in 84% of the pixels, with an average change over the whole city of + 0.096. Around 20% of the pixels show at least one abrupt change. Most abrupt changes (71.5%) were positive, but the negative ones had on average a greater absolute value (−0.170 vs +0.085). However, considering the cumulative impacts during the whole period, 97% of the total change is attributable to gradual changes. The validation proves that abrupt changes successfully capture variations in the extent of vegetation due to land cover changes (e.g., vegetation removal or new greening interventions), while gradual changes can be associated to vegetation growth or decline. We discuss the strengths and limitations of the proposed algorithm, and how the spatially- and temporally-explicit results can be a step forward in the interpretation of urban vegetation dynamics towards an effective monitoring of the impacts of local greening policies.     

How many people should be in the urban forest? A comparison of trail preferences of Vienna and Sapporo forest visitor segments

Differences in trail preferences for social conditions of visitors to forests in Vienna and Sapporo were investigated in 2006 using a standardised image-based stated choice approach. On-site visitors to two comparable peri-urban forests – the Lobau Forest in Vienna, Austria (n=373), and the Nopporo Forest in Sapporo, Japan (n=256) – evaluated the same sets of computer manipulated images depicting 128 trail scenarios with different levels of social stimulation. Latent class segmentations, in three sub-segments of similar sizes, differentiated by partly opposite preferences for social conditions, were derived for both samples. A positive contribution of social stimulation to preferences was found for about 17% of Nopporo and 9% of Lobau respondents, while for close to 50% of Lobau respondents and 38% of Nopporo respondents very low levels of social stimulation were preferred. The results indicate that urban forests should be managed for users with a desire for low social densities as well as a denser social setting providing some levels of social stimulation.     

Diversity and structure in California’s urban forest: What over six million data points tell us about one of the world's largest urban forests

Urban street trees provide many benefits to surrounding communities, but our ability to assess such benefits relies on the availability of high-quality urban tree data. While these data are numerous, they are not available in an easily accessible, centralized place. To fill this gap, we aggregated public and private data into a single, comprehensive inventory of urban trees in California called the California Urban Forest (CUF) Inventory. These data are offered to the public (aggregated to ZIP code) via an online data portal, which at the time of publication contained over 6.6 million urban tree records. In this study, we first describe the assembly and utility of the inventory. Then, we conduct the most comprehensive assessment of the diversity and structure of California’s urban forest to date at statewide, regional, and local spatial scales. These analyses demonstrate that California’s urban forests are highly diverse and among the most diverse urban forests in the world. We present a new and intuitive metric of species diversity, the top diversity or TD-50 index, which represents the cumulative number of species accounting for the top 50 % abundance of trees in an urban forest. We used species abundance data from 81 well-inventoried cities to demonstrate that the TD-50 index was a robust metric of diversity and a good predictor of comprehensive metrics like the Shannon Index. We also found that small-statured trees, such as crape myrtles (Lagerstroemia cv.) dominate California’s urban forests. This aggregated inventory of one of the world's largest urban forests provides the data necessary to assess the structure, diversity, and value of California’s urban forests at multiple spatial scales. The inventory’s presentation to the public and the information that can be gained from its analysis can be a model for urban forest management worldwide.     

Too hot to handle? On the cooling capacity of urban green spaces in a Neotropical Mexican city

Urban areas are particularly vulnerable to climate change due to the Urban Heat Island (UHI) effect, which can be mitigated by urban vegetation through shading and evapotranspiration. Nevertheless, there is still a lack of spatially explicit information on the cooling capacity of green infrastructure for most Latin American cities. In this study, we employed Land Surface Temperature (LST) of the Neotropical Mexican city of Xalapa to (1) analyze its Surface UHI (SUHI) compared to its peri and extra-urban areas, (2) to assess the cooling capacity of urban green spaces larger than 1 ha, and (3) to evaluate the role of green spaces’ size, shape and their surrounding tree cover percentage (Tc) on green spaces cooling range. We evaluated the cooling range of green spaces and their relationships with green spaces metrics and Tc via a linear mixed-effect model and identified threshold values for the variables at 25 m, 50 m, 100 m, and 200 m from the borders of green spaces through Classification and Regression Trees. Xalapa exhibits a SUHI of 1.70 °C compared to its peri-urban area and 4.95 °C to the extra-urban area. Green spaces > 2 ha mitigated heat at ~2 °C and the cooling range was influenced by the size of green spaces ≥ 2.8 ha and Tc > 21% at 50 m and only by Tc surrounding the green spaces at 100 m and 200 m. This shows that the size threshold of urban green spaces should be complemented with the presence of Tc starting at least 50 m to maximize the cooling capacity provided by the green infrastructure. Planning agendas should account for the interaction between the size of green spaces and the cumulative cooling effect of scattered vegetation inside urban areas towards compact green cities to cope with urban warming.     

