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.
The first International Conference on Urban Tree Diversity hosted in June 2014 by the Swedish University of Agricultural Science in Alnarp, Sweden highlighted the need for a better understanding of the current state of urban tree diversity. Here we present and discuss a selection of urban tree diversity themes with the intention of developing and sharing knowledge in a research area that is gaining momentum. We begin by discussing the specific role of species diversity in ecosystem service provision and ecosystem stability. This is followed by exploring the urban conditions that affect
Two devastating insect pests have been introduced to North America and Europe – the Asian longhorned beetle (ALB) (Anoplophora glabripennis) and the citrus longhorned beetle (CLB) (Anoplophora chinensis). These two wood-boring beetles are argued to be one of the most serious threats to the tree landscape since they have a large number of host species and genera. With the aim of creating an up-to-date compilation of these hosts, a systematic review was made of the literature for information on tree species attacked and used by ALB and CLB as hosts for complete life cycle or for feeding. This review revealed that a large number of tree species and genera are liable to be attacked by ALB and CLB. However, based on the findings, the whole picture is still unclear. One reason for this is the lack of transparency in published studies regarding lists of susceptible tree species for ALB and CLB. Another factor that needs to be reported is whether a tree species supports the complete life cycle of the beetles or just feeding by adult beetles. Without this information, species possessing moderate host qualities are at risk of being incorrectly labelled as very good hosts and hence excluded as urban trees.
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