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Tree selection

BY FIONA McWILLIAM
If trees are to do well in our cities and countryside they need to be selected appropriately and nurtured. It is also important to understandthe effect that they will have on their environment and how they will respond to climate change.

Nobody could sensibly object to the claim that trees make places work, look and feel better.

What’s more, according to the Trees & Design Action Group (TDAG) — the independent organisation established in 2007 to facilitate projects ‘promoting the role of the urban forest throughout the UK’ — trees can also help to create conditions for economic success.

Yet Dr Mark Spencer, senior curator of the British and Irish herbarium at the Natural History Museum’s Life Sciences Department, believes there is currently too much ‘fitting in by colour’ when it comes to landscape design in this country, and not enough consideration of the suitability of species and long-term maintenance costs, which should inform which trees are planted and where.

A huge amount of public money is wasted on planting inappropriate trees, he says, and this is often just because they’re too big and/or fast-growing. ‘There’s a huge amount of learning needed for the managing of public space’. The lack of skill and knowledge within some local authorities is extraordinary, he says, and managing parks and parkland is both undervalued and under-skilled, ‘with the core skills of planting and maintenance often lacking’.

According to Emma Hill, a policy consultant with urban tree-planting charity Trees for Cities, up to 25 per cent of newly planted amenity trees fail to establish. ‘The first three to five years are crucial for a tree in terms of watering while they are establishing their root system, and keeping an eye on stakes, ties and guards,’ she says, ‘including making sure these are removed as soon as they are no longer necessary.’

Trees for Cities has a dedicated watering team which spends the first three growing seasons watering trees and checking on their condition. ‘We also put a lot of our resources into engaging the community with the trees in order that they feel a sense of ownership and become our eyes and ears,’ Hill says. ‘If we can reach a tree in trouble quickly we have more of a chance of saving it.’

Where and how a tree is planted are both crucial to its long-term success. Unsurprisingly, Trees for Cities spends a lot of time surveying a potential site prior to choosing which species it plants and where.

Surveying takes account of a site’s spatial constraints, namely its underground utilities, cellars, foundations; overhead fixtures such as electricity and telecommunication cables, CCTV and balconies; and adjacent fixtures, such as site furniture, highways, footways and existing vegetation.

How people and traffic move around a proposed planting site is also an important consideration, as are physical attributes, such as soil pH and structure, aspect, light and shade, drainage, and water availability.

There is no standardised consistent approach to planting and after care, says Hill, which is why the British Standards Institution has proposed a new British Standard — BS8545 Code of Practice for trees from nursery to landscape — which will encompass tree production, despatch, storage, transplanting and maintenance until a tree is independent in the landscape. The standard will, Hill says, ‘recognise all these currently disjointed elements as a continuous process’.

Mark Spencer urges specifiers of trees, including landscape architects, to take a good look at the resources a planting scheme will have access to in the long term, for maintenance, and to realise once and for all that what he calls ‘the colouring-by-number approach’ wastes huge amounts of money. And while appearance is obviously important in urban and suburban settings, he adds, landscape architects need also to examine and understand what else trees are providing, in terms of heat and carbon mitigation, for example, and in catching pollutants such as dust, or reducing noise.

Selecting trees for reducing noise requires the planting of very dense evergreens such as the Leyland cypress. Yet even trees that are thickly vegetated to the ground, and at least 8–10 metres thick, only reduce noise by about 25 per cent. Artificial barriers are, it seems, far more effective.

There is a general assumption that all trees species have a positive impact on air quality, but this is far from straightforward. Lancaster University’s Trees & Sustainable Urban Air Quality report notes that while trees can remove pollutants, particularly nitrogen dioxide and particles from the air, they can also emit VOCs which, in combination with man-made oxides of nitrogen, can contribute to the production of other pollutants, especially ozone and particles that can damage human health.

