I am in my second year of my BA Hons in Garden Design and we are currently studying hard and soft landscaping. We were given an article for further research which I thought was so fascinating that I had to post and share it.
Trees in the Urban Heat Island
Urban Heat Islands were discovered as early as 1810, the main identifying features are an overall increase in temperature over surrounding rural areas, but the most marked increase being in the temperature over night. This occurs because the materials used in buildings have a higher thermal conductivity and heat capacity. Waste heat is also generated through urban usage. Large expanses of tarmac contribute to the urban heat island around the edges of towns and cities. Often the extent of the temperature difference is masked as the recording system for many cities is based in parks – out of the way of normal urban life.
There are issues with this raised temperature:
- The growing season is longer, so a wider range of plants from warmer climates can be grown.
- Change in the localised climate – monthly rainfall is greater downwind of cities, between 48 and 116% mare rainfall, even 20 -40 miles away the rainfall can be 28% greater.
- Wind patterns vary
- Cloud and fog can form in localised areas
- Greater incidence of thunder
- Low air quality – more respiratory disorders
- High death rate in hot periods
There are many ideas about the design of cities, the buildings and the materials used in cities that will reduce the urban heat island effect, in terms of the landscape the use of tree planting is the single most important factor. There is a plan to increase the tree cover in London by 15% by 2025 by planting 2 million trees, an additional 5% more greenspace will be added by innovations like green roofs after that. The benefits of tree planting in urban areas follow
The economic benefits of trees have been understood for a long time. Recently, more of these benefits are becoming quantified. Quantification of the economic benefits of trees helps justify public and private expenditures to maintain them. One of the most obvious examples of economic utility is the deciduous tree planted on the south and west of a building. The shade shelters and cools the building during the summer, but allows the sun to warm it in the winter after the leaves fall. The physical effects of trees–the shade (solar regulation), humidity control, wind control, erosion control, evaporative cooling, sound and visual screening and pollution absorption.
Air pollution reduction
As cities struggle to comply with air quality standards, the ways that trees can help to clean the air should not be overlooked. The most serious pollutants in the urban atmosphere are ozone, nitrogen oxides (NOx), sulphuric oxides (SOx) and particulate pollution. Ground-level ozone, or smog, is created by chemical reactions between NOx and volatile organic compounds (VOCs) in the presence of sunlight. High temperatures increase the rate of this reaction. Vehicle emissions, emissions from industrial facilities, fuel vapours, and chemical solvents are the major sources of NOx and VOCs. Particulate pollution, or particulate matter is made up of microscopic solids or liquid droplets that can be inhaled and retained in lung tissue causing serious health problems. Most particulate pollution begins as smoke or diesel soot and can cause serious health risk to people with heart and lung diseases and irritation to healthy citizens. Trees are an important, cost-effective solution to reducing pollution and improving air quality.
Trees reduce temperatures and smog
With an extensive and healthy urban forest air quality can be drastically improved. Trees help to lower air temperatures and the urban heat island affect in urban areas. This reduction of temperature not only lowers energy use, it also improves air quality, as the formation of ozone is dependent on temperature.
- As temperatures climb, the formation of ozone increases.
- Healthy urban forests decrease temperatures, and reduce the formation of ozone.
- Large shade trees can reduce local ambient temperatures by 3 to 5 °C
- Maximum mid-day temperature reductions due to trees range from 0.04 °C to 0.2 °C per 1% canopy cover increase.
Temperature reduction from shade trees in car parks lowers the amount of evaporative emissions from parked cars. Unshaded car parks can be viewed as miniature heat islands, where temperatures can be even higher than surrounding areas. Tree canopies will reduce air temperatures significantly. Although the bulk of hydrocarbon emissions come from exhaust pipes, up to16% of hydrocarbon emissions are from evaporative emissions that occur when the fuel delivery systems of parked vehicles are heated. These evaporative emissions and the exhaust emissions of the first few minutes of engine operation are sensitive to local microclimate. If cars are shaded in car parks, evaporative emissions from fuel and volatilized plastics will be greatly reduced.
- Cars parked in car parks with 50% canopy cover emit 8% less through evaporative emissions than cars parked in car parks with only 8% canopy cover.
- Due to the positive effects trees have on reducing temperatures and evaporative emissions 50% canopy cover over paved areas is mandatory in Californian car parks.