A review of the concept ‘management’ in relation to urban landscapes and green spaces: Toward a holistic understanding

The management of urban landscapes concerns existing urban open spaces such as public parks, playgrounds and residential green spaces. It involves many different actors and organizations and its practice is of importance for the sustainable development of cities. As a research field, it needs further theoretical development and common definitions. For example, the central term ‘management’ is seldom defined in the relevant literature regarding an urban landscape context, and public participation in management processes is unusual. This paper introduces urban landscape management as an overarching concept that brings together knowledge about management of urban landscapes from fields such as urban forestry, park management and landscape planning. Based on a literature review, a common understanding of management in an urban landscape context is proposed, including organizational and strategic aspects of managerial activities. Our approach is that urban landscapes are ultimately managed to provide user benefits. Urban landscape management can be viewed as a complex process that includes a number of different actors, elements and relationships, mutually affecting each other. This view supports future studies of urban landscape management and its role in sustainable urban development.     

Where to construct new urban green spaces to be at the recommended distance from users and to complement existing ones? A study in five cities of northern Portugal

The proximity to urban green spaces (UGS) is important to facilitate their use by citizens, mainly by the most vulnerable groups in society who are usually unable to walk long distances. Furthermore, there are several ecosystem services from which the population can only benefit if UGS are close. In addition to being important to identify areas without a UGS nearby, it is also important to indicate possible locations for a new UGS, thus avoiding wasting resources by overlapping coverage areas or leaving gaps in coverage. It is also important to understand among all possible locations where the construction of a UGS is most needed. The present study aims (i) to identify possible locations for future UGS and order them by construction priority, (ii) to develop and test a methodology for decision-makers and urban planners and (iii) to contribute to increasing and developing UGS in cities. Considering residential areas, the results reveal a deficiency in UGS coverage in all the cities studied, as only 43.86–57.09% of residential areas have a UGS within 300 m. Construction of proposed UGS will lead to a 12.30–26.15% increase in coverage. The proposed methodology needs further improvement, but it can be a valuable tool for urban planners and decision-makers, and can encourage the construction of more UGS.     

Species richness of hedgerow habitats in changing agricultural landscapes: are α and γ diversity shaped by the same factors?

Understanding the determinants of hedgerow plant diversity in agricultural landscapes remains a difficult task, because the potential drivers affect the complete range of biodiversity components (alpha to gamma diversity). We surveyed herbaceous plant communities (of a height <1.5 m) in 84 hedgerows in the Seine river floodplain of France. Two types of potential drivers for species richness, accounting for landscape mosaic and hedgerow network, were recorded at both hedgerow and site scale. The distribution of species richness through the components of alpha hedgerow diversity (i.e. the average diversity within a habitat) and gamma hedgerow diversity (i.e. the total diversity across habitats) were assessed using additive partitioning methods, while the relationship between species diversity and its potential landscape drivers at both scales was modeled using Generalized Additive Models. Our results indicated that gamma hedgerow diversity is explained by the heterogeneity of the landscape structure, which is correlated with the mosaic of agricultural land use. At this scale, intrinsic properties of the configuration of the hedgerow networks have a weak influence on species richness. Alpha hedgerow diversity is also explained by landscape variables, accounting for both the configuration of agricultural mosaics and hedgerow networks, but to a lesser extent. Time lags for species responses are shown at both scales, and for the two types of drivers. Extinction or colonization debt may be indicated at both scales, while the remnant effects of former practices may also be responsible for such patterns at a local scale. We suggest that hedgerow management should take the specific parameters of both scales into account. At a local scale, management actions should aim to decrease the influence of adjacent land use when the impact is negative, through the implementation of extended buffer zones, while at the landscape and farm scales, agri-environmental schemes should be dedicated to the conservation of specific agricultural land uses.     

