Dear Mr. Manan,

By forests, do you mean peri-urban forests, large parks (e.g. Central Park), green belts????

In any case, you have a lot of ongoing or finished projects and research programs about urban ecosystem services in which you can find information regarding urban forests benefits. For example:

https://www.iucn.org/regions/europe/our-work/local-and-regional-authorities/projects/urbes

https://naturvation.eu/

Also, some useful literature, apart from that of those projects or programs:

https://www.sciencedirect.com/science/article/abs/pii/S1618866717303485

Article Contribution of Ecosystem Services to Air Quality and Climat...

https://www.fs.fed.us/psw/publications/mcpherson/psw_2016_mcpherson001_livesley.pdf

https://www.routledge.com/Urban-Forests-Ecosystem-Services-and-Management-1st-Edition/Blum/p/book/9781771884259

Hope it is useful,

Pedro L. Lomas

Cities are an integral part of the face of the Earth. Although they occupy only 2% of the land area, they are now home to half of the world's population. The main economic, scientific and cultural potential of society is concentrated in cities, so they play an important role in the economic, political and social life of each country individually and of humanity as a whole.

By 2025, the urban population will be 2/3 of the world's. More than half of the citizens live in cities with a population of more than 500 thousand people, and every year the share of the population living in large cities is growing.

Plants that grow in a large city are real "Spartans". The growth of trees here is very difficult due to environmental pollution. Up to 30 tons of various substances fall annually on 1 km2 of a large city, which is 4-6 times more than in rural areas. Scientists believe that a large proportion of deaths in cities around the world is associated with air pollution.

The main cause of photochemical fog is car exhaust gases. A car emits about 10 g of nitrogen oxide per kilometer. Photochemical fog occurs in polluted air as a result of reactions occurring under the influence of solar radiation.

Due to the harvesting of fallen leaves in autumn and snow in winter, in the cold winter period, urban soils are more emasculated and freeze deeper than in woodlands. All this negatively affects the state of the root system of plants.

But not only the microclimate worsens the life of plants in the city. The most important ecological factor in plant life is water. In cities, plants often lack soil moisture due to its runoff into the sewer network.

This explains the fact that the species composition of trees most often planted along roads and on streets is not too diverse. The main breeds in the middle zone are Linden, poplar, maple, chestnut, birch, larch, ash, Rowan, spruce, oak, and about 30 species of shrubs. The latter are often used to create hedges.

What is the role of green spaces in air purification? In the leaves of the tree, chlorophyll grains absorb carbon dioxide and release oxygen. In natural conditions, in summer, a medium-sized tree releases as much oxygen in 24 hours as is necessary for three people to breathe, and 1 ha of green space absorbs 8 liters of carbon dioxide in 1 hour and releases enough oxygen into the atmosphere to support the life of 30 people. Trees clear carbon dioxide from the ground layer of air about 45 m thick.

Among the various types of trees used for urban landscaping, chestnut has special properties. One adult chestnut tree cleans up to 20 thousand m3 of space from incoming exhaust gases.at the same time, unlike many other trees, chestnut decomposes toxic substances almost without harm to its health.

Poplar is also resistant to air pollution. In terms of the amount of carbon dioxide absorbed and oxygen released, 25-year – old poplar exceeds spruce by 7 times, and in terms of air humidity-almost 10 times. So to improve the air, instead of seven firs (three limes or four pines), you can plant one poplar, which also captures dust well.

The foliage of trees actively captures dust and reduces the concentration of harmful gases, and these properties are manifested in different breeds to different degrees. Leaves of elm and lilac (better than poplar leaves) retain dust well. So, planting of 400 young poplars during the summer season catches up to 340 kg of dust, and elm – 6 times more. Acacia, unpretentious fast-growing rosehip and a number of other plants also have similar properties.

On a hot summer day, rising streams of hot air are formed over the heated asphalt and hot iron roofs of houses, carrying with them the smallest particles of dust that remain in the air for a long time. At the same time, descending air flows occur over a Park located somewhere in the center of the city, because the surface of the leaves is much cooler than asphalt and iron. The dust carried away by these descending streams settles on the leaves of the trees of the Park. One hectare of stands of coniferous trees holds up to 40 tons of dust per year, and deciduous-about 100 tons.

In conditions of high air pollution, there are some changes in the phenology of plants, especially those that grow along highways. There is a reduction in the growing season, the timing of flowering and fruit maturation, the degree of flowering and fruiting, the quality and germination of seeds.

For the comfort provided by transport, for a huge number of cars, we pay for the cleanliness of the air. When 1 liter of fuel is burned in the car engine, 200-400 mg of lead gets into the air. In a year, one car can emit up to 1 kg of this metal into the atmosphere. Increased lead content in vegetables and fruits grown near highways, as well as in the milk of cows fed contaminated grass, is a danger to human health.

