News From Eight

Why could 25% of London’s land be covered with trees by 2025?


‘A society grows great when old men plant trees whose shade they know they shall never sit in’ – Greek Proverb

Eight Associates now provides BS5837:2012 Tree Surveys, Arboricultural impact Assessments, Arboricultural Method Statement and Arboricultural Clerk of Works. Valuable trees on a development site are not a hurdle to be overcome – early valuation could mean both a smoother journey through planning and an enlarged design which adapts to trees in close proximity. So, why are trees important to the urban environment?

The Mayor of London’s RE:LEAF work is targeting 25% of London’s land has tree cover by 2025. With the majority of London’s trees in gardens and a quarter populating woodlands, it’s important Londoners understand the future of our tree cover, and the role of trees in the adaptation to future climate scenarios.

There is common consensus that summer and winter temperatures will rise, there will be more dramatic events such as floods and storms and sea levels will rise. The main threats to London from a changing climate are:

1. Overheating: London experiences an Urban Heat Island effect, such that during the 2003 heatwaves temperatures in the centre of London were up to 10 degrees higher than outlying rural areas.

2. Flooding: London is prone to tidal, fluvial (from tributaries and rivers), surface (from rainfall), sewer and groundwater flooding. Climate change will increase the probability of flooding through more rainfall, rising sea levels and increased tidal surges.

3. Drought: climate change is expected to lead to more droughts with summers expected to have more consecutive dry days and longer heat waves. This in turn will demand greater need for urban green spaces to be watered

These impacts will alter the growth of trees and other vegetation, the natural processes within soils and the growth of animals dependent on the vegetation and soils. Species will adapt, some more successfully than others, to the new conditions. In maintained environments, such as many urban greenspaces, this will have a major effect on the choice of plant species to grow, aftercare and management techniques. Pests and diseases may be altered in many ways – the types and species of outbreaks, and the frequency and severity, are likely to be modified.

Trees have an important role to play in adapting London to climate change. Urban trees provide a number of ecosystem services through their lifecycle which target the predicted threats:

1. Overheating: Shading reduces the amount of heat absorbed and stored by buildings, evapotranspiration converts liquid water to water vapour and cools the air by using solar energy that would otherwise result in heating of the air, tree canopies slow down winds thereby reducing the amount of heat lost from a home, especially where conductivity is high (e.g., glass windows).

2. Flooding: Trees reduce stormwater runoff by canopies intercepting and holding rain on leaves, branches and bark, increasing infiltration and storage of rainwater through the tree’s root system, and reducing soil erosion by slowing rainfall before it strikes the soil.

3. Carbon storage and sequestration: trees help mitigate climate change by sequestering atmospheric carbon – trees are comprised of around 50% carbon, their roots and stems acting as long-term store of carbon.

Plus trees improve air quality, biodiversity, human health and wellbeing – and increase surrounding property value. These benefits will be achieved through the long term management of a diverse range of species to future proof the city’s green infrastructure network against drought, pests and diseases.