Funding will be required to evaluate the economic, ecological and environmental value of the Borough of Wirral’s urban forest using the i-Tree Eco survey protocol and model (www.itreetools.org). The project will provide valuable information about the structure, health and future of the urban forest within Wirral and will calculate the value of ecosystem services such as; pollution mitigation, carbon storage and sequestration, and reduction of storm water run-off. The aim of the project is to provide the community and planners with a clear idea of the benefits of managing existing trees as a resource rather than a risk, and the added potential of incorporating urban forests in new structures. The study will be undertaken with the help of volunteers from the local community to provide first-hand information about the value of trees directly to the local community and stakeholders. Support from sponsors will therefore link the sponsor with the local community. It will also improve their environmental footprint and contribute to their corporate social responsibility. The community can identify clearly that the sponsors are contributing towards meeting their environmental obligations.
Air pollution is a public health emergency and poses the biggest environmental health risk to date (WHO, 2016). It is linked to respiratory and cardiovascular illness and mortality (Ferranti et al. 2017), with a more severe effect on vulnerable groups, such as children, the elderly, and people with pre-existing health conditions (Defra, 2017). The damaging effects of air pollution on human health and productivity also have an associated economic cost, which has been valued at around £16 bn per annum for the UK, with a reduction in life expectancy of six months per person (Defra, 2014).
A mounting body of evidence suggests that air pollution is linked to asthma attacks/wheeze (Gasana et al. 2012; Guarnieri & Balmes, 2014; Susan et al. 2018). The UK is amongst the countries with the highest prevalence of asthma in the world and has one of the worst asthma death rates in Europe (Asthma.org.uk). Healthcare expenditure for asthma costs over £1 billion in England and Wales, and over £130 million in Scotland (Mukherjee et al. 2014). As of 2016/17 Wirral had 22,278 patients registered with asthma, 6.49% of Wirral’s population, which is slightly higher than England’s average of 5.94% (Wirral Compendium of Statistics, 2018).
Particulate matter (PM), especially PM2.5, is linked with health effects such as respiratory and cardiovascular morbidity (aggravation of asthma, respiratory symptoms, increase in hospital admissions), mortality from cardiovascular and respiratory diseases and from lung cancer (WHO, 2013). Gowers et al. (2014) estimated local mortality burdens associated with particulate matter and found that of 3,543 deaths (25+) in Wirral in 2010, 166 of them were attributable to anthropogenic PM air pollution. Whilst both the Environmental Health JSNA Report (Wirral Intelligence Service, 2018) and the Wirral Air Quality Annual Status Report (2019) state that PM2.5 levels in Wirral are below the levels set by the European Union and the World Health Organisation, there is actually only 1 monitoring station for PM2.5 located in Victoria Park, Tranmere. This is acknowledged as an urban background site that may not be representative of roadside readings.
Wirral Council launched the Wirral Clean Air Campaign in June 2019 to raise awareness of air pollution and to encourage people to make changes to their behaviour. Other measures to address air quality are smoke control areas, highway maintenance, reducing amount of traffic idling at junctions, air quality meetings, park and ride, electric vehicle charging points and traffic management. The council have also been running the Green Air School Project (GASP), in conjunction with Liverpool John Moores University, to help primary schools measure PM2.5 around the school perimeter. The schools then planted shrubs to act as a barrier to poor air quality. Aside from the GASP project, there is no mention whatsoever of trees and the benefits they provide to communities with regards to air quality.
With this study, we propose to conduct a borough-wide survey of Wirral’s urban forest, using the i-Tree Eco model, to determine how much pollution is removed by Wirral’s trees each year, and to place a monetary value on this ecosystem service. A recent i-Tree survey completed for Southampton’s urban forest reported that Southampton’s trees removed an estimated 90,000 kg of airborne pollutants each year, worth around £534,000 in social damage costs (Mutch et al. 2017). Other i-Tree projects also report yearly removal of pollutants; London’s urban forest removes 2,241,000 kg (worth £126,000,000) (Rogers et al. 2015), Glasgow’s urban forest removes 283,000 kg (worth £1,400,000) (Rumble et al. 2015), Edinburgh’s urban forest removes 195,000 kg (worth £575,313) (Doick et al. 2017) and Wrexham’s urban forest removes 66,000 kg (worth £800,000) (Rumble et al. 2014). This Wirral i-Tree project aims to improve understanding of the current state of Wirral’s urban forest, and to raise awareness of the utility value of urban trees, which are an often-undervalued environmental resource.
