Management of urban forests for climate change adaptation and mitigation helps to ensure the continuing presence of trees in urban landscapes and the provision of the many benefits they provide.
Around 80% of people in England live in urban areas (Environment Agency, 2023). Climate change and the growth of urban populations exacerbate the environmental challenges of urban living such as elevated temperatures, air pollution, flooding, and biodiversity loss. Urban trees can help to alleviate the impacts of these challenges and are also threatened by them. Without climate-conscious planning and management trees and other green infrastructure can be lost as the built environment grows.
An urban forest comprises all the trees in the urban realm – in public and private spaces, along linear routes and waterways and in amenity areas. An urban forest contributes to green infrastructure and the wider urban ecosystem, and provides many different and extensive benefits to society.
Urban forests provide benefits that help to reduce the effects of climate change.
Urban trees provide cooling by both evaporation and shading. Evaporation from inside leaves (transpiration) or from precipitation held on leaves (interception) cools the surrounding air, and this effect can be felt over large areas. Shading is a localised effect and is important along streets, in parks, and for buildings.
Trees planted on floodplains and in river catchments can help to decrease river flooding by slowing the rate of runoff and allowing more time for rain infiltration into the ground. Trees planted in soft and hard landscapes in urban areas help to reduce surface flooding by similarly intercepting rainfall and increasing infiltration.
Urban trees provide rich habitats for microbes, plants, and animals. Urban woodland can create links between scattered wooded areas in the wider landscape.
Trees can help to encourage active travel such as walking and cycling, reducing the emission of air pollutants. They also affect how air pollution disperses, reducing air flow and trapping pollutants in some situations, and reducing exposure to pollutants in others. Trees absorb air pollutants on their surfaces and through their leaves, contributing to other measures to reduce air pollution.
As urban trees grow they sequester carbon (remove carbon from the atmosphere). Urban trees and the soils they grow in also contribute to the carbon stocks of the urban ecosystem. However, the magnitude of carbon sequestration by trees within an urban area is typically small relative to emissions from urban fossil fuel combustion.
Urban forests are fragmented, have multiple owners and stakeholders, have many meanings and purposes for different people, and are subject to a wide range of pressures. Establishing and maintaining a resilient urban forest happens only with effective planning, management, and cooperation.
Maintaining a healthy and diverse urban forest is critical to sustaining delivery of its benefits and requires a good understanding of the urban forest resource, including tree species and age composition, their suitability for their locations, their spatial distribution, and their health.
The diversity and impact of pests and diseases will increase as our climate changes.
The selection of a planting site determines what attributes a tree must have. It also has a direct impact on which benefits and the quantity of benefits the tree provides. Careful site selection is important to ensure that trees are in the right place for all users of the space.
3. Use UK climate projections and the Climate Matching Tool to estimate what the local future climate will be, and, where practical, to inform choice of species and provenance (also see species and stature selection below).
A diverse urban forest can be more resilient to the impacts of climate change. Historical planting has resulted in avenues or lines of trees of the same species and often of the same cultivar, and dominance of urban forests by a few species. These tree populations are thought to be particularly at risk to pests and diseases.
Over recent decades there has been a tendency to plant small stature trees, especially along streets and in new developments, because these trees require less rooting volume and less crown size management. However, small stature trees provide a lower level of benefits than large trees.
Ecosystem disservices are the negative effects of nature on human wellbeing. Disservices from urban trees include unwanted leaf litter that must be cleaned from streets and buildings, damage to structures by roots or drying of shrinkable clay soils, and blocking of light or views.
Urban trees may need to be removed for various reasons, such as causing damage to buildings, making space for development, or because they are in poor condition. Climate change will increase mortality in some urban tree species. Successfully replacing trees requires funding, time, space, careful species selection, and good establishment practices.
Provision of benefits by newly-planted trees depends on their survival and good growth. There is also a financial and carbon cost associated with growing, transporting, and planting trees, and if establishment is not achieved that expenditure is wasted. More frequent droughts and hotter summers make watering and mulching particularly important in the years after planting.
