I’m currently reading a really quite brilliant book entitled Urban Tree Management – for the Sustainable Development of Green Cities, and there is a section where the authors discussed what the main types of land classifications are in the urban environment. A less science-heavy post than normal, hopefully this is both a nice change of pace and thought-stimulating.
Firstly, we can readily observe how urban areas with have what is considered a ‘road traffic area‘. As the name suggests, these areas are situated adjacent to highways, be such highways main routes or residential ones, and the role of trees in this setting may be to: (1) provide amenity value; (2) reduce the speed of traffic; (3) cast shade and cool the highway and surrounding pavements; (4) act as a visual screen; (5) filter out particulate matter; (6) ‘catch’ rainwater in the foliage crown and branching structure so to reduce the stress upon stormwater management systems, and; (7) act as a connective structure between parks and urban woodlands for the benefit of biodiversity. However, these numerous reasons for why trees exist in the urban landscape are pressured by the numerous stressors, such as air pollution, de-icing salts, compaction, impervious surfaces (drought), low humidity, high levels of light reflectance, and high temperatures. In this sense, the roadside tree has a very tough time, and survival rates may be low in the long-term (or short-term), and only those that are truly hardy may be able to reach a good age and retain their dignity in the process.
Alongside the road traffic area, we have the ‘densely built-up area‘. As noted by the authors, and also rather self-explanatory in nature, the two go hand-in-hand; particularly in the inner-city area. Therefore, many of the same stressors apply, and many of the reasons for planting also apply, though because such areas are going to be (at least, in part) residential, considerations must be given to ensuring that the constituent trees ideally don’t trigger allergic reactions in residents (Platanus spp.), emit awful smells (I’m looking at you, Ailanthus altissima), or block light to properties to a marked degree (densely-crowned species, including Quercus spp.). Perhaps, the densely built-up area is even more tricky (in both landscaping and tree survival) than the road traffic area, in terms of landscaping.
Within these densely built-up areas, we may sometimes come across a green roof and / or container plants, to add greenery to the landscape. Trees and plants can, and do, exist upon the rooves of properties in inner-city areas (such as in London, where there was the Derry & Tom’s roof garden, and the Selfridge’s roof garden in Oxford Street), though these areas are very limited in terms of root space and will likely be prone to higher wind speeds (because of the higher elevation and exposed setting). This means that drought is a very real risk, and as such tree species selection may be limited to smaller species, and those which are tolerant of dry conditions.
We then have the ‘sparsely built-up area‘, which may include residential areas (detatched and semi-detatched houses), small parks and green spaces, and ‘green corridors’ that may stretch across these areas. The stressors associated with this area type will be lesser than those in the densely-built up areas, though will still nonetheless exist. Trees planted in private gardens may have more scope to grow and fulfil longer lives, compared to roadside trees that may have more limited rooting space and have to compete with underground services (and the associated trenching that goes alongside repair works). Garden trees will also exist to serve a variety of purposes (depending upon personal preferences), ranging from amenity value and fruit crops, through to ecological value and adding privacy to a dwelling (conifer hedges). Old trees from prior land use (normally agriculture) may also remain, and at times form tree avenues.
Outside of the built-up areas used for residential means, the urban landscape may also be host to industrial parks and commercial parks.Often, these sites are very large, and are covered with large warehouses / units, which all have their own (or shared) car parking areas. For the industrial park, trees that are planted may very well be both quick-growers and reach a large mature size, as a means of softening the visual impact of huge warehouses and the many coming-and-going articulated lorries. On the other hand, whilst commercial areas may utilise trees for similar purposes (amenity and softening), the tree species used are likely to be far more ornamental (perhaps fastigiated cultivars will be used, or sterile varieties of fruit trees). It is therefore unlikely that large trees will be planted, unless there is a central area that can support a large mature tree, or silva-celled systems have been installed complete with underground irrigation systems connected to stormwater reservoirs. For commercial parks that have indoor areas, trees can still be used, though they may be potted, be fake trees (made of plastic), or be small in nature (asides from during the Christmas period, where large conifers may be brought in temporarily).
Back in the outdoors, public parks, ornamental gardens, and cemeteries, will often contain large tree populations. Here, the planting of trees is usually for amenity purposes, though the lack of competition for aerial and rooting space with built structures allows for very large trees to be planted – even avenues. Cemeteries may be host to lines of sequoias, for example, whilst parks and gardens may display unique cultivars of popular tree species, or even contain exotic tree species. Closed cemeteries may, by their very nature, be home to some of the largest trees, mature in age, and some of these may even be very old (ancient) yew trees (Taxus baccata). Public gardens are likely to have many more smaller trees, though the Chelsea Physic Garden in London has some fabulous large mature trees – cork oak (Quercus suber), ginkgo (Ginkgo biloba), magnolia (Magnolia grandiflora), swamp cypress (Taxodium distichum), and sweet gum (Liquidambar styraciflua), to name but a few.
Cutting through towns and cities, we may also come across ports, channels, and waterways. Here, riparian trees that are ideally native (willows, alders), but also non-native species that aren’t invasive (wingnuts), may most often flourish, though because these areas are likely to suffer less markedly from pollution, there is scope for the selection of a wide range of tree species. Despite this, water-borne pollutants may disrupt rooting environments, particularly nearby to ports. Therefore, the scope for tree species selection varies depending upon the context of the site. Where watercourses run through a large park area, perhaps the largest trees may be able to flourish.
Remnants of old woodlands and forests may also grace urban locations, and these are classed as urban forests. These locations may also be man-made however, having been planted on old areas of plotland, parkland, or otherwise. Tree species here may often be exclusively native, though garden escapees may also frequent such areas. Depending upon the popularity and accessibility of the urban forest, soil compaction and understorey disturbance (dogs) may be a particular problem, as may fly-tipping and other pollutants emitted from nearby areas (such as roads). Sporting activities may also take place in urban forests, with off-road biking being a notable activity. Alongside human pressures, the crucial role the urban forests play in retaining relicts of biodiversity (forest islands) must be recognised, and from these ecological hubs trees in the wider landscape may provide connectivity between other urban forest islands. At times, these urban forests may be found within parklands, or adjacent to them.
Brownfield (derelict) sites that have been left undisturbed can also be an important area where trees are supported. Often, the ground is polluted, compacted, and largely unsuitable, so species found here will likely have succeeded naturally and consist of early successional species: alder (Alnus spp.), ash (Fraxinus spp.), birch (Betula spp.), elder (Sambucus nigra), hawthorn (Crataegus spp.), poplar (Populus spp.), and willow (Salix spp.). These regenerating sites may also be important ecologically, and scrubbier areas may support reptiles. Rubble left over from past land use can also be suitable for insects, who utilise these niche habitats for shade and shelter.
Lastly, we have allotment gardens. In these areas, which are usually small plots, large trees are almost certainly not going to exist. Instead, fruit trees may be commonplace, and at times old mature apples (Malus spp.), pears (Pyrus spp.), and plums (Prunus domestica) can be found.
There likely exist many more niche areas within the urban landscape, though hopefully this post has given us all some food for thought. Buy the book (linked below), for many more thought-provoking reads.
Source: Gillner, S., Hoffman, M., Tharang, A., & Vogt, J. (2016) Criteria for species selection: Development of a database for urban trees. In Roloff, A (ed.) Urban Tree Management – for the Sustainable Development of Green Cities. Singapore: Wiley Blackwell.
Arboriculture 