Should this research proposal obtain funding it is hoped that this i-Tree study will provide evidence to inform policy and help local authorities to manage their tree resource as a benefit to be enhanced rather than a risk to be mitigated (Hand & Doick, 2018). Trees and green spaces provide many additional benefits alongside improvements in air quality, such as carbon storage and sequestration (Nowak & Crane, 2001), reduction in surface water runoff (Armson et al. 2013), mitigation of the urban heat island (UHI) effect (Doick et al. 2014; Shishegar, 2014), wildlife habitat (Barth et al. 2015), social cohesion (Peters et al. 2010), and stress reduction and improved mental health (WHO, 2016). A growing body of work in urban forestry identifies urban forests as having a beneficial impact on mental well-being, along with alleviation of depression, anxiety and mood disorders (Bakolis et al. 2018; US Dept of Agriculture, Forest Service, 2018). Additionally, an i-Tree survey will identify issues such as resiliency to climate change and risks to the future sustainability of Wirral’s urban forest (for example a large proportion of the tree population could be wiped out by pests and diseases if overly reliant on only a few tree species) (Hand & Doick, 2018).
This project will utilise the i-Tree Eco model to assess Wirral’s urban forest. I-Tree Eco is a model developed by the USDA Forest Service designed to measure a range of ecosystem services (i.e. pollution removal, carbon sequestration) and to quantify urban forest structure and value to communities, and to date has been applied to UK urban forests in Bridgend, Cardiff, Edinburgh, Glasgow, London, Petersfield, Southampton, Torbay, Tawe Catchment and Wrexham (https://www.forestresearch.gov.uk/research/i-tree-eco/i-tree-eco-projects-completed/) (Table 1) .
Table 1. Results from some of the larger i-Tree projects undertaken in the UK (Treeconomics, 2015; Rumble et al. 2015; Doick et al. 2017; Treeconomics, 2011; Mutch et al. 2017; Rumble et al. 2014)
The study will use the randomised grid method, which involves laying a grid over the study area and picking a random point in each grid square where a plot of size 0.04 ha will be located (Fig 1). Grid squares present at the edge of the study area that do not contain at least 50% of the grid square area are not included. There will be at least 200 plots, giving a plot density of 1 plot per every 56 Ha. The standardised field collection method will be applied to each plot and is outlined in the i-Tree Eco manual (V 6.0) (i-Tree, 2018). Information recorded at the study plots will include: type of land use, percentage distribution of cover present, percentage of the plot that could have trees planted in it, information about the trees (number and species, size, health, amount of light exposure the canopy receives and amount of impermeable surface under the tree) and information about shrubs (<7cm girth and over 1m height).
20 groups of volunteers will undergo a one-day training course run by Treeconomics. A map of each plot will be provided, with the centre of the plot clearly marked. The estimated average time per plot is 2 hours 15 mins (minimum of 2 people). The average distance between plots is around 0.8 miles. Ideally each group of volunteers will be able to commit to surveying a minimum of 10 plots as quality control checks will need to be done on 10% of each group’s plots.
All trees along Princes Boulevard (Higher Bebington, Wirral) were recorded in September 2017 (Fig 2). Each species was identified and measurements were taken of tree stem diameter at breast height (DBH) (1.4 m above the ground), total tree height, crown size, crown health, crown light exposure, direction to closest building and distance to building. Latitude and longitude were also taken for each tree. Standard data collection options for a complete inventory project are outlined in the i-Tree manual (V 6.0) (i-Tree, 2018).
The data is fed into the i-Tree Eco model and several outputs are calculated. Weather data was used from the year 2013, as pollution data is available for that year, and was taken from Tiree weather station (Scotland), the closest station with the most complete record, and the most similar weather data.
Figure 2. Location of trees on Princes Boulevard.
Princes Boulevard has 66 trees, the 3 most common species being Common lime, Hawthorn and London plane, with 9 species overall (Fig 3). The DBH ranged from 6 cm to 99 cm, with 67% of trees having a DBH greater than 50 cm (Fig 4).
The trees of Princes Boulevard remove an estimated 41.79 pounds of air pollution (ozone (O3), carbon monoxide (CO), nitrogen dioxide (NO2), particulate matter less than 2.5 microns (PM2.5) and sulphide dioxide (SO2)) per year, which has an associated value of £86.40.
They are storing 62.67 tons of carbon (C), equivalent to annual C emissions from 44 automobiles, with an associated value of £3, 630. Each year they sequester 2.131 tons of C from the atmosphere, worth £123.
The trees intercept 2,342 cubic feet of water every year, with an associated value of £101.
They have a structural value of £204,000 (the cost of replacing a tree with a similar tree).
Figure 3. Composition of trees in Princes Boulevard.
Figure 4. Number of trees by diameter class
Princes Boulevard is just one street 650 m long but is contributing essential ecosystem services, worth £3940.40, to the residents. Given that Wirral has over 2,400 streets (streetmapof.co.uk) it is not unreasonable to assume that Wirral’s urban forest, which also includes trees in parks, on private property and in cemeteries, is providing a range of ecosystem services worth a considerable amount. The value of the urban forest and the services they provide are often overlooked, so it is hoped that by valuing the quantifiable services provided we can (1) raise awareness of the value of Wirral’s urban trees, especially their role in pollution removal and climate regulation, (2) provide evidence to inform urban tree management and (3) justify investment in their management.
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