The Urban Forest - The Structural Resource

Ground Cover:

Ground cover refers to the types of surface or vegetation within each plot. Within Birmingham, the most common ground cover types are grass (27.2%), building (18.6%), tar (13.4%), and herbs (12.2%). Of the surveyed area, 15% of Birmingham is under tree canopy cover, with 11.8% under shrub cover (note that shrubs are also present under tree cover and so these two figures ‘overlap’).

For context, the average canopy cover for the UK is 16%, though 1coastal and rural areas are often lower and peri-urban areas are often higher. The survey also showed that a further 20.7% of land within the plots could (in theory) be planted with trees. Utilising available space to increase tree canopy cover can improve the provision of ecosystem services such as reducing air pollution and increasing carbon sequestration.

Birmingham City Council have committed to using aerial imaging to determine canopy cover over time. This method differs in resolution and sample size to this study and will return different results.

Land Use

Figure 4 shows the average land cover across Birmingham. Surveyed plots indicate that on average Birmingham’s largest land use is residential (37.5%) and transportation (14.8%). Parks and forests (combined) account for 14.7% of land cover across Birmingham. 1.3% of land in Birmingham is vacant (348.1 ha). This land could potentially be repurposed for tree planting or the creation of new green spaces. Should the 348.1 ha of vacant land be turned over to broadleaved woodland creation, this land could accommodate 556,982 trees (at a spacing of 2.5 x 2.5m/tree). Parkland creation (at a spacing of 25m x 25m/tree) could accommodate 5,570 new trees.

Green spaces make up 25% of land cover in Birmingham; that is higher than the average for Inner London (21%).

Maintaining a species rich urban forest is vital in providing resilience to pests & diseases and climate change:

A diverse urban forest can support a range of pollinators and wildlife, whilst enhancing aesthetic value by providing a variety of colours, textures, and shapes throughout the year. Overall, promoting diversity in urban forests leads to healthier, more resilient ecosystems that provide a wide range of benefits to both humans and the environment.

Many native species are not able to thrive in the artificial environments of our landscaped areas, and the effects of climate change will exacerbate the situation. Maintaining a careful balance of native and non-native species within the population will ensure that habitats are protected whilst providing resilience to our ever-changing climate. Figure 5 shows a dominance diversity curve developed by Hubbell. In this graph, the longer and shallower curves indicate forests with higher diversity and fewer species dominating the population.

Although i-Tree Eco does not yet calculate a valuation of biodiversity it does provide an indication of tree species diversity using diversity indexes. This is important because the diversity of species within Birmingham (both native and non-native) will influence how resilient the tree population will be to future changes, for example, by minimising the overall impact of exotic pests, diseases and climate change. These values are provided in Table 3.

Species

Species/

SHANNON

MENHINICK

SIMPSON

EVENNESS

4.20

3.50

2.80

20.50

0.80

Species: is the number of species sampled.
Species/ha: is the average number of species found per hectare of area sampled.
SHANNON: is the Shannon – Wiener diversity index, which assumes that all species within the area have been sampled. It is an indicator of species richness and has a moderate sensitivity to sample size (on this scale, below 1.5 is considered low and over 3.5 is considered high).
MENHINICK: is the Menhinick’s index. It is an indicator of species richness and has a low sensitivity to sample size and therefore may be more appropriate for comparison between cities. Menhinick’s index is simply the number of species divided by the square-root of the total number of individuals. An index close to 1 or above is considered to be good.
SIMPSON: is Simpson’s diversity index. It is an indicator of species dominance and has a low sensitivity to sample size and therefore may be more appropriate for comparisons between land use types.
EVENNESS: is the Shannon diversity index, which assumes that all species within the area have been sampled. It is an indicator of species evenness and has a moderate sensitivity to sample size and therefore land use and/or cities may not be comparable.

According to most metrics, Birmingham has a good level of diversity. Whilst Birmingham is more diverse than a typical temperate forest, the city still has potential to improve diversity to the level of some other cities in the UK. It is not uncommon for cities to rank highly in diversity often due to non-native tree species. In Birmingham 69.2% of trees are a native species. These species are important for biodiversity and the ecology of the landscape; however, non-native trees will become increasingly important in a changing climate.

Species Richness:

The three most common named species are Silver Birch, Sycamore, and Holly. Some trees were identified at genus level only, these have not been included in this species level analysis to avoid mixing metrics, and are instead included in ‘All Other’.

