The services that trees provide in the urban setting are largely well-recognised, and it can therefore not be debated as to whether tree presence in the urban environment is optional – it’s mandatory. However, because the urban environment is very harsh and vastly different from a natural setting in which trees have adapted (and evolved) to grow, species selection may be limited. On a similar level, species selection may be limited by available space, concerns regarding subsidence, root damage to built structures, and so on. Therefore, a species palette may be quite limited when it comes to selecting the trees to plant, and it may even consist of cultivars that are more ‘fit’ for urban use. Furthermore, it is unlikely that the palette will consist wholly of native tree species, perhaps because they cannot cope with the urban environment, or because they lack the necessary morphological traits required (such as a compact, upright crown). The desire for amenity may also see exotic trees used, as they are inherently likely to be more intriguing.
The limited available options for urban tree species will lead to a limited amount of diversity within an urban landscape, with regards to tree species present (generally-speaking). In particular instances, and notably where avenues are planted, dozens (or hundreds) of trees may exist of the same species, and if they are cultivars or clonally propagated then genetic diversity may also be very limited, if not identical throughout the avenue. Such a lack of diversity, on all levels (from species to intra-species genetics), is always going to be a problem, as the tree population is left at a higher propensity of suffering significantly from a pest or disease outbreak. On the species level, if 50% of an area’s trees are of one particular species, and a pest or disease attacks that species, all are potentially at risk. This risk increases further if the trees wholly (clonal), or only partially (seed sourced from same mother), share genetic properties. Essentially, a street graced with healthy trees one year may be void of trees the next (and we can observe how the emerald ash borer did this to avenues of ash in the US). For this reason, current literature advises that a great diversity of trees are planted in urban areas, to reduce this risk.
With all of this context in mind, we can now look at a study, undertaken across ten major cities over Scandinavia, that looked into the diversity (unfortunately, not on a genetic level) of constituent tree populations (see below map). In total, a little over 190,000 urban trees were featured in the study (not all cities had all their trees recorded, with some cities only possessing accurate inventories for 20% of their tree population). This figure could be split into street trees (123,000) and park trees (67,000).
After analysing all of the tree data, it was found that Tilia spp. (linden or lime) was most frequently used across the ten cities, accounting to a total of nearly 24% of all trees (though, in Helsinki, it accounted for nearly 45%). When grouped with Acer spp., Betula spp., and Sorbus spp., this amounted to nearly 60% of all trees, on average. Evidently, there is a clear reliance upon these four tree genera, which can thus be regarded as ‘staple trees’. In this sense, if any pest or pathogen were to use these genera as hosts, there is great potential for huge levels of damage.
Focussing more intricately on these four genera, it was found that Tilia x europaea (European lime) amounted to 16% of all trees (and was the most common species in five of the ten cities), followed by Acer platanoides and Betula pendula, which both constituted for 9% of the total tree population. In two other cities, Tilia spp. were the dominant species, so in seven of the ten cities limes were most abundant.
With regards to differences between street tree populations and those in parks, the general trend was that parks had a higher diversity of species – though not all cities had records of their park trees; either at all, or had only completed partial records. Arhus had by far the greatest number of park species, with an apparent total of 599 different species (because of a botanical garden). This towered over any other city, as can be seen below. For street tree diversity, cities had an average of around 50-60 different constituent species, though Malmo had the most at 113.
Of the actual diversity between species within streets and parks, Malmo ranked highest with nearly 45% of its park trees consisting of tree species not amounting in numbers to over 2% of the total tree population. This figure dropped to 36% in street scenes, suggesting Malmo’s streets are less diverse but, if particular pests and diseases were to strike, the city may potentially only lose (or have at high risk of mortality) a small segment of its overall tree population. However, only seven different tree species acconted for half of the city’s tree population (higher than all other cities), which could still leave the tree population at marked risk if the ‘wrong’ aggressor were to strike. For the other cities, that risk is even greater.
Across eight of the cities (excluding Oslo and Tampere’s street tree populations), non-native tree species were more common than native tree species. However, in terms of actual tree abundance, native trees outranked non-native trees in all cities besides Arhus’s parks (not surprising, given there were 599 differnet species found in a botanical garden!). From this, we can assert that there is a greater tendency to select non-native tree species, though in lower numbers. Perhaps it is inevitable that the diversity of non-native species will trump native species as well, given the number of non-native species available for selection is vastly higher (for example, the UK only has 33 native tree species, though hundreds of viable non-native species exist for urban planting). Evidently, there is still a focus on ensuring native trees comprise the majority of actual tree numbers, which is beneficial for biodiversity. However, if an exotic pest or disease emerges, these naive trees may be struck hard (ash dieback, for example), and large losses may be incurred.
The authors also found that some tree species were isolated to particular areas (or even a single area). For example, beech (Fagus sylvatica) amounted to 7% of all park trees in Malmo, though these were isolated to one single park. Unfortunately, the exact distribution of tree species wasn’t ascertained in the study, so beyond this example there is no data. However, it does highlight that certain tree species may only be very locally abundant. This may mean that the loss of the tree species isn’t damaging to the city as a whole, as tree cover is still maintained in all other areas, though it does mean that local tree cover is almost wiped-out, and a hugely concentrated inoculum base exists for a pest or pathogen.
In light of all the data provided for the study, one can certainly observe that whilst there is diversity in many cities, there is a tendency for particular genera and species to be selected in great abundance (such as lime; notably Tilia x europaea). Whilst this may be because of the practicality of using such species (survivability, growth form, amenity value, tolerance of poor conditions, etc), if a pest or disease were to use these species as hosts then there is a serious issue at hand. For this reason, it can be suggested that investigations into identifying additional species (and genera) should be undertaken, in an attempt to enrich diversity and lessen the reliance upon a select few genera and species. Of course, such an enriched diversity will come at the utilisation of non-native tree species, which runs the risk of invasive populations forming and damaging surrounding rural and woodland / forest regions. Biodiversity may also suffer, as there are few obligate associations between these exotic trees and native bodiversity.
Source: Sjöman, H., Östberg, J., & Bühler, O. (2012) Diversity and distribution of the urban tree population in ten major Nordic cities. Urban Forestry & Urban Greening. 11 (1). p31-39.