It claims that trees that do not emit the most reactive VOCs, but do have a large leaf surface area, have the best effect on air quality. Trees suited to urban environments which remove the most pollutants without contributing to the formation of new ones include Norway maple, field maple, ash and silver birch.

Oaks, poplars and willows can have detrimental effects on air quality downwind, the report states, so care needs to be taken when planting these species in very large numbers. It concludes though that overall, the effects on air quality of very large-scale planting of almost all tree species in cities would be positive.

The Lancaster report also ranks different tree species by their ability to store carbon. The researchers did not include carbon sequestration in its tree scores for air quality, ‘given the relatively small amount of carbon stored in trees’. (For example, the total amount of carbon stored in the West Midland tree population is said to be equivalent to only 6 per cent of the carbon dioxide emitted to the atmosphere in the region in a single year). Nevertheless, larch, silver birth and poplar score well here, while English oak, field maple, hazel and holly are less impressive.

Specifiers should not ignore changing climatic conditions. According to the Forestry Commission, tree choice should be guided by management objectives and site conditions, and its (and the Environment Agency’s) new decision tool Ecological Site Classification ESC3 helps specifiers to review options likely to be sustainable in the future climate. It uses meteorological and soils data to help specifiers choose from 57 tree species; they can then test their selection against climate change scenarios for 2050 and 2080.

The Right Trees for a Changing Climate tree selector tool hosted on the Greater London Authority’s website has a database of 300 species and cultivars suitable for a variety of urban conditions. The underlying database was prepared by Forest Research in collaboration with the Greater London Authority, the Forestry Commission, Natural England, the Tree Council and the Royal Horticultural Society. The database lists the characteristics of tree species that will be suitable for the predicted climatic conditions that London and other urban areas will experience for the rest of this century.

Hand-in-hand with changing climate are the emerging disease and pest issues described on pages 39–41. Yet landscape architects can and should play an important role in minimising potential problems in the future. Alan Simson, reader in urban forestry and landscape architecture at Leeds Metropolitan University, and a member of the Landscape Institute’s bio-security sub-committee, is eager to encourage specifiers to adopt the Santamour rules for urban planting, set out by the late Frank S Santamour Jr, in his 2002 paper Trees for Urban Planting: Diversity, Uniformity and Common Sense.

The Santamour rules dictate that one should plant no more than 10 per cent of any species, no more than 20 per cent of any genus and no more than 30 per cent of any family at any one site. Santamour, who worked at the US National Arboretum, argued that a broader diversity of trees is needed in our urban landscapes ‘to guard against the possibility of large-scale devastation by both native and introduced insect and disease pests’. Fungi usually attack families, Simson adds, ‘so we need to spread the load’.

Independent arboricultural consultant and TDAG member Jon Heuch advises specifiers to consider both resilience and diversity: ‘A diverse range of trees and shrubs is more likely to be able to cope with specific pests and if different sizes they would likely provide a more varied habitat for wildlife,’ he says. ‘It’s not good news for uniform planting stock for avenues and other linear features as there is a higher risk that a single pathogen could affect all of them.’

For congested space, Heuch suggests that planting fastigiate specimens (with erect, almost parallel branches tapering towards the top) may be useful, ranging from hornbeam (common) to oak (less common) and ‘providing columnar structures with less chance of a spreading monster’.

We need to think strategically too, he adds, and recognise that long-lived trees may not be the right way to enhance green infrastructure. ‘We might need to accept that our trees won’t last as long as in nature,’ he says, ‘and shorter life spans are likely. This is the reality, but people don’t want to accept it.’

Further information
— TDAG Trees in Townscape, A Guide for Decision Makers; www.tdag.org.uk
— Ecological Site Classification (ESC) 3 tool; http://bit.ly/XFIKRn
— Right trees for a Changing Climate database Ash dieback advice; www.forestry.gov.uk/ forestry/INFD-8UDM6S
Possible solution to fungal disease 
— Noise barriers; http://bit.ly/U6hAgb
— Urban Air Quality; http://bit.ly/e7OKGS

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