The ten most common species account for 61.4% of the total population. In total, 76 tree species were recorded in the survey. Increased tree diversity has the potential to minimize the impact upon or destruction of species by specific pathogens and diseases as well as from the effects of climate change. However, there can also be an increased risk to the native tree population and surrounding biodiversity.

Birmingham’s urban forest has a variety of species present, with only 1 species exceeding 10% of the total population. With new tree planting, Santamour’s 10:20:30 tree population diversity rule would therefore be achievable in the near future, indicating that the urban forest has potential to be more resilient to pests and diseases. The most prominent threats from present pests and diseases in the West Midlands are Ash Dieback, with threats to the Oak population such as Acute Oak Decline and Oak Processionary Moth.

The range of tree species in Birmingham is good and the area does not rely too heavily on just a few species. Maintaining a broad tree species diversity through planting selection will help ensure the resilience of this urban forest into the future.

Santamour’s 10-20-30 rule of thumb:

This suggests upper limits for a tree population as follows:

Single species - 10%
Single genus - 20%
Single family - 30%

Many old city park and urban tree populations do not adhere to this rule due to historic plantings, but the rule can help inform future plantings.

Dominance:

Numerous benefits derived from trees are directly linked to the amount of healthy leaf surface area that they have.

A high value shows which species are currently delivering the most benefits based on their population and leaf area. These species currently dominate the urban forest structure and are therefore the most important in delivering benefits.

The Dominance Value is calculated by taking into account the leaf area and relative abundance of the species. In Birmingham the most dominant species are Sycamore, English Oak, and Silver Birch due to having a combination of the largest leaf areas and being a common species.

Certain trees have a high dominance value due to their expansive leaf area even though they represent a relatively low proportion of the population - this can be seen in English Oak, Beech, and Lawson Cypress. The opposite can be true for species with a high population but a smaller leaf area - in this example Silver Birch and Holly.

Species	Leaf area (ha)	Dominance Value
Sycamore	2,749	19.8
English Oak	2807	19.2
Silver Birch	1733	17.9
Ash	2111	15.8
Holly	589	10.7
Lime	1302	9.7
Beech	1339	8.3
Wild Cherry	518	6.1
Field Maple	678	5.5
Lawson Cypress	926	5.5

Table 4: List of the ten most dominant tree species in Birmingham.

Urban Forest Structure:

In this survey, trees were sized by their stem diameter at breast height (DBH) at 1.3m. DBH can be considered a proxy for age, bearing in mind species and potential ultimate size and form.

Trees with a DBH of 7-15 cm constitute 27% percent of the tree population of Birmingham’s urban forest. Larger trees have a greater functional value and provide increased benefits (details of functional value and the resulting benefits are discussed later). It has been estimated in previous studies that a 75cm diameter tree can intercept 10 times more air pollution, can store up to 90 times more carbon and contributes up to 100 times more leaf area to the tree canopy than a 15cm tree.

Size class distribution is also an important factor in managing a sustainable tree population. Having a large population of smaller trees is important as this will ensure that there are enough young trees to replace those older specimens that are eventually lost through old age or disease.

Most regions in England only have 10-20% of trees with a DBH that is greater than 30cm*, but in Birmingham it is 35.1% *Trees in Towns II

Where the goal is to continually maintain tree cover within a landscape, a guiding principle is an inverse J-curve of age going from many young to few mature trees. Forests are unique and there is no ‘one size fits all’ target distribution. However, it is noted that Birmingham will benefit from a greater proportion of larger trees as the tree stock matures, if correctly managed.

Biodiversity of the Urban Forest

Biodiversity is important because it provides a wide range of indirect benefits to humans. However, challenges exist in valuing it because it is difficult to identify and measure the passive, non-use values of biodiversity.

The diversity of species within Birmingham (both native and non-native) will influence how resilient the tree population will be to future changes, such as minimizing the overall impact of exotic pests, diseases, and climate change. A diverse treescape is better able to serve as a habitat for a wide range of creatures, and native trees are important as they are better suited to support other native species. Unfortunately, many native species are not able to thrive in the artificial environments of our landscaped areas, and the effects of climate change will exacerbate the situation, therefore non-native species could become increasingly important for the delivery of benefits in Birmingham.

"The conservation of biodiversity is not just about saving a few species, but about preserving the intricate web of life that sustains us all." Dr. Thomas Lovejoy

Origin of Tree Species

The map below shows the original continent of origin of the tree species found in Birmingham. In total, around 86% of the tree population are native to Europe. Of those, it is expected that a smaller percentage are native to the British Isles, although diversity is key to resilience.

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Birmingham i-Tree Report