Sometimes in the summer you can watch the leaves falling from the trees. The reason for this is the high lead content in the air. Trees are hard to tolerate lead poisoning. The upper threshold of lead concentration for plants has not yet been established. Some plants, such as mosses and larch, absorb it in relatively large quantities, and birch, willow, and aspen – much less. By concentrating lead, plants purify the air. During the growing season, a single tree can accumulate as much lead as it contains in 130 liters of gasoline. A simple calculation shows that at least 10 trees are needed to neutralize the harmful effects of one car.

Green spaces play an important role in noise control. Trees planted between noise sources and residential buildings reduce noise levels by 5-10%. The crowns of deciduous trees absorb up to 26% of the sound energy falling on them. Large woodlands reduce aircraft engine noise levels by 22-56% compared to an open area (at the same distance from the noise source). Even a small layer of snow on the branches of trees increases the absorption of noise.

However, you can get the opposite results if you plant the wrong trees and choose the wrong breeds. For example, planting trees with a dense dense crown along the axis of a street with a busy traffic flow will act as a screen that reflects sound waves towards residential buildings.

Most effectively perform the noise protection functions of planting red elderberry, red oak, and canadian Irga.

Interestingly, the sounds are not absorbed by the leaves of trees. Hitting the trunk, the sound waves break, heading down to the ground, where they are absorbed. The best guardian of silence is considered a fir tree. Even at the most noisy highway, you can live in peace if you protect your home next to green fir trees. And it would be nice to plant chestnuts next to it.

Trees with wide crowns and shrubs planted along sidewalks improve the microclimate of streets.

Trees and shrubs (more than 500 species) emit volatile substances into the air – phytoncides that have the ability to kill microorganisms. Phytoncides, discovered in 1928 by the Soviet scientist B. p. Tokin, have a great impact on the life of plants, speeding up or slowing down their growth and development. Active sources of phytoncides are white acacia, birch, willow, winter and red oaks, spruce, pine, poplar, cherry, etc. It is especially important that phytoncides can kill some pathogens of human and animal diseases. Coniferous forests are harmful for pathogenic microbes. Scientists have found that the coniferous forest has 2 times less bacteria than the deciduous forest. For example, 1 ha of juniper releases about 30 kg of phytoncides per day. Pine phytoncides have a destructive effect on tuberculosis pathogens, and fir, poplar, and oak phytoncides – on diphtheria bacilli. Experiments have shown that in June-July, the phytoncides of common cherry suppress the reproduction of Salmonella, Shigella and inhibit the growth of Staphylococcus, and the phytoncides of Siberian larch suppress the reproduction of Salmonella and inhibit the growth of Shigella.

Tree and shrub plantations weaken the negative effect of winds. But dense planting of green spaces does not perform windproof functions, as it leads to increased turbulence of air flows.

Green spaces during the growing season increase air humidity and stabilize moisture exchange between the earth's surface and the atmosphere. In the shade of the garden on a hot day, the air temperature is 7-8 lower than in the open. If on a summer day the air temperature on the streets is above 30 €8C, then in a Park or square the thermometer shows only 22-24 €8C.

The greatest harm to green spaces in the city is caused by a constant lack of moisture as a result of the predominance of surface runoff over underground. Harmful effects on all types of plantings are caused by soil contamination, especially with heavy metals and salt used in winter to combat ice.

Underground utilities, especially heating and gas pipelines, significantly restrict the planting of green spaces along streets. And drains and sewers themselves are subject to destruction under the influence of tree roots. Therefore, when designing green spaces of streets, it is necessary to take into account the depth of underground communications (the zone of inaccessibility of the root system must be more than 3.4 m).

Trees and shrubs that grow in the city do a great job every day and every hour: they absorb dust and carbon dioxide, produce oxygen, perform sanitary, water protection and noise protection functions, form the microclimate and the peculiar appearance of the city.

The recreational value of green spaces is associated with the organization of optimal recreation. Squares, parks, playgrounds with a variety of plants and compositions, complemented by small architectural forms, decorative water elements (pools, fountains) contribute to the full recreation of the population. Green spaces serve not only as decoration, they are true defenders of people's health.

Article Pilot and Field Studies of Modular Bioretention Tree System ...

Article A review of bioretention components and nutrient removal und...

https://www.nparks.gov.sg/-/media/cuge/ebook/citygreen/cg5/cg5_04.pdf

Apart from air quality improvements as mentioned above, urban forestry also partakes in the improvement of urban stormwater quality.