The volatile components of tarmac evaporate more slowly in shaded areas. The shade not only reduces emissions, but reduces shrinking and cracking so that maintenance intervals can be lengthened.
Active pollutant removal
Trees also reduce pollution by actively removing it from the atmosphere. Leaf stomata, the pores on the leaf surface, take in polluting gases which are then absorbed by water inside the leaf. Some species of trees are more susceptible to the uptake of pollution, which can negatively affect plant growth. Ideally, trees should be selected that take in higher quantities of polluting gases and are resistant to the negative affects they can cause.
A study across the Chicago region determined that trees removed approximately 17 tonnes of carbon monoxide (CO), 93 tonnes of sulphur dioxide (SO2), 98 tonnes of nitrogen dioxide (NO2), and 210 tonnes of ozone (O3) in 1991
Interception of particulate matter
In addition to the uptake of harmful gases, trees also act as filters intercepting airborne particles and reducing the amount of harmful particulate matter. The particles are captured by the surface area of the tree and its foliage. These particles temporarily rest on the surface of the tree, as they can be washed off by rainwater, blown off by high winds, or fall to the ground with a dropped leaf. Although trees are only a temporary host to particulate matter, if they did not exist, the temporarily-housed particulate matter would remain airborne and harmful to humans. Increased tree cover will increase the amount of particulate matter intercepted from the air.
- Large evergreen trees with dense foliage collect the most particulate matter.
- The Chicago study determined that trees removed approximately 234 tonnes of particulate matter less than 10 micrometres in 1991.
- Large healthy trees greater than 75 cm in trunk diameter remove approximately 70 times more air pollution annually (1.4 kg/yr) than small healthy trees less than 10 cm in diameter (0.02 kg/yr).
Biogenic volatile organic compounds
One important thing to consider when assessing the urban forest’s effect on air quality is that trees emit some biogenic volatile organic compounds (BVOCs). These are the chemicals (primarily isoprene and monoterpenes) that make up the essential oils, resins, and other organic compounds that plants use to attract pollinators and repel predators. As mentioned above, VOCs react with nitrogen oxides (NOx) to form ozone. BVOCs account for less than 10% of the total amount of VOCs and BVOCs emitted in urban areas. This means that BVOC emissions from trees can contribute to the formation of ozone. Although their contribution may be small compared with other sources, BVOC emissions could exacerbate a smog problem.
Not all species of trees, however, emit high quantities of BVOCs. The tree species with the highest isoprene emission rates should be planted with caution:
- Liquidambar (Sweetgum)
- Nyssa (Tupelo or Black gum)
- Platanus (Plane)
- Populus (Poplar)
- Quercus (Oak)
- Robinia (Black locust)
- Salix (Willow)
Trees that are well adapted to and thrive in certain environments should not be replaced just because they may be high BVOC emitters. The amount of emissions spent on maintaining a tree that may emit low amounts of BVOCs, but is not well suited to an area, could be considerable and outweigh any possible benefits of low BVOC emission rates.
Trees should not be labeled as polluters because their total benefits on air quality and emissions reduction far outweigh the possible consequences of BVOC emissions on ozone concentrations. Emission of BVOCs increase exponentially with temperature. Therefore, higher emissions will occur at higher temperatures. In desert climates, locally native trees adapted to drought conditions emit significantly less BVOCs than plants native to wet regions. As discussed above, the formation of ozone is also temperature dependent. Thus, the best way to slow the production of ozone and emission of BVOCs is to reduce urban temperatures and the effect of the urban heat island. As suggested earlier, the most effective way to lower temperatures is with an increased canopy cover.
These effects of the urban forest on ozone production have only recently been discovered by the scientific community, so extensive and conclusive research has not yet been conducted. There have been some studies quantifying the effect of BVOC emissions on the formation of ozone, but none have conclusively measured the affect of the urban forest. Important questions remain unanswered. For instance, it is unknown if there are enough chemical reactions between BVOC emissions and NOx to produce harmful amounts of ozone in urban environments. It is therefore, important for cities to be aware that this research is still continuing and conclusions should not be drawn before proper evidence has been collected. New research may resolve these issues.
Trees make little difference to noise levels, it may be that not seeing traffic will change the perception of the noise but a screen has to be 50m wide to screen from noise as leaves are simply not dense enough to screen effectively.
Written by Caroline Jackson, Lecturer at Hadlow College