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25 November 2019

Urban trees: so much more than Carbon sequestration

In July, Science published the article ‘The global tree restoration potential’ which suggested another 0.9 billion hectares of tree canopy cover can be planted in rural areas, which would sequester around 205 gigatonnes of carbon. The aim of course, would be to combat climate change by reducing global levels of CO2.

But urban areas can also play a huge role in the planting project. In this environment, the benefits of trees are not simply restricted to carbon sequestration. When incorporated thoughtfully in designing and planning of buildings and cities, trees can impact heating and cooling across various scales, alter wind patterns and reduce pollution. This article will explain briefly these three benefits, although there are numerous others which are associated with trees in urban environments.

Passive cooling

The major alternative use of trees is for passive cooling, provided by their shade, especially during the summer months. Larger, deciduous trees can intercept large amounts of sunlight that would otherwise have fallen on the building, ensuring that solar heating of the building is reduced. Therefore, the demand for cooling within the building drops, as air and surface temperatures are limited. A key behind reducing cooling requirements is lowering the energy peak demand that buildings experience in early afternoon. If optimised correctly, trees can reduce energy consumed for cooling by 25%. Ensuring maximised energy losses involves planting on westward building facades, reducing afternoon solar radiation that strikes the building. This is one of the key design features behind a new office block in Düsseldorf, which will plant five miles worth of hedges along its tiered facades.

The passive cooling of trees has a substantial impact on the use of air conditioning (AC), and its significant detrimental effects on the environment. One of the negative impacts of using AC is its contribution to the urban heat island (UHI), which coincidently can also be combated by planting trees on a city-wide scale. The UHI phenomenon is caused by the lower albedo of roads and buildings, and their capacity to retain heat, leading to higher urban-rural temperature gradients, especially at night. However, trees shading and evaporative abilities can reduce urban temperatures by up to 5?C. Los Angeles has realised this potential, and aims to reduce city temperatures by 1.5?C by 2050, which will include planting 90,000 trees. However, this issue is not constrained to warmer locations, with the impact of climate change, urban areas across the world will begin to suffer stronger UHI. Although authorities can implement greening projects across public areas, privately owned land can still play a major role in determining the success in reducing the UHI.

Passive heating

For those fortunate enough to lie in the deciduous forest biome, which includes Eastern North America, Japan, Eastern China and Western and Central Europe trees can also improve passive heating of buildings. The once leaf laden branches begin to abscise as temperatures drop in autumn, coinciding with the increase in energy demand for heating. Trees begin to provide less shading, allowing for greater solar radiation to pass through its canopy allowing for increased absorption by the building. Taking advantage of the seasonal cycle would provide financial savings for real estate owners in terms of energy, whilst promoting a sustainable, environmentally friendly heating method.

Beyond trees’ influence on direct solar heating, their ability to form a wind shield has a dramatic effect on the heating requirements of buildings. It was calculated that around a third of a building’s energy consumption is due to wind pressure. The convective cooling of wind increases the energy demand for heating, especially during winter months. But, if considered during designing and planning of buildings, trees may play a vital role in passive heating. In order to gain the most from this benefit, trees must be planted on the dominant windward side of the building. If maximised correctly, trees can reduce heating energy consumption by 18% and heating costs by 15%. This ability of trees highlights the need to consider passive heating as an option, rather than looking to technology for all the solutions.

Wind Patterns

Buildings influence wind patterns on both a microscale (building scale) and mesoscale (city scale) and can increase the speed at which it flows at ground level, especially in street canyons. However, introducing trees into design and planning considerations for both buildings and cities can reduce speeds and increase pedestrian comfort. Tree canopies have the ability to reduce wind speeds at street level, especially where downdraughts from high-rise buildings make walking near their base uncomfortable, and in some instances dangerous. Considering this benefit on a larger scale is key, as cities look to ensure the safety of pedestrians and cyclists are maintained. Take the City of London as an example, it has developed tougher rules regarding building designs which may perpetuate wind speeds. Here, the use of trees may ensure compromise in designs might not be as extreme due to the vegetation’s capability of reducing any indirect adverse effects of the building. Future developments must ensure they look to enhance pedestrian comfort and importantly their safety, with the deployment of trees.

Pollution

Although the discussion around the scale of impact trees have on urban pollution is ongoing, it is certain that they do reduce pollutant levels in comparison to vegetation devoid urban environments. Rates of  particulate matter removal, absorption and dry deposition of other pollutants varies across species.  However, in general all trees can absorb some pollutants, including the key, carbon dioxide, along with nitrous oxides, ammonia and sulphur dioxide. Ensuring this benefit of trees is maximised it is vital to first understand the dominant pollution issue and tailor the species planted in order to see the largest decreases. The reduction in air pollution is enhanced by ensuring the location of trees does not interrupt the airflow, which also disperse pollutants. Architects of the Vertical Forest in Milan took this to the next level, incorporating trees throughout the design, planting on balconies from the ground to top floor. Their aims included reducing the dust which infiltrated apartments and minimising noise pollution.

Real estate must begin to take this ability of trees far more seriously. Reducing pollution near buildings is particularly impactful on occupant health and wellbeing, and can also have a much wider effect at a city level. Never has real estate been able to shape the health of a city by such simple means, extending their economic, environmental and social impacts across a mesoscale.

Conclusion

These four examples alone highlight the true capabilities of trees, and how their influence extends far further than carbon sequestration. Building designs and city planning should now go beyond using trees for simple aesthetics and trend setting, and consider the vegetation for its true worth in developing a sustainable and natural urban environment.