The heritage of urban trees

Trees are an intrinsic part of our cultural and natural heritage, and in the urban environment they are part of the history of how places have developed.

Richmond Park is London’s largest site of special scientific interest. Some of the oaks seen here predate the enclosure of the park in 1637. (Photo: Rob Cowan).

Trees are crucial to our future in the context of the climate and biodiversity emergencies. They create healthy environments, encourage access to greenspace and support wellbeing. The government has set out ambitious plans for increasing tree cover in the England Trees Action Plan 2021–24. Urban areas have a part to play through incorporating trees along streets and integrating greenspaces into new developments.

The mature trees in our urban parks and other green spaces, lining our streets and avenues, were mostly planted from the mid-19th century onwards. Beginning in the 1830s and 1840s, there were calls for the systematic planting of street trees, in part inspired by tree-lined boulevards in continental Europe [1]. At the same time, the movement to provide public parks was gathering momentum, with objectives to improve public health and recreation. These parks and other green spaces, such as arboreta and garden cemeteries, would become green lungs in polluted industrial towns and cities.

The older street trees with which we are most familiar are largely London planes, limes and horse chestnuts. By the time of the creation of garden cities and suburbs in the 20th century, a greater variety of street trees began to be planted. Trees in parks, arboreta, cemeteries and in public and private gardens include diverse species and cultivars. They often relate to trends in planting at different periods, have symbolic associations (in cemeteries in particular) or form part of botanical collections. Trees in parks, green spaces and private gardens make a substantial contribution to our urban forests.

Many trees in urban areas survive from, and can help tell the story of, previous pre-urban landscapes. They may be located in commons or in public parks that developed from earlier landscape parks, or they may have been retained as street trees in areas later developed for housing. The ancient sweet chestnuts in Greenwich Park, survivors of avenues created for Charles II in the 1660s, are one famous example. These trees, collectively part of our living, green heritage, are in some cases as old or older than the historic buildings that surround them. They are also as irreplaceable. Historic trees should be considered in the context of their cultural and design interest and the many environmental benefits that they provide.

Trees planted in 19th-century parks and green spaces were valued for their aesthetic contribution and for the improvements they brought to public health. They still deliver all of this, as do the many trees planted since. But today, we also understand their wider contribution to environmental quality and management. Helping to filter particulates, trees make an important contribution to air quality. They provide shade, helping reduce the impact of high temperatures and urban-heat-island effects. They take up and lock in carbon for the duration of their lifespan. They help reduce flooding by improving soil structures below the hard surfaces of the urban environment. They benefit urban nature, providing food and shelter. They have cultural and heritage value, and they support our physical and mental wellbeing. Together, these benefits can be considered as part of the ecosystem services that trees provide. A study by Forest Research and Defra estimated that the value of trees outside of woodlands, both urban and rural, for the provision of ecosystem services to be approximately £3.8 billion [2].

In the context of rising average and peak temperatures, trees are key, alongside other green infrastructure, to mitigating overheating in urban areas. A recent study found that botanic gardens and parks provided a cooling benefit of nearly 5º C to surrounding areas. Street trees were found to provide nearly 4º C of cooling [3]. A recent Environmental Audit Committee inquiry http://bc-url.com/ctx80a into heat resilience and sustainable cooling recognised the contribution that trees make to passive cooling as part of nature-based solutions to climate change. The committee recommends in its report that the government adopts a range of measures to protect and expand green spaces, particularly in urban areas, and that these must prioritise areas that are disadvantaged and currently under-served by green space.

The greatest benefits are generally provided by mature, large-canopied trees. We need to look after the trees we have, which provide such a wide and important range of benefits already. We need to consider succession: the trees that will take the place of existing trees when they reach the end of their lives. And we need to increase overall tree cover in towns and cities.

When planning for and planting new trees, the adage of ‘right tree, right place’ is important. Careful consideration must be given to planting locations for street trees within urban areas where there may be physical constraints such as underground services. Consideration should also be given to the historic environment. Trees, and particular types of tree, are integral to the character of historic parks and other designed green spaces. There may be considerations such as views and historic character when considering the setting of historic buildings and the character of conservation areas. In some locations there may be archaeological sensitivities to consider.

It is essential to consider existing site conditions and the potential impact of climate change on the resilience of tree species, alongside the interrelated issue of pest and disease threats to tree health. Some tree species may become less tolerant to their situation and more vulnerable to pests and diseases as peak temperatures increase, and through drought and increased rainfall, or a combination of these factors. Increasing species diversity can help to ensure that our urban forests are resilient for the future. It is important, however, not to rely on diversity alone to provide resilience, but to understand species choice in terms of the context in which they will be planted. Historic botanic gardens and arboreta are a valuable resource for studying how tree species and cultivars are faring in different areas and in a changing climate.

Evidence-based tools and publications can assist with tree species choice. The Trees in Design Action Group (TDAG) publication ‘Tree Species Selection for Green Infrastructure: a guide for specifiers’ is a valuable resource. TDAG provides information on a wide range of topics related to urban trees, including recordings of past seminars, on its website. The Green Infrastructure Framework for England also provides principles and standards for trees in green infrastructure.

How trees are planted and managed when becoming established is key to ensuring their long-term health and ability to deliver benefits. While initial investment is required to make sure that urban trees are planted in a way that supports healthy growth and development, the benefits provided over the lifetime of a tree far outweigh the initial investment. Trees also need continual care and maintenance to make sure that they are in optimal health to deliver the benefits they have to offer.

It is important for heritage, landscape, arboriculture and ecological specialists to work together. An integrated approach to managing existing trees and new tree planting is needed, both in the context of local strategies and with relation to individual sites.

References

1 Johnston, Mark (2017) Street Trees in Britain: a history, Windgatherer Press, Oxford
2 www.forestresearch.gov.uk/research/valuing-non-woodlandtrees/
3 www.sciencedirect.com/science/article/pii/S2666675824000262

This article originally appeared as 'Everyone needs trees' in the Institute of Historic Building Conservation’s (IHBC’s) Context 180, published in June 2024. It was written by Erika Diaz Petersen, principal national landscape adviser at Historic England.

--Institute of Historic Building Conservation