Greening residential quarters in China: What are the roles of urban form,socioeconomic factors,and biophysical context?

Residential greenspace has great intracity variations. Its driving factors have been widely investigated in Western countries (e.g., in Europe and North America), where residential areas are mostly privately owned, but few studies have examined the determinants of residential greenspace in China, where land is owned by the government. Taking the subtropical city of Changsha, China, as an example, this study mapped the percent green cover (PGC) in residential quarters with a fine-resolution satellite image. We also investigated its driving factors measured from multisource geospatial data. The results show that (1) the PGC in the residential quarters showed great spatial variation, with an average of 36.6% (0–85.7%) and a standard deviation of 18.3%. (2) Urban form, biophysical context, and socioeconomic factors together explained 49.18% of the variation in the PGC, and they independently explained 22.87%, 11.17%, and 2.31% of the variation, respectively. (3) Residential quarter size, PGC in the surrounding buffer zone, residential quarter age, housing value, and population density significantly and positively impact the PGC, while percent building cover, floor area ratio, and distance to city center had significant negative impacts. The strongest positive and negative impacts came from PGC in the surrounding buffer zone and percent building cover, respectively. Socioeconomic factors, the dominant driving factors in Western countries, show little impact on residential PGC. These findings expand our understanding of the intracity variation in greenspace coverage and the driving factors, which sheds light on the effective planning and management of urban greenspaces in China.     

Suburban habitats and their role for birds in the urban–rural habitat network: points of local invasion and extinction?

Suburban habitats in naturally forested areas present a conundrum in the urban–rural habitat network. Typically, these habitats contain less than half of the native woodland bird species that would exist at these sites if they were not developed. They also contain more total bird species than if these sites were left in a natural state. This apparent contradiction raises the question of “How do suburban habitats function in the urban–rural habitat network?” In this study, we analyze bird distributions on three rural-to-urban gradients in different ecoregions of the United States: Oxford, Ohio; Saint Paul, Minnesota; and Palo Alto, California. All three gradients exhibit similar patterns of extinction of native species followed by invasion of common species and subsequent biotic homogenization with urbanization. This patterning suggests that suburban land uses, those represented by the intermediate levels of development on the gradients, are a point of extirpation for woodland birds as well as an entry point for invasive species into urban systems. Furthermore, there are consistent patterns in the functional characteristics of the bird communities that also shift with intensifying urbanization, providing insight on the possible mechanisms of homogenization and community structure in urban ecosystems including an increase in the number of broods per year, a shift in nesting strategies, a decrease in insectivorous individuals, an increase in granivorous individuals, and a decrease in territoriality. Consequently, it appears that there are specific traits that drive the shift in community composition in response to urban and suburban land use. These results have significant implications for improving understanding of the mechanisms of suburban community ecology and conserving birds in urban habitat networks.     

Dendroctonus micans populations on Picea pungens in the center of a non-outbreak region contain few pathogens,parasites or predators: A new threat for urban forests?

In reviewing the occurrence of the bark beetle Dendroctonus micans in the Czech Republic over the past 100 years, we found that D. micans has been detected at 80 localities with elevations ranging from 158 to 1350 m a.s.l. During 2011–2012, we recorded three local outbreaks on Picea pungens, two of which involved isolated trees in urban areas between houses (always on sites <1 ha). From these locally abundant populations, 125 adults, 5 pupae, and 73 larvae of D. micans were obtained and dissected. Among these dissected specimens, nematodes in the hemolymph were detected in only 5 adults and 11 larvae. No other parasites or pathogens were found in the D. micans specimens. Low numbers of the beetle Rhizophagus grandis, which is a specific predator of D. micans, were detected, and none contained parasites or pathogens. Locally D. micans could constitute a new limited threat to urban trees, including P. pungens, in the Czech Republic and Central Europe. Special attention should be given to solitary P. pungens trees; if D. micans is detected on these trees, defensive measures must be taken to protect uninfested trees in the vicinity.