Birmingham i-Tree Report

Air Pollution Removal:

Poor air quality is a particular problem in many urban areas and along road networks. Air pollution caused by human activity has become a problem since the beginning of the industrial revolution. With the increase in population and industrialization, and the use of transport based on fossil fuels, large quantities of pollutants are produced. The problems caused by poor air quality are well known, ranging from human health impacts to building damage. Trees significantly contribute to improving air quality by reducing air temperature (thereby lowering ozone levels), directly removing pollutants from the air, absorbing them through the leaf surfaces and by intercepting particulate matter (e.g., smoke, pollen, aerosols created in the atmosphere and dusts). They also indirectly reduce energy consumption in buildings, leading to lower air pollutant emissions from power plants.

Particulate matter <2.5 microns (PM2.5) can be incredibly damaging to health, as these particulates are small enough to enter the bloodstream. As such, they have superseded PM10 in importance and policies increasingly focus on reducing PM2.5.

As well as reducing ozone levels, some tree species also emit the volatile organic compounds (VOCs) that lead to ozone production in the atmosphere. The i-Tree Eco software accounts for both reduction and production of VOCs within its algorithms, and the overall effect of Birmingham's trees is to reduce ozone through evaporative cooling; however, this is not valued in this report as there is no UK Social Damage Cost for this pollutant.

Greater tree cover, air pollution concentrations, and leaf area are the main factors influencing pollution filtration and therefore increasing tree planting has been shown to make further improvements in air quality. Furthermore, because filtering capacity is closely linked to leaf area, it is generally the trees with larger canopy potential that provide the most benefits.

It is estimated that trees and shrubs combined remove 80.4 metric tonnes of air pollution, including nitrogen dioxide (NO2), particulate matter less than 2.5 microns (PM2.5), and sulfur dioxide (SO2) per year with an associated value of approximately £6.4 million (based on UK social damage costs published by DEFRA). Total pollution removal per hectare in Birmingham is equivalent to 0.003 tonnes per hectare per year.

Avoided Run-Off

Surface run-off can be a cause for concern in many areas as it can contribute to pollution in streams, wetlands, rivers, lakes, and oceans, as well as adding to flood risks and thereby exacerbating the impacts of Climate Change. During precipitation events, a portion of the precipitation will be intercepted by vegetation (trees and shrubs) while a further portion reaches the ground. Precipitation that reaches the ground and does not infiltrate into the soil becomes surface run-off. Within an urban area, the large extent of impervious surfaces increases the amount of run-off. However, trees are effective at reducing this. Trees intercept precipitation, while their root systems promote infiltration and storage in the soil. Interception slows down rainwater reaching the ground, and some water will be evaporated off without ever touching the ground. The trees of Birmingham help to reduce run-off by an estimated 481,000 cubic meters a year with an associated value of £776,000. English Oak trees intercept the most water, removing a total of 53,300 m3 of water per year, a service worth £86,000. English Oak trees have an expansive canopy to capture/intercept rainfall and are the fourth highest proportion of trees within Birmingham.

Carbon Storage and Sequestration:

Trees can help mitigate climate change by sequestering atmospheric carbon. Since about 50% of wood by dry weight is comprised of carbon, tree stems and roots can store up carbon for decades or even centuries. Over the lifetime of a tree, several tonnes of atmospheric carbon dioxide can be absorbed.

The gross sequestration of Birmingham's trees is approximately 12,800 tonnes of carbon per year (approximately 0.5 tonnes per hectare per year). The value of the carbon sequestered annually is estimated at £12.5 million per year. This value will continue to increase as the trees grow.

Carbon sequestration and storage is a key part of achieving any net-zero target. In 2021, Birmingham city produced a total of 3,865 kt CO2e emissions* (equivalent to approximately 1,050,000 tonnes of carbon), meaning that sequestration by trees account for 1.22% of the total annual emissions. *Department for Energy Security and Net Zero, 2023

The Carbon Balance

The Climate Change Survey 2020 found that 9 out of 10 councils have declared a climate emergency, with approximately 80% setting official targets to become carbon neutral. The West Midlands Combined Authority declared a Climate Emergency in 2019, setting a vision of being carbon neutral by 2041. Birmingham City’s carbon production has been falling quickly over the past few years. However, it still produces around 1.05 million tonnes of carbon each year (2.5 times the carbon storage and 82 times the annual sequestration rate of the trees in Birmingham). The carbon emitted equals approximately 0.92 tonnes per person in Birmingham. This comes from a range of sources, the highest of which are Domestic (36%), Transport (30%), and Industry (18%).

Carbon offsetting is the process by which an organization can prove that through action, the carbon which they produce is subsequently captured and stored for a sufficiently long period to mitigate any environmental damages caused by the initial carbon emission. Invariably, urban forestry can only contribute to the carbon balance - attempting carbon neutrality or ‘net-zero’ goals through urban forestry alone would be highly unadvisable, although it is important to recognize the role it can play in the carbon balance next to other benefits detailed in this report. Increasing carbon sequestration through urban forestry is a long-term solution; it is always recommended that carbon emissions should be reduced and other solutions to sequester and store carbon should be sought alongside urban forestry.

The largest trees sequester the most carbon - gaining in sequestration rate and total carbon stored as they grow to maturity. Depending on the growth rates of species, this can take a long time - it is therefore advisable to consider urban forests on timelines that span decades and centuries.

Care and maintenance should be fundamental to any tree planting program, with best practice followed by the present custodians of Birmingham’s trees and resources strategically deployed to ensure resources are preserved.

As trees and woodlands age, carbon saturation is reached. This is the point when the rate of carbon sequestration becomes balanced with the rate of carbon released through decomposition of organic matter and respiration. As carbon saturation is reached, carbon sequestration will stabilize or decline. The utilization of felled timber can lock up carbon, which would otherwise be returned to the atmosphere, while new tree planting can ensure sequestration can continue.